Chapter 8 Illegitimate Recombination

Abstract

In the absence of any real understanding of ordinary recombination it may seem gratuitous to invoke an even less comprehended event to be known as illegitimate recombination. Yet there are reassociations of nucleotide sequences that apparently do not fit into the framework of classical recombination. These invite attention to yet another attribute of DNA.

General recombination (Chapter 7) involves alignment of genetic homologues (DNA molecules in lower organisms, chromosomes in eukaryotes) and rejoining of material from corresponding segments of two genetically marked parents to give end products whose nucleotide sequences are the same as those in the “parent” molecules, except for the markers. Physical breakage and rejoining of DNA are involved (Taylor, 1965; Meselson, 1967). The process is relatively frequent and extremely accurate in that no losses are incurred. It depends on substantial nucleotide-sequence homology, but its frequency can be modified by the few differences in sequence introduced as genetic markers (Yanofsky et al., 1964; Norkin, 1970). Included under general recombination are examples of unequal crossing over that result from asymmetric pairing between homologues containing duplicated segments (see Hershey, 1970).

Another order of genetic event is grossly perceived as rare, haphazard, and not obviously dependent upon genetic homology. In higher organisms its effects are visible as inversions, deletions, and translocations of parts of chromosomes. In bacteria and bacteriophages it is detected as new juxta-positions of genetic markers, some of which have recently been correlated with rearrangements in DNA made visible in the electron microscope (Hradecna and Szybalski, 1969; Fiandt et...