Retroviridae - General Characteristics

Question 1

What enzyme do Retroviruses use to make DNA intermediates using their RNA genome?

Answer 1

Reverse Transcriptase

Question 2

True/False. RT is an RNA-dependent DNA polymerase that is present within the virions of ALL members of the family.

Answer 2

This is true.

Question 3

What type of infections do Retroviruses typically cause?

Answer 3

Latent infections

Question 4

Describe the basic structure of a Retrovirus virion.

Answer 4

Virions are enveloped.

Three-layered structure:

1. Innermost genome - nucleprotein complex with helical symmetry
2. Surrounded by an icosadhedral capsid
3. This is in turn surrounded by an evelope with glycoprotein spikes

Question 5

Describe the genome of a Retrovirus.

Answer 5

Genome is diploid

Consisting of a dimer of two molecules of linear positive-sense, singlestranded RNA

Genomic RNA has a 3’-polyadenylated tail and 5’-cap

Question 6

Which genes do ALL non-defective retroviruses have?

Answer 6

Gag, pol and env genes

Question 7

Which Retrovirus gene encodes the virion core proteins?

Answer 7

The gag gene

[core proteins: capsid, nucleocapsid, and matrix]

Question 8

Which Retrovirus gene encodes reverse transcriptase and integrase?

Answer 8

The pol gene

Question 9

Which Retrovirus gene encodes the virion envelope proteins?

Answer 9

The env gene

[envelope proteins: surface and transmembrane]

Question 10

How do Retroviruses penetrate the host cell?

Answer 10

By membrane fusion

(less commonly involves receptor-mediated endocytosis)

Question 11

What is a provirus?

Answer 11

Viral DNA that is integrated into host cellular chomosomal DNA

Question 12

How is the provirus integrated into host DNA?

Answer 12

Through the action of virally encoded integrase enzyme and host-cell DNA repair mechanisms

Question 13

The provirus serves as a template for the synthesis of which 2 things?

Answer 13

1. mRNA for protein synthesis
2. Positive-sense, single-stranded, linear DNA

Question 14

Where does assembly and packaging of viral proteins and encapsidation of viral RNA occur for Retroviruses?

Answer 14

Cytoplasm

Question 15

How do mature virions exit the host cell?

Answer 15

By budding through the host cell plasma membrane

Question 16

True/False. Provirus is replicated with host genome an can be passed to daughter cells, resulting in transmission from one generation to the next.

Answer 16

True

Question 17

True/False. Replicatin of retroviruses is accompanied by a low mutation frequency.

Answer 17

FALSE. Replication of retroviruses is accompanied by a HIGH mutation frequency.

[principally due to the lack of a 3’- to 5’ exonuclease proofreading mechanism by RT]

Question 18

What are endogenous retroviruses?

Answer 18

AKA: retro-elements

DNA stretches found widely in genome of most vertebrates that closely resemble retroviruses.

[presumably represent vestiges of retroviral DNA integration events throughout the course of evolution]

Question 19

True/False. Endogenous retroviruses are transmitted only as provirus in the germ line DNA (ova or sperm) from parent to offspring.

Answer 19

This is true.

Question 20

Are endogenous retroviruses pathogenic?

Answer 20

Nope. They are non-pathogenic.

Question 21

True/False. Some RNA tumor viruses belong to the family Retroviridae.

Answer 21

False.

**ALL **RNA tumor viruses belong to the family Retroviridae.

Question 22

How are c-onc/proto-oncogenes inserted into the virus genome? What is the result of this insertion?

Answer 22

By recombination between proviral DNA and host DNA.

Results in a v-onc gene

Question 23

Why are acutely transforming retroviruses directly oncogenic?

Answer 23

They carry an additional viral oncogene (v-onc)

Question 24

How do v-onc genes have the power of unregulated expression?

Answer 24

v-onc genes are separated from the cellular machinery that normally controls gene expression (lacks introns)

Question 25

What controls v-onc genes?

Answer 25

LTRs (large terminal repeats)

These are strong promoters

Question 26

During recombination of acutely transforming retroviruses, the virus genome acquires the proto-oncogene, but which part of its genome does it most commonly lose?

Answer 26

The viral env gene

Question 27

Why must most v-onc containing viruses associate with non-defective viruses to replicate?

Answer 27

Because when the v-onc gene is inserted another viral gene is lost (gag, pol, env). Thus the viruses are unable to synthesize a complete envelope and are replication defective.

Question 28

True/False. Slow/chronic transforming retroviruses do not contain v-onc genes.

Answer 28

This is true.

Question 29

Where do slow/chronic transforming retroviruses integrate their retroviral genes into host chromosomal DNA?

Answer 29

At promotor or enhancer sites that drive the increase in proto-oncogene/c-onc gene expression, leading to malignant transformation of the cell.