Understanding when synapsis takes place is crucial for understanding meiosis, the cell division process that reduces the number of chromosomes by half to produce gametes (sex cells).

Meiosis consists of two successive divisions, Meiosis I and Meiosis II. Synapsis specifically occurs during Prophase I of Meiosis I. Prophase I is further divided into several sub-stages:

• Leptotene: Chromosomes condense and become visible. Each chromosome consists of two sister chromatids.
• Zygotene: This is the stage where synapsis occurs. Homologous chromosomes pair up and align side-by-side. This pairing is very precise, gene for gene, and is facilitated by a protein structure called the synaptonemal complex. The paired homologous chromosomes are now called bivalents or tetrads (since each bivalent has four chromatids).
• Pachytene: Crossing over takes place during this stage. Genetic material is exchanged between non-sister chromatids of homologous chromosomes, leading to genetic recombination. The synaptonemal complex is fully formed.
• Diplotene: The synaptonemal complex dissolves, and the homologous chromosomes begin to separate. However, they remain attached at points where crossing over occurred, called chiasmata.
• Diakinesis: Chromosomes condense further. The nuclear envelope breaks down, and the spindle fibers begin to form, preparing for metaphase I.

When Does Synapsis Occur?

Based on the stages of Prophase I, synapsis, the pairing of homologous chromosomes, distinctly takes place during the zygotene stage. This pairing is a fundamental step for ensuring correct segregation of chromosomes during meiosis I and for allowing crossing over in the subsequent pachytene stage.

Therefore, synapsis is a characteristic event of the zygotene stage of Prophase I.