Chiasma
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- The term "Chiasma" is also sometimes used to refer to the Optic chiasm.
A chiasma (Greek χίασμα, related to the Greek letter Χ) means "crossing." In anatomy, it refers to the point where two (or more) nerves or ligaments cross or intersect. In genetics, it is a technical term for the point where two chromatids are intertwined (interwoven) in a cell.
[edit] Genetics
The chiasmata are thought to be the points where two nonsister chromatids exchange genetic material during chromosomal crossover during meiosis (sister chromatids also form chiasmata between each other, but because their genetic material is identical, it doesn't cause any change in the resulting daughter cells). The chiasmata become visible during the diplotene stage of prophase I of meiosis I, but the actual "crossing-over" of genetic material is thought to occur during the previous pachytene stage. When tetrads, which are composed of two pairs of sister chromatids, begin to split the only points of contact are at the chiasmata.
Chiasma frequency = 2 x recombination frequency
where recombination frequency is
recombination frequency = (No. of recombinants)/(total no. of progeny)
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