Recombination occurs by physical exchange of DNA

KEY TERMS:

• Crossing-over describes the reciprocal exchange of material between chromosomes that occurs during prophase I of meiosis and is responsible for genetic recombination.
• A bivalent is the structure containing all four chromatids (two representing each homologue) at the start of meiosis.
• Chromatids are the copies of a chromosome produced by replication. The name is usually used to describe the copies in the period before they separate at the subsequent cell division.
• A chiasma (pl. chiasmata) is a site at which two homologous chromosomes appear to have exchanged material during 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).
• Heteroduplex DNA (Hybrid DNA) is generated by base pairing between complementary single strands derived from the different parental duplex molecules; it occurs during genetic recombination.

KEY CONCEPTS:

• Recombination is the result of crossing-over that occurs at chiasmata and involves two of the four chromatids.
• Recombination occurs by a breakage and reunion that proceeds via an intermediate of hybrid DNA. 

Genetic recombination describes the generation of new combinations of alleles that occurs at each generation in diploid organisms. The two copies of each chromosome may have different alleles at some loci. By exchanging corresponding parts between the chromosomes, recombinant chromosomes can be generated that are different from the parental chromosomes.

Recombination results from a physical exchange of chromosomal material. This is visible in the form of the crossing-over that occurs during meiosis (the specialized division that produces haploid germ cells). Meiosis starts with a cell that has duplicated its chromosomes, so that it has four copies of each chromosome. Early in meiosis, all four copies are closely associated (synapsed) in a structure called a bivalent. Each individual chromosomal unit is called a chromatid at this stage. Pairwise exchanges of material occur between the chromatids.

The visible result of a crossing-over event is called a chiasma, and is illustrated diagrammatically in Figure 1.31. 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.

The complementarity of the two strands of DNA is essential for the recombination process. Each of the chromatids shown in Figure 1.31 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.

Recombination involves a process in which the single strands in the region of the crossover exchange their partners. Figure 1.32 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 mechanism of course involves other stages (strands must be broken and resealed), and we discuss this in more detail in 15 Recombination and repair, but the crucial feature that makes precise recombination possible is the complementarity of DNA strands. The figure 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 1.31.

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 15 Recombination and repair).