Through meiosis, sexually reproducing organisms create gametes that house half of their genetic material. These gametes are then combined with the gametes of their mate to form the first cell of the offspring organism. 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 process of meiosis looks very similar to the cycle of mitosis by which somatic, or non-reproductive, cells divide. 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. Unlike mitosis, which is cyclical and results in the production of new somatic cells, meiosis ends at the final stage, as the gametes cannot further divide to produce any viable cells.
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.