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Virology 1.5-Genetics
Some terminology from the last part of
Chapter 3
Mutagenesis
The production of mutants
Spontaneous vs. induced
In vivo or in vitro mutagenesis
Wild type (wt or +) vs mutants
Some types of mutations
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Deletion
Insertion
Substitution
Missense/nonsense
Polar
Resistance/dependence
Temperature-sensitive or ts
 (permissive and restrictive)
Leakiness
Conditional lethal
Genetic changes involving large
pieces of nucleic acid
 Reassortment-reshuffling
in segmented
genome viruses
 Recombination
Breakage and reunion-DNA viruses
Copy choice-RNA viruses (template
switching during replication)
Reversion
 Same
site reversion
 Second site reversion (tRNA mutation is
classic example)
Screen vs Selection
 Methods
to identify phenotypic variants
 In
selection, a condition exists such that
only the mutant will be observed
 In
screening-all the viruses are observed
and the operator has to search through all
of them to find the desired virus
Selection vs. Screening
Selection (eg of drug resistant mutants) is
more powerful.
Screening is a differential method and is
more labor-intensive (brute force)
Temperature-sensitive (ts) mutants have to
be screened at both the permissive and
restrictive temperature.
Essential Genes
Could be any gene-but loss of function
abolishes the ability to infect.
A virus with a loss-of-function mutation
in an essential gene is defective.
How can you propagate a defective
mutant virus?
Complementation
Wild type function in a mutant defective
virus restored by a gene provided
from outside the virus.
Helper virus, helper plasmid or
engineered gene can be the source.
Effective in trans only.
Interference
In mixed infections, the presence of two
types of virus reduces the ability of
both to replicate.
Competition for host factors? Disruption
of intracellular milieu?
Not the same as RNA interference.
New and old genetic analysis
Forward genetics-identify a random mutation
and use it to characterize a gene (“brute
force” method).
Reverse gentics-start with a gene sequence
and modify it to create a desired mutation.