Heaphy 7 virology genomes Flashcards
Genomes, sizes types organisation:
+RNA genome virus replication
-RNA genome replication
Segmented/multipartite/ambisense
Small/medium/large DNA genomes
positive +sense
same polarity (nucleotide sequence) as mRNA , can be replicated straight away by ribosomes.
negative -sense
=> can’t be read by ribosomes straight away => RNA copies to +/ve sense for ribosomes to read them
Nucleocapsid:
• Nucleic acid is a polyanion (ionic). Often associates with basic proteins to facilitate packaging, i.e. histones in eukaryotes. but for viruses
Organisation:
- Very condensed genomes
- number of strategies to maximise gene coding capacity, including:
- Overlapping genes(start translating in multiple reading frames & places); using different reading frames, reading both strands
- Differential RNA splicing e.g. 2 splice sites
- No splice = 4 exons
- 1 splice => 1 2nd splice=> 2 proteins
CONSTRAINTS
key event = synthesis of viral proteins by host cell.
• virus must present mRNA to cell => translate & present its genome for packaging by viral proteins. => new particle.
• Control signals must be appropriate to host.
Constraints for an RNA genome in animal cells?
RNA made in nucleus. Template encoded RNA not made in cytoplasm. Cells cannot make DNA or RNA from an RNA template no enzyme. So cannot replicate in nucleus or cytoplasm without other viral factors. Need RNA polymerase made by self
Constraints for a DNA genome?
Enzymes replicating and transcribing DNA are present in nucleus. So either must get the DNA genome to the nucleus or make their own polymerases. In cytoplasm need all machinery for transcribing encoded in virus gene so unlikely.
Eukaryotic cells only translates
monocistronic messages. Virus therefore
• make a polyprotein => cleaved,
• different message for each protein
• polycistronic messages can be read e.g. internal initiation sites can be used
Caliciviruses
+/vestrans
Subgenomic monocistronic mRNAs made by differential RNA splicing. Early messages, mRNA 1 code for non-structural enzymic functions
Picorna
+/ve strand
Single mRNA made and a polyprotein is translated. A viral
protease in the polyprotein cleaves the polyprotein to make
viral proteins
Internal ribosome entry
+/ve strand
Single mRNA made but contains internal ribosome entry sites
+ve sense RNA.
- ss RNA genomes 3.5kb- 30kb.
- (+)sense vRNA directly infectious in absence of virus proteins.
- untranslated region (UTR)
- Both ends of (+)strand eukaryotic virus genomes often modified.
- 5’ end by a small, covalently attached protein or a methylated nucleotide ‘cap’ structure. The 3’ end by polyadenylation. Modifications allow vRNA to be recognised by host cells & to function as mRNA.
untranslated region (UTR)
at 5’ &3’ end of the genome, does not encode proteins. Functionally important in replication. both ends often modified poly…
–ve strand RNA
- more diverse than +stranded viruses.
- larger genomes, more genetic information. Segmentation a common but not universal feature.2+ strands for genome
- Replication more complicated. –ve sense RNA genomes are not infectious as purified RNA.