Clarifying the mechanics of DNA strand exchange in meiotic recombination

Abstract

During meiosis, accurate separation of maternal and paternal chromosomes requires that they first be connected to one another through homologous recombination. Meiotic recombination has many intriguing but poorly understood features that distinguish it from recombination in mitotically dividing cells, and several of these features depend on the meiosis-specific DNA strand exchange protein Dmc1 (disrupted meiotic cDNA1). Many questions about this protein have arisen since its discovery more than a decade ago, but recent genetic and biochemical breakthroughs promise to shed light on the unique behaviours and functions of this central player in the remarkable chromosome dynamics of meiosis.

Figure 1. Connections formed between homologous chromosomes during meiosis

Meiotic cells of most sexual organisms contain two copies of most chromosomes, one from each of the parents (red and blue). After DNA replication, each chromosome comprises a pair of sister chromatids held together by cohesion complexes (green). Sister centromeres (circles) attach as a single unit to microtubules (thin lines) from a spindle pole (not shown). Exchange of chromosome arms between non-sister chromatids yields a chiasma. Dissolution of sister chromatid cohesion along the arms allows the homologues to separate at the first meiotic division (not shown).

Figure 2. DNA events in meiotic recombination

a–c, Presynapsis. Spo11 (elipses) cleaves dsDNA, yielding a covalent Spo11-DNA complex. Endonuclease releases Spo11 bound to a short oligonucleotide, and 5′ DNA strands are degraded to yield 3′ ssDNA tails which are bound by Rad51 and Dmc1 (not shown). d–f, Crossover formation. d, Invasion of ssDNA from one end of the break forms an asymmetric strand exchange intermediate. e, DNA synthesis (dashed line) is primed from the invading 3′ end, the second DSB end is captured and primes DNA synthesis. Ligation yields a pair of Holliday junctions (dHJ). f, Resolution yields a mature product with exchanged flanking DNA. g–h, A noncrossover pathway. Strand invasion (g) and DNA synthesis (h) are inferred but have not been directly detected. A transient strand invasion complex may be dissociated, perhaps by DNA helicases, allowing newly synthesized DNA to anneal to complementary ssDNA on the other side of the break (i). Further DNA synthesis and ligation yield a mature noncrossover product.

Figure 3. The DNA strand exchange reaction

a, Three stages of strand exchange: presynaptic helical filament on ssDNA (left); synaptic complex with homologous dsDNA (middle); post-synaptic stage with the original ssDNA fully base-paired with the complementary strand from the dsDNA donor (right). b, c, Typical in vitro DNA strand exchange assay systems. b, Strand transfer from a linear duplex to a circular nucleoprotein filament. c, D-loop assay. Invasion of a short nucleoprotein filament into a negatively supercoiled circular duplex, creating a bubble, or D-loop.

Figure 4. Oligomeric structure of Dmc1 nucleoprotein complexes

a, b, Electron micrographs of Dmc1-DNA complexes (from refs , , with permission). Scale bars, 50 nm. a, Stacks of octameric Dmc1 rings on DNA. b, A helical Dmc1-ssDNA filament, similar to structures created by other RecA family members. c, A Dmc1 ring and a right-handed helical nucleoprotein filament. The dynamic relationship between rings and helical filaments is not known, but it is possible that filaments are formed by the opening and deposition of Dmc1 rings.