Mechanism of Crossing Over

Mutual exchange of blocks of homologous genes between a pair of homologous chromosomes is known as crossing over.

Crossing over is the exchange of corresponding segments of genetic material between non-sister chromatids of homologous chromosomes during meiosis, leading to new combinations of genes in gametes.

It is an important event of genetic recombination and plays a major role in producing variation among offspring.

Stage and occurrence:

• Crossing over occurs during prophase I of meiosis I.
• The actual exchange takes place specifically during the pachytene stage, when homologous chromosomes are fully paired.
• The points of exchange become visible later as chiasmata during the diplotene stage.

• During prophase I, homologous chromosomes come close and pair with each other by synapsis.
• Since each chromosome has already duplicated, the homologous pair forms a bivalent or tetrad consisting of four chromatids.
• In pachytene, non-sister chromatids break at corresponding points and exchange equal segments.
• The exchanged segments rejoin with the opposite chromatid, producing recombinant chromatids.
• In diplotene, the homologous chromosomes begin to separate but remain attached at the crossing points called 'chiasmata'.

A-G. Process of crossing over during meiosis I:

• Produces genetic variation in gametes and offspring, increasing diversity in a population.
• Provides basis for gene mapping and linkage maps using recombination frequency.
• Gives evidence for linear arrangement of genes on chromosomes.
• Important in plant/animal breeding to obtain useful recombinant combinations.

• Crossing over is the exchange of genetic segments between non-sister chromatids of homologous chromosomes.
• It occurs in pachytene of prophase I and becomes visible as chiasmata in diplotene.
• It produces recombinant chromatids and increases genetic variation.
• It is important for evolution, linkage studies, and gene mapping.