Which of the following describes meiosis II in humans?
A) 4N -> 2N
B) 2N -> 2N
C) 2N -> 1N
D) 1N -> 2N E) 1N -> 1N
The Correct Answer and Explanation is:
The correct answer is C) 2N -> 1N.
Explanation of Meiosis II
Meiosis is a type of cell division that reduces the chromosome number by half and results in the formation of gametes (sperm and eggs) in humans and other sexually reproducing organisms. It consists of two successive divisions: meiosis I and meiosis II.
In meiosis I, homologous chromosomes are separated, resulting in two cells, each with a haploid (1N) number of chromosomes, but each chromosome still consists of two sister chromatids (thus, they are technically still considered diploid, or 2N, at this point).
Meiosis II is similar to mitosis. It follows meiosis I without a preceding round of DNA replication. In meiosis II, the two haploid cells from meiosis I undergo a second division:
- Prophase II: The nuclear envelope breaks down, and the spindle apparatus forms. The chromosomes, still consisting of two sister chromatids, become more condensed.
- Metaphase II: Chromosomes line up along the metaphase plate, similar to what occurs in mitosis.
- Anaphase II: The sister chromatids are pulled apart toward opposite poles of the cell. This is the key step where the chromosome number is halved.
- Telophase II and Cytokinesis: Nuclear envelopes reform around the separated chromatids, now considered individual chromosomes, and the cells divide, resulting in a total of four haploid (1N) cells.
Thus, during meiosis II, the chromosome number transitions from 2N (where each chromosome has two chromatids) in the haploid cells produced by meiosis I to 1N, as each chromatid is separated into individual chromosomes. The end result of meiosis II is four non-identical haploid gametes, each with half the number of chromosomes (1N) compared to the original diploid cell (2N) from which they originated. This reduction is essential for maintaining the species’ chromosome number across generations during fertilization.