In 1876, a German zoologist Oscar Hertwig (1842–1922) first described the process of meiosis during his study of sea urchin eggs. His theory, which stemmed from the observation of two nuclei in one egg, stated that these nuclei most likely developed due to the integration of substances found within a spermatozoan and an egg. Another German biologist named Walther Flemming also investigated cellular division and the distribution of chromosomes into daughter nuclei in 1882. These observations, along with work by American geneticist Walter Sutton in 1902, helped us to understand the pattern and movement of chromosomes during meiosis and how it differed from mitosis in somatic cells. It wasn’t until 1905, however, when the researchers J. B. Farmer and J. S. Moore published their work, that the term meiosis was first coined.

In many ways, meiosis is similar to mitosis, but there are some important differences. These differences stem from the function of each type of cell division. The function of mitosis is to produce two genetically identical cells. The function of meiosis is to produce genetically unique haploid gametes.

How do the chromosome numbers change during meiosis? The term "n" (or "1n") refers to the number of chromosomes in a haploid cell of a given species, and the term "2n" is equal to the number of chromosomes in a diploid cell of a given species. For example, in humans, 1n=23, and 2n= 46. The names of the stages in meiosis are similar to those in mitosis, but meiosis consists of two rounds of division called meiosis I and meiosis II. The goal of meiosis I is to separate homologous chromosomes and reduce the chromosome number by half. The goal of meiosis II is to separate sister chromatids. Figure 1 shows the separation of homologous chromosomes, sister chromatids, and alleles for a single chromosome pair during meiosis.

Meiosis I begins with prophase I, (Figure 2) during which DNA synthesis takes place. Each of the chromosomes has been copied, and the sister chromatids that were produced are held together at the centromere. Sister chromatids are considered part of the same chromosome when they are connected by a centromere, so the total chromosome number is determined by counting the number of centromeres. The sister chromatids are joined via protein complexes called cohesins. The cohesin complexes exist all along the lengths of the sister chromatids. Recent evidence suggests that cohesin proteins bind sister chromatids by encircling them in a ring-like fashion. Two unique and important things happen in prophase I, which may take days or even longer to complete. The nuclear envelope begins to breakdown and nuclear chromatin starts to condense into individual chromosomes each made up of two sister chromatids. Homologous pairs of chromosomes in which each chromosome is made up of two sister chromatids then bind to each other in a process called synapsis. Synapsis occurs via a protein structure called the synaptonemal complex. These pairs of homologous chromosomes are made up of four sister chromatids, called a tetrad or bivalent. During synapsis, large sections of chromosomes are transferred between non-sister chromatids. This is termed crossing over.

Crossing over involves the exchange of genes between sister chromatids of homologous chromosomes. What this means is that the paternal and maternal chromosomes that are organized randomly during independent assortment are already genetically different from the parental chromosomes. Once synapsis is complete, the synaptonemal complex disassembles, and the chromosomes begin to partially separate. The paired homologous chromosomes exchange segments of non-sister chromatids at points of contact called chiasmata. The chiasmata themselves become visible during the latter portion of prophase I. Crossing over is important for the proper segregation of the homologous chromosomes during meiosis; the chiasmata keep homologous chromosomes together during prophase. If they aren't tethered in this way, there is a greater chance that both homologs will segregate into the same daughter cell, causing chromosome missegregation and aneuploidy (the wrong number of chromosomes) after division.

Another important step during prophase I is the duplication of centrosomes in animals, or microtubule organizing centers (MTOCs) in plants. These structures are vital in ensuring that the chromosomes are distributed appropriately into the resulting daughter cells. Each centrosome assembles a complex of microtubules that will function as tracks for the homologs to move along. The microtubules (also known as spindle fibers) connect to each homologous chromosome at the kinetochore. The kinetochore acts as a motor unit and provides the force that propels each homologous chromosome across the cell during meiosis.

Each tetrad now moves along its microtubule attachments so that it is lined up along the metaphase plate. This is an equatorial division within the cell that splits it into two halves. One homologous chromosome is now facing one pole of the cell, while the other homologous chromosome faces the opposite pole (Figure 3). The alignment of each tetrad along the metaphase plate is entirely random in terms of which homologous chromosome is attached to which pole, contributing to the genetic diversity seen in the resulting daughter cells.

The microtubules connected to the kinetochore of each homologous chromosome begin to shorten and pull the homologous chromosomes towards opposite poles (Figure 4). The sister chromatids of each chromosome remain tightly connected. The movement of each homologous chromosome is controlled by the kinetochore microtubules. The microtubules that aren't connected to the kinetochores become longer, which causes the centrosomes to migrate further apart.

Once the homologous chromosomes have finished migrating to opposite poles of the cell, telophase I begins (Figure 5). The result is a complete haploid set of chromosomes at each pole. Each chromosome in the haploid set is made up of two sister chromatids. A nuclear envelope forms around the chromosomes as they decondense. Now that the haploid sets of chromosomes are located on opposite poles of the cells, cytokinesis may begin. Cytokinesis is the division of the cell cytoplasm into two new daughter cells. Once this process has finished, the cells may enter into interkinesis or a resting period prior to undergoing meiosis II. No further DNA replication occurs during this time. Telophase I and interkinesis are typically not seen in most plant species, because they move directly from Anaphase I into Prophase II.

Each daughter cell that formed during meiosis I now undergoes meiosis II. The goal of meiosis II is to separate the sister chromatids. This is the same goal as in mitosis, so meiosis II and mitosis are very similar. Meiosis II and mitosis do not reduce the chromosome number. In prophase II, a spindle apparatus forms (Figure 6). If the nucleolus and nuclear envelope reformed during interkinesis, they now disappear. The sister chromatids do not exchange any further DNA.

In metaphase II, the chromosomes align along the metaphase plate (Figure 7). As in mitosis, the kinetochore of one sister chromatid is attached to spindle fibers from one pole, and the kinetochore of the other sister chromatid is attached to spindle fibers from the opposite pole (Figure 8).

In anaphase II, (Figure 9) the centromeres of sister chromatids separate. This enables each sister chromatid to move to opposite poles of the cell. The kinetochores that are bound to the microtubules facilitate this movement. At this point, each sister chromatid becomes a chromosome.

Finally, in telophase II (Figure 10), each chromosome uncoils, and the nuclear envelope begins to form. The spindle microtubules also begin to disassociate and are no longer visible. Cytokinesis then occurs producing two daughter cells that are genetically unique from one another. At the end of meiosis II, four daughter cells are formed, with each containing a haploid set of chromosomes.

How does meiosis work in humans? Human males are born with spermatogonial stem cells, and spermatogenesis begins to produce spermatocytes by meiotic division at the age of puberty. This is in contrast to females in which meiosis begins much earlier, during the fetal stages of development. In the female fetus, primordial germ cells that have finished their migration into the ovary immediately begin to undergo meiosis and form primary oocytes. Meiotic prophase is arrested before birth, and then meiosis doesn’t resume until the female reaches puberty.

In females, the products of meiosis are not all equivalent. When meiosis begins again after puberty, each month one of the primary oocytes is selected for ovulation. The first meiotic division produces two “cells,” a secondary oocyte and a very small structure called a polar body. The polar body has very little cytoplasm but contains one copy of the chromosomes. The polar body is eventually reabsorbed into the female’s ovary. The secondary oocyte completes meiosis II after fertilization, producing a second polar body and a mature egg (Figure 11).