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.
Benson FE, Baumann P, West SC.; ''Synergistic actions of Rad51 and Rad52 in recombination and DNA repair.''; PubMedEurope PMCScholia
Masson JY, Davies AA, Hajibagheri N, Van Dyck E, Benson FE, Stasiak AZ, Stasiak A, West SC.; ''The meiosis-specific recombinase hDmc1 forms ring structures and interacts with hRad51.''; PubMedEurope PMCScholia
Murayama Y, Kurokawa Y, Mayanagi K, Iwasaki H.; ''Formation and branch migration of Holliday junctions mediated by eukaryotic recombinases.''; PubMedEurope PMCScholia
Oliver-Bonet M, Turek PJ, Sun F, Ko E, Martin RH.; ''Temporal progression of recombination in human males.''; PubMedEurope PMCScholia
Okorokov AL, Chaban YL, Bugreev DV, Hodgkinson J, Mazin AV, Orlova EV.; ''Structure of the hDmc1-ssDNA filament reveals the principles of its architecture.''; PubMedEurope PMCScholia
Santucci-Darmanin S, Walpita D, Lespinasse F, Desnuelle C, Ashley T, Paquis-Flucklinger V.; ''MSH4 acts in conjunction with MLH1 during mammalian meiosis.''; PubMedEurope PMCScholia
Hayashi K, Yoshida K, Matsui Y.; ''A histone H3 methyltransferase controls epigenetic events required for meiotic prophase.''; PubMedEurope PMCScholia
Oliver-Bonet M, Campillo M, Turek PJ, Ko E, Martin RH.; ''Analysis of replication protein A (RPA) in human spermatogenesis.''; PubMedEurope PMCScholia
Snowden T, Acharya S, Butz C, Berardini M, Fishel R.; ''hMSH4-hMSH5 recognizes Holliday Junctions and forms a meiosis-specific sliding clamp that embraces homologous chromosomes.''; PubMedEurope PMCScholia
Barlow AL, Hultén MA.; ''Crossing over analysis at pachytene in man.''; PubMedEurope PMCScholia
Sheridan SD, Yu X, Roth R, Heuser JE, Sehorn MG, Sung P, Egelman EH, Bishop DK.; ''A comparative analysis of Dmc1 and Rad51 nucleoprotein filaments.''; PubMedEurope PMCScholia
Johnson FB, Lombard DB, Neff NF, Mastrangelo MA, Dewolf W, Ellis NA, Marciniak RA, Yin Y, Jaenisch R, Guarente L.; ''Association of the Bloom syndrome protein with topoisomerase IIIalpha in somatic and meiotic cells.''; PubMedEurope PMCScholia
Berg IL, Neumann R, Lam KW, Sarbajna S, Odenthal-Hesse L, May CA, Jeffreys AJ.; ''PRDM9 variation strongly influences recombination hot-spot activity and meiotic instability in humans.''; PubMedEurope PMCScholia
Cohen PE, Pollack SE, Pollard JW.; ''Genetic analysis of chromosome pairing, recombination, and cell cycle control during first meiotic prophase in mammals.''; PubMedEurope PMCScholia
Baumann P, West SC.; ''Heteroduplex formation by human Rad51 protein: effects of DNA end-structure, hRP-A and hRad52.''; PubMedEurope PMCScholia
Baudat F, Buard J, Grey C, Fledel-Alon A, Ober C, Przeworski M, Coop G, de Massy B.; ''PRDM9 is a major determinant of meiotic recombination hotspots in humans and mice.''; PubMedEurope PMCScholia
Cannavo E, Marra G, Sabates-Bellver J, Menigatti M, Lipkin SM, Fischer F, Cejka P, Jiricny J.; ''Expression of the MutL homologue hMLH3 in human cells and its role in DNA mismatch repair.''; PubMedEurope PMCScholia
Myers S, Bowden R, Tumian A, Bontrop RE, Freeman C, MacFie TS, McVean G, Donnelly P.; ''Drive against hotspot motifs in primates implicates the PRDM9 gene in meiotic recombination.''; PubMedEurope PMCScholia
Baumann P, Benson FE, West SC.; ''Human Rad51 protein promotes ATP-dependent homologous pairing and strand transfer reactions in vitro.''; PubMedEurope PMCScholia
Sartori AA, Lukas C, Coates J, Mistrik M, Fu S, Bartek J, Baer R, Lukas J, Jackson SP.; ''Human CtIP promotes DNA end resection.''; PubMedEurope PMCScholia
Kondo E, Horii A, Fukushige S.; ''The interacting domains of three MutL heterodimers in man: hMLH1 interacts with 36 homologous amino acid residues within hMLH3, hPMS1 and hPMS2.''; PubMedEurope PMCScholia
Thorslund T, Esashi F, West SC.; ''Interactions between human BRCA2 protein and the meiosis-specific recombinase DMC1.''; PubMedEurope PMCScholia
Scully R, Chen J, Plug A, Xiao Y, Weaver D, Feunteun J, Ashley T, Livingston DM.; ''Association of BRCA1 with Rad51 in mitotic and meiotic cells.''; PubMedEurope PMCScholia
Inagaki A, Schoenmakers S, Baarends WM.; ''DNA double strand break repair, chromosome synapsis and transcriptional silencing in meiosis.''; PubMedEurope PMCScholia
Enomoto R, Kinebuchi T, Sato M, Yagi H, Kurumizaka H, Yokoyama S.; ''Stimulation of DNA strand exchange by the human TBPIP/Hop2-Mnd1 complex.''; PubMedEurope PMCScholia
Barlow AL, Benson FE, West SC, Hultén MA.; ''Distribution of the Rad51 recombinase in human and mouse spermatocytes.''; PubMedEurope PMCScholia
Bocker T, Barusevicius A, Snowden T, Rasio D, Guerrette S, Robbins D, Schmidt C, Burczak J, Croce CM, Copeland T, Kovatich AJ, Fishel R.; ''hMSH5: a human MutS homologue that forms a novel heterodimer with hMSH4 and is expressed during spermatogenesis.''; PubMedEurope PMCScholia
Golub EI, Gupta RC, Haaf T, Wold MS, Radding CM.; ''Interaction of human rad51 recombination protein with single-stranded DNA binding protein, RPA.''; PubMedEurope PMCScholia
The gene encoding SPO11 shares sequence similarity to TopoVI, a type II topoisomerase. SPO11 dimers cleave both strands of DNA. Each subunit of the dimer remains covalently attached to the 5' end of one strand of DNA via a phosphodiester linkage to a conserved tyrosine residue of SPO11. In addition to SPO11, work from budding yeast has shown a total of 7 proteins essential for double strand break formation. The mammalian ortholog of Mei4 (S. cerevisiae) as well as a mammalian-specific gene called Mei1 are essential to formation of meiotic double strand breaks.
Two RecA homologs, RAD51 and the meiosis-specific DMC1, coat single-stranded 3' ends of DNA produced by resection of double-strand breaks (Barlow et al. 1997, Masson et al. 1999, Sehorn et al. 2004, Sheridan et al. 2008, Okorokov et al. 2010). RAD51 and DMC1 interact and colocalize to the same early recombination nodules (Masson et al. 1999). Knockouts of DMC1 abolish recombination and synapsis therefore RAD51 is not sufficient for recombination. 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.
Meiotic Holliday junctions are cleaved to yield either crossovers or non?crossovers (gene conversions). The resolvase or resolvases responsible for cleavage are unknown but a resolvase complex may include SLX4 and/or GEN1. 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.
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 of single strands 3' to the attached SPO11. The 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 in somatic cells 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.
The 3' end of the invading strand is extended by an unknown DNA polymerase and the extended strand is then ligated back to the original homolog, generating a double Holliday junction. MSH4 and MSH5 form heterodimers which bind Holliday junctions and, in the presence of ATP, slide along the parental duplexes (Bocker et al. 1999, Snowden et al. 2004, Snowden et al. 2008). MSH4 is present at hundreds of meiotic nodules during late zygotene but only about 10% of these nodules become crossovers (Oliver-Bonet et al. 2005). Bloom Syndrome protein (BLM) and Topoisomerase IIIa (TOP3A) are also present and may promote homologous recombination repair without crossing over (Johnson et al. 2000, Wu et al. 2000).
The gene encoding SPO11 shares sequence similarity to TopoVI, a type II topoisomerase. SPO11 dimers cleave both strands of DNA. Each subunit of the dimer remains covalently attached to the 5' end of one strand of DNA via a phosphodiester linkage to a conserved tyrosine residue of SPO11. In addition to SPO11, work from budding yeast has shown a total of 7 proteins essential for double strand break formation. The mammalian ortholog of Mei4 (S. cerevisiae) as well as a mammalian-specific gene called Mei1 are essential to formation of meiotic double strand breaks.
As inferred from experiments in vitro with mouse Prdm9, human PRDM9 methylates histone H3 dimethylated at lysine-4 to yield histone H3 trimethylated at lysine-4.
PR-domain containing 9 (PRDM9) protein is a meiosis specific histone H3 lysine 4 (H3K4) methyltransferase, with a zinc finger domain at the C-terminus. Meiotic recombination hotspots in humans and mice are known to be sites for histone modification. PRDM9 has been shown to affect recombination profiles and meiotic recombination hotspot activity, by binding specific sequence motifs within or close to recombination hotspots (Baudat et al. 2010, Myers et al. 2010), and reorganizing chromatin structure. Variation within this protein has been proven to negatively affect human male fertility, with certain patients harboring variants at the PRDM9 locus exhibiting azoospermia. PRDM9 recognizes a specific sequence motif, but also acts at human hotspots lacking the motif, suggesting it is capable of acting in cis to regulate hotspot activity. 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.
Following double strand break (DSB) formation and strand resection, RAD51 and DMC1 coat single-stranded 3' ends of DNA and catalyze the search for homology and strand invasion into the DNA duplex of the chromosomal homolog (Baumann et al. 1996, Barlow et al. 1997, Benson et al. 1998, Baumann and West 1999, Masson et al. 1999, Sehorn et al. 2004, Murayama et al. 2008). The invading strand displaces the original strand of the chromosomal homolog creating a D-loop structure. Other proteins present in the complex are inferred from cytology (Barlow et al. 1997, Oliver-Bonet et al. 2005, Oliver-Bonet et al. 2007).
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.
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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.
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.
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 in somatic cells 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.