DNA Recombination:- Recombination during Meiosis Is Initiated with Double-Strand Breaks
A likely pathway for homologous recombination during meiosis is outlined in Figure 1a. The model has four key features. First, homologous chromosomes are aligned. Second, a double-strand break in a DNA molecule is enlarged by an exonuclease, leaving a single strand extension with a free 3-hydroxyl group at the broken end (step 1). Third, the exposed 3 ends in vade the intact duplex DNA, and this is followed by branch migration (Fig. 2) and/or replication to create a pair of crossover structures, called Holliday junctions (Fig. 1a, steps 2 to 4). Fourth, cleavage of the two crossovers creates two complete recombinant products (step 5).
In this double-strand break repair model for re combination, the 3 ends are used to initiate the genetic exchange. Once paired with the complementary strand on the intact homolog, a region of hybrid DNA is created that contains complementary strands from two different parent DNAs (the product of step 2 in Fig. 1a). Each of the 3 ends can then act as a primer for DNA replication. The structures thus formed, Holliday inter mediates (Fig. 1b), are a feature of homologous genetic recombination pathways in all organisms. Homologous recombination can vary in many details from one species to another, but most of the steps out lined above are generally present in some form. There are two ways to cleave, or “resolve,” the Holliday inter mediate so that the two recombinant products carry genes in the same linear order as in the substrates—the original, unrecombined chromosomes (step 5 of Fig. 1a). If cleaved one way, the DNA flanking the region containing the hybrid DNA is not recombined; if cleaved the other way, the flanking DNA is recombined. Both outcomes are observed in vivo in eukaryotes and prokaryotes.
The homologous recombination illustrated in Figure 1 is a very elaborate process with subtle molecular consequences for the generation of genetic diversity. To understand how this process contributes to diversity, we should keep in mind that the two homologous chromosomes that undergo recombination are not necessarily identical. The linear array of genes may be the same, but the base sequences in some of the genes may differ slightly (in different alleles). In a human, for example, one chromosome may contain the allele for hemoglobin A (normal hemoglobin) while the other contains the allele for hemoglobin S (the sickle-cell mutation). The difference may consist of no more than one base pair among millions. Homologous recombination does not change the linear array of genes, but it can determine which alleles become linked together on a single chromosome.
FIGURE 1 Recombination during meiosis. (a) Model of double-strand break repair for homologous genetic recombination. The two homologous chromosomes involved in this recombination event have similar sequences. Each of the two genes shown has different alleles on the two chromosomes. The DNA strands and alleles are colored differently so that their fate is evident. The steps are described in the text. (b) A Holliday intermediate formed between two bacterial plasmids in vivo, as seen with the electron microscope. The intermediates are named for Robin Holliday, who first proposed their existence in 1964.
FIGURE 2 Branch migration. When a template strand pairs with two different complementary strands, a branch is formed at the point where the three complementary strands meet. The branch “migrates” when base pairing to one of the two complementary strands is bro ken and replaced with base pairing to the other complementary strand. In the absence of an enzyme to direct it, this process can move the branch spontaneously in either direction. Spontaneous branch migration is blocked wherever one of the otherwise complementary strands has a sequence nonidentical to the other strand.
