When in the cell cycle is the number of chromosomes normally reduced? Page 96 of the Study Guide for Campbell Biology, 11th Edition has the following question: > 6. A reduction in the number of chromosomes per cell occurs > > a. During meiosis I > > b. During meiosis II > > c. During mitosis > > d. During the S phase of interphase > > Page 451 states that the answer is a. I thought that the number of chromosomes at the end of meiosis I per cell equals the number of chromosomes in the parent cell (although sister chromatids remain attached), whereas the number of chromosomes at the end of meiosis II per cell equals half the number of chromosomes of the parent cell. If so, isn't the answer b?

It is the other way around. Meiosis I (or reductional division) splits chromosome _pairs_ so each cell gets half of the chromosomes of the parent. Meiosis II (or equational division) splits double-chromatid chromosomes (making two single-chromatid chromosomes), so cells retain the number of chromosomes, like in mitosis.

* * *

If more clarification needed:

* before the S phase: 23 pairs of chromosomes, 46 single-chromatid chromosomes
* before the meiosis I: 23 pairs of chromosomes, 46 double-chromatid chromosomes
* before the meiosis II: 23 double-chromatid chromosomes without their homologous other
* after the meiosis II: 23 single-chromatid chromosomes without their homologous other
* after fertilization = before the S phase



(I'm talking about humans here.)