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Explain the stages of prophase I of meiosis.

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Hint: It is the first stage of meiotic division and is subdivided into five phases based on chromosomal behavior. This phase is typically longer and more complex when compared to prophase of mitosis.

Complete answer:
Sexual reproduction includes the fusion of two gametes. Gametes are produced from specialised diploid cells. This specialised form of cell division which reduces the number of chromosomes by half and produces four daughter haploid cells is known as meiosis.

Meiosis involves two stages called meiosis I and meiosis II. Meiosis I is initiated after the replication of the parental chromosomes in order to create identical sister chromatids in phase S. Prophase I is subdivided into Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

In the leptotene stage, the chromosomes gradually become visible underneath the light microscope. Chromosomal compaction occurs in leptotene. In the second stage of prophase, which is called zygotene, the chromosomes begin to pair together, and this association process is called synapsis. The complex formed by a pair of synapsed homologous chromosomes is known as a bivalent or a tetrad.

The four chromatids of each bivalent chromosome become distinct at the pachytene stage and clearly appear as tetrads. This stage is marked by the presence of recombination nodules, the sites where the homologous chromosomes cross over between non-sister chromatids. Crossing over between the two chromosomes leads to a recombination of genetic material. The recombination of homologous chromosomes is completed towards the end of pachytene leaving the chromosomes joined at the crossover sites.

The start of diplotene is recognised by the breakup of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to differentiate from each other except at crossover sites.

Diakinesis is the last stage of prophase I. This is indicated by chiasmata terminalisation. The chromosomes are completely condensed during this process, and the meiotic spindle is assembled in preparation for the separation of the homologous chromosomes. The nucleolus disappears by the end of diakinesis, and even the nuclear shell breaks down. This stage marks the transition to metaphase.

Note: Meiosis includes pairing of homologous chromosomes and recombination between the homologous non-sister chromatids. Meiosis II is the stage where four haploid cells are produced.