Flashcards in Chapter 5 Deck (196)
Viruses are a unique group of biological entities known to infect ____ type of cell, including bacteria, algae, fungi, protozoa, plants, and ___.
___ water can contain about 10 million viruses per milliliter. Human feces probably contain ____ times that many.
A variegated tulip gets its beautiful colors from a ___ infection.
The French scientist Louis Pasteur was on the right track when he postulated that rabies was caused by a living thing smaller than ____, and in 1884 he was able to develop the first ____ for rabies.
Pasteur also proposed the term ____ to denote this special group of infectious agents.
The first substantial revelations about the unique characteristics of viruses occurred in the _____.
D. Ivanovski and M. Beijerinick showed that a disease in ____ was caused by a virus. (tobacco mosaic virus)
Friedrich Loeffler and Paul Frosh discovered an animal virus that causes ______ in cattle.
Early researchers found that when infectious fluids from _____ were passed through porcelain filters deigned to trap bacteria, the filtrate remained _____. This result proved that an infection could be caused by a cell-free fluid containing agents smaller than bacteria and thus introduced the concept of ____.
Years of experimentation were required to show that viruses were ____ particles with a definite size, shape, and chemical composition.
Using special techniques, ___ can be cultured in laboratories.
Thanks to new genomic techniques, including ______ and "next-generation" nucleic acid sequencing techniques, we are getting a much clearer picture of the number and variety of viruses on earth.
The ____ structure and behavior of viruses have led to debates about their connection to the rest of the microbial world.
One viewpoint holds that since viruses are unable to multiply independently from the ___, they are not living things but should be called infectious molecules.
Another viewpoint proposes that even though viruses do not exhibit most of the life processes of cells, they can direct them and thus are certainly more than inert and ____ molecules.
This debate has greater philosphical than practical importance when discussing disease because viruses are agents of disease and must be dealt with through control, therapy, and _____, whether we regard them as living or not.
In keeping with their special position in the biological spectrum, it is best to describe viruses as either active or ____.
Recent discoveries suggest that viruses have been vital in forming ____ and other life forms as they are today.
Scientists believe that approximately ___% of the human genome consists of sequences that come from viruses that have incorporated their genetic material ___ into human DNA.
Bacterial DNA also contains ___% to ___% viral sequences.
10% to 20%
Viruses are different from their host cells in size, _____, and physiology.
Viruses are a type of obligate intracellular parasites that cannot multiply unless they invade a ____ and instruct its genetic an metabolic machinery to make and release quantities of new viruses.
specific host cell
Viruses are obligate intracellular parasites of bacteria, protozoa, fungi, algae, ____, and animals.
It is estimated that virus particles makes up ___times the amount of prokaryotes on Earth's surface.
Viruses are ubiquitous in nature and have a ___ on development of biological life.
Viruses are ultramicroscopic in size ranging from ____ nm up to ___ nm (diameter)
20 nm to 450 nm
Viruses are not ___; structure is very compact and economical.
Viruses do not independently fulfill the characteristics of ___.
The basic structure of viruses consist of a ______ or (_____) surrounding nucleic acid core.
protein shell or capsid
The nucleic acid in viruses can be either DNA or RNA, but not _____.
The nucleic acid in viruses can be either DNA or RNA that is ____ or ____.
single stranded or double stranded
Molecules on virus surface impart _____ for attachment to host cells.
Viruses multiply by taking control of host cell's _____ and regulating the synthesis and assembly of new viruses.
Viruses lack ____ for most metabolic processes.
Viruses lack machinery for synthesizing ___.
For many years, the animal viruses were classified mainly on the basis of their hosts and the kind of diseases they ___.
Newer systems for naming viruses also take into account the actual ____ of the virus particles themselves, with only partial emphasis on ____ and disease.
The main criteria presently used to group viruses are _____, chemical composition, and similarities in genetic make up.
In 2012, the International Committee on the Taxonomy of viruses issued a report on the classification of viruses. The committee listed ___ orders with ____ families and 143gfenera, plus another 71 families with 278 genera not yet assigned to any order.
Previous to 2000, there had been only a ____ recognized order of viruses.
As a group, viruses represent the ____ infectious agents (with some unusual exceptions to be discussed later).
Viruses are dwarfed by their host cells: More than ____ bacterial viruses could fit into an average bacterial cell, and more than ___ million pollio viruses could be accommodated by an average human cell.
Animal viruses range in size from the small ____ (around 20 nm) to the newly discovered _______, that are about the same size as common bacterial cell (1 um).
Some cylindrical viruses are relatively long (800 nm or .8um in length) but so narrow in diameter that their ____ is still limited without the high magnification and resolution of an electron microscope.
Viral architecture is most readily observed through special ___ in combination with electron microscopy.
Viruses bear no real resemblance to ___, and that they lack any of the protein-synthesizing machinery found in even the simplest cells.
Viruses molecular structure is composed of regular, repeating subunits that give rise to their _____ appearance.
The general plan of virus organization is the utmost in simplicity and ___.
Viruses contain only those parts needed to invade and control a host cell: an ____ coating and a ____ containing one or more nuclei acid strands of either DNA or RNA, and sometimes one or two _____.
All viruses have a protein ____, or shell, that surrounds the nucleic acid in the central core.
Together the capsid and the nucleic acid are referred to as the _________.
Members of 13 of the 20 families of animal viruses possess an additional covering external to the capsid called an _____, which is usually a modified piece of the host's cell membrane.
Viruses that consist of only a nucleocapsid are cconsidered ______.
Both naked and enveloped viruses possess proteins on their outer surfaces that project from either the _________ or ________.
nucleocapsid or the envelope
_____ are molecules that allow viruses to dock with their host cells and project from the outer surface of either a nucleocapsid or the envelope.
The enveloped viruses differ from naked viruses in the way that they enter and leave a _____.
A fully formed virus that is able to establish an infection in a host cell is often called a ___.
The simplest virus is a ____.
An envelope usually has special ____ inserted into it.
When a virus particle is magnified several hundred thousand times the capsid appears as the most ______ feature.
In general, each capsid is constructed from identical subunits called _____ that are constructed from protein molecules.
The _____ spontaneously self-assemble into the finished capsid.
Depending on how the capsomers are shaped and arranged, this assembly results in two different types: ____ and icosahedral.
The simpler ____ capsids have rod-shaped capsomers that bond together to form a series of hollow discs resembling a bracelet. During the formation of the _____, these discs link with other discs to form a continuous helix into which the nucleic acid strand is ____.
The nucelocapsids of _____ viruses are very rigid and tightly wound into a cylinder shaped package. An example is the tobacco mosaic virus, which attacks tobacco leaves (right).
______ nucleocapsids are more flexible and tend to be arranged as a looser helix within the envelope. This type of morphology is found in several _____ human viruses, including influenza, measles, and rabies.
These capsids form an ______ a three dimensional, 20 sided figure with 12 evenly spaced corners. The arrangement of the capsomers vary from one virus to another. Some viruses construct the capsid from a single type of capsomer, while others contain several types of capsomers.
____ is an example of a nake icosahedral virus.
Two very common viruses, hepatitis B virus and the herpes simplex virus both possess ______.
____, found in the viruses that infect bacteria , may have multiples types of proteins and take shapes that are not symmetrical. They are never enveloped.
When _____ (mostly animal) are released from the host cell, they take with them a bit of its membrane system in the form of an envelope.
With enveloped viruses, some viruses bud off the cell membrane; others leave via the ______ or the endoplasmic reticulum.
Whichever avenue of escape, the viral ____ differs significantly from the host's membranes.
In the envelope, some or all of the regular membrane proteins are replaced with special _____.
Some of the envelope proteins attach to the capsid of the virus, and glcoproteins remain ____ on the outside of the envelope.
These protruding molecules, called ___ when they are on enveloped viruses, are essential for the attachment of viruses to the next host cell.
Because the envelope is more ____ than a capsid, enveloped viruses are pleomorphic (of variable shape) and range from spherical to filamentous in shape.
The sum total of the genetic information carried by an organism is called its _____.
Viruses, although neither alive nor cells, carry genetic information by ____.
Unlike cells, which contain both DNA and RNA, viruses contain ____ DNA or RNA _____.
bot not both
Because viruses pack into a tiny space all of the genes necessary to instruct the host cell to make _____, the number of viral genes is quite small compared with that of a cell.
Viruses posses only the genes needed to ____ host cells and redirect their activity.
Bacteria and human cells have a much larger amount of genes which allow cells to carry out the complex _____ necessary for independent life.
Antibiotics ____ work on viruses because antibiotics cannot alter functions or processes that do not exist in viruses. (no cytoplasmic membranes to attack or no protein synthesis)
DNA viruses can have single stranded (ss) or double stranded (ds) DNA; the dsDNA can be arranged linerly or in ______.
RNA viruses can be double stranded but are more often ____.
Single stranded RNA genomes that are ready for immediate translation into proteins are called _____.
positive sense RNA
Other RNA genomes have to be converted into the proper form to be made into proteins, and these are called _____.
negative sense RNA
RNA genomes may also be ____ meaning that the individual genes exist on separate pieces of RNA.
A special type of RNA virus is called a ____, these viruses are distinguished by the fact that they carry their own enzymes to create _____ out of their RNA.
Variola virus causes _____ and is a double stranded DNA virus.
Herpes simplex II causes _____ and is a double stranded DNA virus.
Parvovirus causes ____ (skin condition) and is single stranded DNA.
Influenza virus causes ____ and is a single stranded (-) viruses.
Poliovirus causes ____ and is a single stranded (+) RNA virus.
Rotavirus causes _____ and is a double stranded RNA.
HIV causes ___ and is a single stranded RNA reverse transcriptase.
In all cases, these tiny strands of genetic material carry the blueprint for viral _____ and ______.
structure and functions
Basically, viruses are genetic parasites because they cannot multiply until their nucleic acid has reached the ____ of the host cell. At the minimum they must carry genes for synthesizing the ___ and genetic material, for regulating the actions of the host and the packaging of the mature virus.
In addition to the protein of capsid, the proteins and lipids of the envelopes, and the nucleic acid of the core, viruses can contain _______ for specific operations within their host cells.
Viruses may come with preformed enzymes that are required for _____.
Examples of preformed enzymes include _____ that synthesize DNA and RNA, and the replicases that copy RNA.
The AIDS virus comes equipped with _____ for snythesizing DNA from RNA.
Viruses completely lack the genes for synthesis of _____ enzymes. This deficiency has little consequence, because viruses have adapted to completely take over their hosts' ____ resources.
Some viruses can actually carry away ___ from their host cell. Like arenaviruses that pack along host ribosomes, and retroviruses that ____ the host's tRNA molecules.
The process of viral _______ is an extraordinary biological phenomenon. The nature of this cycle dictates the way the virus is _____ and what it does to its host, the responses of the immune defenses, and human measures to control viral infections
Viruses are minute ____ that seize control of the synthetic and genetic machinery of cells.
The phases of the life cycle of animal viruses are
penetration and uncoating
release from the host cell
The length of the entire multiplication cycle varies from 8 hours in polioviruses to ___ hours in some herpesviruses.
Because a virus can invade its host cell only through making an exact fit with a specific host molecule, the range of hosts it can infect is _____. This is known as the ____.
The host range, may be highly restricted as in the case of ____, which infects only liver cells of humans; moderately restrictive like the poliovirus, which infects intestinal and ___ cells of primates (humans, apes, monkeys); or broad like the rabies virus, which can infect various cells of all mammals.
Cells that lack compatible virus ____ are resistant to absorption and invasion by that virus.
This ____ explains why human liver cells are not infected by the canine hepatitis virus and dog liver cells cannot hos the human hepatitis A virus.
The host range also explains why viruses usually have ___ specificities called tropisms for certain cells in the body. The hepatitis B virus targets the liver, and the mumps virus targets the salivary glands.
The configuration of the spike s on viruses have a complementary fit for ____. This process in which the virus lands on the cell and plugs into receptors is termed ____.
Animal viruses exhibit some impressive mechanisms for entering a host cell. The flexible cell membrane of the host is penetrated by the ___ or its ____.
In penetration by _____, the entire virus is engulfed by the cell and enclosed in a vacuole or vesicle. When enzymes in the vacuole dissolve the envelope and capsid, the virus is said to be ____.
___ is a process that releases the viral nucleic acid.
The exact manner of uncoating varies, but in most cases, the virus fuses with the wall of the ____. Another means of entry involves direct fusion of the ____ with the host membrane (in influenza and mumps).
When the viral envelope is fused directly during uncoating, the envelope merges directly with the cell membrane, thereby liberating the ______ into the cell's interior.
In general, the DNA viruses (except poxviruses) enter the host cell's ____. and are replicated and assembled there. With few exceptions (such as retroviruses), RNA viruses are replicated and assembled in the ____.
The synthesis of new genomes and mRNAs for translation differ among the various types of RNA and ____.
The retroviruses turn their RNA genomes into ____. This step is accomplished by a viral enzyme called _____ and has important implications in infections with these viruses, one of which is HIV.
In the life cycle of dsDNA viruses, the synthesis phase is divided into two parts. During the early phase, viral DNA enters the ___, where several genes are transcribed into a messenger RNA. The newly synthesized RNA transcript then moves into the cytoplasm to be translated into ____ (enzymes) needed to replicate the viral DNA; this replication occurs in the nucleus. The host's own DNA polymerase is often involved, though some viruses (herpes) have their own. During the late phase, other parts of the viral genome are transcribed and translated into proteins required to form the capsid and ___. The new viral genomes and capsids are assembled, and the mature viruses are released by budding or cell deginteration. In some viruses, the viral DNA becomes silently integrated into the host's genome by ___ at a particular site on the host genome. This integration may later lead to the transformation of the host cell into a cancer cell and the production of a tumor.
____, this step actually puts together the new viruses using the "parts" manufactured in the synthesis process: new capsid and new nucleic acids.
___ is the viral release from the host cells.
The number of viruses released by infected cells is ____, controlled b factors such as size of the virus and health of the host cell.
About 3,000 to 4,000 virions are released from a single cell infected with poxviruses, whereas a poliovirus-infected cell can release over ____ virions.
If even a small number of these virions happen to meet another susceptible cell and infect it, the potential for rapid viral proliferation is ____.
____ effects are defined as virus-induced damage to the cell that alters its microscopic appearance.
Individual cells can undergo gross changes in shape or size, or develop ____ changes due to viruses.
It is common to find _____, or compacted masses of viruses or damaged cell organelles, in the nucleus and cytoplasm.
Examination of cells and tissues for cytopathic effects is an important part of the ____ of viral infections.
One very common CPE (cytopathic effect) is the fusion of multiple ___ into single large cells containing multiple nuclei. These ____ are a result of some viruses ability to fuse membranes. The respiratory virus, syncytial virus is even named for this effect.
Accumulated damage from a virus infection kills most host cells, some cells maintain a carrier relationship, in which the cell harbors the virus and is not immediately lysed. These so called _____ infections can last from a few weeks to teh remainder of teh host's life.
Viruses can remain latent in the cytoplasm of a host cell, or can incorporate into the ___ of a host.
When viral DNA is incorporated into the DNA of teh host, it is called a ____.
One provirus, the measles virus, can remain hidden in brain cells for many years, causing progressive damage and ____.
loss of function
Several types of viruses remain in a _______ state, periodically becoming reactivated. Examples include herpes simplex virus (cold sores and genital herpes) and herpes zoster virus (chickenpox and shingles). Both viruses can go into latency in ____ cells and later emerge under the influence of various stimuli to cause recurrent symptoms.
Some animal viruses enter a host cell and permanently alter it genetic material, leading to ____.
Experts estimate that up to ___% of human cancers are caused by viruses. These viruses are termed oncogenic, and tehir effect on the cell is called ____.
In microbial genetics, the transfer of genetic material contained in "naked" DNA fragments from a donor cell to a competent recipient cell.
Viruses that cause cancer in animals can act in several different ways. The virus can carry genes that directly cause the cancer. In some other cases, the virus produces proteins that induce a loss of _____ in the cell, leading to cancer.
Transformed cells have an increased rate of growth; alterations in chromosomes, changes in the cell's ____; and the capacity to divide for an indefinite period, unlike normal animal cells.
Mammalian viruses capable of initiating tumors are called ____.
Some oncoviruses are ____ viruses such as papillomavirus (genital warts associated with cerival cancer), herpesviruses (epstein-barr virus, and hepatitis B virus (liver cancer).
A virus related to HIV - HTLV I is also involved in ____. These finding have spurred a great deal of speculation on the possible involvement of viruses in cancers and other diseases such as multiple sclerosis.
A virus that specifically infects bacteria.
Fredrick Twort and Felix d'Herelle discovered bacterial viruses in 1915, it first appeared that the bacterial host cells were being eaten by some unseen parasite; hence the name _____.
Most bacteriophages contain ____ DNA, although _____ DNA and RNA types exist as well.
double stranded DNA
single stranded DNA
Every bacterial species is parasitized by various ____ bacteriophages.
Bacteriophages are of great interest to medical microbiologists because they often make the bacteria they infect more _____ for humans.
The most widely studied bacteriophages are those of the intestinal bacterial ______ especially the ones known as the T-even phages such as T2 and T4. They have an icosahedral capsid head containing DNA, a central tube (surrounded by a sheath), collar,, base plate, tail pins and fibers, which in combination make an efficient package for infecting a bacterial cell.
T-even bacteriophages go through similar stages as the ____ viruses.
T-even bacteriophages ____ to host bacteria using specific receptors on the bacterial surface. Although the entire phage does not enter the host cell, the nucleic acid penetrates the host after being injected through a rigid tube the phage inserts through the _____ and wall. This eliminates the need for uncoating. Entry of the nucleic acid causes the cessation of host cell DNA replication and protein syntheis. Soon the host cell machinery is used for viral ____ and synthesis of viral proteins. As the host cell produces new phage parts, the parts spontaneously ____ to bacteriophages.
An average size Escherichia coli cell can contain up to 200 new phage units at the end of the assembly period. Eventually, the host cell becomes so packed with viruses that it ____ thereby releasing the mature virions. This process is hastened by viral enzymes produced late in the infection cycle that digest the ____, thereby weakening it. Upon release, the virulent phages can spread to other susceptible bacterial cells and begin a new cycle of infection.
lyses - splits open
Bacteriophage infection may result in lysis of the cell. When this happens it is said that the bacteriophage has been in the ____ phase or cycle.
Alternatively, bacteriophages can be less obviously damaging, in a cycle called the _____ cycle.
In 2008, a new type of virus was discovered, _____. They parasitize other viruses that are infecting the same host cell they infect, using genes from other (usually larger) viruses for their own replication and production. Even though these are parasites of viruses, note that they must be in a ___, along with their host virus.
While special DNA phages, called ____ phages, can participate in a lytic phase, they also have the ability to undergo adsorption and penetration into the bacterial host and not undergo replication or release immediately. Instead, the viral DNA enters an inactive _____ state, reminiscent of the provirus state in animal viruses, during which it is inserted into the bacterial chromosome. This viral DNA will be retained by teh bacterial cell and copied during its normal cell division so that they cell's progeny will also have the temperate phage DNA.
This condition, in which the host chromosome carries bacteriophage DNA, is termed _____ because viral particles are not produces, the bacterial cells carrying temperate phages do not lyse and they appear entirely normal.
On occasion, in a process called ____, the prophage in a lysogenic cell will be activated and progress directly into viral replication and the lytic cells.
Lysogeny is a less deadly form of parasitism than the full _____ and is though to be an advancement that allows the virus to spread without killing the host.
Bacteriophages are just now receiving their due as important ___ of biological life.
Scientist believe that there are more bacteriophages than ____ forms of life in the biosphere combined.
Viral genes linger in human, animal, plant, and bacterial genomes in huge numbers. As such, viruses can contribute what are essentially ____ traits to the bacteria, so much so that it could be said that all bacteria-indeed all organisms are really hybrids of themselves and the viruses that infect them.
Many bacteria that infect humans are lysogenized by ______. Occasionally bacteriophage genes in the bacterial chromosome cause the production of toxins or _____that cause pathology in the human.
When a bacterium acquires a new trait from its temperate phage, it is called _____. The phenomenon was first discovered in the 1950s in the bacterium that causes diptheria, Corynebacterium diptheriae. The diphtheria toxin responsible for the deadly nature of the disease is a bacteriophage product.
Scientists have developed methods, which include inoculation of lab-bred animals and embryonic bird tissues and cell _____.
The primary purposes of viral cultivation are to _______ and _______ viruses in clinical specimens, prepare viruses for vaccines, and do detailed research on viral structure, multiplication cycles, genetics, and ______.
isolate and identify
effects on host cells
Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits are the usual choices for ____ cultivation of viruses.
Invertebrates (insects) or nonhuman primates are occasionally used for animal ____.
Because viruses can exhibit host specificity, certain animals can propagate a given virus more ____ than others.
A bird egg containing an embryo provides an _____ and self-supporting unit, complete with its own sterile environment and nourishment. It also furnishes several embryonic tissues that readily support ____. Chicken, duck, and turkey eggs are the most common choices for inoculation. The virus must be injected through the egg shell, usually by drilling a hole or making a small window.
The most important early discovery that led to easier cultivation of viruses in the lab was the development of a simple and effective way to grow populations of isolated animal cells in culture. These type of ____ cultivation systems are termed cell culture, or tissue culture.
Animal cell cultures are grown in sterile chambers with special media that contain the correct nutrients required by animal cells to survive. The cultured cells grow in the form of a ____, a single confluent sheet of cells that supports viral multiplication and permits close inspection of the culture for signs of ___.
For the first time in 2012, the FDA approved for general use a cell culture based vaccine for the seasonal ____.
One way to detect the growth of a virus in culture is to observe ____ and lysis of infected cells in the monolayer of cells.
The areas where virus-infected cells have been destroyed show up as __, well defined patches in the cell sheet called plaques.
_____ are essentially the macroscopic manifestation of cytopathic effects (CPEs).
The same technique is used to detect and count bacteriophages, because they also produce ____ when grown in soft agar cultures of their host cells (bacteria). These develop when the viruses released by an infected host cell radiate out to adjacent host cells. As new cells become infected, they die and release more viruses, and so on. As the process continues, teh infection spreads, gradually and ____ from teh original point of infection, causing the macroscopic appearance of round, clear spaces that correspond to areas of dead cells.
Not all noncellular infectious agents are ___.
One group of unusual forms even smaller and simpler than viruses, is implicated in chronic, persistent disease in humans and animals. These diseases are called ____because the brain tissue removed from affected animals resembles a sponge. This infection has a long period of latency (usually several ___) before the first clinical signs appear. Signs range from mental derrangement to loss of _____. The disease are progressive and universally fatal.
A common feature of spongiform encephalopathy is the deposition of distinct protein fibrils in the _____.
Researchers have hypothesized that these fibrils are the agents of the disease (spongiform encephalopathy) and have named them ______.
Creutzfeldt-Jakob disease afflicts the central nervous system of humans and causes gradual degeneration and ____.
death - spongiform encephalopothy
Bovine spongiform encephalophathy (BSE) or ________ was recently the subject of fears and a crisis in Europe when researchers found evidence that the disease could be acquired by humans who consumed contaminated beef. This was the first incidence of prion disease transmission from animals to humans.
mad cow disease
Several hundred Europeans developed symptoms of a variant form of Creutzfeldt jakob disease, leading to strickt controls on exporting _____ and ____ products.
cattle and beef
Prions are composed primarily of ____ and no nucleic acids.
_____ viruses are actually dependent on other viruses for replication.
Th adeno-associated virus (AAV) is an example of a _____ virus because i can infect cells that are infected with other viruses or that have had their DNA disrupted through other means.
The delta agent, is a naked circle of RNA that is expressed only in the presence of hpe B anc can worsen the severity of liver damage is an example of a ____.
Plants are also parasitized by viruslike agents called _____ that differ from ordinary viruses by being very small (about one tenth the size of an average virus and being composed of only naked strands of RNA, lacking a capis or any other type of coating.
Viroids are significant pathogens in several _____ important plants, including tomatoes, potatoes, cucumbers, citrus tress, and chrysanthemums.