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A-Level Biology OCR A

Crossing Genetic Forming Chromatids Describe Process Explain Produces

This note explains the process of crossing over, detailing how homologous chromosomes form bivalents, entangle at chiasmata, break and rejoin, forming recombinant chromatids and thus leading to genetic variation.

Crossing over involves homologous chromosomes pairing (bivalents), entangling at chiasmata, breaking/rejoining to form recombinant chromatids, increasing genetic variation.

Front Describe the process of crossing over and explain how it produces genetic variation.
Back Crossing over:
  • Homologous chromosomes pair up, forming a bivalent 
  • Non-sister chromatids become entangled at points called chiasmata
  • They break off and re-join, sometimes resulting in an exchange of DNA forming recombinant chromatids with different combinations of alleles and therefore genetic variation

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