In Chapter 20, we shall see that both too much and too little homologous recombination can lead to cancer in humans, the former through repair using the “wrong” template (as described above) and the latter through an increased mutation rate caused by inefficient DNA repair. Clearly, a delicate balance has evolved that keeps this process in check on undamaged DNA, while still allowing it to act efficiently and rapidly on DNA lesions as soon as they arise.
Not surprisingly, mutations in the components that carry out and regulate homologous recombination are responsible for several inherited forms of cancer. Two of these, the Brca1 and Brca2 proteins, were first discovered because
Figure 5–51 Structure of a portion of the Rad52 protein. This doughnutshaped structure is composed of 11 subunits. Single-strand DNA has been modeled into the deep groove running along the protein surface. Rad52 helps load Rad51 onto single-strand DNA to form the nucleoprotein filament that carries out strand exchange. Rad52 also acts later to re-form the double helix and complete the homologous recombination reaction. (From
M.R. Singleton et al., Proc. Natl Acad. Sci. USA 99:13492–13497, 2002. With permission from National Academy of Sciences.) mutations in their genes lead to a greatly increased frequency of breast cancer. Because these mutations cause inefficient repair by homologous recombination, accumulation of DNA damage can, in a small proportion of cells, give rise to a cancer. Brca1 regulates an early step in broken-end processing; without it, such ends are not processed correctly for homologous recombination and instead are repaired inaccurately by the nonhomologous end-joining pathway (see Figure 5–45). Brca2 binds to the Rad51 protein, preventing its polymerization on DNA, and thereby maintaining it in an inactive form until it is needed. Normally, upon DNA damage, Brca2 helps to bring Rad51 protein rapidly to sites of damage and, once in place, to release it in its active form onto single-strand DNA.
We have seen that homologous recombination comprises a group of reactions— including broken-end processing, strand exchange, limited DNA synthesis, and ligation—to exchange DNA sequences between two double helices of similar nucleotide sequence. Having discussed its role in accurately repairing damaged DNA, we now turn to homologous recombination as a means to generate DNA molecules that carry novel combinations of genes as a result of the deliberate exchange of material between different chromosomes. Although this occasionally occurs by accident in mitotic cells (and is often detrimental), it is a frequent and necessary part of meiosis, which occurs in sexually reproducing organisms such as fungi, plants, and animals.
Here, homologous recombination occurs as an integral part of the process whereby chromosomes are parceled out to germ cells (sperm and eggs in animals). We discuss the process of meiosis in detail in Chapter 17; in the following sections, we discuss how homologous recombination during meiosis produces chromosome crossing-over and gene conversion, resulting in hybrid chromosomes that contain genetic information from both the maternal and paternal homologs (Figure 5–53). Crossing-over and gene conversion are both generated by homologous recombination mechanisms that, at their core, resemble those used to repair double-strand breaks.
Meiotic Recombination Begins with a Programmed Double-Strand Break
Homologous recombination in meiosis starts with a bold stroke: a specialized protein (called Spo11 in budding yeast) breaks both strands of the DNA double helix in one of the recombining chromosomes (Figure 5–54). Like a topoisomerase, Spo11, after catalyzing this reaction, remains covalently bound to the broken site of gene site of conversion crossover