10 - Retrovirus & Hepadnavirus Replication Flashcards
1
Q
Retroviridae
A
- Enveloped (+)ssRNA genomes
- USe RT to synthesise cDNA intermediate
- Includes HIV and HTLV
- Exist in human genomes in endogenous or exogenous form
2
Q
Endogenous form of retroviral genomes
A
- Common in the genomes of humans (4-8%)
- Ancient relics of germ line infection that occurred millions of years ago
3
Q
Exogenous
A
Horizontally transmitted, infectious viruses (eg HIV) that integrate dsDNA, complimentary to their genome, into a host
chromosome for gene expression
4
Q
Retrovirus endogenisation and human endogenous retrovirus (HERV) formation
A
- During replication, retroviral RNA is reverse-transcribed into a double stranded DNA (dsDNA) provirus and integrated into the cellular genome
- All current human retroviruses target somatic cells, showing a horizontal transmission from an infected individual to new hosts
- The exogenous retroviruses that gave rise to HERVs were
also able to infect germ line cells
5
Q
Key components of HIV
A
- gp120-gp41
- tRNA lys
- Integrase
- Capsid (cone shaped)
- Matrix
- Reverse Transcriptase
- Nucleocapsid
- Diploid genome
6
Q
Diploid retroviral genome
A
- Diploid RNA genome (both copies identical)
- Coated with nucleoproteins
- Bound to tRNA, that is bound
to viral RNA at the primer binding site PBS and serves as primer for reverse transcription
7
Q
High genetic variability of HIV-1
A
- Due to fast replication cycle and high mutation rate
- May cause the generation of many viral variants in a single infected patient during a single day
8
Q
Major HIV group
A
Group M (main)
9
Q
HIV replication cycle part 1
A
- HIV-1 infects CD4+ T cells via binding of surface gp120 to CD4 receptor
- This triggers a conformational change that initiates gp120 binding to a co-receptor CCR5/CXCR4
- This triggers rearrangement of the gp120-gp41 complex such that the fusion peptide of gp41 inserts into the plasma membrane and the two fuse.
- The HIV nucleocapsid releases into the cytoplasm, releasing the viral RNA genome and enzymes (RT, integrase, protease) into the cytoplasm
- RT converts ssRNA into dsDNA which is transported into the nucleus where it integrates into a host chromosome using viral integrase
10
Q
HIV replication part 2
A
- Viral genes are transcribed by host pol II to produce eukaryotic mRNA and mRNA is transported to cytoplasm where translation occurs
- Some proteins enter the nucleus to affect gene expression
- Major structural proteins that form the internal parts of the virion are synthesised as polyproteins Gag, Gag-pol and Env.
- Protease cleaves Gag/Gag-pol polyproteins to produce infectious virus particles with a morphologically distinct core
- HIV-1 nef/vpu mediate translocation of cell-surface CD4 to the cytoplasm/lysosol allowing new virions to leave the cell
11
Q
Receptor binding in HIV entry
A
gp120 binds to CD4
12
Q
Two major domains of retrovirus RT enzyme
A
p66 and p51
13
Q
Two active sites of RT
A
- The polymerase active site
- The RNase H active site
14
Q
RNase H
A
- Binds to RNA in duplex (either RNA:RNA or RNA:DNA) and makes internal cleavages to break apart RNA template
- Essential in synthesis of complimentary dsDNA
15
Q
First 5 steps of reverse transcription in HIV-1
A
- (First strand (-) cDNA synthesis is initiated in a 5’ direction.
- RNase H activity degrades the small fragment of complimentary template RNA
- This allows for the first jump to occur. Complimentary R
(R’) within the newly synthesised (-)cDNA is complimentary to R in the 3’ end of the RNA template, allowing for RT extension - (-)cDNA extension occurs towards the 5’ end of the template
- With RNase H activity degrading the template RNA, leaving behind only the RNA complimentary to PPT
