What Does "crossing Over" Mean in Meiosis?


Quick Answer

"Crossing over" in meiosis refers to the process by which segments of DNA on nonsister, or homologous, chromatids break off and recombine on the other homologous chromatid. This process becomes a source of variation in egg and sperm cells.

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Full Answer

Each chromatid from a nonsister chromatid pair comes from a different parent. While they carry the same genes, the genes may be in different forms. Crossing over is so precise that no genes or parts of genes are lost in the process. The genes that break off on one nonsister chromatid are replaced by the segment coming from the other nonsister chromatid.

Crossing over occurs during prophase I, the first stage in the formation of egg and sperm cells called meiosis. Segments of nonsister chromatids overlap at certain points called chiasmata. At chiasmata, segments of DNA break off on each homologous chromatid, switch places and then reattach on the other nonsister chromatid. The result of crossing over is that different homologous chromatids are produced; these nonsister chromatids are no longer the exact ones passed down from the parents.

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