Crossing Over: Stage of Meiosis

Correct Answer: Pachytene Stage of Prophase-I

From the notes: Crossing over = exchange of segment between non-sister chromatids of homologous chromosomes in the pachytene stage of Meiosis-I (Prophase-I).

Stages of Prophase-I (Mnemonic: LZPDD)

• Leptotene: Chromosomes begin to condense
• Zygotene: Homologous chromosomes begin to pair (synapsis); synaptonemal complex forms
• Pachytene: Full synapsis complete; crossing over occurs ✔; chiasmata form
• Diplotene: Bivalents begin to repel; chiasmata become visible
• Diakinesis: Chromosomes maximally condensed; nuclear envelope breaks down

Significance

Crossing over is the 2nd biggest cause of genetic variation (after segregation). It creates new combinations of alleles on chromosomes, which is important for evolution and for breaking up unfavourable linkages in breeding programmes.