Prophase I is subdivided into five phases, viz. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
Leptotene: During this stage, the chromosomes become gradually visible under a light microscope. Compaction of chromosomes continues throughout this phase.
Zygotene: Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes. The formation of synapsis is accompanied by the formation of the synaptonemal complex.
Pachytene: Recombination nodules appear. These nodules are the sites at which crossing over takes place between non-sister chromatids of the homologous chromosomes.
Diplotene: Synapotnemal complex is dissolved at this stage. The recombined homologous chromosomes of the bivalent separate from each other; except at the site of crossing over. The X-shaped structures; thus formed; are called chiasmata.
Diakinesis: Chiasmata is terminated at this stage.