Lecture 7 - Virology Genomes Flashcards
What are the three types of RNA genome?
Negative sense
Positive sense
Ambi sense
What are the possibilities for a viral genome?
Dna Rna Single stranded Double stranded Circular Segmented
What does negative sense genome mean?
The mRNA is in opposite polarity to the viral genome, meaning it can not be read by ribosomes
What is meant by a positive sense genome?
The mRNA is in the same polarity as the viral RNA genome meaning it can be directly read by the ribosomes.
What size are viral genomes?
3500base pairs -2.5 million base pairs long
What is the maximum size of an RNA genome?
30kb
Why are RNA virus genomes smaller?
Viral RNAP is very error prone meaning that replication fidelity limits the size of the genome.
Where do RNA viruses usually replicate?
The cytoplasm
Where do DNA genomes usually replicate?
The nucleus
What does nucleic acid often associate with to facilitate packaging?
Basic proteins
What are the strategies to maximise gene coding capacity utilised by viruses?
Overlapping genes
Using different reading frames
Differential RNA splicing
Reading both strands
What are the constraints for an RNA genome in animal cells?
Template encoded RNA is not made in the cytoplasm
Host Cells can not make DNA or RNA from an RNA template
How do RNA viruses conquer their constraints?
The use of viral factors
What are the constraints for a DNA genome?
Enzymes replicating and transcribing DNA are present in the nucleus therefore just get the DNA into the nucleus or make their own polymerases
How can a virus replicate in the cytoplasm?
They must have all their functions with them so they can function as mRNA in the cell.
Generally eukaryotic cells only translate monocistronic messages therefore viruses must:
Make a polyprotein which can be cleaved
Have a different message for each protein
Make sure that internal initiation sites can be used
Is positive sense vRNA directly infectious?
Yes
How is the ends of eukaryotic postive sense RNA viruses modified?
5’ end is capped with a methylated nucleotide
3’ end is tailed with polyadenylation
What are the characteristics of negative sense RNA virus genomes?
More diverse than positive strand
Larger genomes
Segmentation is common
Replication is more complicated
Outline the replication process for a negative sense rna virus
They must contain accompanying virions
First the genome is copied with the viruses own RNAP
These form either
A positive sense transcript used directly as mRNA
A double stranded molecule known as the replicative intermediate which serves as a template for further rounds of mRNA synthesis
What is a segmented genome in a virus?
When genomes have 2 or more pieces of nucleic acid packaged in the same particle
Describe the lamda phage
Linear double stranded DNA genome
T4 phage
Large linear ds DNA genome
Genome exhibits terminal redundancy
What is an adenovirus?
A virus which is genetically very similar to the host cells which they infect.
