Viral Genetics and Evolution Flashcards
Viral genomes are ______ changing
Continuously
Segmented genomes
Transcribed to produce monocistronic mRNAs
- advantage: various proteins can be produced in different amounts, rather than in a constant ratio
- more control
Non-segmented genomes
Produce polycistronic mRNA, which is translated to form a polyprotein, processed by proteolytic cleavage to form the mature gene products
- either make everything, or nothing
_____ viruses are stronger than ____ viruses
DNA; RNA
- due to proofreading ability
Spontaneous mutations
Arise naturally during viral replication
Mutation rate
10^8 - 10^11 per base per cycle for DNA viruses
10^3 - 10^4 per base per cycle for RNA viruses
1 progeny = 1 _______
Mistake
- not all mutations will have an effect due to redundancy of the genetic code
What ensures that a mutation will survive?
If the mutation does not cripple essential viral function
What allows most mutations to have no effect on viral protein or function?
The redundancy of the genetic code
Point mutation
One base is replaced by another or insertion/deletion of a base
Epitope
Smallest unit that can stimulate antibody binding
Recombination
Exchange of genetic information between two genomes
- leads to creation of a new virus –> only happens between 2 viruses that are similar in structure
Reassortment
If a virus has a segmented genome and if 2 variants of that virus infect a single cell
- only occurs in segemented RNA viruses
- ex; orthomyxoviridae, arenaviridae, birnaviridae, bunyaviridae, and reoviridae
Definition
- segmented genomes: transcribed to produce monocistronic mRNA
- nonsegmented genomes: produce polycistronic mRNA, –> translated to form a polyprotein –> processed by proteolytic cleavage to form the mature gene products
Multiplicity reactivation
Apply to a cell infected with 2 or more viruses of the same strain, each with a different mutated gene
- ex: dsDNA viruses are inactivated with UV and high multiplicity of infection, the virus may complement others = a new genotype or regenerating the wild type virus
Defective viruses
Lack the full complement of genes necessary for a complete infectious cycle
- need another virus to provide the missing functions (helper virus)
Examples of defective viruses
- retroviruses need related viruses as a helper
- hepatitis delta virus (RNA) needs hepatitis virus (DNA) as a helper
Phenotype mixing
When 2 different viruses infect a cell, progeny viruses may contain coat components of both parents
- no alteration in genetic material
- only the outside coat has changed
Pseudotype
Phenotype mixing is a life cycle of envelope-defective retroviruses
- has genome of the defective parental virus but the envelope glycoproteins of the helper virus
- new virus as a result of phenotype mixing
Antigenic drift
Point mutations
Antigenic shift
Genetic reassortment and generate a new subtype
- more dramatic change
