Gametes are produced by the process of mitosis or meiosis

Meiosis to produce gametes is a process that requires two cell divisions. Before the first division, the cell's Chromosomes are replicated as they do in mitosis to create sister chromatids for each chromosome attached at a centromere. Homologous chromosome pairs condense and align at the equator of the cell and spindle fibers form attached to the chromosomes centromeres. One of each of the homologous pairs will be moved to either pole of the cell by the spindle fibers to be passed to the daughter cell which ends meiosis 1. In meiosis 2 the sister chromatids align again at the equator of their respective daughter cells from meiosis 1 , and split apart to move to the cell's opposite poles so that in each daughter cell, there is a haploid gene number and therefore a gamete.

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Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females.

During meiosis one cell divides twice to form four daughter cells.
These four daughter cells only have half the number of chromosomes of the parent cell – they are haploid.
Meiosis produces our sex cells or gametes (eggs in females and sperm in males).

Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II):

Meiosis I

1. Interphase:

The DNA in the cell is copied resulting in two identical full sets of chromosomes.
Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division.
During interphase, microtubules extend from these centrosomes.

2. Prophase I:

The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.
Each chromosome is composed of two sister chromatids containing identical genetic information.
The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.
The pairs of chromosomes may then exchange bits of DNA in a process called recombination or crossing over.
At the end of Prophase I the membrane around the nucleus in the cell dissolves away, releasing the chromosomes.
The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.

3. Metaphase I:

The chromosome pairs line up next to each other along the centre (equator) of the cell.
The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them.
The meiotic spindle fibres attach to one chromosome of each pair.

4. Anaphase I:

The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.
In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II.

5. Telophase I and cytokinesis:

The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new nuclei.
The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.

Meiosis II

6. Prophase II:

Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids).
In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.
The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes.
The centrioles duplicate.
The meiotic spindle forms again.

7. Metaphase II:

In each of the two daughter cells the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell.
The centrioles are now at opposites poles in each of the daughter cells.
Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.

8. Anaphase II:

The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle.
The separated chromatids are now individual chromosomes.

9. Telophase II and cytokinesis:

The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new cell nuclei.
This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis.
Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid):
- in males, these four cells are all sperm cells
- in females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs).

Illustration showing the nine stages of meiosis.
Image credit: Genome Research Limited

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Figure 2: Examples of polytene chromosomes

Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.

Although he did not know it, Walther Flemming actually observed spermatozoa undergoing meiosis in 1882, but he mistook this process for mitosis. Nonetheless, Flemming did notice that, unlike during regular cell division, chromosomes occurred in pairs during spermatozoan development. This observation, followed in 1902 by Sutton's meticulous measurement of chromosomes in grasshopper sperm cell development, provided definitive clues that cell division in gametes was not just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan cell division, a process referred to as reductive division. As a result of this process, each gamete that Sutton observed had one-half the genetic information of the original cell. A few years later, researchers J. B. Farmer and J. E. S. Moore reported that this process—otherwise known as meiosis—is the fundamental means by which animals and plants produce gametes (Farmer & Moore, 1905).

The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else. Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells.

To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example (Hirsch, 1963). Each pair consists of two homologues: one maternal and one paternal. Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:

Pair 1: A and a
Pair 2: B and b
Pair 3: C and c

When these chromosome pairs are reshuffled through independent assortment, they can produce eight possible combinations in the resulting gametes:

A B C
A B c
A b c
A b C
a B C
a B c
a b C
a b c

A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete. Thus, in the previous example of three chromosome pairs, the calculation is 23, which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is (2n)2, which results in some fairly large numbers.

But what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce 223 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is (223)2 possible combinations. However, some of these combinations produce the same genotype (for example, several gametes can produce a heterozygous individual). As a result, the chances that two siblings will have the same combination of chromosomes (assuming no recombination) is about (3/8)23, or one in 6.27 billion. Of course, there are more than 23 segregating units (Hirsch, 2004).

While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2n calculation. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation.

While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction, genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.