Prophase II is the first phase of meiosis II and differs from mitotic prophase by what key factor?

Prophase II is the first phase of meiosis II and differs from mitotic prophase by what key factor?

• A. The chromosome number is reduced by half during meiosis I
• B. Spindle formation occurs earlier
• C. Crossing over occurs
• D. The nuclear envelope remains intact

Answer: (A)

The key idea is that meiosis I reduces the chromosome number, so prophase II starts in cells that are haploid. After meiosis I, the cell has only one set of chromosomes, each still consisting of two sister chromatids. That makes prophase II resemble mitotic prophase in the way chromosomes condense and the spindle forms, but the cellular context is different: you’re dealing with a haploid complement rather than a diploid one, and there’s no homologous chromosome partner to pair with or to undergo crossing over. Crossing over occurs in prophase I, not prophase II, and the nuclear envelope breaks down in prophase II just as it does in mitotic prophase. Therefore, the defining distinction from mitotic prophase is that meiosis II begins after chromosome number has already been halved in meiosis I, leaving haploid cells to proceed with the separation of sister chromatids.

The key idea is that meiosis I reduces the chromosome number, so prophase II starts in cells that are haploid. After meiosis I, the cell has only one set of chromosomes, each still consisting of two sister chromatids. That makes prophase II resemble mitotic prophase in the way chromosomes condense and the spindle forms, but the cellular context is different: you’re dealing with a haploid complement rather than a diploid one, and there’s no homologous chromosome partner to pair with or to undergo crossing over. Crossing over occurs in prophase I, not prophase II, and the nuclear envelope breaks down in prophase II just as it does in mitotic prophase. Therefore, the defining distinction from mitotic prophase is that meiosis II begins after chromosome number has already been halved in meiosis I, leaving haploid cells to proceed with the separation of sister chromatids.