Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.
Genetic recombination is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes ; & (2) intrachromosomal recombination, occurring through crossing over.
Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two hybridized nucleic acid strands, using energy from ATP hydrolysis. There are many helicases, representing the great variety of processes in which strand separation must be catalyzed. Approximately 1% of eukaryotic genes code for helicases.
RecQ helicase is a family of helicase enzymes initially found in Escherichia coli that has been shown to be important in genome maintenance. They function through catalyzing the reaction ATP + H2O → ADP + P and thus driving the unwinding of paired DNA and translocating in the 3' to 5' direction. These enzymes can also drive the reaction NTP + H2O → NDP + P to drive the unwinding of either DNA or RNA.
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids.
A Holliday junction is a branched nucleic acid structure that contains four double-stranded arms joined. These arms may adopt one of several conformations depending on buffer salt concentrations and the sequence of nucleobases closest to the junction. The structure is named after Robin Holliday, the molecular biologist who proposed its existence in 1964.
Sister chromatid exchange (SCE) is the exchange of genetic material between two identical sister chromatids.
DNA mismatch repair protein Mlh1 or MutL protein homolog 1 is a protein that in humans is encoded by the MLH1 gene located on chromosome 3. It is a gene commonly associated with hereditary nonpolyposis colorectal cancer. Orthologs of human MLH1 have also been studied in other organisms including mouse and the budding yeast Saccharomyces cerevisiae.
Spo11 is a protein that in humans is encoded by the SPO11 gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic recombination. Its active site contains a tyrosine which ligates and dissociates with DNA to promote break formation. One Spo11 protein is involved per strand of DNA, thus two Spo11 proteins are involved in each double stranded break event.
Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.
Bloom syndrome protein is a protein that in humans is encoded by the BLM gene and is not expressed in Bloom syndrome.
Exonuclease 1 is an enzyme that in humans is encoded by the EXO1 gene.
DNA topoisomerase 3-alpha is an enzyme that in humans is encoded by the TOP3A gene.
MutS protein homolog 4 is a protein that in humans is encoded by the MSH4 gene.
Crossover junction endonuclease MUS81 is an enzyme that in humans is encoded by the MUS81 gene.
DNA mismatch repair protein Mlh3 is a protein that in humans is encoded by the MLH3 gene.
RecQ-mediated genome instability protein 1 is a protein that in humans is encoded by the RMI1 gene.
Fanconi anemia, complementation group M, also known as FANCM is a human gene. It is an emerging target in cancer therapy, in particular cancers with specific genetic deficiencies.
The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.
Synthesis-dependent strand annealing (SDSA) is a major mechanism of homology-directed repair of DNA double-strand breaks (DSBs). Although many of the features of SDSA were first suggested in 1976, the double-Holliday junction model proposed in 1983 was favored by many researchers. In 1994, studies of double-strand gap repair in Drosophila were found to be incompatible with the double-Holliday junction model, leading researchers to propose a model they called synthesis-dependent strand annealing. Subsequent studies of meiotic recombination in S. cerevisiae found that non-crossover products appear earlier than double-Holliday junctions or crossover products, challenging the previous notion that both crossover and non-crossover products are produced by double-Holliday junctions and leading the authors to propose that non-crossover products are generated through SDSA.
