Meiosis is the process by which sexually reproducing organisms create gametes that house half of their genetic material, to be combined with the genetic material of a mate. It allows for stable reproduction producing two half sets of the genome so that, through reproduction, a complete functional genome is built in the offspring.
Meiosis occurs in all eukaryotes that utilize sexual reproduction. In animals, gametes are produced, such as eggs and sperm. For plants and fungal species, meiosis results in spores. Prokaryotes and some parthonogenic eukaryotes do not use meiosis and instead are capable of reproducing without mates, whether through self-fertilization or binary fission.
The first two steps in the process are identical to those of mitosis, and result in replicated chromosomes. After the S phase, meiosis deviates to produce haploid cells, or cells with only one of each pair of the parent organism's chromosomes.
Growth Phase (G1) - The cell goes through a rapid growth in anticipation of future division. Proteins are rapidly produced to fill the cell to capacity. In this stage, the cell is 2N, as it has two of each chromosome.
Synthesis Phase (S) - The genetic material is replicated. The cells are still 2N, as the number of chromosomes is identical to G1, but each chromosome is doubled in size and has a pair of sister chromatids.
Prophase I - Here DNA is exchanged between homologous chromosomes through recombination. Similar stretches of DNA cross and break, leaving each chromosome slightly reorganized. This is often described as a step that introduces genetic diversity into offspring.
Metaphase I - Pairs of homologous chromosomes line up along the metaphase plate, with kinetochore microtubules attaching to the kinetochores of each chromosome.
Anaphase I - The microtubules shorten, pulling homologous chromosomes apart and to opposite ends of the cell.
Telophase I - With the chromosomes on opposite ends of the cells, the cell splits down the middle. This creates two cells, each with only one of each chromosome (haploid, N).
Prophase II - Chromatids thicken in preparation for another division.
Metaphase II - Spindle fibers attach to the kinetochores of each chromosome again.
Anaphase II - Centromeres holding the sister chromatids together are broken and the fibers pull each chromatid to opposite ends of the cell.
Telophase II - The cell splits again, leaving 4 total new cells, each with one sister chromatid of each chromosome (haploid, N). These are the gametes or spores for reproduction.
In female mammals, the process of meiosis results in one functional gamete, called an egg, and a set of non-functional polar bodies. This process is known as oogenesis and occurs as soon as germ cells move to the ovary in the developing embryo. The eggs are then stored until they begin to be released during the menstrual cycle later in life.
For males, each meiotic process creates 4 functional sperm. This process is known as spermatogenesis and occurs throughout the organisms lifespan.