Meiotic recombination (Homo sapiens)
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Description
Meiotic recombination exchanges segments of duplex DNA between chromosomal homologs, generating genetic diversity (reviewed in Handel and Schimenti 2010, Inagaki et al. 2010, Cohen et al. 2006). There are two forms of recombination: non-crossover (NCO) and crossover (CO). In mammals, the former is required for correct pairing and synapsis of homologous chromosomes, while CO intermediates called chiasmata are required for correct segregation of bivalents.
Meiotic recombination is initiated by double-strand breaks created by SPO11, which remains covalently attached to the 5' ends after cleavage. SPO11 is removed by cleavage of single DNA strands adjacent to the covalent linkage. The resulting 5' ends are further resected to produce protruding 3' ends. The single-stranded 3' ends are bound by RAD51 and DMC1, homologs of RecA that catalyze a search for homology between the bound single strand and duplex DNA of the chromosomal homolog. RAD51 and DMC1 then catalyze the invasion of the single strand into the homologous duplex and the formation of a D-loop heteroduplex. Approximately 90% of heteroduplexes are resolved without crossovers (NCO), probably by synthesis-dependent strand annealing.
The invasive strand is extended along the homolog and ligated back to its original duplex, creating a double Holliday junction. The mismatch repair proteins MSH4, MSH5 participate in this process, possibly by stabilizing the duplexes. The mismatch repair proteins MLH1 and MLH3 are then recruited to the double Holliday structure and an unidentified resolvase (Mus81? Gen1?) cleaves the junctions to yield a crossover.
Crossovers are not randomly distributed: The histone methyltransferase PRDM9 recruits the recombination machinery to genetically determined hotspots in the genome and each incipient crossover somehow inhibits formation of crossovers nearby, a phenomenon called crossover interference. Each chromosome bivalent, including the X-Y body in males, has at least one crossover and this is required for meiosis to proceed correctly. View original pathway at Reactome.
Meiotic recombination is initiated by double-strand breaks created by SPO11, which remains covalently attached to the 5' ends after cleavage. SPO11 is removed by cleavage of single DNA strands adjacent to the covalent linkage. The resulting 5' ends are further resected to produce protruding 3' ends. The single-stranded 3' ends are bound by RAD51 and DMC1, homologs of RecA that catalyze a search for homology between the bound single strand and duplex DNA of the chromosomal homolog. RAD51 and DMC1 then catalyze the invasion of the single strand into the homologous duplex and the formation of a D-loop heteroduplex. Approximately 90% of heteroduplexes are resolved without crossovers (NCO), probably by synthesis-dependent strand annealing.
The invasive strand is extended along the homolog and ligated back to its original duplex, creating a double Holliday junction. The mismatch repair proteins MSH4, MSH5 participate in this process, possibly by stabilizing the duplexes. The mismatch repair proteins MLH1 and MLH3 are then recruited to the double Holliday structure and an unidentified resolvase (Mus81? Gen1?) cleaves the junctions to yield a crossover.
Crossovers are not randomly distributed: The histone methyltransferase PRDM9 recruits the recombination machinery to genetically determined hotspots in the genome and each incipient crossover somehow inhibits formation of crossovers nearby, a phenomenon called crossover interference. Each chromosome bivalent, including the X-Y body in males, has at least one crossover and this is required for meiosis to proceed correctly. View original pathway at Reactome.
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at resected DSB
endssingle-stranded DNA
complexbreak with 3' single strand
breaksAnnotated Interactions
at resected DSB
endsat resected DSB
endssingle-stranded DNA
complexsingle-stranded DNA
complexsingle-stranded DNA
complexSPO11 forms a dimer and each subunit cleaves a single strand of DNA, thus creating a double-strand break. After cleaving DNA, a SPO11 subunit remains covalently attached to each 5' end via a tyrosine residue. SPO11 is removed from the DNA by cleavage and exonucleolytic digestion of single strands 3' to the attached SPO11. The overall products are a resected 5' end (protruding 3' overhang) and a covalent complex of SPO11 with an oligonucleotide. Two size classes of oligonucleotide are observed: 12 to 26 nucleotides and 28 to 34 nucleotides. The enzyme responsible for excision of SPO11:oligonucleotide in mammals is inferred to be MRE11 in the MRE11:RAD50:NBS1:CtIP complex based on conservation of the reaction mechanism across yeast, plants, and animals (Sartori et al. 2007).
In fission and budding yeast the Mre11:Rad50:Xrs2/Nbs1 (MRN/MRX) complex is required for removal of SPO11. In human somatic cells the MRN complex together with CtIP resects double-strand breaks but the role of the MRN complex in mammalian meiosis, though essential, is unclear (Sartori et al. 2007).
After excision of SPO11:oligonucleotide the recessed 5' end is further resected by unknown exonucleases.
SPO11 forms a dimer and each subunit cleaves a single strand of DNA, thus creating a double-strand break. After cleaving DNA, a SPO11 subunit remains covalently attached to each 5' end via a tyrosine residue. SPO11 is removed from the DNA by cleavage and exonucleolytic digestion of single strands 3' to the attached SPO11. The overall products are a resected 5' end (protruding 3' overhang) and a covalent complex of SPO11 with an oligonucleotide. Two size classes of oligonucleotide are observed: 12 to 26 nucleotides and 28 to 34 nucleotides. The enzyme responsible for excision of SPO11:oligonucleotide in mammals is inferred to be MRE11 in the MRE11:RAD50:NBS1:CtIP complex based on conservation of the reaction mechanism across yeast, plants, and animals (Sartori et al. 2007).
In fission and budding yeast the Mre11:Rad50:Xrs2/Nbs1 (MRN/MRX) complex is required for removal of SPO11. In human somatic cells the MRN complex together with CtIP resects double-strand breaks but the role of the MRN complex in mammalian meiosis, though essential, is unclear (Sartori et al. 2007).
After excision of SPO11:oligonucleotide the recessed 5' end is further resected by unknown exonucleases.
These specific sequence motifs also appear to be species specific, as the degenerate 13-bp motif associated with 40% of human hotspots does not function in chimpanzees, probably as a result of the rapidly evolving zinc finger domain (Myers et al. 2010). Subtle changes in the zinc finger array in humans can have global effects on recombination throughout the human genome, enhancing or decreasing the activity of a hotspot, or even creating entirely new hotspots (Berg et al. 2010). In addition to its role in regulating recombination hotspot activity, PRDM9 also appears to have a role in maintaining stability within the human genome, as variation in the PRDM9 gene can lead to large-scale genomic rearrangements and minisatellite instability in humans.
Two classes of crossovers have been defined: class I crossovers are dependent on the MutL homologs, MLH1 and MLH3, while class II crossovers are dependent on the MUS81-EME1 endonuclease. Class I crossovers constitute 90-95% of all crossovers, and correspond to meiotic nodules that contain MLH1and MLH3. These arise as a subset of the many hundreds of MSH4/MSH5-positive meiotic nodules that arise at the time of double Holliday junction formation. What happens to all the other meiotic nodules is not clear, but they most likely follow a second pathway that results in non-crossovers (or gene conversions). MLH1 and MLH3 form heterodimers that repair mismatches in duplex DNA. In mouse, MLH1 is required for crossovers but not for non?crossover resolution of Holliday junctions. About 10% of early meiotic nodules are somehow selected to become Class I crossover events, possibly by first losing BLM (and probably associated TOP3A), and acquiring MLH1 and MLH3.
The selection of sites for class II crossovers follows an, as yet, unknown pathway, but almost certainly stems from the same initiating D-loop intermediate.
In the process known as crossover interference, the presence of a crossover nodule inhibits formation of nearby crossover nodules so that crossovers are not clustered and each chromosome bivalent has at least one crossover. In mouse, crossover interference is seen among nodules at two stages: RPA?containing nodules during late zygonema and MLH1?containing nodules during pachynema. Class II crossovers are not subject to interference constraints.
Immunocytology shows the RPA heterotrimer arrives at recombination nodules with or after RAD51 and DMC1 (Golub et al. 1999, Oliver-Bonet et al. 2005, Oliver-Bonet et al. 2007)). (In mitotic recombination RPA precedes RAD51.)
BRCA1 and BRCA2 are found extensively distributed on synaptonemal complexes. Results from human cells and knockout mice indicate that BRCA2, RAD51C, and TEX15 participate in loading RAD51 and DMC1 onto single-stranded DNA (Thorslund et al. 2007). BRCA1 participates in loading RAD51 but not DMC1 (Scully et al. 1997).
The kinase ATM is also localized to double-strand breaks where it phosphorylates histone H2AX.
In human spermatocytes about 350 early recombination nodules form but only about 10% will continue on to make crossovers. The remaining 90% are believed to be resolved by synthesis-dependent strand annealing, which transfers short segments of DNA (about 0.2-2.0 kilobases) between homologs.
break with 3' single strand
breaksbreak with 3' single strand
breaks