synapsis

Before two matching chromosomes can swap parts, they first have to find each other and line up side by side, gene for gene. That careful pairing is synapsis: during prophase of meiosis I, each chromosome locates its homologous partner and zips up alongside it along their whole length.

Synapsis is held in place by a protein scaffold called the synaptonemal complex, which acts like a zipper running between the two homologs. With the partners aligned at matching positions, the cell can carry out crossing over precisely, exchanging equivalent segments rather than mismatched ones.

This pairing is unique to meiosis; it does not happen in ordinary mitotic division. The close alignment of synapsis is also what later produces a tetrad, the four-chromatid structure of two paired homologs, and sets the stage for the chiasmata that hold the pair together until separation.