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What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?

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Ans. These are the different stages of meiotic prophase:

(a) Leptotene: During the stage of leptotene, the chromosomes are slowly visible under a light microscope. The compression of chromosomes continues until the end of this stage. The sister chromatids which are tightly bound so that they cannot be differentiated from each other.

(b) Zygotene: Zygotene is the stage where the chromosomes start forming into pairs.This process of formation is called synapsis. Formation of synapsis goes along with the formation of synaptonemal complexes. Each pair formed in this process is called a bivalent or tetrad.

(c) Pachytene: In the stage of pachytene, the bivalent chromosomes get detached.Reconnection of the nodules starts to appear on the tetrads. The recombination nodule is the site at which the crossing over of the non-sister chromatid takes place. Crossing over happens between non-sister chromatids of the homologous chromosomes.Crossing over helps in the exchange of genes between two homologous chromosomes.

(d) Diplotene: In the stage of diplotene, the synaptonemal complex starts to break down. The reconnected chromosomes begin to segregate from each other. But they are connected at the site of crossing over. This makes an X-shaped structure called chiasmata.

(e) Diakinesis: In the Diakinesis stage, the Chiasmata is terminated. The nucleolus disappears in this process, and so does the nuclear membrane.

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