Prophase I:
Prophase in meiosis-I is typically longer and more complex than the prophase in meiosis II. Prophase I is subdivided into five phases, viz. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
Leptotene:
• During this stage, the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout this phase.
Zygotene:
• Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes.
• Formation of synapsis is accompanied by the formation of the synaptonemal complex.
• The synaptonemal complex by a pair of homologous chromosomes is called a bivalent or a tetrad.
Pachytene:
• Bivalent chromosomes clearly appear as tetrads, at this stage.
• Recombination nodules appear. These nodules are the sites at which crossing overtakes place between non-sister chromatids of the homologous chromosomes.
• Exchange of genetic materials between two homologous chromosomes takes place during crossing over. This leads to the recombination of genetic materials on the two 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.
• Meiotic spindles are formed to prepare the homologous chromosomes for separation.
• Nucleolus disappears and nuclear envelope breaks down by the end of diakinesis.