CELL DIVISION: MEIOSIS The most common kind of cell division in cells possessing chromosomes is mitosis, but there is another type of cell division called meiosis . Meiosis, a nuclear and cell division process, is highly limited in cell type and time of occurrence. In this lesson, you'll learn more about meiosis and its purpose. MEIOSIS New cells are produced in two main ways. The first way is mitosis, which provides two exact copies of the parent cell. Mitosis is important for cell growth, replacement, and repair. The second type of cell division is meiosis, which is for the production of gametes or sex cells, such as eggs or sperm. Meiosis is the process of sexual reproduction. Because meiosis produces gametes, it takes place in the gonads (sex organs) of the organism. Gametes are normally haploid . A haploid cell is one that only has half a set of chromosomes, or only one member of each homologous pair . When the nuclei of two gametes undergo fertilization, each gamete contributes a "set" of chromosomes. A fertilized egg will contain one set from each gamete and therefore from each parent. All but one pair of the chromosomes (the sex chromosomes) will have a matching equivalent in the other set. A fertilized egg will then be diploid like all other cells because it has a complete set of homologous pairs of chromosomes (half a set from each gamete). Diploid cells occur in human and many other organisms' body cells (except for the gametes). During meiosis, the diploid number of chromosomes is reduced by half, forming haploid cells. Thus, meiosis (named after the Greek word for "less") is a reduction of chromosomes. The diploid parent cell duplicates its DNA, divides into two daughter cells, and these two daughter cells then divide to produce four haploid daughter cells. Meiosis thus provides a mechanism by which the chromosome number may be kept the same from one generation to the next.
CELL DIVISION PROCESS Meiosis is a cell division process that requires two nuclear divisions. So the cell division process of meiosis is divided into two parts: meiosis I and meiosis II. The stages under each part of meiosis are similar to the stages of mitosis. But just remember that meiosis is a division producing haploid cells while mitosis is a division producing the same exact type of cell as the parent. As you review the stages of meiosis, think about how they are similar to mitosis. A similarity of all cells is the interphase stage of the cell's life. INTERPHASE The period in the life of a cell between cell divisions is known as interphase . In interphase, no evident activity is associated with cell division, and the genetic material (DNA) in the cell nucleus occurs in a thin, threadlike form known as chromatin.
- Fall '17