Mitosis takes a diploid cell and creates a nearly exact copy. Mitosis has two main functions: (1) it leads to the creation of all of the somatic (body) cells in humans and other living organisms; (2) in organisms that undergo asexual reproduction, diploid parent cells undergo mitosis to create identical daughter copies of themselves. Mitosis creates a daughter cell with chromosomes that are identical to the chromosomes in its parent cell.

But humans and most other complex plants and animals each have a unique set of chromosomes. This diversity of chromosomes is the result of sexual reproduction, which involves the contribution of the genetic material from not one, but two parents. During sexual reproduction the father’s haploid sperm cell and the mother’s haploid ovum (egg) cell fuse to form a single-celled diploid zygote that then divides billions of times to form a whole individual.

In order for sexual reproduction to take place, however, the parents first need to have haploid sperm or ova, also called sex cells, germ cells, or gametes. Meiosis is the name for the special type of cell division that produces gametes.

Unlike the single-cell division of mitosis, meiosis involves two cellular divisions: meiosis I and meiosis II. Each stage of meiosis runs through the same five stages as discussed in mitosis. During the first round of division, two intermediate daughter cells are produced. By the end of the second round of meiotic division (meiosis II), the original diploid (2n) cell has become four haploid (n) daughter cells.

Meiosis I is quite similar to mitosis. However, there are a number of crucial differences between meiosis I and mitosis, all of which will be outlined in the discussion of each individual stage below.

Just as in mitosis, the cell undergoes DNA replication during this intermediate phase. After replication, the cell has a total of 46 chromosomes, each made up of two sister chromatids joined by a centromere.

The major distinction between mitosis and meiosis occurs during this phase. In mitotic prophase, the double-stranded chromosomes line up individually along the spindle. But in meiotic prophase I, chromosomes line up along the spindle in homologous pairs. Then, in a process called synapsis, the homologous pairs actually join together and intertwine, forming a tetrad (two chromosomes of two chromatids each, or four total chromatids). Often this intertwining leads the chromatids of homologous chromosomes to actually exchange corresponding pieces of DNA, a process called crossing-over or genetic reassortment. Throughout prophase I, sister chromatids behave as a unit and are identical except for the region where crossover occurred.

After prophase I, the meiotic cell enters metaphase I. During this phase, the nuclear membrane breaks down, allowing microtubules access to the chromosomes. Still joined at their crossover regions in tetrads, the homologous pairs of chromosomes, with one maternal and one paternal chromosome in each pair, align at the center of the cell via microtubules, as in mitotic metaphase. The pairs align in random order.

Anaphase I differs slightly from its mitotic counterpart. In mitotic anaphase, sister chromatids split at their centromeres and are pulled apart toward opposite poles. In contrast, during anaphase I, the centromeres do not split: the entire maternal chromosome of a homologous pair is pulled to one end, and the paternal chromosome is pulled to the other end.

During telophase I, the chromosomes arrive at separate poles and decondense. Nuclear membranes re-form around them. The cell physically divides, as in mitotic cytokinesis.

Meiosis I results in two independent cells. One cell contains the maternal homologous pair, with a small segment of the paternal chromosome from crossover. The other cell contains the paternal homologous pair, likewise with a small segment of the maternal chromosome. Despite the small region of crossover in the chromosomes of each cell, the maternal sister chromatids are still quite similar, as are the paternal sister chromatids. Both cells formed by meiosis I contain a haploid amount of DNA.

The cells produced in meiosis I are different from those produced in mitosis because both haploid members of the meiotic pair derive from random assortments of either the maternal or paternal chromosomes from each homologous pair (with the exception of the small crossover sections). In mitosis, the cellular division separates sister chromatids and results in diploid cells containing one maternal and one paternal copy in each diploid pair.

The cells produced by meiosis I quickly enter meiosis II. These cells do not undergo DNA replication before entering meiosis II. The two cells that move from meiosis I into meiosis II are haploid—each have 23 replicated chromosomes, rather than the 46 that exist in cells entering both mitosis and meiosis I.

Meiotic division II occurs through the familiar phases from meiosis I and mitosis. To distinguish the phases, they are called prophase II, metaphase II, anaphase II, and telophase II. One important difference between the events of meiosis I and II is that no further genetic reassortment takes place during prophase II. As a result, prophase II is much shorter than prophase I. In fact, all of the phases of meiosis II proceed rapidly.

During meiosis II, chromosomes align at the center of the cell in metaphase II exactly the way they do in mitotic metaphase. In anaphase II, the sister chromatids separate, once again in the same fashion as occurs in mitotic anaphase. The only difference is that since there was no second round of DNA replication; only one set of chromosomes exists. When the two cells split at the end of meioisis II, the result is four haploid cells.

Of the four haploid cells, one cell is composed completely of a maternal homologue, another of a maternal homologue with a small segment of paternal DNA from crossover in meiosis I, another complete paternal homologue, and a final paternal homologue with a small segment of maternal DNA from crossover in meiosis I. These four haploid cells are the gametes, the sperm or egg cells, that fuse together in sexual reproduction to create new individuals.