Heteroduplexes and Genetic Recombination
Having considered the existence and use of genetic recombination, we are ready to consider how it comes about. Genetic recombination yields a precise cut and splice between two DNA molecules. Even if one DNA
molecule were to have been cut, it is hard to imagine how an enzyme could know where to cut the other DNA duplex so as to produce the perfect splices that genetics experiments show occur. The difficulty can be largely overcome by a mechanism utilizing the self-complementary double-stranded structure of DNA. A denatured portion of one duplex could anneal to a denatured portion of complementary sequence from the other duplex (Fig. 8.12). This would hold the two DNA molecules in register while the remainder of the recombination reaction proceeded.
The life cycle of yeast permits a direct test of the model outlined above. A diploid yeast cell undergoes recombination during meiosis, and the two meiotic cell divisions yield four haploid spores. These four spores can be isolated from one another and each can be grown into a colony or culture. In essence, the cells of each colony are identical copies of each of the original recombinants, and the cells can be tested to determine the genetic structure of the original recombinants. If one of a pair of homologous chromosomes contains a mutation and the other does not, generally two of the four resulting spores will contain the mutation and two will not.
Consider the situation resulting from melting portions of the du-plexes and base pairing between complementary strands of two homolo-gous yeast chromosomes in the process of genetic recombination. The region of pairing may include the mutation. Then a heteroduplex forms that contains the mutant sequence on one strand and the wild-type sequence on the other (Fig. 8.13). As discussed, mispaired bases are subject to mismatch repair and, if it occurs, the yeast repair system in this case has no apparent reason to choose one strand to repair in preference to the other. Therefore strands may be correctly or incorrectly repaired, so the final outcome could be three copies of the wild-type or mutant sequence and one copy of the other in the meiosis from a single yeast cell. In total, a single yeast cell can produce one or three progeny spores containing the marker from one of the original chromosomes. This phenomenon is called gene conversion. It is experi-mentally observed and consequently it is reasonable to expect that pairing between complementary strands of recombinant partners oc-curs during recombination. Without heteroduplex formation and mis-match repair, there is no easy way to generate any ratio other than 2:2.
Figure 8.13 Gene conversion in yeast. A diploid (A/a) undergoes meiosis,which produces heteroduplexes A-a that are both repaired to A-A.