Prophase I staging During meiosis I, in which substage do homologous chromosomes undergo synapsis (pairing via the synaptonemal complex)?

Introduction / Context:
Prophase I of meiosis is subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis. Each stage features distinct chromosome behaviors that enable recombination and proper segregation.

Given Data / Assumptions:

• Synapsis is the intimate pairing of homologs mediated by the synaptonemal complex.
• Crossing over occurs after synapsis is established.

Concept / Approach:
Leptotene: chromosomes begin to condense. Zygotene: homologs synapse. Pachytene: synapsis is complete, and crossing over occurs. Diplotene: synaptonemal complex disassembles; chiasmata visible. Diakinesis: further condensation and terminalization.

Step-by-Step Solution:

Verification / Alternative check:
Standard meiosis schematics label zygotene as the synapsis phase.

Why Other Options Are Wrong:

• Leptotene precedes pairing; chromosomes are thread-like.
• Pachytene follows synapsis and features crossing over.
• Diplotene/diakinesis involve desynapsis and preparation for metaphase I.

Common Pitfalls:
Confusing synapsis (pairing) with recombination (exchange), which mainly occurs in pachytene.

Final Answer:
Zygotene