Retroviridae - General Characteristics Flashcards
What enzyme do Retroviruses use to make DNA intermediates using their RNA genome?
Reverse Transcriptase
True/False. RT is an RNA-dependent DNA polymerase that is present within the virions of ALL members of the family.
This is true.
What type of infections do Retroviruses typically cause?
Latent infections
Describe the basic structure of a Retrovirus virion.
Virions are enveloped.
Three-layered structure:
- Innermost genome - nucleprotein complex with helical symmetry
- Surrounded by an icosadhedral capsid
- This is in turn surrounded by an evelope with glycoprotein spikes
Describe the genome of a Retrovirus.
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
Which genes do ALL non-defective retroviruses have?
Gag, pol and env genes
Which Retrovirus gene encodes the virion core proteins?
The gag gene
[core proteins: capsid, nucleocapsid, and matrix]
Which Retrovirus gene encodes reverse transcriptase and integrase?
The pol gene
Which Retrovirus gene encodes the virion envelope proteins?
The env gene
[envelope proteins: surface and transmembrane]
How do Retroviruses penetrate the host cell?
By membrane fusion
(less commonly involves receptor-mediated endocytosis)
What is a provirus?
Viral DNA that is integrated into host cellular chomosomal DNA
How is the provirus integrated into host DNA?
Through the action of virally encoded integrase enzyme and host-cell DNA repair mechanisms
The provirus serves as a template for the synthesis of which 2 things?
- mRNA for protein synthesis
- Positive-sense, single-stranded, linear DNA
Where does assembly and packaging of viral proteins and encapsidation of viral RNA occur for Retroviruses?
Cytoplasm
How do mature virions exit the host cell?
By budding through the host cell plasma membrane
True/False. Provirus is replicated with host genome an can be passed to daughter cells, resulting in transmission from one generation to the next.
True
True/False. Replicatin of retroviruses is accompanied by a low mutation frequency.
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]
What are endogenous retroviruses?
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]
True/False. Endogenous retroviruses are transmitted only as provirus in the germ line DNA (ova or sperm) from parent to offspring.
This is true.
Are endogenous retroviruses pathogenic?
Nope. They are non-pathogenic.
True/False. Some RNA tumor viruses belong to the family Retroviridae.
False.
**ALL **RNA tumor viruses belong to the family Retroviridae.
How are c-onc/proto-oncogenes inserted into the virus genome? What is the result of this insertion?
By recombination between proviral DNA and host DNA.
Results in a v-onc gene
Why are acutely transforming retroviruses directly oncogenic?
They carry an additional viral oncogene (v-onc)
How do v-onc genes have the power of unregulated expression?
v-onc genes are separated from the cellular machinery that normally controls gene expression (lacks introns)
What controls v-onc genes?
LTRs (large terminal repeats)
These are strong promoters
During recombination of acutely transforming retroviruses, the virus genome acquires the proto-oncogene, but which part of its genome does it most commonly lose?
The viral env gene
Why must most v-onc containing viruses associate with non-defective viruses to replicate?
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.
True/False. Slow/chronic transforming retroviruses do not contain v-onc genes.
This is true.
Where do slow/chronic transforming retroviruses integrate their retroviral genes into host chromosomal DNA?
At promotor or enhancer sites that drive the increase in proto-oncogene/c-onc gene expression, leading to malignant transformation of the cell.
