CBG20 Flashcards
What is a virus?
a small non cellular agent largely consisting of nucleic acid with a protein case, requiring a host cell for reproduction.
* generally not considered alive.
Why was a virus originally considered not to be alive?
as it had a capsid, no ribosomes, no respiration no metabolism small size self assembly = not alive.
Give two examples of viruses which go against the original defintion of ‘alive’
Pandora virus has no capsid.
mimivirus viruses larger than bacteria
What are the characterisics of an RNA genome?
tend to be smaller *can be bigger than 30Kb
doesnt have proof reading capabilities of dna
consequently higher mutation rate
consequently energy off offspring different to parent
therefore is a quaisispecies. - virus does not exist as a single sequence but as a pool of sequences.
This is why its hard to make vaccine for eg. HIV
What do viruses with small genomes tend to have?
overlapping genes
frame shift
code for more than 1 protein.
What is the genome diversity of viruses?
- DNA or RNA
- ds/ss
- circular (bacterial) or linear (animal)
- (+)/(-) sense.
- segmented/non segmented
What is a (+) sense virus?
analagous to mRNA
translated directly
What is a (-) sense virus?
needs to become (+) sense before translated which influences the life cycle.
What is a segmented genome?
genome on lots of pieces of nucleic acid, each segment usually codes for one protein and found together in a capsid.
* however all segments are not required to be in the same virion for the virus to be infectious.
What are the advantages of a segmented genome for the bacteria?
allows for dramatic genetic changes by reassortment of its genome.
Allows it to undergo antigenic shift - viruses can exchange genes upon coinfection
Describe the influenza virus
negative sense virus with segmented RNA genome
7/8 segments depending on the virus.
evolve by mutation, recombination and reassortment.
What is a virion?
RNA/DNA and capsid
Virion partical simplicity is not a reflection of genomes
- sometimes envelope of lipids that surround protein coat.
When does antigenic shift occur?
give an example
When RNA segments are exchanged between viral strains in a secondary host.
eg. avian H3 and human H2
H= Hemagglutin type of envelope.
What kind of genomes do retroviruses have?
RNA positive sense non segmented genomes enveloped virus evolve by mutation and recombination
What kind of RNA are most RNA viruses?
Give an exception
ss
apart from reovirus
*RNA viruses loop so are double stranded in places.
What kind of DNA are most DNA viruses
Give an exception
ds
exceptions are some small viruses (less than 4000bp)
eg. parvovirus
What is SV40 an example of?
a circular dsDNA virus
= found in monkeys and humans
- cells develop unusual number of vaculoles = potential to cause tumours.
What do the simplist virus have?
Give an example?
a genome surrounded by a capsid (coat)
eg. TMV
Tobacco mosaic virus - + rna virus, infects plants
mosaic like molting and discolouration in leaves
*first virus ever discoverd
the capsid protects the nucleic acid.
How can viruses be viewed?
EM
ussually too small for light
What happens if RNA of virus is put into a test tube with capsid monomers and shaken?
self assembly.
What is a zoonotic virus?
animal virus that can be transmitted to humans
- ussually worse for new host.
- viralence decreases over time
- new virus = high mortality rates as havent been exposed to it before.
What can capsid proteins form?
one or more icosahedral conformations
What is WNV
West Nile Virus
mosquito born zoonotic arbovirus (transmitted by arthopod vector)
virus can be transmitted from virus to bird and back to virus as birds= carriers dont ussually die.
humans are dead end carriers
virus in mammals is ussually not sufficient to be transmited back to mosquito.
Describe the mimivirus
related to virus like pox large 900 genes genome 1mbp blurs distinction between cellular and life viruses
