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Obligate intracellular parasitic molecules; replication can only occur within another living cell, considered non-living; 5,000 known to man


Central Core of a Virus

- Genome: The nucleic acids found inside a virus will store information that will direct the host cell to replicate the viral parts (numerous times)
- Enzymes: Some are carried along with the nucleic acid; used to allow entry to a cell or to replicate its own nucleic acids (e.g., DNA and RNA polymerase)



Outer protein coat found on all viruses; constructed from identical building blocks (capsomeres) which are composed of smaller protein molecules



Form viral capsids spontaneously, forming viruses into nearly perfect geometric shapes; rod-shaped form a helical capsid



Capsid shape; 3-dimensional, 20-sided polygon with 12 evenly spaced corners



Proteins that project outward into the environment and are essential for viruses to attach to a host cell


Enveloped Viruses

Viruses that have a bilayer surrounding their capsid


Naked Virus

Type of virus that does not have an envelope


Functions of Capsids and Envelopes

- Both protect the nucleic acid from various enzymes and chemicals outside the host cell
- Both allow the virus to attach to host cells and help the virus penetrate the host cell


Host Range

The organisms or cell types a virus can infect; usually limited to only a few cell types or species or even strains


Productive State

New viruses are being produced by the host cell:
- Host cell destroyed by lysis when new viruses are released (lytic cycle)
- Host cell continues to divide while releasing viruses


Latent State

No new viruses are produced by host cell:
- Viral nucleic acid is incorporated into host chromosome
- Host cell continues to divide, replicating viral DNA with each new cell (lysogenic cycle)



Broadest category into which viruses are divided into:
- Those that contain DNA (7 families)
- Those that contain RNA (13 families)
- Family names end with Latin root: viridae (capitalized and underlined)


Characteristics Used to Classify Viruses into Families

- Shape of capsid
- Overall viral size
- DNA/RNA viruses
- Presence of cell envelope
- Type of host cell it will infect
- How the virus effects the host cell (e.g., productive, latent)



Viruses that infect bacteria


Adsorption (Bacteria)

The bacteriophage's spikes on the capsid (all are naked) attach to bacterial receptor proteins; viruses are not capable of locomotion, they must collide with a host cell by chance; this must occur before any other productive phase of infection can occur


Penetration (Bacteria)

Entrance of viral nucleic acids into cytoplasm of bacterial host cell:
- The enzyme lysozyme digests its way through a small portion of bacterial cell wall
- Viral nucleic acids are injected through the cell membrane of the host
- Viral nucleic acid enters cytoplasm but capsid remains on the outside of the cell


Transcription of Viral DNA and Production of New Viral Parts (Bacteria)

- DNA of virus is transcribed and translated using the machinery of the host cell
- Phage-induced proteins are produced
- The phage DNA is replicated and phage capsid is produced
- All virus particles are produced independently of one another, including all DNA, capsomeres, binding proteins, etc.


Phage-Induced Proteins

Contain new genes that are unique to the virus and not the host cell; code for the production of proteins that perform specific viral functions



An enzyme that breaks apart and destroys host cell DNA; essential for the replication of viral DNA and capsid


Maturation (Bacteria)

Assembling virus particles into mature viruses:
- Some particles spontaneously assemble themselves
- Others use enzymes to join the particles together


Mature Viruses are Released (Bacteria)

- Lysozyme is produced and begins to digest cell wall from inside out
- Osmotic pressure causes the cell to rupture expelling the mature viruses


Burst Size

The number of phages released per cell when it lysis due to viral infection (approximately 200)


Adsorption (Animal)

A recognition process between a virus and a host cell that results when the virus attaches to the external surface of the host cell


Receptor Sites

Specific proteins found on the cell membrane of a host that a virus attaches to; may have to attach to two (virus may have more than one type of spike); normally used by the host cell for other functions


Penetration/Uncoating (Animal)

Entrance of the virus into the host cell



Process by which cells take up particles by enclosing them in a vesicle pinched off from the cell membrane; virus is engulfed in a vesicle of the host cell's making



A process in which enzymes found within the host cell will dissolve the capsid and envelope (if present) and release the nucleic acid strands into the cytoplasm



The viral envelope fuses with the cell membrane of the host; only occurs with enveloped viruses; only capsid and genome enters host's cytoplasm; capsid is uncoated releasing the nucleic acid


Transcription of Viral DNA and Production of New Viral Parts (Animal)

The copying and expression of the viral genome using the hosts equipment that results in the production of various virus components:
- The viral genome is expressed and copied
- Viral enzymes, necessary for the production of viral parts, are produced
- Viral parts are constructed, including capsomeres and spikes


Maturation (Animal)

Assembly of individual viral parts into a whole virus:
- Viral enzymes and nucleic aced strands are surrounded by capsomere components
- If the virus is enveloped, then spikes are placed within the cell membrane of the cell


Release (Animal)

Escape from the host cell to the environment; burst size - 3,000 to 50 million from one cell



Process enveloped viruses use to escape a cell:
- A vesicle (cell membrane) is placed around the virus by the cell (membrane contains spikes)
- Viruses are released slowly without the sudden destruction of the cell
- Can be destructive (not always) to the cell because of the accumulation of damage to cell membrane or the membrane of organelles; can shut down metabolism and genetic expression; toxicity of viral components


Virulent Viruses

Viruses that will always be productive and cause cell lysis; a rapid, severe, and destructive virus


Latent Virus

A nonactive virus which is in a dormant state within a cell; do not lyse host cells; incorporate their DNA directly into the chromosomes of the host cell (lysogenic cycle); e.g., herpes virus



A bacteriophage that is latent (lamda virus infects E. coli)



An animal virus that is latent (HIV, herpes)


Temperate Viruses

Viruses that are capable of causing lysis or becoming latent; whether the virus will lysis the cell or become latent is usually due to chance but modifying environmental conditions may play a role in which type is more often expressed


Lysogenic Conversion

With a latent virus, some genes are expressed and the living host cell acquires new proteins, and hence, new characteristics that it does not normally have


Repressor Gene

A viral gene that is expressed by a latent virus; codes for a repressor protein that prevents transcription of a gene that produces an excise enzyme


Excise Enzyme

An enzyme that has the ability to remove viral DNA from a host chromosome; once viral DNA is removed from the host DNA, the virus becomes virulent and new viruses are produced and assembled



The process of converting a latent virus into a virulent one; Lysogenic cycle ----> Lytic cycle


Acute Infections

Viral infections that cause disease symptoms for short periods of time:
- Are productive
- Can cause extensive tissue damage and cell death, but usually localized
- Host organism can recover and does not usually die from exposure
- Much of the damage can be repaired after recovery
- The body can develop life-long immunity
- Ex. mumps, measles, influenza (flu), cold viruses


Persistent Infections

A specific virus is present within the host for long periods of time (life-long in some cases):
- Can be productive or latent depending on the state of the disease
- Disease symptoms may not be apparent
- The host is a persistent source of infective viruses
- Ex. hepatitis C, HIV


Late Complications Following Acute Infection

A chronic infection where a person develops an acute viral disease, recovers, and years later will relapse (possibly with other disease symptoms); e.g, acute measles, 1-10 years later some develop subacute sclerosing panencephalitis (SSPE), which is a fatal brain disorder


Latent Infections

A chronic infection where a person develops an acute viral disease, recovers, and days, months, or years later will develop another, similar infection:
- Cannot be detected until disease onset
- e.g., herpes simplex virus 1 causes cold sores, fever blisters; as the virus heals, some viruses leave the mucous membrane of the mouth and enter neurons (will not harm); viruses are prevented from replicating
- Stress, menstruation, fever, sunburn, can all trigger the movement of viruses from the neuron, reentering the mucosa and cause more blisters``


Chronic Infections

The virus is present at all times, where the person may or may not show symptoms of the disease, but the host is continually affected; e.g., hepatitis B & C, after an acute phase (nausea, fever) the virus persists and can slowly cause cirrhosis of the liver


Slow Infections

The virus number slowly increases over a long period of time, finally accumulating to a lethal point where the fatal disease is caused; e.g., HIV... no symptoms... AIDS



Abnormal condition of unregulated cell division



A mass of newly dividing cells:
- Benign: cells remain within a localized region
- Malignant: the cells migrate to other locations (metastasize)


Proto-Oncogenes (accelerator)

Type of regulatory gene that activates gene transcription and increases the rate of cell division; if this gene is always turned on then the cell can become a cancer cell


Tumor Suppressor Genes (brake)

Type of regulatory gene that suppresses cell division; if this gene is turned off, then a cell can become a cancer cell


How Regulatory Genes are Altered

- Mutation: cause 80% of all human cancer (50% on tumor suppresor, 30% on proto-oncogenes)
- Viruses: cause 15% of all human cancers (probably more)
- The rest may result from genetic predisposition



Viruses that invade a host and transform it into a cancer cell; latent and will incorporate its genetic material into the DNA of the host cell



Genes of viral DNA that disrupt the normal functioning of proto-oncogenes in host cells and produce cancer cells (proto-oncogenes permanently turned on)



Cancer causing virus; single strand of RNA; uses enzyme reverse transcriptase to form a double stranded DNA molecule and then it is incorporated into host chromosome; these viral proteins can modify host cell function



A circular molecule of RNA without a capsid:
- Don't produce proteins
- Cause several economically important plant diseases
- None known to infect animals
- 1/10 the size of the smallest plant virus
- How they cause disease is a mystery but one theory is that it interacts in some way with the host genome, changing the expression of the host genes to cause disease



Infectious agent composed only of protein:
- Affect the central nervous system
- Normal cellular protein are destroyed by the host cell proteases
- Infectious agent's proteins are not easily broken down and accumulate in the cell
- Not known exactly how it causes disease but one theory is that the infectious agent's proteins attach to normal proteins and convert their shape into the agent's shape



An enzyme that breaks down proteins


Induction Triggers

Often unknown, but usually related to:
- decreased immunity
- stress
- fever
- UV exposure