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  Deletion  Insertion  Substitution  Missense/nonsense  Polar  Resistance/dependence  Temperature-sensitive or ts (permissive and restrictive) (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) 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.