Crossing over occurs during prophase I of meiosis.

Quick Scoop

  • During prophase I, homologous chromosomes pair up closely in structures called tetrads, which allows non-sister chromatids to physically exchange DNA segments (crossing over).
  • This exchange produces new combinations of alleles, increasing genetic variation in gametes formed after meiosis.
  • Crossing over is a defining feature of meiosis I and is essentially completed before the cell proceeds to metaphase I.

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