SH8: Virus genomes Flashcards
What is the range of size of virus genome?
3,500 nucleotides long too 2,500,000 nucleotides long
Why are viral RNA genomes smaller in size than viral DNA genomes?
Viral RNA polymerases are error prone compared to DNA polymerases which are much more accurate. By making RNA genome that is smaller it reduces the chance of error (replication fidelity).
Why do DNA viruses tend to replicate in the nucleus?
All the enzymes needed for DNA synthesis is in the nucleus
Due to the condensed nature of virus genomes, how is the genome maximised?
Overlapping genes, on a double strand the same point for each strand codes for a different gene, different reading frames, differential RNA splicing (gaining different genetic coding from the same strand of genome)
What is meant by monocistronic and polycistronic?
Monocistronic- one RNA transcript codes for one protein.
Polycistronic- one mRNA codes for several proteins.
Viruses genomes are polycistronic. This genome will not be read in a eukaryotic host cell which is monocistronic. What do the viruses have to do to ensure all proteins are made by the host cell?
- Make a polyprotein (coding for all the different proteins of the virus particle) which can be cleaved
- Present a different message for each code using splicing
- Make sure the polycistronic messages can be read by using internal initiation sites
What is segmented genome?
Segmented genome has 2 or more segments of nucleic acid packaged into the same particle- orthomxyo viruses
What is multipartite genome?
Multipartite genomes are segmented but each segment is contained in different viral particles. To establish a successful infection, the cell must be infected by call particles contraining different nucleic acid segments- bipartite comoviruses
What is ambience genome?
Ambisense genomes are both positive and negative- arenavirus
How is single stranded positive RNA viral genome modified to fit in with the mRNA of the host cell?
Both ends of the viral RNA strand are often modified (5’ cap, methylated nucleotide or 3’ tail, poladenylation).
What is a replicative intermediate (RI) or replicative form (RF)?
Occurs in the replication of positive and negative single stranded RNA virus genome. When polymerase copies the positive/negative RNA strand into the opposing charged RNA strand, in order to make a copy of the original strand. This is to replicate the genome
Describe small DNA genome.
Small DNA genomes (bacteriophage m13) is positive single stranded circular strand. 90% of the genome is coding. Virus infects cell, DNA polymerase copies it to the double stranded DNA form (RF), mRNA’s made by copying the double stranded DNA. When gene 5 is coded for, it stops proteins being made and allows more genetic code to be made- it is called ‘rolling circle replication’ as the DNA polymerase continues to move around the long piece of DNA copying it making more DNA.
Describe medium DNA genomes.
Medium DNA genomes (phage lambda) is double stranded linear DNA. It has cohesive sequences at both the 5’ and 3’ end. Once entered into the cytoplasm of a host cell, it circularises forming ‘concatemers’ (circular DNA), as it has sticky ends so is complimentary. mRNA and proteins are made from circular form. Later moves to rolling circle replication, replicating both strands.
Describe large DNA genome.
Large DNA genomes (phage T4), are linear double stranded DNA. Genome exhibits terminal redundancy, meaning repeated units of the same sequences surrounding the core genome. Uses an RNA primer to initiation DNA synthesis. Gets rid of RNA primer at 5’ end once synthesized, so genome gets shorter. If genome was continuously synthesized then eventually all terminally redundant sequences would be eaten into and the core genome would be gotten rid of. Concatemers are formed to stop the eating of the core genome.
