Viral Genetics and Reassortment
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Direct answer
Viral genetics and reassortment describe how viruses change their genomes. Point mutation and recombination alter a single genome, whereas reassortment swaps whole gene segments between two segmented viruses infecting the same cell. Most RNA viral polymerases lack proofreading, so RNA viruses mutate quickly, and reassortment in segmented viruses such as influenza is the mechanism behind antigenic shift.
What you must remember
- Point mutation: Error-prone RNA-dependent RNA polymerases introduce single-base changes, which accumulate fastest in RNA viruses and drive antigenic drift.
- Recombination: Two genomes present in one cell exchange sequence by template switching, a process well documented in poliovirus and coronaviruses.
- Reassortment: Entire gene segments are exchanged between two segmented viruses in one cell, as seen in influenza, rotavirus and reoviruses.
- Complementation: A defective virus is rescued by a helper virus in the same cell, and the classic example is hepatitis delta virus using hepatitis B surface antigen.
- Phenotypic mixing: The genome of one virus is packaged in the coat of another, so the coat changes for that generation while the genome stays the same.
- Multiplicity reactivation: Two inactivated virions, each carrying lethal damage, can yield a viable virus when they infect the same cell.
- Antigenic drift: Gradual point mutation in the haemagglutinin antigenic sites causes seasonal influenza epidemics and the need for yearly vaccine updates.
- Antigenic shift: Reassortment between human and animal influenza strains can create a novel subtype against which most people have little immunity.
Where the exam tries to catch you
Most errors come from attributing a sudden antigenic change to the wrong mechanism.
- Trap: Calling antigenic shift the result of point mutation Correct: Point mutation drives antigenic drift, whereas shift requires reassortment of whole segments between two different viruses.
- Trap: Calling hepatitis delta virus an independent infection Correct: It is a defective agent that needs hepatitis B surface antigen for its envelope, which is complementation.
- Trap: Assuming any two viruses in one cell can reassort Correct: Reassortment needs segmented genomes, and most DNA viruses carry one continuous genome, so the event is limited to segmented RNA viruses.
Frequently asked questions
Why does influenza change antigenically more often than many DNA viruses?
Influenza RNA polymerase is error-prone, so point mutations accumulate quickly in the haemagglutinin gene and produce antigenic drift. Its segmented genome also allows reassortment, which can cause the larger jump called antigenic shift.
What is the difference between antigenic drift and antigenic shift?
Antigenic drift is gradual point mutation in the haemagglutinin antigenic sites, which causes seasonal epidemics and yearly vaccine updates. Antigenic shift is a sudden change caused by reassortment between human and animal influenza strains, and it can cause pandemics.
Which hepatitis virus is the classic example of complementation?
Hepatitis delta virus is the classic example of complementation, because this defective agent needs hepatitis B surface antigen to form its envelope. It depends on a helper hepatitis B infection in the same host to complete its life cycle.
Why is recombination well documented in coronaviruses?
Coronaviruses carry a single long positive-sense RNA genome that undergoes template switching during replication, which allows recombination between co-infecting strains. Influenza, by contrast, relies mainly on reassortment because its genome is split into separate segments.
Does phenotypic mixing produce a permanent change in a virus?
Phenotypic mixing does not produce a permanent genetic change, because progeny carry the genome of their original parent inside the borrowed coat. Only that generation shows the altered phenotype, whereas mutation and reassortment change the genome itself.