Exam 1- Virus replication process

Question 1

What are the 3 mechanisms for synthesis of viral mRNAs by DNA viruses

Answer 1

• For dsDNA viruses that replicate in the nucleus, transcription of viral mRNAs is carried out by cellular RNA polymerase II.
• single-stranded DNA viruses are first converted to dsDNA by cellular DNA polymerase, then mRNAs are transcribed by cellular RNA pol II.
• For large dsDNA ciruses that replicate int he cytoplasm, viral mRNAs are transcribed by a viral transcriptase

Question 2

What is the enzyme In the host cell that is responsible for cellular mRNA transcription ?

Answer 2

RNA polymerase II

Question 3

How do DNA viruses regulate their gene expression?

Answer 3

They use temporal regulation.

Immediate early

Early genes

Late genes.

Question 4

During DNA replication, what are the advantages of temporal regulation of gene expression?

Answer 4

• gives viruses an advantatge when competing with host cell for gene expression
• Highly antigenic structural proteins are not expressed until genome replication is complete and ready for packaging and release

Question 5

For genome replication in DNA viruses, what are the strategies used for initiation of DNA synthesis by cellular DNA polymerase

Answer 5

DNA polymerases cannot initiate DNA synthesis without a primer.

• complementary termini (repeats/hairpins)
• Protein primer attached covalently to 5’-termini
• circular DNA genome

Question 6

What are pathways for synthesis of viral mRNA’s by RNA viruses?

Answer 6

• RNA viruses utilize virus-encoded enzymes for replication of RNA by one of two unique biochemical pathways”
• RNA-dependent- RNA synthesis (RNA replication)
• RNA-dependent - DNA - sysnthesis (reverse transcription - used by retrovirusess)
• Neither pathway is normally found in uninfected cells and enzymes used in the pathways must be encoded by the virus and expressed during infection
• RNA viruses with different types of genomes have necessarily evolved different routes to the production of viral mRNA.

Question 7

What are the mechanisms of regulation fo gene expression in RNA viruses

Answer 7

• in contrast to DNA viruses, there is no temporal difference in gene expression
• RNA viruses employ multiple strategies to compete with tht host cells for gene expression at the level of translation.

Question 8

What are the mechanisms RNA viruses use to inhibit and/or bypass cellular protein synthesis:

Answer 8

1. interference with formation fo the 5’-methyl guanosine cap on the cellular mRNA
2. Removal of the 5’-cap from the cellular messenger RNA molecules and using it on the virus’ own messenger RNA
3. Utilize internal ribosome entry sites that directly bind eIF-4G to initiate translation (hops into the middle)

Question 9

What are the steps of genome replication in RNA viruses

Answer 9

• Genome replication by RNA viruses is defined as the production of progeny viruse genomes. This requires the action of virus encoded RdRp
• Replication of viral RNA requires first the synthesis of complementary RNA, which then serves as a template for making more NRA.
• Retroviruses are unique RNA viruses that replicate through a DNA intermediate. This requires a virus-encoded enzyme, reverse transcriptase, which uses an RNA template to synthesize DNA.

Question 10

What are some important features regarding RdRp

Answer 10

• Either packaged into the virion and transported into the newly-infected cell, or synthesized very soon after infection.
• Lack proof reading and have a high error rate during genome replication
• Excellent targets for anti-viral drugs

Question 11

What strategies are used to maximize viral coding capacity

Answer 11

• polycistronic mRNAs- a single mRNA that encodes for more than one protein
• Splicing/alternative splicing of mRNA
• Alternate translational start sites
• ooverlapping reading frames
• translational frameshifting

Question 12

How do polycistronic messenger RNA’s function

Answer 12

Viral proteins are translated as precursor polyproteins, which are subsequently cleaved by viral or cellular proteases.

Question 13

How are envelope glycoproteins synthesized?

Answer 13

These are often translated as precursor polyproteins and undergo several modification before incorporation into the virion.

• Single peptide on N-terminus, which targets protein to the E.R. and Golgi
• Modified by addition of carbohydrates
• Cleaved by cellular proteases

Initial sugar residues are added in the ER and further processing of the carbohydrate moieties occur int he golgi.

Question 14

Following translation and post-translational modifications, the virus proteins and genomic nucleic acid must co-localize and assemble into a virus particle by what steps:

Answer 14

1. Formation fo individual structural units of protein shell
2. Self-assembly of the protein shell through interactions among structural units
3. Selective packaging of nucleic acid genome and other essential viral components
4. Acquisition of an envelope- only for enveloped viruses.

Question 15

Where are RNA viruses assembled

Answer 15

cytoplasm

Question 16

Where are DNA viruses assembled

Answer 16

Nucleus (except Poxviruses)

Question 17

What happens to the cell when a non enveloped virus leaves the cell

Answer 17

The cell must die (cellular lysis) and the cell membrane must degrade sufficiently that the virions are released.

Question 18

How do enveloped viruses leave the host cell?

Answer 18

the process of budding occurs. The viral components assemble near the viral nucleocapsid.

This is not necessarily lethal to the cell, though it often is.

Question 19

Define Susceptible

Answer 19

the cell is sensitive to infection by a particular virus. Largely a function of the presence or absence of receptors. Does not necessarily imply a productive infection

Question 20

Define permissive

Answer 20

Cell is able to support complete replication of a particular virus. This is a function fo the internal biochemistry of the cell, not the presence or absence of recptors.

Question 21

Define a productive infection

Answer 21

An infection fo a cell by a virus that results in the production of progeny virions

Question 22

Define cytocidal

Answer 22

virus replication results in damage to cell, resulting in death of the cell

Question 23

Define a persistent infection

Answer 23

virus infection that lasts a long time

Question 24

Define Latent

Answer 24

Virus infection in which viral genome is maintained, but few viral gene products are made and no virus is produced.

Question 25

List features of cytolytic infection

Answer 25

1. cytolytic viruses kill the cells in which they replicate
2. cytolytic viruses induce productive infection fo susceptible cells
3. The resulting cytopathic effect can usually be observed by light microscopy
4. The nature of the CPE may be characteristic of a particular virus, and can aid in diagnosis.

Question 26

what are the mechanisms of cell damage.

Answer 26

1. Inhibition of cellular metabolism
2. Toxicity of viral gene products
3. Alteration of cell membrane
4. Immune-mediated cytolysis
5. Damage to the cytoskeleton

Question 27

What are the types of cytopathic effects?

Answer 27

1. Cell death- virus- induced lysis is often a strategy for virus release and happens late in infection after production of progeny virions.
2. Formation fo syncytial or “giant” cells: Fusion of a virus infected cell with a neighboring cell can result in formation of multinucleated cell, often called polykaryocyte or giant cell.
3. Inclusion bodies. These can be intranuclear or intracytoplasmic, large or small etc.

• Viral inclusion bodies may be viral components or the result of virus- induced degenerative changes.

Question 28

What type of infection is a non-lytic infection?

Answer 28

Persistent infections. The virus production coexists with the cell without dramatically altering it.

Question 29

What are potential side effects of a non-cytocidal infection?

Answer 29

• Hormone production
• Cellular communication / immune function

(dysregulation of the cell signaling cascade, or causing immunodeficiency)

• Neuronal function

Question 30

Define Oncogene

Answer 30

Cellular or viral gene whose products are able to transform eukaryotic cells so that they will begin to grow like tumor cells

Question 31

Define Proto-oncogene

Answer 31

A gene in a normal eukaryotic cell that is believed to have given rise to an oncogene in a transforming virus

Question 32

Define Transduction

Answer 32

Transfer by a virus of cellular genes from one organism or cell to another

Question 33

In order for viral replication, what stage of cellular replication does the host cell need to be in ?

Answer 33

S1 phase to accomidate the need for elevated levels of cellular polymerases, transcriptional activating proteins etc.