Meiosis I

Meiosis Listeni/maɪˈoʊsɨs/ is a specialized type of cell division which reduces the chromosome number by half. This process occurs in all sexually reproducing eukaryotes (both single-celled and multicellular) including animals, plants, and fungi.

In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids attached at a centromere. In meiosis I, homologous chromosomes pair with each other and can exchange genetic material in a process called crossing over. The homologous chromosomes are then pulled apart into two new separate daughter cells, each containing half the number of chromosomes as the parent cell. At the end of meiosis I, sister chromatids remain attached and may differ from one another if crossing-over occurred. In meiosis II, the two cells produced during meiosis I divide again. During this division, sister chromatids detach from one another and are separated into four total daughter cells. These cells can mature into gametes, spores, pollen, and other reproductive cells.

Because the number of chromosomes is halved during meiosis, it allows gametes to fuse (i.e. fertilization) to form a zygote containing a mixture of paternal and maternal chromosomes. Thus, meiosis and fertilization facilitate sexual reproduction with successive generations maintaining the same number of chromosomes. For example, a typical diploid human cell contains 23 pairs of chromosomes (46 total, half of maternal origin and half of paternal origin). Meiosis produces haploid gametes with one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes.