Saltatory DNA Replication (Molecular Biology)

Particles of SV40 virus are produced in permissive simian cells. Infection of nonpermissive rodent cells by SV40 results in integration of the viral genome into the host chromosome. The transformed rodent cells have no free SV40 genomes, but SV40 genomes are produced when these nonpermissive cells are fused with permissive simian cells. The chromosomes from the monkey cells provide replicative functions not present in the rodent cells, making SV40 DNA replication possible in the heterokaryons. Repeated rounds of initiation of DNA replication from the integrated SV40 replication origins result in polytenization of that region (see Polytene Chromosome), which may liberate a linear DNA duplex containing the integrated SV40 genome. The liberated DNA will then recombine with itself to form a circular viral genome. In cell lines with tandem duplications of the integrated SV40 DNA, the efficiency of infectious virus formation is increased because of the terminal duplication of the liberated linear DNA. In the absence of tandem duplication, cyclization by nonhomologous recombination may form mutants with the deletion of viral DNA or insertion of host DNA (1).