Meiosis is a process in cell division during which the number of chromosomes decreases to half the original number by two divisions of the nucleus, resulting in the production of gametes. Meiosis is often confused with mitosis, the reproduction of somatic, or body, cells, such as skin or blood cells. While both processes involve the replication (or formation) of new cells, there are some major differences between them.
- Mitosis is the division of somatic (body) cells, while meiosis is the formation of gametes (sex cells).
- The daughter (new) cells resulting from mitosis have the same number of chromosomes as the parent cells. Meiosis generates daughter cells with half the number of chromosomes.
- Mitosis can happen in asexually reproducing organisms. Meiosis only happens in sexually reproducing ones.
- Mitosis involves one nuclear division (division of the nucleus). Meiosis involves two.
The main events that occur during meiosis are similar to mitosis, but the end result is different.
Meiosis is divided into different stages. It is important to remember that these steps are for illustrative purposes. The actual process is never-ending and forms a cycle. Breaking the process into distinct steps is useful for analyzing what is happening along the way. In meiosis there are two cell divisions. The second division results in each cell having half the number of chromosomes of the original cell. This way, when the sperm and egg join during fertilization, the correct diploid number (2n) is reached.
The steps of meiosis are as follows:
- Interphase—This is a period of growth and duplication of chromosomes. It is not technically a step in meiosis; rather, it precedes meiosis and ensures the cell is ready to divide. Each chromosome is made of two sister chromatids held together at the centromere, a region of DNA where the chromatids are joined. Because all of the genetic material is being replicated, sex cells spend most of their time in this stage.
- Prophase I—This is also a long stage of the process. The replicated chromosomes condense, or become smaller, and pair with homologues in a process called synapsis, which is the alignment of homologous chromosomes during prophase I of meiosis. Homologous chromosomes are identical pairs of chromosomes which pair together during synapsis of meiosis I in diploid organisms, and are present after interphase. This forms a tetrad, two pairs of homologous chromosomes (which share the same genes). For example, at this time, fruit flies have four tetrads. Once the tetrad is formed, regions of each pair trade places. This exchange of DNA between homologous chromosomes during meiosis is called crossing-over and serves to increase genetic diversity of the offspring. Also, the nuclear envelope, which surrounds the nucleus, breaks down, and the spindle fibers form. A spindle fiber is a microtubule that attaches to chromosomes and pulls them to opposite poles of a cell during cell division.
- Metaphase I—The homologous pairs attach at their centromeres to the spindle fibers and then line up along the equator, or central plane, of the cell.
- Anaphase I—The spindle fibers pull one chromosome from each tetrad toward each end, or pole, of the cell. Each chromosome still has two chromatids.
- Telophase I—The homologous chromosome pairs gather into nuclei, the regions containing DNA. Each pole now has a chromosome set. Telophase I occurs simultaneously with cytokinesis, or the pinching off of the cytoplasm to form two new cells.
There is only a brief interphase between telophase I and prophase II. It is used mostly as a resting stage for the cells and no chromosome duplication happens.
- Prophase II—A spindle apparatus containing the spindle fibers forms, the chromosomes condense again, and the nuclear membrane that surrounds them dissolves.
- Metaphase II—Paired chromatids line up in the equator of the cell.
- Anaphase II—The centromeres divide, and the sister chromatids begin to separate. Separated chromosomes move to opposite ends of the cell.
- Telophase II—The chromosomes come together into nuclei. Cytokinesis occurs, resulting in four haploid daughter cells in the male. Each of these cells is different genetically than the original because of the crossing over that occurred in prophase I. In female animals, there is an uneven distribution of the genetic material. This results in a viable haploid (n) cell and smaller structures called polar bodies. A polar body is a nonviable cell produced at the end of meiosis in females. The function of polar bodies is to sequester an appropriate amount of genetic material away from the viable haploid cell. These are usually reabsorbed by the female's body, although some species retain them.