10. Recombination occurs by physical exchange of DNA

1.10 Recombination occurs by physical exchange of DNA

Key terms defined in this section

Bivalent is the structure containing all four chromatids (two representing each homologue) at the start of meiosis.
Breakage and reunion describes the mode of genetic recombination, in which two DNA duplex molecules are broken at corresponding points and then rejoined crosswise (involving formation of a length of heteroduplex DNA around the site of joining).
Chiasma (pl. chiasmata) is a site at which two homologous chromosomes appear to have exchanged material during meiosis.
Crossing-over describes the reciprocal exchange of material between chromosomes that occurs during meiosis and is responsible for genetic recombination.
Hybrid DNA is another term for heteroduplex DNA.

Genes on the same chromosome show genetic linkage because they are present on the same (very long) molecule of DNA. Linkage is revealed in the progeny of a genetic cross when the proportion of recombinant genotypes (where an allele of one parent is found with an allele of the other parent) is less than the number of parental genotypes (with the same combination of alleles as either parent). Genes on different chromosomes segregate independently, of course, as predicted by Mendel, giving 50% parental and 50% recombinant progeny. The proximity of two loci is measured by the per cent recombination between them (in formal terms a map distance of 1 centiMorgan = 1% recombination). When pairwise combinations of loci on the same chromosome are tested in genetic crosses, loci close to one another are linked, as defined by a map distance <50 cM. Loci that are farther apart recombine at the limit of 50%. But a linkage map corresponding to the chromosome can be generated by extending a series of genetic crosses in which in effect two loci >50 cM apart are connected because they show linkage to a locus between them.

Recombination results from a physical exchange of chromosomal material. This is visible in the form of the crossing-over that occurs during meiosis. Early in meiosis, at the stage when all four copies of each chromosome are organized in a bivalent, pairwise exchanges of material occur between the closely associated (synapsed) chromatids.

Figure 1.23 Chiasma formation is responsible for generating recombinants.

The visible result of a crossing-over event is called a chiasma, and is illustrated diagrammatically in Figure 1.23. A chiasma represents a site at which two of the chromatids in a bivalent have been broken at corresponding points. The broken ends have been rejoined crosswise, generating new chromatids. Each new chromatid consists of material derived from one chromatid on one side of the junction point, with material from the other chromatid on the opposite side. The two recombinant chromatids have reciprocal structures. The event is described as a breakage and reunion. Its nature explains why a single recombination event can produce only 50% recombinants: each individual recombination event involves only two of the four associated chromatids.

A crucial concept in the construction of a genetic map is that the distance between genes does not depend on the particular alleles that are used, but only on the genetic loci. The locus defines the position occupied on the chromosome by the gene representing a particular trait. The various alternative forms of a gene Xthat is, the alleles used in mapping Xall reside at the same location on its particular chromosome. So genetic mapping is concerned with identifying the positions of genetic loci, which are fixed and lie in a linear order. In a mapping experiment, the same result is obtained irrespective of the particular combination of alleles.

The complementarity of the two strands of DNA is essential for the recombination process. Each of the chromatids shown in Figure 1.23 consists of a very long duplex of DNA. For them to be broken and reconnected without any loss of material requires a mechanism to recognize exactly corresponding positions. This is provided by complementary base pairing.

Figure 1.24 Recombination involves pairing between complementary strands of the two parental duplex DNAs.

Figure 1.13 Denatured single strands of DNA can renature to give the duplex form.

Recombination involves a process in which the single strands in the region of the crossover exchange their partners. Figure 1.24 shows that this creates a stretch of hybrid DNA in which the single strand of one duplex is paired with its complement from the other duplex. The process accomplishes a result analogous to the denaturation and renaturation shown previously in Figure 1.13. The mechanism of course involves other stages (strands must be broken and resealed), and we discuss this in more detail in 14 Recombination and repair, but the crucial feature that makes precise recombination possible at all is the complementarity of DNA strands. Figure 1.24 shows only some stages of the reaction, but we see that a stretch of hybrid DNA forms in the recombination intermediate when a single strand crosses over from one duplex to the other. Each recombinant consists of one parental duplex DNA at the left, connected by a stretch of hybrid DNA to the other parental duplex at the right. Each duplex DNA corresponds to one of the chromatids involved in recombination in Figure 1.23.

The formation of hybrid DNA requires the sequences of the two recombining duplexes to be close enough to allow pairing between the complementary strands. If there are no differences between the two parental genomes in this region, formation of hybrid DNA will be perfect. But the reaction can be tolerated even when there are small differences. In this case, the hybrid DNA has points of mismatch, at which a base in one strand faces a base in the other strand that is not complementary to it. The correction of such mismatches is another feature of genetic recombination (see 14 Recombination and repair).