Meiosis is a type of cell division in eukaryotic organisms that results in the production of gametes, or sex cells. In humans, the gametes are sperm (spermatozoa) in males and eggs (ova) in females.

The key characteristic of a cell that has undergone meiosis is that it contains a haploid number of chromosomes, which in humans is 23. Whereas the vast majority of the human body's trillions of cells divide by mitosis and contain 23 pairs of chromosomes, for 46 in all (this is called the diploid number), gametes contain 22 "regular" numbered chromosomes and a single sex chromosome, labeled as X or Y.

Meiosis can be contrasted with mitosis in a number of other ways. For example, at the onset of mitosis, all 46 chromosomes assemble individually along the line of eventual division of the nucleus. In the process of meiosis, the 23 pairs of homologous chromosomes in every nucleus line up along this plane.

The big-picture view of the role of meiosis is that sexual reproduction ensures the maintenance of genetic diversity in a given species. This is because the mechanisms of meiosis ensure that every gamete produced by a given person contains a unique combination of DNA from that person's mother and father.

Genetic diversity is important in any species because it serves as a safeguard against environmental conditions that could wipe out an entire population of organisms or even a whole species. If an organism happens to have inherited traits that render it less susceptible to an infectious agent or other threat, even one that may not exist at the time the organism comes into being, then that organism and its offspring stand a better chance of survival.

Meiosis and mitosis in humans begin the same way – with an ordinary collection of 46 newly replicated chromosomes in the nucleus. That is, all 46 chromosomes exist as a pair of identical sister chromatids (single chromosomes) joined at a point along their length called the centromere.

In mitosis, the centromeres of the replicated chromosomes form a line across the middle of the nucleus, the nucleus divides and each daughter nucleus contains a single copy of all 46 chromosomes. Unless errors occur, the DNA in each daughter cell is identical to that of the parent cell, and mitosis is complete after this single division.

In meiosis, which occurs only in the gonads, two successive divisions occur. These are named meiosis I and meiosis II. This results in the production of four daughter cells. Each of these contains a haploid number of chromosomes.

This makes sense: the process begins with a total of 92 chromosomes, 46 of which are in sister-chromatid pairs; two divisions is sufficient to reduce this number to 46 after meiosis I and 23 after meiosis II. Meiosis I is the objectively more interesting of these, since meiosis 2 is really just mitosis in everything but its name.

The distinguishing and vital features of meiosis I are crossing over (also called recombination) and independent assortment.

As with mitosis, the four distinct phases/stages of meiosis are prophase, metaphase, anaphase and telophase – "P-mat" being a natural way to remember these and their chronological sequence.

In prophase I of meiosis (each stage receives a number matching the meiosis sequence it belongs to), the chromosomes condense from the more diffuse physical arrangement they lie in during interphase, the collective name for the non-dividing portion of a cell's life cycle.

Then, the homologous chromosomes – that is, the copy of chromosome 1 from the mother and chromosome 1 of the father, and similarly for the other 21 numbered chromosomes as well as the two sex chromosomes – pair up.

This allows for crossing over between material on homologous chromosomes, a sort of molecular open-market exchange system.

Prophase I of meiosis includes five distinct substages.

• Leptotene: The 23 paired and duplicated homologous chromosomes, each one of which is called a bivalent, condense. In a bivalent, the chromosomes sit side by side, forming a rough XX shape, with each "X" consisting of the sister chromatids of one parental chromosome. (This comparison has nothing to do with the sex chromosome labeled "X"; it is intended only for purposes of visualization). 
• Zygotene: The synaptonemal complex, the structure that holds paired chromosomes together and promotes genetic recombination, begins to form. This process is called synapsis.
• Pachytene: At the beginning of this step, synapsis is complete. This step, notably, can last for days.
• Diplotene: In this stage, the chromosomes begin to de-condense, and much cell growth and transcription occurs.
• Diakinesis: This is where prophase 1 morphs into metaphase 1.

Crossing over, or genetic recombination, is essentially a grafting process in which a length of double-stranded DNA is excised from one chromosome and transplanted onto its homolog. The spots at which this occur are called chiasmata (singular chiasma) and can be visualized under a microscope.

This process ensures a greater degree of genetic diversity in offspring because the exchange of DNA between homologs results in chromosomes with a new complement of genetic material.

• On average, two or three crossover events occur on each pair of chromosomes during meiosis I.

In this phase, bivalents line up along the midline of the cell. The chromatids are bound together by proteins called cohesins.

Critically, this arrangement is random, meaning that a given side of the cell has an equal probability of including either the maternal half of the bivalent (i.e., the two maternal chromatids) or the paternal half.

• The number of possible different arrangements in the cell of the 23 chromosome pairs is 223 or about 8.4 million, which represents the number different possible gametes that can be generated during meiosis. Since each gamete must fuse with a gamete of the opposite sex to create a fertilized human egg, or zygote, this number must be squared again to determine the number of genetically distinct humans that can result from a single fertilization – almost 70 trillion, or about 10,000 times the number of people currently alive on Earth. 

In this phase, homologous chromosomes separate and migrate to opposite poles of the cell, moving at right angles to the line of cell division. This is accomplished by the pulling action of microtubules that originate from centrioles at the poles. In addition, the cohesins are degraded in this phase, which has the effect of dissolving the "glue" holding the bivalents together.

Anaphase of any cell division is rather dramatic when seen through a microscope, as it involves a great deal of literal, visible motion within the cell.

In telophase I, chromosomes complete their journeys to to the opposite poles of the cell. New nuclei form at each pole and a nuclear envelope forms around each set of chromosomes. It is helpful to think of each pole as containing non-sister chromatids that are alike but no longer identical owing to crossing-over events.

Cytokinesis, the division of an entire cell as opposed to the division of its nucleus alone, takes place and produces two daughter cells. Each of these daughter cells contains a diploid number of chromosomes. This sets the stage for meiosis II, when the chromatids will again be separated during a second cell division to produce the required 23 in each sperm and egg cell at the conclusion of meiosis.

Related meiosis topics:

• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
• Haploid cells
• Diploid cells