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Genetic Recombination

Norton D. Zinder


The small single-stranded DNA phages, such as ϕX 174 (icosahedral phage, IP) or f1 (filamentous phage, FP), were appropriate materials with which to attempt the study of the mechanism of genetic recombination. The small size of their DNA should make intermediate forms in recombination readily separable from host DNA. Their few genes should allow markers to be distributed over the whole genome. Even their single-stranded nature should not create problems since immediately upon entry into their host cell the DNA becomes double-stranded (Denhardt 1975). The major difficulties are that their DNAs are ring molecules and that breakage of the rings renders them nonviable. In addition, for rings to recombine and yield a monomer at least two events may be required.

Still, a review of segregation data from single-burst genetic crosses of these phages is extremely interesting. From any single event one tends to recover one genetically intact parent (even when it is marked with mutations scattered about the ring) and a recombinant for a selected pair of genes, most of which recombinants contain the markers of the other parent. This somewhat remarkable finding must be telling us something about the mechanism of genetic recombination. We will refer to this hereafter as the “one parent-one recombinant” yield.

Genes and Marker Mutations
The first studies of genetic recombination in these small phages were performed by Zahler (1958). At the time, the only fact known about these phages was that they were small. He had only a few plaque-morphology mutants and concluded from...

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