Botany - Cell Cycle and Cell Division Question with Solution | TestHub

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.