Mechanism of Crossing Over

Crossing over unfolds through four sequential, microscopically recognisable stages within prophase I of meiosis. The first stage, synapsis (also called syndesis), begins at the zygotene stage, when the two members of each homologous chromosome pair align intimately side by side along their entire length, forming a paired structure called a bivalent; this pairing can start from the middle of the chromosome (procentric synapsis), from the telomeric ends (proterminal synapsis), or from no fixed point at all (random synapsis). In the second stage, tetrad formation, each of the two paired homologous chromosomes duplicates into two identical sister chromatids that stay joined at a shared centromere, so that the bivalent as a whole now consists of four chromatids, called a tetrad. In the third stage, the actual crossover, which happens during the pachytene sub-stage, the non-sister chromatids of the homologous pair make physical contact at one or more points called chiasmata (singular chiasma); at each chiasma the two chromatids break and rejoin in a reciprocal exchange, swapping equal and corresponding DNA segments between them, a process now understood to be organised by a protein scaffold called the synaptonemal complex. In the final stage, …