What is synapsis?

• A. Crossing over during meiosis
• B. Replication of chromosomes
• C. Separation of sister chromatids
• D. Pairing of homologous chromosomes

Answer: (D)

Synapsis is the pairing of homologous chromosomes during early meiosis, when the homologs come together and align along their lengths. This happens in prophase I after chromosomes have condensed and replicated, and the pairing is held together by protein structures to form a tetrad of four chromatids. This close alignment sets the stage for crossing over between non-sister chromatids, which increases genetic variation in the resulting gametes. The definition described here focuses on the pairing of homologous chromosomes, not on replication, which occurs earlier in the cell cycle, or on the separation of sister chromatids, which happens later. Crossing over is a consequence of synapsis, not its basic definition.