Let's take a closer look at the process of meiosis. As you know, the G1, S, and G2 phases precede meiosis. The end result of these phases is that the maternal chromosome (the chromosome received from the mother) becomes two sister chromatids and the paternal chromosome (the chromosome received from the father) replicates to form its own two sister chromatids. Therefore, inside the gamete-producing cells of a male there are homologous pairs from his father (paternal) and homologous pairs from his mother (maternal) lining up side by side. The same is the case for a female. Watch this video to see what happens next. As you watch, try to identify the key differences between the processes of meiosis and mitosis.
Unlike mitosis, meiosis occurs in two broader phases, known as meiosis 1 and meiosis 2. At the beginning of meiosis 1, two duplicated chromosomes lay side by side. Then, in prophase 1, the chromatids belonging to the homologous pairs swap portions of themselves. This exchange is called crossing over and is what causes the chromatids to be non-identical chromatids. The exchange of chromatid sections is also why a baby has some characteristics of its mother, and some of its father. During metaphase 1 the spindle fibers pull the pairs of chromosomes until they line up in the center of the cell. During anaphase 1 these pairs are separated. Then the new nuclei form and the cell separate into two cells. In humans, each of these new cells contains 23 chromosomes. Both cells then enter meiosis 2. In meiosis 2 the process that occurred in meiosis 1 repeats itself...from prophase to telophase. At the end of meiosis 2, there are four haploid cells instead of two.
In meiosis I, the first stage of meiosis, the two parts of the duplicated chromosome in a germ line or reproductive cell lie side by side to form a tetrad-like structure. Then the chromatids belonging to the homologous pairs "swap" portions of themselves. This physical exchange of chromosomes--generally called crossing over--causes the chromatids to become non-sister (non-identical) chromatids, which changes the genetic nature of the maternal and paternal sex cells. It is due to this physical exchange of chromosome pieces during cross over that babies are born with some characteristics that belong to the father and some that belong to the mother.
Another important event that occurs in meiosis I is the actual division of the cell. In meiosis, the homologous chromosomes, or homologs, separate as intact pairs into two different cells. This division reduces the number of chromosomes in one cell--a once diploid cell becomes two haploid cells. Each cell contains a maternal pair and a paternal pair of sister chromatids. Notice, however, that the maternal pair has paternal portions--due to cross-over--and vice versa.
But how is it possible that four haploid cells are produced from one diploid cell? After the division that occurs in meiosis I, meiosis II proceeds and each of the two diploid cells containing 46 chromosomes divides again, creating a total of four cells containing 23 chromosomes each.
Question
Red foxes have 34 chromosomes in their body cells. Figure out the number of chromosomes that will occur in the haploid (sperm) cells of a red fox male after meiosis is over.
