Expert: John Locke - 10/13/2007

Question
Hi, i understand the basic principle of the meiosis vs mitosis, and the differences and similarities that they share but please could you explain meiosis I compared to meiosis II, are they of the same cycle or can they be seperate, does meiosis II have to follow meiosis I? Thank you.

Answer
Thanks for using AllExperts, Tiffiny. Meiosis, as you already know, occurs in two nearly identical phases that may be separated by some period of time. Meiosis I and II are both necessary components of the overall cycle of meiosis; that is, meiosis is not complete until the germ cells involved have undergone both meiosis I and II in that order (to answer your second question, meiosis I must occur before meiosis II). Meiosis II can't occur without meiosis I, nor are germ cells completely mature until they have undergone both meiosis I and II. Sperm cells, for example, are not capable of fertilizing egg cells until they have undergone both meiosis I and II. Why is this? The two stages, while they occur in the same order and have the same components, involve different distributions of genetic material. That distribution isn't complete until meiosis II has finished.

In meiosis I, homologous chromosome pairs are pulled apart to opposite sides of the cell (that is, each daughter cell receives a single duplicated set of each chromosomes). In meiosis II, sister chromatids are separated to form haploid gametes (that is, the duplicated sets of chromosomes received from meiosis I are themselves pulled apart: each haploid gamete receives a single chromatid).

If you have any further questions about meiosis--which is a bit more complicated than mitosis, as you certainly know--I'd be happy to answer them.