CSIM 1.21 Viruses and Population level Flashcards
What are the theories of virus origins?
- Regressive hypothesis
- Progressive hypothesis
- Virus first hypothesis
Not mutually exclusive
Describe the regressive hypothesis of viral origins
- Viruses were previously independent life forms
* These life forms lost many functions and only retained what they needed for a parasitic lifestyle
Describe the progressive hypothesis of viral origins
Viruses come from intracellular functional assemblies of macromolecules that gained the capacity to move from cell to cell
Describe the virus first hypothesis of viral origins
What backs up this hypothesis
Viruses evolved from primitive pre-biotic self-replicating molecules and evolved simultaneously to cellular evolution
Viruses can be traced back to before the first known common ancestor in the tree of life. There was an ‘RNA world’ floating in the ocean before cells came about, and there may have been some kind of replication mechanism here (IMG 51)
What are the mechanisms of viral evolution?
- Mutation
- Recombination
- Reassortment
- Selection
Describe which viruses have the higher mutation rate
- High turnover viruses
- RNA viruses
- Those with smaller genome sizes
Why do RNA viruses have a higher rate of mutation than DNA viruses?
Because RNA polymerase has no proof-reading capability, like DNA polymerase has
What is a quasi species?
A ‘spectrum’ of mutant RNA viruses derived from the dominant master copy, due to the absence of proofreading
What are the types of viral mutation?
IMG 52
By the two types of antigenic drift:
Type A is faster:
• Has both genetic drift and genetic shift
Type B is slower
• Has only genetic drift
Describe what is meant by recombination and which viruses undergo this
Usually RNA viruses
DNA viruses:
• Two types of the same virus will split their DNA at the same point and ‘swap’ chunks of DNA
RNA viruses:
• ‘Copy choice’ IMG 53
• During RNA replication, the RNA polymerase will begin replicating from one RNA template, and then ‘switch’ to the same point on a different viral RNA template
• This RNA is then transcribed giving a different daughter virus
Describe what is meant by reassortment
- Viruses often have different segments to its genome, and isn’t just one long piece of genetic information
- Viruses can therefore swap different segments into daughter viruses
(NB: this is what leads to H1N1, etc.)
IMG 54
How does zoonosis occur in influenza (e.g. swine flu)?
Through reassortment:
• One influenza adapted to another species (e.g. birds) infects the same cell as a second virus adapted to humans
• Reassortment can then occur, whereby a virus is produced which is adapted to replicating in human cells, but contains antigens which human immune systems are naive to
IMG 55 & 56
Describe what is meant by genetic drift and genetic drift
Genetic drift:
• Slow accumulation of mutations due to copying errors and immune selection
• Causes seasonal variability
Genetic drift:
• Major genetic changes from recombination or reassortment
• Causes pandemics
Describe what is meant by selection
Positive and negative selective pressures for certain variants
How does PCR check for a specific virus’ DNA?
The primer used to replicate the template will be complimentary if the correct DNA is used
