Chromosomal crossover (or crossing over) is the process by which two chromosomes pair up and exchange sections of their DNA. This often occurs during prophase 1 of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed until near the end of prophase 1. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. The result of this process is an exchange of genes, called genetic recombination. Chromosomal crossovers also occur in asexual organisms and in somatic cells, since they are important in some forms of DNA repair.
A double crossing over Recombination involves the breakage and rejoining of parental chromosomes Crossing over was first described, in theory, by Thomas Hunt Morgan. The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931 Meiotic recombination initiates with double-stranded breaks that are introduced into the DNA by the Spo11 protein. One or more exonucleases then digest the 5’ ends generated by the double-stranded breaks to produce 3’ single-stranded DNA tails. The meiosis-specific recombinase Dmc1 and the general recombinase Rad51 coat the single-stranded DNA to form nucleoprotein filaments.The recombinases catalyze invasion of the opposite chromatid by the single-stranded DNA from one end of the break. Next, the 3’ end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single-stranded DNA generated from the other end of the initial double-stranded break. The structure that results is a cross-strand exchange that is known as a Holliday junction. Another word for crossing-over is chiasmata which refers to the contact between two chromatids that will soon undergo crossing-over. The Holliday junction is a tetrahedral structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure.
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