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PROPHASE I

Prophase I is the first and the longest stage of the first division of meiosis. It takes almost 90 per cent of all time, which is necessary for meiosis. They distinguish five stages in prophase I.

• Leptotene (the stage of fine fibers) is the beginning of condensation of chromosomes; in its entirety it resembles the early prophase of mitosis, but differs by thinner chromosomes and larger nuclei.
• Zigotene (the stage of interfusing fibers) is convergence and the beginning of conjugation of homologous chromosomes; by the end of it all homologues join into bivalents.
• In the pachytene (the stage of thick fibers) crossing-over occurs.
• Diplotene (the stage of double fibers) begins with mutual repulsion of homologues and emerging of chiasmata; in diplotene in the great majority of organisms further helix formation in chromosomes and reduction of the number of nucleoli take place. This is the longest period of prophase I.
• For diakinesis (the stage of isolation of double fibers) decreasing of the number of chiasmata and considerable compactness of bivalents are characteristic.