Viruses Flashcards

Question

The term “virus” is derived from the Latin word vīrus referring to

Answer 1

poison and other noxious liquids

Answer 2

❖ The branch of science which deals with the study of viruses is called virology. ❖ The term “virus” is derived from the Latin word vīrus referring to poison and other noxious liquids. ❖Viruses can infect all types of life forms including multicellular organisms to unicellularorganisms. ❖Many of human, animal and plant diseases are caused by them. Even the recently appeared pandemic “COVID-19 ” is also caused by virus. ❖ In 1884, the French microbiologist Charles Chamberland invented a filter (known today as the Chamberland filter or the Pasteur-Chamberland filter) with pores smaller than bacteria. Thus, he could pass a solution containing bacteria through the filter and completely remove them. ❖ In 1892, the Russian biologist Dmitri Ivanovsky first Dmitri Ivanovsky discoveredvirus in an infected tobacco plant. C. Chamberland ❖ In 1898, the Dutch microbiologist Martinus Beijerinck observed that this “agents” multiplied only in cells that were dividing. He called it a contagium vivum fluidum (soluble living germ) and re-introduced the word virus. F. Twort M. Beijerinck M. Stanley ❖ The first images of viruses were obtained upon the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll. ❖ The Tobacco Mosaic Virus was the first to be crystallized by Wendell Meredith Stanley in 1935 and its structure could therefore be explained in detail. ❖ By the end of the 19th century, viruses defined in terms of their infectivity, ability to be filtered, and their requirement for living hosts. Viruses had been grown only in plants and animals. ❖ In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages. ❖ The second half of the 20th century was the golden age of virus discovery and most of the over 2,000 recognized species of animal, plant, and bacterial viruses were discovered during these years. ❖ Microbiologist Félix d'Herelle described viruses that, when added to bacteria on an agar plate, would produce areas of dead bacteria.

Answer 3

❖ Viruses are a cellular, non-cytoplasmic infectious agents. Therefore, a unit of virus is referred to as ‘a virus particle’ ratherthan ‘a virus cell’. ❖ They are smallerthan bacteria and can pass through bacteriological filter. ❖ They are consisting mainly of a nucleic acid surrounded by a protein envelope called capsid. ❖ They are devoid of the sophisticated enzymatic and biosynthetic machinery essential for independent activities of cellular life. Therefore, they can grow only inside suitable living cells. ❖ These virusesdo not grow, neitherrespire nor metabolize, butthey reproduce. ❖ Viruses may even be crystallized much like molecules although some kind of viruses can only be purified but not crystallized . ❖ A virus cannot contain both DNA and RNA. Therefore, virus is called either ‘DNA virus’ or ‘RNA virus’ depending on whetherit contains the nucleic acid DNA or RNA. ❖ Viruses are transmissible from disease to healthyorganisms. ❖ All viruses are obligate parasites and can multiply only within the living host cells. ❖ Viruses are host specific that they infect only a single species and definite cells of the host. ❖ They are highly resistantto germicides and extremes of physical conditions. ❖ Viruses are called connective link between living and non living

Answer 4

They possess genetic material i.e., either DNA or RNA. ✓ They can undergo mutation. ✓ They show irritability. ✓ They are capable to reproduce ✓ They can increase their number. ✓ They can be transmitted from one host to another. ✓ They react to heat, chemicals and radiations. ✓ These develop resistantto antibiotics.

Answer 5

✓They can be crystallized. ✓ They are inert outside the host. ✓ Lack cell membrane and cell wall. ✓ Lack of cytoplasm and organelles. ✓ They do not show cell division. ✓ They can't grow in size, shape. ✓ They don't possess sort of nutrients. ✓ They don't respire and excrete. ✓ Don't undergo their own metabolism. ✓ Lack any energy producing system.

Answer 6

Holmes (1948) included all viruses in a single order Virales which were divided into three suborders: * Phagineae : This sub-order includes viruses infecting bacteria i.e., bacteriophage. * Phytophagineae : It includes viruses infecting plants. * Zoophagineae : It includes viruses infecting animals

Answer 7

: Composed of a single type of capsomer stacked around a central axis to form a helical structure, which may have a central cavity, or hollow tube. E.g: TMV

Answer 8

❖ Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication. ❖ Viruses can be placed in one of the seven following groups: Group I ds-DNA viruses (e.g. Adenoviruses,Herpesviruses, etc) Group II ss-DNA viruses (Parvoviruses) Group III ds- RNA viruses (e.g. Reoviruses) Group IV Positive-sense ss- RNA viruses (Coronaviruses,Picornaviruses, etc) Group V Negative-sensess-RNAviruses (e.g. Orthomyxoviruses,Rhabdoviruses) Group VI Reverse transcribing diploidss-RNA viruses (e.g. Retroviruses) Group VII Reverse transcribing circulards-DNA viruses (e.g. Hepadnaviruses) ds=double-stranded, single-stranded=ss, double-stranded=ds, Group I & II are DNA virus, group III, IV & V are RNA virus, GroupVI & VII are Reverse transcribing virus

Answer 9

*: Most animal viruses are icosahedral or near-spherical with icosahedral symmetry. E.g: Adenovirus

Answer 10

This is an isosahedron elongated along one axis and is a common arrangement of the heads of bacteriophages.

Answer 11

: Some virus envelop themselves in a modified outer lipid bilayer known as a viral envelope. E.g: HIV

Answer 12

These viruses possess a capsid that is neither purely helical nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall. E.g: Bacteriophage

Answer 13

* Viruses display a wide diversity of sizes. * In general, viruses are much smaller than bacteria. * Most viruses that have been studied have a diameter between 20 and 350 nanometres. * They are smaller than bacteria. * Some are slightly larger than protein and nucleic acid molecules * Some are about of the same size (small pox virus) as the smallest bacterium and some virus (virus of lymphogranuloma, 300-400 um) are slightly larger than the smallest bacterium.

Answer 14

* Envelope protein: Enveloped viruses contain glycoprotein which differ from virus to virus. * Nucleocapside protein: Viral capsids are made up totally of protein of identical subunits (promoters). E.g: capsids contain single type of protein in TMV. * Core protein: Protein found in the nucleic acid is known as core protein. * Viral enzyme: In animal viruses especially in the enveloped viruses, many virion specific enzymes have been characterized. E.g: RNase, reverse transcriptase in retrovirus.

Answer 15

❖ Lipid provide flexibility to the shape. ❖ The spikes attached to the outer surface of the envelope are made up of glycoproteins

Answer 16

A substantial amount of carbohydrate specified by rather host cell (arbovirus) or viral genome (vaccinia virus) is found in viral envelope. For example galactose, mannose, glucose, glucosamine, galactosamine are found in influenza virus, parainfluenza virus .

Answer 17

Viruses contain either DNA or RNA fortheir genetic information. ❖ Viruses containing DNA are called Deoxyviruses, whereas, having RNA called Riboviruses. ❖ In general, all plant viruses have ss-RNA. ❖ Animal viruses have either single or (rarely) ds-RNA or ds-DNA. ❖ Bacterial viruses contain mostly ds-DNA but can also have ss-DNA or RNA. ❖ Insect viruses contain RNA and only a few have DNA. ❖ DNA of some bacterial and animal viruses is circular but in others it is like RNA. ❖ DNA viruses cause human diseases, such as chickenpox, hepatitis B, and some venereal diseases, like herpes and genital warts. ❖ Mutations in RNA viruses occur more frequently than in DNA viruses. ❖ This causes them to change and adapt more rapidly to their host. ❖ Human diseases caused by RNA viruses include hepatitis C, measles, and rabies.

Answer 18

❖ Viral populations do not grow through cell division, because they are acellular. ❖ Virus use the machinery and metabolism of a host cell to produce multiple copies of themselves. ❖ During the process of viral replication, a virus induces a living host cell to synthesize the essential components forthe synthesis of new viral particles. ❖ The particles are then assembled into the correct structure, and the newly formed virions escape from the cell to infect other cells. ❖ The host cell is forced to rapidly produce thousands of identical copies of the original virus. ❖ Replication between viruses is varied and depends on the type of genes involved. ❖ Most DNA viruses assemble in the nucleus; ❖ Most RNA viruses develop solely in cytoplasm.

Answer 19

* Attachment * Penetration * Uncoating * Replication * Assembly * Release

Answer 20

❖ Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. ❖ This specificity determines the host range and type of host cell of a virus. ❖ For example, HIV infects a limited range of human leucocytes. ❖ This is because its surface protein, gp120, specifically interacts with the CD4 molecule—a chemokine receptor— which is most commonly found on the surface of CD4+ T-Cells. ❖ This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. ❖ Attachment to the receptor can induce the viral envelope protein to undergo changes that result in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow the virus to enter.

Answer 21

❖ In this process viral capsid is removed: This may be by degradation by viral enzymes or host enzymes or by simple dissociation. ❖ the end-result is the releasing of the viral genomicnucleic acid.

Answer 22

❖ Virions enter the host cell through receptormediated endocytosis or membrane fusion. This is often called viral entry. ❖ The infection of plant and fungal cells is different from that of animal cells. ❖ Plants have a rigid cell wall made of cellulose, and fungi one of chitin, so most viruses can get inside these cells only after trauma to the cell wall. ❖ However, nearly all plant viruses (such as tobacco mosaic virus) can also move directly from cell to cell, in the form of singlestranded nucleoprotein complexes, through pores called plasmodesmata. ❖ Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. However, since bacterial cell walls are much less thick than plant cell walls due to their much smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell across the cell wall, while the viral capsid remains outside.

Answer 23

❖ It involves synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive sense RNA viruses), viral protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by early or regulatory proteinexpression. ❖ This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins.

Answer 24

❖ Following the structure-mediated self-assembly of the virus particles, some modification of the proteins often occurs. ❖ Viruses such as HIV, modification occurs after the virus has been released fromthe host cell.

Answer 25

❖ Viruses can be released from the host cell by lysis, a process that kills the cell by bursting its membrane and cell wall. ❖ This is a feature of many bacterial and some animal viruses and called lytic cycle. ❖ Some viruses undergo a lysogenic cycle. ❖ In lysogenic cycle, viral genome is incorporated by genetic recombination into a specific place in the host's chromosome. ❖ The viral genome is then known as a "provirus" or, in the case of bacteriophages a "prophage". ❖ Whenever the host divides, the viral genome is also replicated. ❖ The viral genome is mostly silent within the host. ❖ At some point, the provirus or prophage may give rise to active virus, which may lyse the host cells. ❖ Enveloped viruses (e.g., HIV) typically are released from the host cell by budding. ❖ During this process the virus acquires its envelope, which is a modified piece of the host's plasma or other, internal membrane.

Answer 26

❖ Viruses are known to infect both plant cells, animal cells and bacteria. ❖ Viruses are obligate intracellular parasites they must develop direct methods of transmission, between hosts, in order to survive. ❖ The mobility of animals increases the mechanisms of viral transmission. ❖ The plants remain immobile and thus viruses must rely on environmental factors to be transmitted between hosts

Answer 27

* Mechanical transmission by rubbing leaves together, injecting plant extract, by action of animals etc. * Vegetative and graft transmission through rhizomes, bulbs, corns, tubers etc. * Pollen transmission; when pollens consisting for viruses fall on stigma of female plants, they germinate and eventually facilitate the virus to infect the ovules of plants. * Seed transmission; very rare * Nematode transmission * Fungal transmission * Insect vector transmission * Dodder transmission; dodder are the trailer or climber parasitic plant which grow forming bridge between two plant

Answer 28

❖ In the plants, the cell wall forms a tough barrier between the intracellular components and the extracellular environment, which has to be penetrated. ❖ The plant viruses relying on the wind and soil to transmit seeds as well as vectors. ❖ Vectors either transmit the virus propagative transmission, which results in an amplification of the virus by replication. ❖ Common vectors include Bacteria, Fungi, Nematodes, arthropods and arachnids. ❖ Furthermore, human intervention, including grafting and experimental mechanical damage, physically damages the cell wall, contributes to the array of transmission routes. ❖ The virus commonly uses these methods to be passed from one host to another. ❖ However, the virus is dependent upon physical damage, generated naturally by the wind and feeding of vectors or by human intervention

Answer 29

❖ Viral infections often develop into systemic infections as a means of transmission. ❖The virus often infects many tissues, if not the whole plant. ❖ There are a variety of methods the virus can use to spread throughout the organism ❖ The most common route use the vascular system, known as the xylem and phloem, and the plasmodesmata, which interconnect adjacent cells. ❖ The common mechanism involve by different virus is expressing proteins which coat the virus and interact with the structure of the plasmodesmata. ❖ The array of proteins expressed by the different viruses may act differently but all achieve a similar goal, passage between adjacent cells.

Answer 30

❖ Plant viruses are viruses that affect plants and obligate intracellular parasites. ❖ Plant viruses can be pathogenic to higher plants. ❖ Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. ❖ They rarely have an envelope. ❖ The great majority have an RNA genome, which is usually small and single stranded (ss), but some viruses have double-stranded (ds) RNA, ssDNA or dsDNA genomes. ❖ Tobacco mosaic virus (TMV), the first virus to be discovered. ❖ Plant viruses are grouped into 73 genera and 49 families. However, these figures relate only to cultivated plants, which represent only a tiny fraction of the total number of plant species. ❖ Viruses in wild plants have been relatively little studied. ❖ To transmit from one plant to another and from one plant cell to another, plant viruses must use strategies that are usually different from animal viruses. ❖ Plants do not move, and so plant-to-plant transmission usually involves vectors (such as insects). ❖ Plant cells are surrounded by solid cell walls, therefore transport through plasmodesmata is the preferred path for virions to move between plant cells. ❖ Plants have specialized mechanisms for transporting mRNAs through plasmodesmata, and these mechanisms are thought to be used by RNA viruses to spread from one cell to another. ❖ Plant defenses against viral infection include, among other measures, the use of siRNA in response to dsRNA. ❖ Most plant viruses encode a protein to suppress this response. ❖ Plants also reduce transport through plasmodesmata in response to injury

Answer 31

❖ Through sap: Viruses can be spread by direct transfer of sap (E.g., TMV, potato viruses and cucumber mosaic viruses )by contact of a wounded plant with a healthy one. Such contact may occur: 1. By damage caused by tools 2. By hands 3. By naturally 4. By an animal feeding on the plant. ❖ Insect: Plant viruses transmitted by a vector, most often insects (E.g., leafhoppers).Class Rhabdoviridae, has been proposed to actually be insect viruses that have evolved to replicate in plants. The chosen insect vector of a plant virus will often be the determining factorin that virus's host range. Depending on the way, plant viruses are classified as: 1. Non-persistent: Viruses become attached to the distal tip of the stylet of the insect and on the next plant it feeds on, it inoculates it with the virus. 2. Semi-persistent: Virus entering the foregut of the insect. 3. Persistent: Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands. There are two sub-classes of persistent viruses: a. Propagative: Propagative viruses are able to replicate in both the plant and the insect (and may have originally been insect viruses). b. Circulative: Must escape the insect gut and spread to neighboring organs to reach the salivary glands for transmission. ❖ Nematodes: Soil-borne nematodes also transmit viruses. They acquire and transmit them by feeding on infected roots. The virions attach to the stylet (feeding organ) or to the gut when they feed on an infected plant and can then detach during later feeding to infect other plants. E.g., Tobacco Ringspot Virus and Tobacco Rattle Virus. ❖ Plasmodiophorids: A number of virus genera are transmitted, both persistently and nonpersistently, by soil borne zoosporic protozoa. These protozoa are not phytopathogenic themselves, but parasitic. Transmission of the virus takes place when they become associated with the plant roots. Examples Polymyxa graminis, and Polymyxa betae which transmits Beet necrotic yellow vein virus. Plasmodiophorids also create wounds in the plant's rootthrough which other viruses can enter. ❖ Seed and pollen borne viruses: In this mode of transmission, the seed is infected in the generative cells and the virus is maintained in the germ cells and sometimes, butless often, in the seed coat. E.g., Bean common mosaic virus. When the growth and development of plants is delayed because of situations like unfavorable weather, there is an increase in the amount of virus infections in seeds. Little is known about the mechanisms involved in the transmission of plant viruses via seeds, although it is known that it is environmentally influenced and that seed transmission occurs because of a direct invasion of the embryo * via the ovule * by an indirect route with an attack on the embryo * by infected gametes. ❖ Direct plant-to-human transmission: The virus may be transmitted through the infected human. This is a very rare and highly unlikely event as, to enter a cell and replicate, a virus must bind to a receptor on its surface, and a plant virus would be highly unlikely to recognize a receptor on a human cell. E.g., Pepper Mild Mottle Virus (PMMoV) may have moved on to infect humans

Answer 32

❖ Plant viruses are extremely small and can only be observed under an electron microscope. ❖ The structure of a virus is given by its coat of proteins, which surround the viral genome. ❖ Assembly of viral particles takes place spontaneously. ❖ Over 50% of known plant viruses are rod-shaped (flexuous or rigid). ❖ They are usually between 300–500 nm lengths with a diameter of 15–20 nm. ❖ Protein subunits can be placed around the circumference of a circle to form a disc. ❖ In the presence of the viral genome, the discs are stacked, then a tube is created with room for the nucleic acid genome in the middle. ❖ The second most common structure amongst plant viruses are isometric particles. ❖ They are 25–50 nm in diameter. In cases when there is only a single coat protein, the basic structure consists of 60 T subunits, where T is an integer. ❖ Some viruses may have 2 coat proteins that associate to form an icosahedral shaped particle. ❖ There are three genera of Geminiviridae that consist of particles that are like two isometric particles stuck together. ❖ A very small number of plant viruses have, in addition to their coat proteins, a lipid envelope. ❖ This is derived from the plant cell membrane as the virus particle buds off from the cell.

Answer 33

rhabdoviruses. At least nine types of rhabdovirus cause economically important diseases in species including salmon, pike, perch, sea bass, carp and cod.

Answer 34

Hepatovirus, picornaviridae Human Fecal-oral Hepatitis

Answer 35

Viruses are simple and acellular infectious agents. ❑ They can infect all types of life forms, from multicellular organisms to unicellular organisms. ❑ Viruses are microscopic obligate cellular parasites, generally much smaller than bacteria. They lack the capacity to thrive and reproduce outside of a host body. ❑ Regressive Hypothesis, Escaped Gene Theory and Co-evolution Hypothesis explain the origin of viruses. ❑ The branch of science which deals with the study of viruses is called virology. ❑ The term “virus” is derived from the Latin word vīrus referring to poison. ❑ In 1892, Ivanovsky first discovered virus in an infected tobacco plant. ❑ Martinus Beijerinckcalled introduced the word virus. ❑ The Tobacco Mosaic Virus was the firstto be crystallized by Stanley in 1935. ❑ Félix d'Herelle described viruses that, when added to bacteria on an agar plate, would produce areas of dead bacteria. ❑ Viruses of all shapes and sizes consist of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope. ❑ They are devoid of the sophisticated enzymatic and biosynthetic machinery essential for independent activities of cellular life. ❑ Viruses are classified into five groups based on morphology. ❑ Many viruses attach to their host cells to facilitate penetration of the cell membrane, allowing their replication inside the cell Non-enveloped viruses can be more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses. ❑ The virus core contains the small single- or double-stranded genome that encodes the proteins that the virus cannot get from the host cell. ❑ Viral populations do not grow through cell division, because they are acellular. ❑ Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. ❑ A virus cannot contain both DNA and RNA. Therefore, virus is called either ‘DNA virus’ or ‘RNA virus’. ❑ Viruses are called connective link between living and non living. ❑ The main purpose of isolation and purification of viruses is to isolate and identify the virus for clinical specimen and research work. ❑ Viruses show lytic or lysogenic mode of life cycle. ❑ Transmission of plant viruses may carried through: mechanically, vegetative and graft transmission, pollen transmission, seed transmission, nematode transmission, fungal transmission, insect etc. ❑ The definition of a disease is an illness or sickness with specific, well-defined symptoms that affects a person, plant or animal. ❑ The diseases may spared through: food, water, air, infection, life style etc. KEY POINTS OF THE LECTURE ❑ Most plant viruses are rod-shaped and rarely have an envelop

Answer 36

❑ Most plant viruses are rod-shaped and rarely have an envelope. ❑ The protein discs forming a tube surrounding the viral genome; isometric particles are another common structure in plant virus. ❑ Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV), Barley yellow dwarf virus (BYDV) , Tomato spotted wilt virus (TSWV), Prunus necrotic ring spot virus (PNRV), Potato spindle tuber viroid (PSTV), Tomato yellow leaf curl virus (TYLCV) etc are some example of plant viruses. ❑ Genetic host resistance, control the vectors, cultural practices, chemical applications and biological control, government regulatory measures etc are some control measurements of the plant viral diseases. ❑ Animal viruses may be divided into two class: 1. Vertebrates and 2. Invertebrates. ❑ The viruses of vertebrates are causes infections of humans and animals and are called clinical virology and veterinary virology, respectively. ❑ Humans cannot be infected by plant or insect viruses, but they are susceptible to infections with viruses from other vertebrates, called viral zoonotic infections. ❑ Viruses are important pathogens of livestock and cause diseases such as foot-and-mouth disease and bluetongue. ❑ Many viruses,including caliciviruses, herpesviruses, adenoviruses and parvoviruses, circulate in marine mammal populations. ❑ Prevention of environmental contamination, control of intermediate host, vectors and reservoirs, isolation of sick animals, quarantine for newly purchased animal, vaccination etc are some important control measurements of the animal viral diseases.

Answer 37

❑ Animal virus: Any virus capable of infecting one or moree animal species. ❑ Assembly: The gathering and replication of viruses within a cell by using the metabolism of the host organism. ❑ Attachment: The condition where the capsid proteins of the virus bind to certain receptors of the host organism. ❑ Bacteriophage: Any virus that infects and replicates within bacteria or archaea. ❑ Capsid: The outer shell of protein that encloses and protects the genetic material of a virus. ❑ Capsomere: A subunit of the viral capsid which self-assembles with other capsomeres to form the capsid. ❑ Dalton (Da): A unit of length frequently used to describe the size of a virus or viral particle. ❑ Envelope: A lipid casing that surrounds the capsid that covers a virus. A viral envelope assists the virus in infiltrating the cells of the host organism. ❑ Gene Expression: An activity where information from a gene is made into functional gene material. ❑ Genome Replication: The reproduction of genetic material, particularly that in the structure of DNA. ❑ Icosahedral: Having the symmetry of an icosahedron. ❑ Kilobase (kb): One kilobase is equal to 1000 base pairs. ❑ Latent Infection: A viral infection that exists in dormancy and does not exhibit symptoms. ❑ Maturation: The phase during replication at which a virus becomes infectious. ❑ mRNA: A form of ribonucleic acid which carries copied genetic information from DNA to the cell ribosome. ❑ Neucleocapsid: The composition of a virus that includes the DNA, RNA, and the capsid protein cover. ❑ Penetration: The process of the virus entering the cell of the host organism, causing infection.

Answer 38

Receptor: A specific type of molecule found on a cell membrane that a virus is able to attach to. ❑ Release: The process of the death of a host cell that discharges a virus. ❑ Replication: Any of the various processes by which a virus reproduces. ❑ Uncoating: A condition when the protein capsid of the virus is unsheathed due to enzymes of the cells of the host organism. ❑ Vector: Insects, such as mosquitoes or ticks, that carry disease from one organism to another. ❑ Viral disease: Any disease that occurs when an organism's body is invaded by infectious viral particles of one or more pathogenic viruses which attach to, enter, and parasitize susceptible cells. ❑ Viral envelope: A lipid casing present in some viruses which surrounds the capsid and helps to penetrate the host's cell wall. ❑ Virion: A singular, stable particle that is the independent form in which a virus exists while not inside an infected cell or in the process of infecting a cell. Virions are the products of a completed viral replication cycle; upon release from the infected cell, they are fully capable of infecting other cells of the same type. ❑ Virions: A virus particle, which invades the cells of a host organism, causing infection. ❑ Virology: The study of viruses and virus-like agents, which seeks to understand and explain their structure, classification, evolution, and mechanisms of infection, as well as the diseases they cause, techniques to isolate and culture them, and their use in research and therapy. Virology is often considered a subfield of microbiology or of medical science. ❑ Virus Attachment Protein: A specific protein found on a virus in charge of fixating to the receptor. ❑ Virus attachment protein: Any protein which helps to facilitate the binding of a virus to a receptor on a host cell

Answer 39

inside living cells

Answer 40

the complete infectious virus particle

Answer 41

Vector: viral genome that is engineered to serve as a tool to replicate and express genes.

Answer 42

* Invasions, wars, and immigrations can introduce viral diseases to different populations * Some viral infections have had major global effects: * Influenza type A is believed to be responsible for global flu outbreaks in 1918, 1957 and 1968 * 16th century Spanish explorers brought small pox, measles and influenza viruses to the New World * While flu is caused by influenza A and B viruses, a type of coronavirus called SARS-CoV-2 was responsible for COVID-19 global outbreaks in recent year

Answer 43

* Vaccines against viruses enhance the immune response of the host * Immune cells are exposed to viral proteins or deactivated virus * Immune system creates specific memories * Immune system is enhanced and “ready” to defend against the virus * Scientists are investigating methods that use viruses for treatment of diseases such as cancer and Alzheimer's disease

Answer 44

* Some viruses have enzymes inside the virion. All ss- RNA viruses with negative polarity have the enzyme transcriptase ( RNA dependent RNA polymerase) inside virions. * Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase.

Answer 45

* An icosahedral is a polygon with 12 vertices (corner), 20 facet (sides) and 30 edges. * Each facet is an equilateral triangle. * Icosahedral capsid is the most stable and found in human pathogenic virus eg. Adenovirus, Picornavirus, Papovavirus, herpes virus etc. * Icosahedral capsid are of two types; Pentagon; Pentagonal capsomere at the vertices Hexagon; Hexagonal capsomere at the vertice

Answer 46

* The capsomere and nucleic acid are wined together to form helical or spiral tubelike structure. * Most of the helical viruses are enveloped and all are RNA viruses. * The typical virus with helical symmetry is tobacco mosaic virus (TMV), which is a RNA virus with 2130 identical capsomeres arranged in a heli

Answer 47

* Some virus are more complex, being composed of several separate capsomere with separate shape and symmetry. * They do not have either icosahedral or helical symmetry due to complexity of their capsid structure. Eg. Pox virus, Bacteriophage. Binal symmetry: it is a type of complex symmetry * Some viruses such as T-phage (T2,T4 etc) have compex symmetry including head and tail * The most complicated virus in terms of structure are some bacteriophage which possess icosahedral head and helical tail. Such structure is called binal symmetry

Answer 48

Virus contains either DNA or RNA as genetic material but not both. Virus which contains DNA as genetic material are called DNA virus and those containing RNA are called RNA virus. * Unlike other living cell where ds DNA is always a genetic material, a viral genome may consist of linear or circular ds DNA, single stranded DNA, ss linear RNA or ds linear RNA. * Examples; Reo virus is a RNA virus which contains ds RNA genome. Parvovirus contains ss DNA, Papovavirus contains ds circular DNA as genetic material

Answer 49

a. the cell DNA directs the production of new viruses

Answer 50

Answer: C) Protein Explanation: The outer covering of a virus is called the capsid, which is made of protein subunits called capsomeres. The capsid protects the viral genetic material (DNA or RNA) and helps in attachment to host cells. Some viruses also have an additional lipid envelope, but the main structural component of the viral "head" (capsid) is protein. ✔ Since the viral outer structure is primarily protein-based, option C is correct.

Answer 51

Answer: D) Viruses reproduce only within living cells. Explanation: Viruses are obligate intracellular parasites, meaning they can only reproduce inside a host cell. Because they hijack the host's cellular machinery, destroying the virus often means damaging the host's own cells. Bacteria, on the other hand, can be targeted with antibiotics that disrupt bacterial structures (e.g., cell walls, ribosomes) without harming human cells. Antiviral drugs are harder to develop because they must selectively inhibit viral replication without killing the host cell. ✔ Since viruses replicate only inside living cells, they are much harder to cure, making option D correct. Why the Other Options Are Incorrect: A) Viruses are smaller than bacteria → Incorrect Size does not determine difficulty of treatment. Some small bacteria (e.g., Mycoplasma) can be treated with antibiotics, while large viruses (e.g., Poxviruses) are still hard to cure. B) Viruses are non-living → Incorrect While viruses are considered non-living outside a host, this does not explain why they are harder to treat. The real challenge is their intracellular replication. C) Viruses cannot be killed using sterile techniques → Incorrect Sterilization (e.g., heat, UV, disinfectants) can kill viruses outside the body, but this does not help treat viral infections inside the body. ✔ Final Answer: D (Viruses reproduce only within living cells).

Answer 52

Lungs – Causing respiratory infections like pneumonia and bronchitis. Example: Influenza virus, SARS-CoV-2 (COVID-19), Respiratory Syncytial Virus (RSV). Liver – Leading to hepatitis and liver cancer. Example: Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV). Central Nervous System (CNS) – Causing neurological disorders. Example: Rabies virus, Poliovirus, Herpes Simplex Virus (HSV), West Nile Virus. Intestines – Leading to gastrointestinal infections and diarrhea. Example: Rotavirus, Norovirus, Adenovirus.

Answer 53

Certain viruses are oncogenic, meaning they can cause cancer by integrating their genetic material into host cells and triggering uncontrolled cell division. About 20% of all human cancers are linked to viral infections. Examples include: Human Papillomavirus (HPV) – Causes cervical, anal, and throat cancers. Hepatitis B and C Viruses (HBV & HCV) – Cause liver cancer (hepatocellular carcinoma). Epstein-Barr Virus (EBV) – Associated with Burkitt’s lymphoma and nasopharyngeal carcinoma. Human T-cell Leukemia Virus (HTLV-1) – Causes adult T-cell leukemia.

Answer 54

Viruses are a major cause of child mortality in low-income regions, particularly due to respiratory and gastrointestinal diseases. The lack of access to vaccines, clean water, and proper medical care worsens the situation. Respiratory tract infections: Pneumonia caused by Influenza virus, RSV, Measles virus kills many children. Gastrointestinal infections: Diarrheal diseases caused by Rotavirus, Norovirus, Adenovirus result in severe dehydration, a leading cause of death in children under five.

Answer 55

Vaccination (e.g., HPV vaccine, Hepatitis B vaccine, Rotavirus vaccine). Improved sanitation and clean water to prevent gastrointestinal infections. Better healthcare access for early diagnosis and treatment of viral infections. Antiviral treatments (e.g., for HIV, HBV, HCV) to reduce cancer risk and mortality.

Answer 56

containing particles in the oceans are viruses, making them the most abundant biological entities in marine environments. 15 times more abundant than Bacteria and Archaea, which were previously thought to be the most dominant life forms in the ocean.

Answer 57

Viruses kill 20-40% of marine microbes daily, which includes bacteria, archaea, and even plankton. This massive viral-induced mortality is known as the Viral Shunt, where viruses lyse (burst) microbial cells, releasing their organic content into the surrounding water.

Answer 58

When marine microbes are killed by viruses, their cellular contents—including carbon, nitrogen, and phosphorus—are released as dissolved organic matter (DOM). This DOM is then consumed by phytoplankton and other microorganisms, fueling the base of the ocean’s food chain.

Answer 59

Phytoplankton are microscopic marine plants that perform photosynthesis, producing 50% of the world's oxygen and forming the foundation of marine food webs. By recycling nutrients, viral activity supports phytoplankton growth, indirectly sustaining higher trophic levels like fish, whales, and seabirds.

Answer 60

Viruses influence global carbon cycles by controlling microbial populations. Viral lysis releases carbon into the ocean, where it can either be: Recycled in surface waters, supporting new microbial life. Sequestered in deep waters, impacting long-term carbon storage and climate regulation.

Answer 61

ERVs and Evolution Some ERVs may have contributed to immune system evolution. Others may regulate gene expression, acting as genetic "switches." Scientists believe ERVs played a role in shaping mammalian and primate evolution.

Answer 62

Human Immunodeficiency Virus (HIV) causes AIDS by attacking the immune system, particularly CD4+ T cells, leading to immune suppression. Hepatitis G Virus (GBV-C) is a virus that infects humans but does not cause significant disease on its own. Beneficial Effect: Studies show that HIV-infected individuals who also have persistent GBV-C infection experience slower disease progression and a lower risk of AIDS-related death. Possible Mechanisms: Cytokine Modulation: GBV-C might alter immune signaling, reducing inflammation and immune exhaustion. CD4+ T Cell Protection: GBV-C may prevent HIV from binding to its target cells. Interference with HIV Replication: It may block HIV from efficiently replicating inside host cells.

Answer 63

Murine herpesvirus is a type of latent virus that remains in the host without causing active disease. Listeria monocytogenes is a bacterial pathogen that causes foodborne infections and severe illness. Yersinia pestis is the bacterium responsible for the plague (Black Death). Beneficial Effect: Mice that are latently infected with certain herpesviruses show increased resistance to bacterial infections. Possible Mechanisms: Immune System Priming: The latent virus may keep the immune system in a heightened state of alert, making it better prepared to fight bacterial invaders. Macrophage Activation: Herpesviruses may stimulate macrophages, the immune cells that engulf and destroy bacteria. Interferon Production: Persistent viral infection may lead to continuous production of interferons, molecules that help fight bacterial infections.

Answer 64

Viral interference occurs when one virus affects the ability of another virus or pathogen to establish an infection or cause disease. Cross-protection can happen when an immune response triggered by one pathogen protects against another. Other Examples: Flavivirus Cross-Protection: Dengue virus infection can sometimes reduce the severity of Zika virus infection (both belong to the Flavivirus family). Influenza and Rhinovirus: In some cases, rhinovirus (common cold) infections can temporarily reduce the risk of influenza, possibly by blocking viral entry or stimulating immune responses.

Answer 65

ERVs are ancient viral sequences embedded in human DNA. They originated from retroviruses, which insert their genetic material into the host genome. Unlike active viruses, most ERVs are "fossils" and no longer infectious due to mutations and gene silencing. ERVs make up 5-8% of human DNA, far more than the 1-2% occupied by protein-coding genes How Did They Get into Our DNA? Millions of years ago, retroviruses infected germ cells (sperm or egg cells). The viral genetic material integrated into the human genome. These changes were passed down from generation to generation. Over time, mutations and regulatory mechanisms silenced most ERVs.

Answer 66

Some ERV elements remain active and are suspected of contributing to diseases, including: Neurological diseases: ERVs are linked to conditions like multiple sclerosis (MS) and schizophrenia. Autoimmune disorders: Some ERV proteins may trigger immune system overactivation. Cancer: ERVs have been found in melanoma, breast cancer, and other tumors, possibly contributing to tumor development.

Answer 67

Bacteriophages (or phages) are viruses that infect and kill bacteria. Their study played a crucial role in shaping modern genetics and molecular biology. Key Contributions of Bacteriophage Research Discovery of DNA as the Genetic Material The Hershey-Chase Experiment (1952) used bacteriophages to show that DNA, not protein, carries genetic information. This finding laid the groundwork for genetics and biotechnology. Understanding Gene Regulation The Lac Operon Model (studied using phages) led to the discovery of how genes turn on and off. This helped in understanding diseases and drug development. Birth of Recombinant DNA Technology Phages contributed to the development of gene cloning, which led to modern genetic engineering. CRISPR and Gene Editing The CRISPR-Cas system (a bacterial defense against phages) inspired CRISPR gene-editing technology, which is now used for genetic modifications and medical treatments. Bacteriophage Studies and Nobel Prizes Salvador Luria, Max Delbrück, and Alfred Hershey won the 1969 Nobel Prize in Medicine for their discoveries about phage genetics.

Answer 68

ERVs Have Beneficial Functions While some ERVs may be harmful, others play essential roles in human biology. A key example is placental development: Syncytin: A protein derived from an ancient ERV is crucial for the formation of the placenta. It helps create the syncytiotrophoblast, a layer of cells that allows nutrient and gas exchange between the mother and fetus. Without ERVs, human pregnancy might not be possible.

Answer 69

TMV crystallization demonstrated that viruses are not "living organisms" but molecular entities, consisting of proteins and nucleic acids. Paved the Way for X-ray Crystallography The ability to crystallize TMV allowed scientists to use X-ray diffraction to study its molecular structure. This technique later led to solving the structure of DNA (Watson & Crick, 1953). Advanced the Field of Structural Virology TMV’s structure helped in understanding how viruses assemble and led to modern vaccine development. Impact on Medicine and Biotechnology The structural biology methods used for TMV paved the way for studying other viruses, including those that cause diseases in humans, such as HIV, coronaviruses, and influenza.

Answer 70

These postulates provide a systematic method for determining whether a microorganism causes a specific disease: Association – The suspected pathogen must always be found in diseased individuals and not in healthy ones. Isolation – The pathogen must be isolated from the diseased host and grown in a pure culture (a controlled environment outside the host). Causation – The isolated pathogen, when introduced into a healthy, susceptible host, must reproduce the same disease seen in the original host. Reisolation – The same pathogen must be re-isolated from the newly infected host, confirming it is the same organism that caused the disease. These postulates provided scientific proof that microbes were the cause of infectious diseases, forming the basis of germ theory.

Answer 71

Anthrax (Bacillus anthracis) Koch proved that Bacillus anthracis caused anthrax, a deadly disease affecting cattle and humans. He demonstrated that anthrax spores could remain dormant in soil for years, reactivating under favorable conditions. Tuberculosis (Mycobacterium tuberculosis) He identified Mycobacterium tuberculosis as the cause of tuberculosis (TB). This discovery earned him the Nobel Prize in Physiology or Medicine in 1905. Cholera (Vibrio cholerae) Koch also isolated Vibrio cholerae, proving that it caused cholera.

Answer 72

While Koch’s postulates were groundbreaking, modern microbiology has identified exceptions: Viruses and obligate intracellular pathogens (e.g., Chlamydia, Rickettsia, HIV) cannot be cultured on artificial media. Asymptomatic carriers (e.g., Typhoid Mary, HIV carriers) may not always show disease symptoms. Polymicrobial infections involve multiple pathogens working together, making it hard to isolate a single causative agent.

Answer 73

Ivanovsky, a Russian scientist, studied tobacco mosaic disease, a condition that damaged tobacco plants. He used unglazed porcelain filters designed to trap bacteria but found that the disease-causing agent passed through the filter. This suggested that the pathogen was smaller than any known bacterium, but Ivanovsky did not conclude it was a new type of infectious agent.

Answer 74

Beijerinck, a Dutch microbiologist, conducted similar experiments on tobacco mosaic disease. He confirmed that the agent could pass through filters that trapped bacteria. More importantly, he proposed that it was not a bacterium but a new infectious agent. He called it a "contagium vivum fluidum" (contagious living fluid) because he believed it was a liquid agent, not a solid particle. This was the first conceptual leap in virology, even though Beijerinck misunderstood the nature of viruses.

Answer 75

Friedrich Loeffler and Paul Frosch, former students of Robert Koch, studied foot-and-mouth disease, a deadly livestock illness. They found that the causative agent also passed through bacterial filters. However, unlike Beijerinck, they hypothesized that viruses were solid, submicroscopic particles, not liquids. This was the first direct evidence that viruses were distinct from bacteria.

Answer 76

Much smaller than any known bacteria. Passed through standard filters that trapped bacteria. Could not be cultured in artificial media. Replicated only inside their host organisms, meaning they needed living cells to multiply.

Answer 77

Scientists diluted extracts from infected tobacco plants into a sterile solution and observed that no new infectious agents appeared. However, when the diluted solution was introduced into healthy plants, the plants developed the disease. This demonstrated that the disease was caused by an infectious entity rather than a bacterial toxin, since toxins cannot multiply or reproduce.

Answer 78

Unlike bacteria, which multiply readily in nutrient solutions, viruses failed to grow in solutions that support bacterial growth. Instead, they only replicated when inside living host cells. This confirmed that viruses were distinct from bacteria and had a unique host-dependent replication cycle.

Answer 79

) Martinus Beijerinck observed that the causative agent of tobacco mosaic disease could: Pass through filters that retained bacteria. Be transmitted from plant to plant. Multiply only inside host organisms. He named it "contagium vivum fluidum" (contagious living fluid) because he believed it was a liquid infectious agent rather than a solid particle.

Answer 80

Scientists began calling these new infectious agents "ultrafilterable viruses" because they passed through filters designed to trap bacteria. The term "virus" comes from the Latin word for "poison". Over time, the term "virus" was simplified and became the standard name for these infectious agents.

Answer 81

The First Discovered Virus The discovery of TMV is often attributed to Dimitrii Ivanovsky (1892), who first observed its ability to pass through bacterial filters. However, Beijerinck (1898) was the first to propose that TMV was a new infectious agent, laying the foundation for virology.

Answer 82

The serial transmission experiments, along with the failure of viruses to grow outside a host, confirmed that viruses were distinct from bacteria. Beijerinck’s "contagium vivum fluidum" concept marked the beginning of virology, and the term "virus" eventually became the standard name for these unique infectious agents. The discovery of Tobacco Mosaic Virus (TMV) in 1892 remains a milestone in microbiology.

Answer 83

The discovery of bacteriophages (viruses that infect bacteria) was a pivotal moment in microbiology and laid the groundwork for molecular biology. 1. The First Recognition of Bacteriophages Frederick Twort (1915) Twort was testing whether smallpox virus could grow on simple nutrient media. He found bacterial contaminants, but some appeared transparent—indicating bacterial lysis (destruction). He suspected that an unknown invisible agent was responsible but did not investigate further. Félix d’Hérelle (1917) d’Hérelle independently discovered these bacteria-killing agents. He named them "bacteriophages" (from Greek phagein, meaning "to eat"), because they appeared to consume bacteria. Unlike Twort, d’Hérelle actively experimented with bacteriophages, proving they were replicating viruses rather than chemical toxins. 2. The Significance of Bacteriophage Research The study of bacteriophages transformed virology and genetics, leading to major breakthroughs in understanding: How viruses replicate inside host cells. Gene regulation and genetic exchange. The structure of DNA and the genetic code. Bacteriophage research established the field of molecular biology, influencing major discoveries like: The Hershey-Chase Experiment (1952) – Proved that DNA (not protein) is the genetic material by using phages. The Lac Operon Model – Uncovered gene regulation mechanisms. The Discovery of Restriction Enzymes – Enabled the rise of genetic engineering and biotechnology. 3. Bacteriophages and Their Impact on Medicine d’Hérelle pioneered bacteriophage therapy, proposing their use to treat bacterial infections before antibiotics were developed. Phage therapy is now being revisited as a solution to antibiotic resistance. Conclusion The discovery of bacteriophages by Twort and d’Hérelle was a turning point in microbiology, leading to the birth of molecular biology and advances in genetics, virology, and biotechnology. Their work reshaped our understanding of viruses and gene regulation, influencing modern medicine and genetic research.

Answer 84

Structure: Bacteriophage T4 is a complex virus with an intricate tail and tail fibers. It has an icosahedral head that contains double-stranded DNA (dsDNA). The tail fibers recognize and attach to bacterial cell walls, injecting viral DNA into the host. Host: Infects Escherichia coli (E. coli) bacteria. Significance: It was a model system in molecular biology, leading to discoveries in DNA replication and genetic exchange.

Answer 85

Structure: TMV has a rod-shaped, helical capsid composed of repeating protein subunits. It lacks an envelope, making it highly stable in harsh environments. Host: Infects tobacco plants and other crops, causing mosaic-like discoloration. Significance: TMV was the first virus discovered (Ivanovsky, 1892) and the first virus crystallized (Stanley, 1935), leading to breakthroughs in structural virology.

Answer 86

Structure: VSV is enveloped, meaning it has a lipid membrane derived from its host cell. It has a bullet-shaped, helical nucleocapsid containing negative-sense single-stranded RNA (-ssRNA). Host: Infects livestock and sometimes humans. Significance: VSV is used as a model virus for studying RNA virus replication and vaccine development.

Answer 87

Rotavirus has a non-enveloped, icosahedral structure, with three layers of protein shells. It contains double-stranded RNA (dsRNA) as its genome. Host: Infects humans and animals, causing severe diarrhea in children. Significance: A leading cause of gastroenteritis worldwide; vaccines have significantly reduced its mortality rate.

Answer 88

Viruses span a broad size range, from small viruses (~30 nm, like poliovirus) to larger ones (~200 nm, like herpesvirus). They are smaller than bacteria (1-10 µm) but larger than ribosomes (~20 nm). Visualization Techniques: Light Microscopy: Can see plant/animal cells and bacteria. Electron Microscopy: Required to observe viruses. X-ray Crystallography & NMR Spectroscopy: Used for protein structure determination at the atomic level.

Answer 89

Plant viruses, such as Tobacco Mosaic Virus (TMV), were among the first viruses to be studied due to their simple transmission. Transmission can occur when infected plant extracts are applied to a scratch or wound on a healthy plant. Even a single infectious particle can be enough to cause visible lesions on plant leaves, allowing scientists to measure viral concentration. This property made plant viruses ideal for early virology studies.

Answer 90

Some human and animal viruses could be studied by infecting laboratory animals. Scientists developed methods to quantify viruses based on their lethal dose (LD50) or infectious dose (ID50). Max Theiler’s work (1930) was groundbreaking: He successfully transmitted yellow fever virus to mice, allowing for further study. He developed an attenuated (weakened) strain of the virus, which became the basis for the highly effective yellow fever vaccine, still in use today.

Answer 91

Once specific viruses were linked to particular host organisms, researchers were able to: Grow viruses in large quantities for experimental studies. Analyze their physical and chemical properties. Understand their life cycle and impact on hosts, including: Incubation period (time between infection and symptoms). Symptoms of infection and disease progression. Tissue and organ damage caused by viral replication.

Answer 92

Understanding how viruses depend on hosts led to vaccine development. It also helped researchers develop antiviral drugs, which work by blocking viral entry, replication, or release from host cells. The study of viral infections in controlled lab conditions paved the way for modern virology, immunology, and epidemiology.

Answer 93

The ICTV is the primary organization responsible for the formal classification of viruses. It follows a hierarchical system similar to biological classification, but viruses are not classified into domains or kingdoms like cellular life. Hierarchy of Virus Classification (ICTV) Realm – Highest taxonomic level (e.g., Riboviria for RNA viruses). Kingdom – Groups viruses based on broader genetic relationships. Phylum – Further subdivisions within kingdoms. Class – Defines relationships at a finer level. Order – Ends with "-virales" (e.g., Herpesvirales). Family – Ends with "-viridae" (e.g., Herpesviridae). Subfamily – Ends with "-virinae" (e.g., Alphaherpesvirinae). Genus – Ends with "-virus" (e.g., Simplexvirus). Species – Given a unique descriptive name (e.g., Human alphaherpesvirus 1 for Herpes Simplex Virus 1). The ICTV classification considers: Genome type (DNA or RNA, single or double-stranded). Capsid symmetry (helical, icosahedral, complex). Presence of an envelope. Replication strategy. Host range and pathogenicity. The ICTV system is the most widely accepted classification method and is updated regularly as new viruses are discovered.

Answer 94

Developed by David Baltimore (1971), this system classifies viruses based on their genetic material and how they replicate. It divides viruses into seven groups: The Seven Baltimore Groups Group I: dsDNA Viruses (Double-stranded DNA) Example: Adenovirus, Herpesvirus, Poxvirus. Group II: ssDNA Viruses (Single-stranded DNA) Example: Parvovirus (B19). Group III: dsRNA Viruses (Double-stranded RNA) Example: Rotavirus (Reoviridae). Group IV: (+) ssRNA Viruses (Positive-sense single-stranded RNA) Example: Poliovirus, Dengue virus, SARS-CoV-2. Group V: (–) ssRNA Viruses (Negative-sense single-stranded RNA) Example: Influenza virus, Rabies virus, Ebola virus. Group VI: ssRNA-RT Viruses (Single-stranded RNA with reverse transcriptase) Example: HIV (Retrovirus). Group VII: dsDNA-RT Viruses (Double-stranded DNA with reverse transcriptase) Example: Hepatitis B virus. Strengths of the Baltimore System: Reflects fundamental differences in viral replication. Helps in understanding antiviral drug targets. Simple and widely used in molecular virology. Limitations: Does not consider morphology or host range. Does not reflect evolutionary relationships as well as ICTV.

Answer 95

classifies viruses based on: Nature of genetic material (DNA or RNA). Capsid symmetry (Helical, Icosahedral, Complex). Presence of an envelope. Diameter of the virion. Mode of replication. While pioneering, the LHT system is now outdated and largely replaced by ICTV and Baltimore classification.

Answer 96

This system classifies viruses based on the type of host they infect: Animal viruses (e.g., Herpesviruses, Influenza virus). Plant viruses (e.g., Tobacco Mosaic Virus, Cauliflower Mosaic Virus). Bacteriophages (e.g., T4 Phage, Lambda Phage). Archaeal viruses (e.g., Sulfolobus turreted icosahedral virus). While simple and useful for medical and agricultural studies, this system does not reflect genetic or structural relationships.

Answer 97

Virus Nomenclature (Naming) Viruses are named using different conventions depending on the system and purpose. (A) Naming According to ICTV Species names follow a descriptive format (e.g., Human alphaherpesvirus 1). Family names use the -viridae suffix (e.g., Herpesviridae). Genus names use the -virus suffix (e.g., Simplexvirus). Orders use the -virales suffix (e.g., Herpesvirales).

Answer 98

Some viruses are named based on the disease they cause: Hepatitis A, B, C viruses – Cause hepatitis. Human Immunodeficiency Virus (HIV) – Causes AIDS. Measles virus – Causes measles.

Answer 99

Plant viruses are often named after the plants they infect: Tobacco Mosaic Virus (TMV) – Infects tobacco plants. Barley Yellow Dwarf Virus – Infects barley. Animal viruses may be named after the tissue they affect: Respiratory Syncytial Virus – Affects respiratory cells. Neurotropic Viruses – Infect the nervous system (e.g., Rabies virus).

Answer 100

Some viruses are named after the location where they were first identified: Ebola virus – Named after the Ebola River (Democratic Republic of Congo). Zika virus – Discovered in the Zika Forest (Uganda). Marburg virus – First identified in Marburg, Germany.

Answer 101

Some viruses are named based on their morphology: Coronavirus – Named for its crown-like spikes ("corona"). Rhabdovirus – Has a bullet shape (Greek rhabdos, meaning rod). Picornavirus – Means "small RNA virus" (pico = small, RNA = genetic material).

Answer 102

Obligate Intracellular Parasites Viruses cannot reproduce outside a host cell. They lack independent metabolism and must hijack host machinery for replication. Non-Cellular Structure Viruses are not made of cells and do not have organelles or cytoplasm. They consist of nucleic acid (DNA or RNA) enclosed in a protein coat. Size Viruses range from 20 nm to 300 nm, much smaller than bacteria. Examples: Picornaviruses (Poliovirus, Hepatitis A Virus) ~ 30 nm. Poxviruses (Variola Virus - Smallpox) ~ 200-300 nm. Genomic Diversity Viral genomes can be DNA or RNA, single-stranded (ss) or double-stranded (ds). Can be linear, circular, or segmented. Structure Capsid (Protein Coat): Protects viral genetic material. Envelope (Lipid Membrane): Present in some viruses, derived from host membranes. Host Specificity Viruses infect specific hosts, including bacteria, plants, animals, and humans. Some viruses, like rabies, have a broad host range. Replication Mechanism Viruses use either DNA or RNA-dependent replication. Some, like retroviruses (HIV), use reverse transcriptase to convert RNA into DNA.

Answer 103

cosahedral 20-sided symmetric capsid Poliovirus (Picornaviridae), Adenovirus Helical Rod-like structure, genome wraps around capsid proteins Tobacco Mosaic Virus, Rabies virus Complex Multiple structural components Bacteriophage T4, Smallpox virus

Answer 104

a genome of RNA or DNA surrounded by a protein shell called a capsid

Answer 105

they are viral particles that lack cytoplasm, are NOT cells, for they generally lack metabolic activity and cannot replicate outside of a host

Answer 106

the form and structure of the virus

Answer 107

Enveloped Viruses Have a lipid membrane acquired from the host HIV, Influenza, Herpesvirus Non-Enveloped Viruses Only have a capsid (no lipid membrane) Poliovirus (Picornaviridae), Adenovirus

Answer 108

a capsid is a protein shell of a virus, enclosing its genetic material main capsids: - helical - icosahedral

Answer 109

Helical is rodlike/threadlike while icosahedral is spherical and has 20 equilateral triangular facets

Answer 110

synthesize protein directly from their genomic RNA sequence

Answer 111

obligate intracellular

Answer 112

the cell or organism/species type that a virus is capable of infecting

Answer 113

the ability of a virus to infect a specific subset of cells

Answer 114

viral genome tricks host cell into making viruses and can only reproduce inside cells

Answer 115

virus hijacks the cells transcription and translation machineries to express - early genes - middle genes - late genes

Answer 116

Double stranded DNA and single stranded RNA (positive/negative)

Answer 117

+RNA is exactly same as mRNA so it can synthesize proteins (translation) directly from their genomic RNA sequence = quick take over

Answer 118

- replication is error prone (high rates of mutation) - single stranded and double stranded - air-prone -HIV

Answer 119

- double stranded - circular - smallpox

Answer 120

single-stranded RNA viruses able to transcribe their RNA into double stranded DNA (looks similar to host cell)

Answer 121

structurally and functionally diverse viruses that infect only bacteria

Answer 122

bacteriophage replicates and ruptures the host cell

Answer 123

bacteriophage DNA is incorporated into host genome, passing itself on to the subsequent generation

Answer 124

attachment, penetration, assembly, maturation, release

Answer 125

1. attachment 2. penetration 3. integration 4. propagation 5. induction

Answer 126

during integrated portion of lysogenic cycle, some viral genes may be expressed

Answer 127

it is latent/chronic a viral infection that spikes at one point and then stays dormant before spiking again

Answer 128

slow gradual mutations over time

Answer 129

rapid recombination event that leads to different strains of a virus

Answer 130

Capsid + genome

Answer 131

Stable in hostile environment * Released by lysis of host cells * Examples: – Adeno-associated Virus (AAV) – Adenovirus B19

Answer 132

The Baltimore classification system Based on: – Genetic contents – Replication strategies of viruses * Seven classes: 1. dsDNA viruses 2. ssDNA viruses 3. dsRNA viruses 4. (+) sense ssRNA viruses (codes directly for protein) 5. (-) sense ssRNA viruses 6. RNA reverse transcribing viruses 7. DNA reverse transcribing viruses

Answer 133

Distinguishing characteristics of viruses * Obligate intracellular parasites * Extreme genetic simplicity * Contain DNA or RNA * Replication involves disassembly and reassembly * Replicate by "one-step growth”

Answer 134

Respiratory transmission – Influenza A virus * Faecal-oral transmission – Enterovirus * Blood-borne transmission – Hepatitis B virus * Sexual Transmission – HIV * Animal or insect vectors – Rabies virus

Answer 135

Targeting of the virus to specific tissue and cell types * Receptor Recognition – CD4+ cells infected by HIV – CD155 acts as the receptor for poliovirus

Answer 136

Satellites – Contain nucleic acid – Depend on co-infection with a helper virus * Viroids – Unencapsidated, small circular ssRNA molecules that replicate autonomously * Prions – No nucleic acid – Infectious protei

Answer 137

18 Century – smallpox, Edward Jenner * 1840 Jacob Henel, plant viruses * 1892 TMV – Ivanowski * 1898 Foot and Mouth disease – Loeffler&Frosch * 1901 yellow fever, Walter Reed * 1917 Bacterophages F.W.Twort

Answer 138

* Viruses are inert (nucleoprotein ) filterable Agents * Viruses are obligate intracellular parasites * Viruses cannot make energy or proteins independent of a host cell * Viral genome are RNA or DNA but not both. * Viruses have a naked capsid or envelope with attached proteins * Viruses do not have the genetic capability to multiply by division. * Viruses are non-living entities

Answer 139

Heat & Cold There is great variability in the heat stability of different viruses. Icosahedral viruses tend to be stable, losing little infectivity after several hours at 37 °C. Enveloped viruses are much more heat-labile, rapidly dropping in titer at 37 °C. Viral infectivity is generally destroyed by heating at 50–60 °C for 30 minutes, though there are some notable exceptions (eg, hepatitis B virus, polyomaviruses). Stabilization of Viruses by Salts Many viruses can be stabilized by salts in concentrations of 1 mol/L; ie, the viruses are not inactivated even by heating at 50 °C for 1 hour. The mechanism by which the salts stabilize viral preparations is not known. Viruses are preferentially stabilized by certain salts. MgCl2, 1 mol/L, stabilizes picornaviruses and reoviruses; MgSO4, 1 mol/L, stabilizes orthomyxoviruses and paramyxoviruses; and Na2SO4, 1 mol/L, stabilizes herpesviruses. pH Viruses are usually stable between pH values of 5.0 and 9.0. Radiation Ultraviolet, x-ray, and high-energy particles inactivate viruses. The dose varies for different viruses. Photodynamic Inactivation Viruses are penetrable to a varying degree by vital dyes such as toluidine blue, neutral red, and proflavine. These dyes bind to the viral nucleic acid, and the virus then becomes susceptible to inactivation by visible light. Ether Susceptibility Ether susceptibility can be used to distinguish viruses that possess an envelope from those that do not. Detergents Nonionic detergents—eg, NP40 and Triton X-100—solubilize lipid constituents of viral membranes. The viral proteins in the envelope are released (undenatured). Anionic detergents, eg, sodium dodecyl sulfate, also solubilize viral envelopes; in addition, they disrupt capsids into separated polypeptides. Formaldehyde Formaldehyde destroys viral infectivity by reacting with nucleic acid. Formaldehyde has minimal adverse effects on the antigenicity of proteins and therefore has been used frequently in the production of inactivated viral vaccines.

Answer 140

Penetration - The process by which a virus penetrates the cell’s membrane barriers and gains access to the cytoplasm. * Two basic processes: – Direct penetration of the plasma membrane – Endocytosis and subsequent penetration of the endocytic vesicle membrane * For enveloped viruses, penetration usually involves a membrane fusion step. * Penetration by nonenveloped viruses is less well understood. * Coreceptors may be involved in penetration.

Answer 141

* Uncoating is the process by which the viral genome is released from the nucleocapsid into the cytoplasm. * This allows the genome to begin to function either in the cytoplasm or in the nucleus of the infected cell. * Closely linked to Penetration.

Answer 142

* Viral gene expression – Pre-early or immediate early genes (complex DNA viruses): Cell cycle disruption, activation of other viral genes. – Early genes: genome replication and other functions. – Late genes: viral structural proteins * Viral genome replication – Various replication mechanisms are used, depending on genome type (RNA or DNA), replication location (nucleus or cytoplasm) and type of virus. * Host macromolecular synthesis is inhibited by most, but not all viruses.

Answer 143

* The process of assembling new virions from virion subunits. * Occurs in nucleus or cytoplasm, depending on virus type. * Enveloped viruses usually acquire membranes by budding through a cellular membrane. * ―Self-assembly‖ - Components of many smaller viruses will spontaneously assemble into virions. Assembly of large or complex virions requires energy (ATP) and/or assembly proteins.

Answer 144

* Self Assembly – Capsid protein Aggregation into stable state around nucleic acid

Answer 145

* Release - Process by which progeny virions are released from the host cell. * Viruses budding from the plasma membrane - these are released as part of the assembly process. * Viruses assembled intracellularly - may utilize cellular secretory pathways (e.g., herpesviruses) or may depend on cell disruption (lysis) for release. * Mechanisms responsible for lysis are not well understood.

Answer 146

A virus Viruses may be defined as acellular organisms whose genomes consist of nucleic acid, and which obligatory replicate inside host cells using host metabolic machinery and ribosomes to form a pool of components which assemble into particles called VIRIONS, which serve to protect the genome and to transfer it to other cells. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. But unlike still simpler infectious agents, viruses contain genes, which gives them the ability to mutate and evolve. Over 5,000 species of viruses have been discovered yet.

Answer 147

– serves to protect genetic information inside viral particle and permit attachment to appropriate host. Outer Envelope- complex viruses also have a quasi-membrane or envelope (proteins and phospholipids) around the Capsid to enhance adhesion to an appropriate host Capsid Construction- made of identical protein subunits termed capsomeres The protein spikes on viruses surface that allow it to bind and fuse with host cells.

Answer 148

1. Size: Most phages range in size from 24- 200 nm in length. 2. Head or Capsid: Some are icosahedral others are filamentous. The head is composed of proteins. Inside the head is found the nucleic acid. The head acts as the protective covering for the nucleic acid. 3. Tail: Many but not all phages have tails attached to the phage head. The tail is a hollow tube through which the nucleic acid passes during infection. In the more complex phages the tail is surrounded by a contractile sheath which contacts during infection of the bacterium. At the end of the tail the more complex phages have a base plate and one or more tail fibers attached to it. The base plate and tail fibers are involved in the binding of the phage to the bacterial cell

Answer 149

All bacteriophages have a lytic or infectious cycle, in which the virus, incapable of replicating itself, injects its genetic material into a bacterium. By pirating its host’s enzymes and protein-building capacities, the virus can reproduce and repackage, making about 100 new copies before it bursts from and destroys the bacteria. Some bacteriophages, however, behave differently when they infect a bacterium. The injected genetic material instead integrates itself into its host DNA, passively replicating with it to be inherited by bacterial daughter cells. In about 1 in 100,000 of these lysogenic cells, the viral DNA spontaneously activates and starts a new lytic cycle

Answer 150

1. Type of nucleic acid; 2. Is the nucleic acid double or single stranded: 3. Presence or absence of nuclear envelope.

Answer 151

:-The protein shell, or coat, that encloses the nucleic acid genome. A capsid is almost always made up of repeating structural subunits that are arranged in one of two symmetrical structures, a helix or an icosahedron. The functions of protein capsid are a- Protect the viral nucleic acid, b- Participate in the viral infection, and c- Share the antigenicity.

Answer 152

: In some animal viruses, the nucleocapsid is surrounded by a membrane, called an envelope, made up of a lipid bilayer, and is comprised of host-cell lipids. It also contains virally encoded proteins, often glycoproteins which are trans-membrane proteins. These viral proteins serve many purposes, such as *binding to receptors on the host cell, *playing a role in membrane fusion and cell entry, etc. Enveloped viruses are formed by budding through cellular membranes, usually the plasma membrane but sometimes an internal membrane such as the ER, Golgi, or nucleus. In these cases, the assembly of viral components (genome, capsid, matrix) occurs on the inside face of the membrane. This ability to bud allows the virus to exit the host cell without lysing or killing the host. In contrast, non-enveloped viruses, and some enveloped viruses, kill the host cell in order to escape.

Answer 153

1-Viruses influenza are spread through the air by droplets of moisture when people cough or sneeze. 2- Viruses such as Hepatitis A virus are transmitted by the fecal–oral route, which involves the contamination of hands, food and water. 3- Rotavirus is often spread by direct contact with infected children. 4-The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex. 5- Dengue virus, are spread by blood-sucking insects.

Answer 154

As obligate intracellular parasites, Virus must enter and replicate in living cells in order to “reproduce” themselves. This “growth cycle” involves specific attachment of virus, penetration and un-coating, nucleic acid transcription, protein synthesis, maturation and assembly of the virions and their subsequent release from the cell by budding or lysis There are six basics, overlapping stages in the life cycle of viruses in living cell . * Attachment is the binding of the virus to specific molecules on the surface of the cell. * Penetration follows attachment; viruses penetrate the host cell by endocytosis or by fusion with the cell. Virions are either engulfed into vacuoles by “endocytosis” or the virus envelope fuses with the plasma membrane to facilitate entry * Uncoating happens inside the cell when the viral capsid is removed and destroyed by viral enzymes or host enzymes, thereby exposing the viral nucleic acid. * Replication of virus particles is the stage where a cell uses viral messenger RNA in its protein synthesis systems to produce viral proteins. The RNA or DNA synthesis abilities of the cell produce the virus's DNA or RNA. * Assembly takes place in the cell when the newly created viral proteins and nucleic acid combine to form hundreds of new virus particles. * Release occurs when the new viruses escape or are released from the cell. Most viruses achieve this by making the cells burst, a process called lysis. Other viruses such as HIV are released more gently by a process called budding.

Answer 155

genetic material, either RNA or DNA a protein coat that surrounds the genetic material

Answer 156

reservoir.

Answer 157

invade or attach to a new host cell.

Answer 158

presence of specific receptors on the host cell surface

Answer 159

viruses have nucleic acids, which undergo mutations.

Answer 160

type of disease caused type of replication viral structure type of nucleic acid present

Answer 161

can vary between less than an hour to many years.

Answer 162

the host cell

Answer 163

no single common ancestor has been identified by scientists.

Answer 164

adhering to a receptor molecule on the cell's surface.

Answer 165

making holes in the cell exterior and killing the cell budding out of the cell by exocytosis

Answer 166

genetic material of the virus enters the cell.

Answer 167

kill the cell immediately.

Answer 168

lytic; lysogenic

Answer 169

Viral penetration into a host cell may not cause immediate death of the host cell.

Answer 170

phage therapy

Answer 171

lytic cycle- The host cell is killed as new virus particles are released. lysogenic cycle- Viral DNA can insert into the host chromosome.

Answer 172

-genetic material of virus is replicated -prophage may be formed

Answer 173

-cell starvation -DNA damage

Answer 174

A capsid is a protein shell that surrounds and protects the nucleic acid (DNA or RNA) of a virus. It’s made up of protein subunits called capsomeres. The capsid plays a key role in: Protecting the genetic material. Helping the virus attach to and enter host cells. This is found in all viruses, making it the most accurate and general answer to the question.

Answer 175

Some viruses, especially animal viruses like HIV and influenza, have a lipid envelope outside their capsid. The envelope is derived from the host cell membrane as the virus buds off. However, not all viruses have an envelope. Viruses that lack an envelope are called non-enveloped or naked viruses. So while it's a structure some viruses form around their capsid, it is not typically around the nucleic acid directly.

Answer 176

A virulent virus immediately hijacks the host cell machinery to replicate itself. It causes cell lysis (destruction of the cell) to release new viral particles. In bacteria, a virulent virus refers to a lytic phage.

Answer 177

A lytic phage infects bacteria and follows the lytic cycle: Injects its DNA. Takes over host machinery. Produces new viruses. Destroys the host cell (lysis).

Answer 178

A viroid is a small, circular RNA molecule that infects plants, not bacteria or humans. It does not encode proteins and does not infect Vibrio cholerae.

Answer 179

A lysogenic bacteriophage is a virus that infects bacteria but instead of killing the cell right away, it integrates its DNA into the host genome. This viral DNA is called a prophage. That viral DNA can carry genes (like toxins). In Vibrio cholerae, a specific lysogenic phage called CTXϕ (CTX phi) carries the cholera toxin gene (ctxA and ctxB). When the phage infects a harmless V. cholerae, it inserts its DNA into the bacterial genome, turning it into a toxin-producing strain. 🔥 This is what causes cholera disease—the bacterium secretes the cholera toxin, which causes massive fluid loss and diarrhea.

Answer 180

In phage lambda, once it infects a bacterial cell, it expresses early genes almost immediately. These early genes encode regulatory proteins like: cI (lambda repressor) → promotes lysogeny Cro → promotes lytic cycle The balance between these proteins determines which pathway the phage will follow: If cI wins: phage integrates into the genome → lysogenic. If Cro wins: phage begins making new virus particles → lytic. So, the expression of early genes directly controls this decision.

Answer 181

it can infect many cell types or species What is host range? A virus's host range refers to the spectrum of hosts (organisms or cells) it can infect. Some viruses have a narrow host range (e.g., only humans). Others have a broad host range, meaning they can infect: Multiple cell types within one species, and/or Multiple species (sometimes even across different taxonomic groups).

Answer 182

This describes the zoonotic spillover process — where viruses jump from animals to humans, often after mutation or recombination. Examples: HIV (from primates) SARS-CoV-2 (likely from bats via an intermediate host) Ebola (from fruit bats or primates) Influenza (from birds or pigs) The new host (e.g., humans) lacks immunity or defense mechanisms, making the virus more likely to cause severe disease or outbreaks. This unpredictability and severity make emerging viruses a major public health concern.

Answer 183

enveloped viruses

Answer 184

Viruses are highly specific about what cells they infect — this is called tissue tropism. Why? Because to enter a host cell, a virus must bind to specific receptor proteins on the cell's surface. These receptors act like locks, and the virus must have the right “key” (ligand or capsid protein). Different tissues express different surface receptors, so a virus that targets nerve cells (like rabies) will bind to a receptor that’s only present on neurons, not on skin or nose cells.

Answer 185

A) Double-stranded DNA (dsDNA) — ✅ Legitimate Many viruses have double-stranded DNA genomes, similar to cellular life. Examples: Herpesviruses (e.g., Herpes simplex virus) Adenoviruses Poxviruses (e.g., Variola virus – smallpox) 🧬 B) Single-stranded DNA (ssDNA) — ✅ Legitimate Some viruses have single-stranded DNA genomes. These are typically small viruses. Examples: Parvoviruses (e.g., Canine parvovirus) Bacteriophage φX174 🧬 C) Double-stranded RNA (dsRNA) — ✅ Legitimate Though not common, some viruses do have double-stranded RNA genomes. They often have segmented genomes (separate RNA pieces). Examples: Reoviruses (e.g., Rotavirus) 🧬 D) Single-stranded RNA (ssRNA) — ✅ Legitimate This is a very common genome type for viruses. ssRNA viruses can be: Positive-sense (+RNA): acts like mRNA (e.g., Poliovirus, SARS-CoV-2) Negative-sense (−RNA): must be converted to mRNA (e.g., Influenza, Rabies virus) Examples: Coronaviruses Flaviviruses (e.g., Dengue, Yellow Fever) Paramyxoviruses (e.g., Measles)

Answer 186

HIV interacts with normal CCR5 surface receptors and uses them to gain entry to the cell's interior. How does HIV use CCR5? HIV primarily infects CD4+ T cells. To enter these cells, the virus needs to bind: To the CD4 receptor. And to a coreceptor, most commonly CCR5 (or sometimes CXCR4). HIV’s gp120 surface protein binds to CD4, then CCR5, triggering fusion with the cell membrane and entry. 🧬 What is the CCR5-Δ32 mutation? This is a 32-base pair deletion in the CCR5 gene. It produces a nonfunctional receptor that is: Not present on the cell surface. Invisible to HIV. People who are: Homozygous for the mutation (Δ32/Δ32) are highly resistant to infection by HIV-1. Heterozygous (one copy of the mutation) may get infected but often experience slower disease progression.

Answer 187

It is a surface protein which provides entry to the cell interior. ✅ Explanation: 🦠 What is Hemagglutinin (H)? Hemagglutinin (HA) is a surface glycoprotein found on influenza viruses. It plays a critical role in the virus’s ability to infect host cells. The “H” number (like H1, H5, etc.) refers to the specific type of hemagglutinin. 🧬 Function of Hemagglutinin: Attachment: Hemagglutinin binds to sialic acid residues on the surface of host epithelial cells (especially in the respiratory tract). Fusion: After binding, HA triggers the fusion of the viral envelope with the host cell membrane, allowing the viral RNA to enter the host cell. This is the first step in viral infection — getting the virus into the cell.

Answer 188

Viral infection requires that proteins of the capsid can bind to specific surface proteins on the host cell. Cells of different tissues have different types of surface proteins. ✅ Explanation: Viruses are highly specific in the types of cells they infect — this specificity is known as tissue tropism or cell tropism. The main reason for this is receptor compatibility. 🔬 Here's what happens: For a virus to infect a cell, it must attach to the cell surface. This attachment occurs when viral surface proteins (often on the capsid or envelope) bind to specific receptor proteins on the surface of the host cell. Different tissues express different surface proteins, so a virus like rabies, which targets neurons, can only bind to receptors present on nerve cells, such as the nicotinic acetylcholine receptor.

Answer 189

✅ C) without a nucleus ✅ Explanation: The word “Prokaryote” comes from Greek roots: “Pro-” (πρό) = before or without “-karyote” / “-karyon” (κάρυον) = nucleus or nut/kernel So, “Prokaryote” literally means “before nucleus” or “without a nucleus.” 🔬 What is a Prokaryote? Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and membrane-bound organelles. Their genetic material floats freely in the cytoplasm, usually in a region called the nucleoid. Includes: Bacteria Archaea

Answer 190

✅ B) endospore ✅ Explanation: An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria to survive in harsh environmental conditions, such as: Extreme heat or cold UV radiation Desiccation (drying out) Chemicals and disinfectants Lack of nutrients 🦠 Key facts about endospores: Formed inside the bacterial cell (hence the prefix “endo-”). Common in Gram-positive bacteria, such as: Bacillus spp. Clostridium spp. (e.g., C. botulinum, C. difficile, C. tetani) Can remain viable for years and “reactivate” when conditions improve — a process called germination.

Answer 191

Newly replicated viruses exit the host cell through one of two main processes: 1. Lysis (Lyse) Common in non-enveloped (naked) viruses The virus bursts open (lyses) the host cell, killing it and releasing new viral particles. Example: Bacteriophages and some animal viruses This is part of the lytic cycle. 2. Exocytosis (Budding) Common in enveloped viruses The virus acquires a piece of the host’s cell membrane, forming a viral envelope. The virus buds off the cell or uses the host's exocytosis machinery to leave without immediately killing the cell. Example: Influenza, HIV, Herpesvirus

Answer 192

Pili are hair-like, protein-based structures that extend from the surface of many prokaryotic cells — especially bacteria. There are two main types of pili, and each has specific functions: 1. Attachment (Fimbriae or Adhesive Pili) Help bacteria stick to: Surfaces (like rocks, teeth, or medical devices) Host tissues (important in infections) Other bacteria These pili are shorter and more numerous than flagella. 🦠 Example: E. coli uses fimbriae to attach to the intestinal lining. 2. Conjugation (Sex Pilus) Involved in horizontal gene transfer between bacteria. A sex pilus forms a bridge between two bacterial cells, allowing the transfer of plasmid DNA (e.g., antibiotic resistance genes). This process is called bacterial conjugation.

Answer 193

What type of virus is this? Answer: Retrovirus 🧬 How do we know it's a retrovirus? From the diagram, we can see: RNA: Retroviruses carry their genetic material as RNA, not DNA. Reverse Transcriptase: This enzyme is unique to retroviruses. It converts viral RNA into DNA after the virus infects a host cell. Capsid: Protein shell that protects the RNA. Viral Envelope: A lipid membrane derived from the host cell. Viral Proteins: Help with cell recognition and entry. 🧠 Famous Example of a Retrovirus: HIV (Human Immunodeficiency Virus) HIV is the classic example of a retrovirus and shows all the components labeled in your image.

Answer 194

Halophiles are a type of archaea (archaebacteria) that thrive in extremely salty environments, such as: Salt lakes (e.g., the Dead Sea) Salt flats Salt mines Pickling brines The word "halophile" comes from Greek: “Halo-” = salt “-phile” = loving So it literally means “salt-loving”.

Answer 195

C) peptidoglycan in cell walls of eubacteria ✅ Explanation: The key structural difference between eubacteria and archaebacteria lies in their cell walls: Eubacteria (true bacteria) have peptidoglycan in their cell walls. Peptidoglycan is a complex polymer of sugars and amino acids. It provides structural support and shape to the bacterial cell. Archaebacteria do NOT have peptidoglycan. Instead, they have pseudopeptidoglycan or other unique molecules in their cell walls. This is one reason they are classified as a separate domain of life

Answer 196

This virus is responsible for causing: infections in bacteria (not humans) So, to fill in the blank: This virus is responsible for causing infections in bacteria. 🔬 Key clues from the diagram: Icosahedral capsid: A geometric shape typical of many viruses. Fibers: Used by bacteriophages to attach to bacterial cell walls. Genetic material: Typically DNA, injected into the host bacterium. No envelope – typical of non-enveloped bacteriophages like T4 phage. 🦠 What it infects: Bacteriophages (or phages) infect bacteria, not humans. They are used in research and therapy, especially against antibiotic-resistant bacteria.

Answer 197

Cyanobacteria (also known as blue-green algae, though they are bacteria, not algae) are: Photosynthetic prokaryotes Among the earliest organisms to perform oxygenic photosynthesis (producing oxygen as a byproduct) Crucial contributors to the Great Oxygenation Event (~2.4 billion years ago), which transformed Earth's atmosphere by introducing free oxygen 🌍 Why are they important? Before cyanobacteria, Earth's atmosphere was anaerobic (no oxygen). Cyanobacteria used sunlight, water, and carbon dioxide to make food — and released oxygen. This allowed for: The formation of the ozone layer Evolution of aerobic (oxygen-using) organisms

Answer 198

Transduction is the process by which bacteriophages (viruses that infect bacteria) transfer DNA from one bacterial cell to another. There are two main types of transduction: Generalized Transduction: A phage accidentally packages a piece of host bacterial DNA instead of its own viral DNA. When it infects another bacterium, it injects the donor bacterial DNA into the new host. Specialized Transduction: Happens with lysogenic phages. When a prophage is incorrectly excised from the host genome, it carries along adjacent host genes. These genes get transferred to the next bacterial host upon infection.

Answer 199

B) whether their genome is DNA or RNA and whether that DNA or RNA is single or double stranded. ✅ Explanation: Viruses are primarily classified based on the type and structure of their genetic material — a system known as the Baltimore Classification. This system groups viruses by: Type of nucleic acid: DNA or RNA Strandedness: Single-stranded (ss) or double-stranded (ds) Polarity (for RNA viruses): Positive-sense (+RNA) or negative-sense (−RNA)

Answer 200

C) thermoacidophiles ✅ Explanation: Thermoacidophiles are a type of archaea that thrive in: Extremely hot temperatures (thermo = heat) Very acidic environments (acidophiles = acid-loving) These organisms are commonly found in: Hot springs (like those in Yellowstone) Hydrothermal vents on the ocean floor Geysers and volcanic areas They are extremophiles — organisms that live in conditions too extreme for most life.

Answer 201

Methanogens are archaea that produce methane (CH₄) as a byproduct of their metabolism. They live in anaerobic (oxygen-free) environments such as: Swamps and marshes The guts of ruminant animals (e.g., cows, sheep) Landfills Sewage treatment plants 🌍 Why methanogens contribute to the greenhouse effect: Methane (CH₄) is a potent greenhouse gas — over 25 times more effective at trapping heat in the atmosphere than carbon dioxide (CO₂) over a 100-year period. Methanogens are responsible for producing a significant portion of atmospheric methane, contributing to global warming and climate change.

Answer 202

A) Initiate transcription in the same manner True Archaea and Eukarya start transcription similarly, using TATA-binding proteins and RNA polymerases that resemble each other. This is one reason Archaea are considered more closely related to Eukarya than to Bacteria. ✅ B) Have similar types of RNA True Both domains have similar rRNA sequences and RNA polymerase components, more alike than those in Bacteria. ✅ C) Share some of the same ribosomal proteins not found in bacteria True Archaea and Eukaryotes share unique ribosomal proteins that are absent in Bacteria, again showing a closer evolutionary relationship. ❌ D) Have nucleated cells False → ✅ Correct answer (EXCEPT) Only Eukarya have true nuclei, which are membrane-bound. Archaea, like Bacteria, are prokaryotes, meaning they lack a nucleus.

Answer 203

C) maturation ✅ Explanation: The lytic cycle is the process by which a virus infects a host cell and forces it to produce and release new virus particles, ultimately destroying the host cell. Here’s a breakdown of the five main stages of the lytic cycle: 1. Attachment (A) The virus attaches to specific receptors on the host cell surface. 2. Penetration (not listed in the options) The virus injects its nucleic acid (DNA or RNA) into the host cell. 3. Biosynthesis (B) The viral genome is replicated, and viral proteins (like capsid and enzymes) are synthesized using the host's machinery. BUT the components are still separate at this stage. 4. Maturation (✅ C — Correct Answer) The newly made viral genomes and proteins are assembled into complete viruses (virions). This is when hundreds of new viral particles are put together. 5. Release (D) The host cell lyses (bursts), releasing the newly assembled viruses to infect other cells.

Answer 204

are highly resistant, dormant structures formed by certain bacteria (like Bacillus and Clostridium) to survive harsh environmental conditions such as: Heat UV radiation Desiccation (drying) Disinfectants Nutrient deprivation But they are not a form of reproduction.

Answer 205

Bacterial Certain lactic acid bacteria (e.g., Lactobacillus) ferment lactose in milk to form lactic acid, which causes the milk to curdle and form curd/yogurt.

Answer 206

Historically, bacteria were grouped with plants because of their cell wall, but modern classification places them in their own domain. While they can synthesize vitamins, this isn't why they were considered plant-like. So (a) is the best historical justification.

Answer 207

autotrophic nutrition and unicellular sex organs Spirogyra is a green alga with: Autotrophic nutrition (photosynthetic) Unicellular reproductive structures That’s why it's classified as an alga.

Answer 208

c) both (a) and (b) Food preservation involves: Killing microbes (e.g., through heating or chemical preservatives) Inactivating them (e.g., refrigeration or drying) So both are used.

Answer 209

✅ (a) A protein shell and nucleic acid A virus is made up of genetic material (DNA or RNA) enclosed in a protein shell called the capsid. Some viruses also have an envelope, but it's not a basic requirement.

Answer 210

Capsid The capsid is the protein coat that protects the viral genome and aids in host recognition.

Answer 211

(d) It's an infectious and fully formed viral particle A virion is a complete virus particle that can infect a host.

Answer 212

✅ (c) Either DNA or RNA A virus has either DNA or RNA, never both, which is unique compared to living organisms.

Answer 213

✅ (a) Megavirus chilensis Discovered after Mimivirus, Megavirus is one of the largest viruses known, with a complex genome.

Answer 214

✅ (a) They are individual units of the capsid Capsomeres are protein subunits that make up the capsid.

Answer 215

✅ (b) They are spike-like projections on enveloped viruses Peplomeres (like H and N proteins in influenza) help in host cell recognition and attachment.

Answer 216

✅ (d) Both a and b Icosahedral symmetry involves 20 triangular faces, and the capsomeres can be pentagonal or hexagonal.

Answer 217

The envelope contains lipids (from host), proteins, and glycoproteins for binding.

Answer 218

✅ (a) T4 bacteriophage T4 phage has a complex structure: an icosahedral head and a helical tail.

Answer 219

✅ (a) TMV (Tobacco Mosaic Virus) TMV is the classic example of a helical, rod-shaped virus

Answer 220

✅ (a) It's an RNA virus TMV contains single-stranded RNA and infects plants.

Answer 221

✅ (d) Either DNA or RNA Same as Q4: Viruses use either nucleic acid, never both.

Answer 222

✅ (a) Rod-shaped TMV has a rigid, rod-like helical capsid.

Answer 223

✅ (c) ds DNA phage T2 (like T4) is a double-stranded DNA virus that infects E. coli.

Answer 224

✅ (b) Circovirus Circoviruses have tiny circular DNA genomes, as small as ~1.8 kb.

Answer 225

✅ (c) Peplomers Repeats Q7: Used for attachment to host cells, found on enveloped viruses.

Answer 226

✅ (c) Viroid Viroids are naked RNA molecules that infect plants.

Answer 227

Beijerinck and LoefflerMartinus Beijerinck (1898) Dutch microbiologist Studied Tobacco Mosaic Disease Coined the term “virus” to describe the infectious agent that passed through filters that trapped bacteria Concluded the agent was not a bacterium, but a new form of infectious particle — what we now know as a virus ✅ Friedrich Loeffler (along with Paul Frosch) In 1898, they demonstrated that foot-and-mouth disease in cattle was caused by a filterable agent — again, a virus. This was the first animal virus discovered.

Answer 228

BeijerinckMartinus Beijerinck, a Dutch microbiologist, is credited with: Studying Tobacco Mosaic Disease in the late 1800s. He filtered the sap from infected plants and found that the infectious agent could pass through filters that trapped bacteria. He concluded that the disease was caused by something smaller than bacteria — an infectious, living fluid. He called it “Contagium vivum fluidum”, which is Latin for “contagious living fluid.” This was one of the earliest conceptual definitions of a virus, even before viruses were ever seen under a microscope.

Answer 229

It is outside a host cell It is not yet replicating It can infect a host cell when the opportunity arises Key features of a virion: Contains genetic material (DNA or RNA) Enclosed in a protein coat (capsid) May also have a lipid envelope (in enveloped viruses) Structurally complete and capable of initiating infection

Answer 230

A. TRUE ✅ Explanation: Viruses are acellular and not truly living organisms. They lack the cellular machinery required for independent life — including metabolic enzymes. 🔬 Key Points: Viruses do not carry out metabolism on their own. They do not have enzymes for: Respiration Protein synthesis ATP production Instead, they hijack the host cell's machinery to replicate and produce proteins. Some viruses may carry specialized enzymes (e.g., reverse transcriptase in retroviruses), but these are for replication, not metabolism. ✅ So the statement “Metabolic enzymes are absent in viruses” is completely TRUE.

Answer 231

The term “bacteriophage” (meaning “bacteria eater”) was coined by Félix d’Herelle in 1917. He observed viruses that infected and destroyed bacteria, specifically Escherichia coli (E. coli). These viruses were later called bacteriophages, or simply phages. 🔬 Who did what? Frederick Twort (1915) — First observed a mysterious agent that killed bacteria, but didn’t fully identify it as a virus. Félix d’Herelle (1917) — Independently discovered the same phenomenon and coined the term “bacteriophage.”

Answer 232

None A virion is the complete, infectious virus particle, and it consists of: A capsid (the protein shell) Either DNA or RNA as its genetic material — never both In some viruses, an additional lipid envelope surrounding the capsid

Answer 233

C. Neuraminidase ✅ Explanation: Neuraminidase is an enzyme that helps certain viruses, especially influenza viruses, to: Break down sialic acid residues on the surface of the host cell. This action helps the virus to: Enter the host cell more easily by penetrating the cell surface. Exit the cell after replication, aiding in viral release and spread. 🧬 Where is neuraminidase found? On the surface of influenza viruses, alongside hemagglutinin. This is why flu strains are labeled like H1N1: H = Hemagglutinin N = Neuraminidase

Answer 234

Explanation: The nucleic acid content of a virus (either DNA or RNA) typically makes up a small percentage of the total virus particle's mass — often around 5–6%, depending on the virus type. The majority of a virus’s mass is from the capsid proteins and, in some cases, the lipid envelope. So even though the nucleic acid is the core functional component, it's a minor physical portion of the virion. 📊 Typical Composition of a Simple Virus: Component Approximate Percentage Nucleic acid (DNA or RNA) ~5–6% Capsid proteins ~90% Envelope (if present) Varies (adds more lipids and glycoproteins)

Answer 235

Capsomeres are the individual protein subunits that come together to form the capsid, which is the protective protein shell surrounding the viral nucleic acid. These subunits self-assemble into geometric structures like: Icosahedrons (20-sided) Helical tubes (as in TMV) 🧬 Role of Capsomeres: Protect the viral genome (DNA or RNA) Determine the virus’s shape and symmetry Help in host cell recognition in some viruses

Answer 236

Boiling at 100°C (212°F) is a method commonly used to kill microorganisms, and it is generally lethal to both bacteria and viruses under normal conditions.

Answer 237

A. Will it die ✅ Explanation: The capsid is the protein shell that: Protects the viral genetic material (DNA or RNA) Helps in attachment to the host cell Aids in injecting the viral genome into the host 🔬 So what happens if the capsid is removed? The virus will lose its structural integrity It cannot infect a host cell without the capsid It becomes non-functional and non-infectious — essentially, it’s “dead” Note: Viruses are not truly alive, but in this context, “will it die” means it can no longer function or replicate. ❌ Why the other options are incorrect: B. Will it germinate – ❌ Viruses do not germinate; they replicate only inside host cells. C. Will it remain normal – ❌ No, without the capsid, the virus cannot survive or function. D. Will it change in nature – ❌ It won’t change into another form; it will just be inactive or destroyed. ✅ Final Answer: A. Will it die

Answer 238

Although viruses are not considered fully "alive," they do exhibit some living characteristics, especially inside a host cell. The most compelling evidence of their living nature is: ➤ Their nucleic acids (DNA or RNA) can direct protein synthesis — specifically, they can instruct the host cell to make viral proteins, including the capsid. This ability to replicate and control cellular machinery is a hallmark of life

Answer 239

Nucleoproteins: These are complexes of nucleic acids (DNA or RNA) and proteins. Both bacteria and viruses contain nucleoproteins to some extent, but there's a major difference when it comes to histones. 🧬 Histones: Histones are positively charged proteins that help organize and package DNA into a compact structure (chromatin). Histones are found in: Eukaryotes (like humans, plants, fungi) Some archaea But: ❌ Bacteria do not have true histones (though they have histone-like proteins for DNA packaging). ❌ Viruses do not have histones at all. 📌 So among the options: A. Bacteria – ✅ Have nucleoproteins and histone-like proteins. B. Viruses – ❌ Have nucleic acid and capsid proteins but no histones. C. Both (a) and (b) – ❌ Incorrect, since viruses lack histones. D. None of these – ❌ Incorrect, since bacteria do contain nucleoproteins. ✅ Final Answer: A. Bacteria Let me know if you’d like a chart comparing DNA packaging in bacteria, viruses, and eukaryotes!

Answer 240

The ICTV is the primary organization responsible for the formal classification of viruses. It follows a hierarchical system similar to biological classification, but viruses are not classified into domains or kingdoms like cellular life. Hierarchy of Virus Classification (ICTV) Realm – Highest taxonomic level (e.g., Riboviria for RNA viruses). Kingdom – Groups viruses based on broader genetic relationships. Phylum – Further subdivisions within kingdoms. Class – Defines relationships at a finer level. Order – Ends with "-virales" (e.g., Herpesvirales). Family – Ends with "-viridae" (e.g., Herpesviridae). Subfamily – Ends with "-virinae" (e.g., Alphaherpesvirinae). Genus – Ends with "-virus" (e.g., Simplexvirus). Species – Given a unique descriptive name (e.g., Human alphaherpesvirus 1 for Herpes Simplex Virus 1). The ICTV classification considers: Genome type (DNA or RNA, single or double-stranded). Capsid symmetry (helical, icosahedral, complex). Presence of an envelope. Replication strategy. Host range and pathogenicity. The ICTV system is the most widely accepted classification method and is updated regularly as new viruses are discovered.

Answer 241

Developed by David Baltimore (1971), this system classifies viruses based on their genetic material and how they replicate. It divides viruses into seven groups: The Seven Baltimore Groups Group I: dsDNA Viruses (Double-stranded DNA) Example: Adenovirus, Herpesvirus, Poxvirus. Group II: ssDNA Viruses (Single-stranded DNA) Example: Parvovirus (B19). Group III: dsRNA Viruses (Double-stranded RNA) Example: Rotavirus (Reoviridae). Group IV: (+) ssRNA Viruses (Positive-sense single-stranded RNA) Example: Poliovirus, Dengue virus, SARS-CoV-2. Group V: (–) ssRNA Viruses (Negative-sense single-stranded RNA) Example: Influenza virus, Rabies virus, Ebola virus. Group VI: ssRNA-RT Viruses (Single-stranded RNA with reverse transcriptase) Example: HIV (Retrovirus). Group VII: dsDNA-RT Viruses (Double-stranded DNA with reverse transcriptase) Example: Hepatitis B virus. Strengths of the Baltimore System: Reflects fundamental differences in viral replication. Helps in understanding antiviral drug targets. Simple and widely used in molecular virology. Limitations: Does not consider morphology or host range. Does not reflect evolutionary relationships as well as ICTV.

Answer 242

Nature of genetic material (DNA or RNA). Capsid symmetry (Helical, Icosahedral, Complex). Presence of an envelope. Diameter of the virion. Mode of replication. While pioneering, the LHT system is now outdated and largely replaced by ICTV and Baltimore classification.

Answer 243

Type Characteristics Examples Enveloped Viruses Have a lipid membrane acquired from the host HIV, Influenza, Herpesvirus Non-Enveloped Viruses Only have a capsid (no lipid membrane) Poliovirus (Picornaviridae), Adenovirus

Answer 244

Viruses are classified based on the type of nucleic acid (DNA or RNA) they use for replication. 1. DNA Viruses Contain DNA as genetic material. Can be single-stranded (ssDNA) or double-stranded (dsDNA). Examples: dsDNA Viruses: Herpesvirus, Adenovirus, Poxvirus. ssDNA Viruses: Parvovirus B19. 2. RNA Viruses Contain RNA instead of DNA. Can be positive-sense (+) or negative-sense (-). Examples: (+ ssRNA Viruses): Poliovirus, Hepatitis A Virus (Picornaviridae), SARS-CoV-2. (- ssRNA Viruses): Influenza virus, Rabies virus. dsRNA Viruses: Rotavirus.

Answer 245

Some viruses prefer specific host cells and are named accordingly. Host Cell Type Examples Human Viruses HIV (Human Immunodeficiency Virus), HPV (Human Papillomavirus) Animal Viruses Feline Immunodeficiency Virus (FIV), Avian Influenza Virus Simian Viruses Simian Immunodeficiency Virus (SIV), Monkeypox Virus Bacteriophages T4 Phage (infects bacteria)

Answer 246

Viruses are also classified by how they spread between hosts. 1. Arthropod-Borne Viruses (Arboviruses) Transmitted by mosquitoes, ticks, or flies. Examples: Dengue virus (Flaviviridae). Yellow fever virus (Flavivirus). Zika virus (Flavivirus). 2. Respiratory Transmission Spread through airborne droplets. Examples: Influenza virus. Coronavirus (SARS-CoV-2). Rhinovirus (common cold virus). 3. Fecal-Oral Transmission Spread through contaminated food and water. Examples: Hepatitis A Virus (HAV). Rotavirus. Norovirus. 4. Bloodborne Transmission Spread via blood contact, needles, or sexual transmission. Examples: HIV (Human Immunodeficiency Virus). Hepatitis B and C Viruses (HBV, HCV).

Answer 247

Viruses show preference for specific tissues or organs. Tissue Tropism Virus Example Respiratory Tropism Influenza virus, Adenovirus Gastrointestinal Tropism Rotavirus, Norovirus, Enterovirus Neurotropic Viruses Rabies virus, Poliovirus Hepatotropic Viruses Hepatitis A, B, C Viruses

Answer 248

A virus is a non-cellular, infectious agent that consists of genetic material (DNA or RNA) enclosed within a protein coat (capsid) and, in some cases, a lipid envelope. Viruses lack independent metabolism and can only replicate inside living host cells, making them obligate intracellular parasites. Key Characteristics of Viruses: Acellular (non-living outside a host) Require a host cell to reproduce Possess either DNA or RNA, but never both Do not have ribosomes or metabolic machinery Highly diverse in structure and genetic composition

Answer 249

Host-Specific – Infect specific hosts (bacteria, plants, animals, humans). Obligate Parasites – Depend entirely on host cell machinery for replication. High Mutation Rates – Some viruses, like HIV and influenza, mutate rapidly, making them difficult to control. Can Cause Diseases – Viruses are responsible for diseases like influenza, polio, HIV/AIDS, COVID-19, rabies, and hepatitis.

Answer 250

Suggests that viruses evolved from mobile genetic elements (such as transposons or plasmids) that escaped from host cells. These elements later gained the ability to infect other cells. Evidence: Some viral genes resemble host genes.

Answer 251

Proposes that viruses evolved from once-free-living cellular organisms that lost their ability to survive independently. Some bacteria, like Rickettsia and Chlamydia, rely on host cells similarly to viruses. Evidence: Some giant viruses (e.g., Mimivirus, Pandoravirus) contain genes typical of cellular life.

Answer 252

Suggests that viruses existed before cellular life, evolving alongside early self-replicating molecules. Viruses may have played a role in the evolution of cellular life. Evidence: Some viral genetic elements are older than cellular genes.