Lecture 6 Flashcards
Why is it important for viruses the have small genomes?
Prokaryotic viruses - need to replicate as fast as bacteria
Eukaryotic viruses- need to fit into virion
Why are RNA virus genomes generally smaller than DNA’s?
- RNA is fragile
- RNA has higher mutation rate (due to RNA polymerase having higher error rate)
Why is mutation both good and bad for viruses?
To escape host defenses but may have a inhibitory mutation
What is the HIV mutation rate?
1 in 1,000 - 10,000
Why is this mutation rate considered high?
HIV replicated fast so within only a few days, many replictions have occured and hence many mutations have happened in quite a small genome - meaning the HIV has changed substantially
When can viruses interact?
During super infections - infection with more than one mutant
What can virla interaction lead to?
Complementation- Viral gene products of two viruses that leads to increased production for one or both virus. They remain genetically unchanged
Recmobination- Interactions between genomes resulting in genes that were not present in either viral parent genome
Describe the genome organisation of an example type 1 virus
Herpesvirus
Unique long (UL) and unique short (US) regions bounded by inverted repeats. These inverted repeats allows rearrangement of these regions. Genomic clustering of genes exressed in different stages of lifecycle. Cicular genome only tethered to host genome. Lytic cycle transactivator, RTA, which is inhibited by latent tethering protein LANA1. Cellular homologues.
Describe the genome organisation of an example type II virus
M13
Genome organised into frequently and infrequently transcribed regions
Host RNA and DNA polymerases converts +ve ssDNA into covalantly closed dsDNA called the replicative form. Host RNA polmeryase transcibes viral RNA from -ve DNA. Viral g2p protein nicks the RF DNA at the origin of replication and new +ve DNA is made and converted to RF DNA for further transcription. When enough g5p has been made g2p action is inhibited by coating +ve DNA in g5p for release.
Describe the genome organisation of an example type III virus
Rotavirus
Fragmented genome in 11 pieces encoding 12 proteins. 5’ cap but no poly-A tail; instead UGACC conserved sequence in all fragments. The fragmented structure with UGACC allows reassortment.
Transcription of DNA happens in lysosome, releasing +ve RNA strands for protein synthesis.
Describe the genome organisation of an example type IV virus
HCV
Essentially an mRNA (+ve ssRNA). Clustering of structural and non-structural. Very different to the complex HBV.
Describe the genome organisation of an example type V virus
Usually larger genome than +ve sense. Commly segments and can be ambisense.
Ebola
Uncomplicated genome but can infect many cell types.
Influenza
8 segments
Describe the genome organisation of an example type VI virus
HIV
Truly diploid. Rely on host polymerase. Requires cellular tRNAlys for initation of reverse transcription. Thier +ve strand RNA is not used as mRNA. Fifteen proteins. Three clustered segments; envelope, capsid and replication. Long terminal repeats at ends of genome; facilitate integration into host chromatin, 5’ end serves as promoter, 3’ mediates polyadenylation. Each LTR contains three regions: U3, R and U5. U5 is essentially for HIV transcription.
What is tat?
Trans-activator of transcription - an viral protein of HIV which enhances transcription by 100-fold; bing to tar element and enhancing elongation
What is nef?
Negative regualtory factor - inhibits viral transcription by binding LTR but also inhibits host transcription to prepare cell for HIV transcription at right time. Also has immunomodulatory function.
