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What is Prophase of meiosis-I, TRUE statement is/are and explain in detail?
Prophase-I of Meiosis I- Is long & complex phase that differ considerably from mitotic prophase and is customarily divided into five sub- stages. 1. Leptotene: - During leptotene, homologous chromosomes (maternal and paternal copies of the same chromosomes), replicated in a preceding S phase and each consisting of sister chromatids joined at the centromere. Cytochrome begin to condense 2. Zygotene: - During zygotene, the homologous chromosomes initiate pairing / synapsis. The pairs of synapsed homologous are known as Bivalent. The sex chromosomes also sta to synapse during zygotene. 3. Pachytene: - When synapsis is complete for all chromosomes, the cell is said to be in pachytene. Each Bivalent look like a single thick structure, but is actually two pairs of sister chromatids held together by the synaptonemal complex. Genetic recombination between non-sister chromatids is completed at this point, with sites where it has occurred, appearing as recombination nodules in the center of the synaptonemal complex. 4. Diplotene: - During diplotene, the synaptonemal complex disassembles and pairs of homologous chromosomes, now much shoened, separated, except where crossing over has occurred (chiasmata). This process is called disjunction. In the ovaries: - Primary oocyte gets arrested in diplotene by the fifth month is utero and each remains at this stage until the period before ovulation. (up to 50 years) 5. Diakinesis: - It is the prometaphase of the first meiotic division. The chromosomes, still Bivalents, become even shoer and thicker. They gradually attach to spindle & become aligned at a metaphase plate.