When two different viruses infect same cell, genetic recombination can take place
Intramolecular recombination: exchange of nucleic acid sequences between different but closely related viruses during replication. Hybrid virus so generated breeds true. ds DNA viruses & RNA viruses (picorna viridae, coronaviruses, togaviruses)
Reassortment: occurs in viruses having segmented genome (arenaviridae, bunyaviridae, influenza A virus, rota virus). In a single cell infected by two related viruses there is exchange of segments.
Reactivation: Is of two types
Cross-reactivation/marker rescue: when a cell is infected with an active virus and a related inactive virus, progeny possessing one or more genetic traits of inactive virus may be produced. When epidemic strain (A2) of influenza virus is grown with the standard strain (AO) inactivated by UV irradiation a progeny may be produced which has antigenic character of A2 but growth character of AO. Used for vaccine production for influenza virus
Multiplicity reactivation: when a cell is infected with two or more virus particles of the same strain, each of which has suffered a lethal mutation in a different gene, live virus may be produced. Different virions suffer from damage to different genes by UV radiation, so that from total genetic pool it may be possible to obtain full complement of undamaged genes. Therefore UV radiation not used for production of inactivated virus vaccines
Protein encoded by one virus allows a different virus to replicate in doubly infected cells. Eg, HDV (defective virus) and HBV (helper virus)
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