Meiosis

Mitosis is immediately followed by cytokinesis, the division of the cell organelles and cytoplasm into two daughter cells. Some cells have cytokinesis happening at the same time as mitosis, while others have a separation of the two processes—something that’s called “endoreplication”, which means there are multiple nuclei within the cells, at least for a period.

MEIOSIS

Meiosis is a type of cell division that divides the cell but reduces the number of chromosomes in half, creating four haploid cells called gametes from a single totipotent stem cell. Meiosis happens in all sexually-reproducing single-celled and multicellular organisms (including animals, fungi, and plants). When the cell has errors in meiosis, there is aneuploidy, which is the lack of complete separation between chromosomes in the meiotic process so that some of the haploid cells do not have a complete set of chromosomes, while others have too many. Figure 19 shows what meiosis is like:

During meiosis, DNA replication is followed by two rounds of cell division without replication, so that four haploid cells are created. The two main aspects of meiosis are referred to as meiosis I and meiosis II. Before meiosis can begin, the DNA is replicated in the S phase so that there are two identical sister chromatids. After this happens, there is a prolonged G2-like phase called meiotic prophase. During this time, there is genetic recombination, in which there is cutting and splicing of the NA so that genetic information is exchanged during crossovers. This ensures that the haploid cells are unique and contain different combinations of genes. In female animals, there is elimination of three of the four haploid gametes so that just one gamete survives to produce the single ovum. This is not the case with male animals.

Meiosis I involves the segregation of chromosomes in order to produce two haploid cells; the process is also called reductional division because the number of chromosomes is divided in half. Meiosis II, on the other hand is referred to as equational division,

which is similar to mitosis in that the sister chromatids are separated. This ultimately involves the creation of 4 haploid daughter cells. Recombination occurs in prophase I of meiosis I. This creates the genetic variation seen in the cell. Meiosis II is most similar to mitosis and has prophase II, metaphase II, anaphase II, and telophase II.