Sperm cell development and oocyte development involve meiosis. This kind of cell division occurs only in the gonads. It consists of two consecutive nuclear divisions without a second replication of the genetic material between the divisions. Four daughter cells are produced, and each has half as many chromosomes as the parent cell.

The normal chromosome number in human cells is 46. This number is called a diploid or a 2n number of chromosomes. The chromosomes consist of 23 pairs. Each pair of chromosomes is called a homologous pair. One chromosome of each homologous pair is from the male parent, and the other is from the female parent. The chromosomes of each homologous pair look alike, and they contain genes for the same traits.

In sperm cells and oocytes the number of chromosomes is 23. This number is called a haploid or n number of chromosomes. Each gamete contains one chromosome from each of the homologous pairs. Reduction of the number of chromosomes in sperm cells or oocytes to an n number is important. When a sperm cell and an oocyte fuse to form a fertilized egg, each provides an n number of chromosomes, which reestablishes a 2n number of chromosomes. If meiosis did not occur, each time fertilization occurred the number of chromosomes in the fertilized oocyte would double. The extra chromosomal material would be lethal to the developing offspring.

The two divisions of meiosis are called meiosis I and meiosis II. The stages of meiosis have the same names as these stages in mitosis, that is, prophase, metaphase, anaphase, and telophase; but there are distinct differences between mitosis and meiosis.

Before meiosis begins, all the deoxyribonucleic acid in the chromosomes is duplicated. At the beginning of meiosis each of the 46 chromosomes consists of two sister chromatids connected by a centromere. In prophase of meiosis I the chromosomes align with their homologous pairs near the middle of the cell. This process is call synapsis. Because each chromosome consists of two chromatids, the pairing of the homologous chromosomes bring two chromatids of each chromosome close together, an arrangement called a tetrad. Occasionally part of a chromatid of one homologous chromosome breaks off and is exchanged with part of another chromatid from the other homologous chromosome of the tetrad. This exchange of genetic material is called crossing over. Crossing over allows the exchange of genetic material between maternal and paternal chromosomes.

During synapsis homologous pairs of chromosomes line up near the enter of the cell undergoing meiosis. However, for each pair of homologous chromosomes, the side of the cell on which the maternal or paternal chromosome is located is random. The way the chromosomes align during synapsis results in the random assortment of maternal and paternal chromosomes in the daughter cells during meiosis. Crossing over and the random assortment of maternal and paternal chromosomes are responsible for the large degree of diversity in the genetic composition of sperm cells and oocytes produced by each individual.

During anaphase I the homologous pairs are separated to each side of the cell. As a consequence, when meiosis I is complete, each daughter cell has one chromosome from each of the homologous pairs. Each of the 23 chromosomes in each daughter cell consists of two chromatids joined by a centromere.

It is during the first meiotic division that the chromosome number is reduced from 2n number (46 chromosomes or 23 pairs) to an n number (23 chromosomes or one from each homologous pair). The first meiotic division is threfore called a reduction division.

The second meiotic division is similar to mitosis. The chromosomes, each consisting of two chromatids, line up near the middle of the cell. Then the chromatids separate at the centromere, and each daughter cell receives one of the chromatids from each chromosome. When the centromere separates, each of the chromatids is called a chromosome. Consequently, each of the four daughter cells produced by meiosis contains 23 chromosomes.

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