Recombination

Recombination is the principal mechanism through which variation is introduced into populations. For example, during meiosis, the process that produces sex cells (sperm or eggs), homologous chromosomes—one derived from the mother and the equivalent from the father—become paired, and recombination, or crossing-over, takes place. The two DNA molecules are fragmented, and similar segments of the chromosome are shuffled to produce two new chromosomes, each being a mosaic of the originals. The pair separates so that each sperm or egg receives just one of the shuffled chromosomes. When sperm and egg fuse, the normal set of two copies of each chromosome is restored.

There are two forms of recombination, general and site-specific. General recombination typically involves cleavage and rejoining at identical or very similar sequences. In site-specific recombination, cleavage takes place at a specific site into which DNA is usually inserted. General recombination occurs among viruses during infection, in bacteria during conjugation, during transformation whereby DNA is directly introduced into cells, and during some types of repair processes. Site-specific recombination is frequently involved in the parasitic distribution of DNA segments throughout genomes. Many viruses, as well as special segments of DNA called transposons, rely on site-specific recombination to multiply and spread. The two processes are described in greater detail below.