Genetic Recombination

Genetic Recombination

Genetic crosses between two strains, each containing a mutation in the same gene, are occasionally observed to yield nonmutant, that is, wild-type, progeny. This is the result of a crossover in which the two parental DNA molecules are precisely broken, exchanged, and rejoined.

This section describes phenomenon of recombination and the next section describes experiments that make use of recombination to order genetic markers on chromosomes.

Alignment of the pairs of homologous chromosomes during meiosis has long been known through observations with light microscopes. The subsequent movement of visually identifiable sites on the chromosomes suggested that homologous chromosomes were broken and portions exchanged in a process called crossing over or recombination. Although recombination was first observed in eukaryotic cells, it appears to be almost ubiquitous. Even simple bacterial viruses can engage in genetic

Figure 8.7 Co-infection of a single cell by phage of two different genotypespermits recombination to produce output phage of wild type in addition to output phage of the parental types.

recombination. The bacterial phage experiments were particularly im-portant to the development of the field because phage provided a simple and small system with few variables, a high sensitivity for recombinants, and a very short generation time. These properties permitted many experiments to be done rapidly and inexpensively.

Phage mutants can easily be isolated that yield plaque morphologies different from the normal or wild-type. This is done simply by plating mutagenized phage on cells and locating the occasional different plaque. Nonsense mutations in essential phage genes may be identified as phage that grow only on nonsense-suppressing host cells. Genetic recombination between phage can be revealed by co-infection of cells with two mutants at a sufficiently high multiplicity of infection that each cell is infected with both types of phage (Fig. 8.7). Some of the progeny phage are found to carry alleles from both of the input phage! Since the phage carry only one DNA copy, such progeny have to be recombinants carrying some of the genetic information from each of the parental phage types. The discovery of genetic recombination in phage by Del-brück and Hershey opened the way for intensive study of the phenome-non of genetic recombination at the molecular level.