During the zygotene stage of prophase I, chromosomes start pairing together in a process called synapsis. The paired chromosomes are referred to as homologous chromosomes. This process of association is accompanied by the formation of a complex structure called the synaptonemal complex.

The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. These are more clearly visible during the pachytene stage. During this stage, the four chromatids of each bivalent become distinct and recombination nodules appear, facilitating crossing over between non-sister chromatids.

The beginning of diplotene is recognised by the dissolution of the synaptonemal complex. The recombined homologous chromosomes of the bivalents tend to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. In the final stage of prophase I, diakinesis, chiasmata undergo terminalisation.

Diagram: A diagram illustrating these stages would show the pairing of homologous chromosomes during synapsis in zygotene, the distinct four chromatids forming a bivalent (tetrad) in pachytene, and the X-shaped chiasmata formed at the sites of crossing over during diplotene.