AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

EVENTS OF CROSSING OVER

• Crossing over occurs during Meiosis of Gametogenesis.
  • During Zygotene stage of Prophase - I of Meiosis, the homologous chromosomes (genetically identical chromosomes) move towards each other. This phenomenon of pairing of homologous chromosomes is called Synapsis.
  • Each homologous chromosome of a bivalent split longitudinally and form two identical sister chromatids and results in four chromosomes. This stage is called tetrad stage.
  • Crossing over involves the breakage and exchange of parts between two non-sister chromatids of a homologous pair of chromosomes. This produces an X-like structure at the point of exchange of the chromatid segments. This structure is called Chiasma (plural chaismata).
  • After the completion of crossing over, the non-sister chromatids start to repel each other. They separate from each other from the centromere towards chiasma and to the end of tetrad. The separation is called Terminilisation. Due to this terminilisation the homologous chromosomes are separated.
  • Each event of crossing over produces two recombinant chromatids called as crossover chromatids and two non crossover chromatids (original chromosomes).