Eukaryotic cells, which are all the cells that do not belong to the prokaryotic organisms in the bacteria and archaea domains, make copies of themselves by replicating their genetic material and then splitting in two from the inside out.

This, however, is unlike the simple division of cell contents called binary fission seen in prokaryotes. It comes in one of two forms: mitosis and meiosis.

Mitosis is the simpler of these two related cell-division processes and is similar to binary fission in that it is a single division that results in the formation of two genetically identical daughter cells with the same diploid number of chromosomes as the parent cell (46 in humans).

Meiosis, however, encompasses two successive divisions, resulting in four daughter cells with a haploid chromosome number (23 in humans); these daughter cells are genetically distinct from the parent cell and from each other.

Both mitosis and meiosis start with a diploid parent cell that splits into daughter cells. The diploid number results from the fact that each cell includes one copy of each chromosome (numbered one through 22 in humans, plus one sex chromosome) from the organism's mother and one from the father. These copies of each chromosomes are known as homologous chromosomes and are found only in the domain of sexual reproduction.

Because the cell has replicated its chromosomes earlier in the cell cycle, the genetic material at the onset of mitosis or meiosis includes 92 individual chromatids, arranged in identical pairs of sister chromatids joined at a structure called a centromere to create a duplicated chromosome.

• Sister chromatids are not homologous chromosomes. 

In addition, both processes can be divided into four substages, or phases: prophase, metaphase, anaphase and telophase, with mitosis finishing after one round of this scheme and meiosis proceeding through a second.

The essential characteristics of the respective phases of both mitosis and meiosis in humans are:

• Prophase: Chromatin condenses into 46 chromosomes. 
• Metaphase: Chromosomes are aligned on the cell midline, or equator. 
• Anaphase: Sister chromatids are pulled to opposite poles of the cell. 
• Telophase: Nuclear envelope forms around each set of daughter nuclei.

After this separation of the nucleus and its contents, cytokinesis, the division of the entire parent cell, follows in short order.

Because meiosis includes two rounds of this, these are neatly termed meiosis I and meiosis II. Meiosis I thus includes prophase I, metaphase I and so on and accordingly for meiosis II. It is during prophase I and metaphase I of meiosis that the events that ensure genetic diversity in offspring occur. These are called crossing over (or recombination) and independent assortment respectively.

Mitosis is the process by which an organism's cells are continually replenished after they die as a result of physical trauma from the outside or natural aging from within. It therefore occurs in every eukaryotic cell, although turnover rates differ markedly between tissue types (e.g., muscle cell and skin cell turnover are typically very high, while heart cell turnover is not).

Meiosis, on the other hand, occurs only in specialized glands called the gonads (testicles in males, ovaries in females).

Also, as noted, mitosis has one round of phases that gives rise to two daughter cells, whereas meiosis has two phases and gives rise to four daughter cells. It helps to organize these schemes if you keep in mind that meiosis II is simply a mitotic division. Also, neither phase of meiosis involves the replication of any new genetic material. DNA replication is a result of the one-two punch recombination of and independent assortment.

The daughter cells that result from meiosis are called gametes. Males produce gametes called sperm (spermatocytes), whereas females produce gametes known as egg cells (oocytes). Human males have one X sex chromosome and one Y sex chromosome, so sperm cells contain either a single X or a single Y chromosome. Human females have two X chromosomes and thus all of their egg cells have a single X chromosome.

In the end, each daughter cell of meiosis is genetically "half-identical" to its parent no matter the result, yet is distinct from not only the parent cell but other daughter cells as well.

In prophase I, not only do chromosomes become more condensed, but homologous chromosomes line up side by side to form tetrads, or bivalents. A single bivalent thus contains the sister chromatids of a given labeled chromosome (1, 2, 3 and so on up to 22) along with those of its homologous chromosome.

Crossing over involves the swapping of lengths of DNA between adjoining non-sister chromatids in the middle of the bivalent. Although errors occur in this process, they are quite rare. The result is chromosomes that are very similar to the originals yet clearly distinct in their DNA composition.

In metaphase I of meiosis, the tetrads line up along the metaphase plate, preparing to be pulled apart in anaphase I. But whether the female contribution to the tetrad winds up on a given side of the metaphase plate or whether the male contribution winds up in its place instead is purely a matter of chance.

If humans had only one chromosome, then a gamete would wind up with either the derivative of the female homolog or the derivative of the male homolog (both of which are likely to have been modified by crossing over). So there would be two possible combinations of chromosomes in a given gamete.

If humans had two chromosomes, the number of possible gametes would be four. Since humans have 23 chromosomes, a given cell can give rise to 223 = almost 8.4 million distinct gametes as a result of independent assortment in meiosis 1 alone.

While meiosis is the engine driving genetic diversity in eukaryotic reproduction, mitosis is the force that allows everyday, moment-to-moment survival and growth. The human body contains trillions of somatic cells (that is, cells outside the gonads that cannot undergo meiosis) that must be able to respond to changing environmental conditions through various repair mechanisms.

Without mitosis to give the body new cells to work with, this would all be moot.

Mitosis unfolds at vastly different rates throughout the body. In the brain, for example, adult cells almost never divide. The epithelial cells on the surface of the skin, on the other hand, typically "turn over" every few days.

When the cells divides, it may then differentiate into more specialized cells as a result of specific intracellular signals, or it may continue to divide in a way that retains its original composition but the capacity for differentiation on command. In bone marrow, for example, stem cell mitosis yields daughter cells that can develop into red blood cells, white blood cells and other kinds of blood cells.

The "differentiable" but not-yet-specialized cells are known as stem cells, and they are vital in medical research as scientists continue to discover new techniques to prod cells to divide into specifically determined tissues rather than persist along their "natural" course.

Related topics:

• Why Is Mitosis a Form of Asexual Reproduction?