Meiosis, also known as reduction division, is the process of cell division by which diploid (2n) organisms create the haploid (n) gametes needed for sexual reproduction. Diploid organisms have two full sets of chromosomes, which exist in pairs known as homologous pairs. Each homologous pair contains a paternal and a maternal copy of the chromosome. These homologous pairs were created upon the fusion of gametes, or sex cells, during sexual reproduction. Meiosis creates the gametes, sperm and egg, needed for this process. In addition to the creation of gametes, meiosis also creates genetic variation in a species while maintaining the chromosome number of said species from one generation to the next.
During meiosis several mechanisms exist that shuffle the genetic cards. As meiosis begins the homologous pairs find one another and embrace in a process called synapsis, forming structures called tetrads. During synapsis, the sister chromatids which compose the maternal and paternal chromosomes form chiasma. A chiasmata is a location where the non-sister chromatids of a maternal and paternal pair cross over one another, weaken, break and exchange pieces. The result is deemed crossing over and can occur up to ten times for a given homologous pair. Another mechanism resulting in variation is the random alignment of the maternal and paternal chromosomes along the midplane of the cell. The subsequent separation of the pair will lead to a random assortment of these chromosomes into daughter cells. The number of possible chromosomal combinations is 2^n where n equals the haploid chromosome number. For example, in humans there are 23 chromosomes in our sex cells. Therefore, there are over 8 million possible combinations of chromosomes due to the random assortment described above. Variation is further introduced by random fertilization of a specific egg with a specific sperm.
The process of meiosis itself involves two rounds of cell division called Meiosis I and Meiosis II. Meiosis I involves four stages that differ significantly from mitosis, the form of cell division involved in growth and tissue repair. During Prophase I, the first stage, synapsis, chiasmata formation, and crossing over occurs. During Metaphase I, the meiotic spindle moves the tetrads or homologous pairs to the midplane. Anaphase I separates the homologous pairs to opposite poles of the spindle. During Telophase I, nuclei reform around each chromosomal grouping and two daughter cells are formed. These cells, at this point, are haploid in terms of chromosome number, but have twice the amount of DNA since each chromosome is still composed of sister chromatids. To rectify this Meiosis II follows which progresses like mitosis. The end result is four genetically varied, haploid sex cells.
It is important to note that the process of meiosis described above is known as gametic meiosis and primarily occurs in animals. Two other forms of meiosis occur in other organism groups. Sporic meiosis occurs in plants and some forms of algae. The result of sporic meiosis is the formation of haploid cells called spores. This form of meiosis is involved in the most complex sexual life cycle known as alternation of generations. A third type of meiosis, called zygotic meiosis, occurs in fungi and some forms of algae. It involves the division of the diploid zygote into haploid cells.
Whether meiosis is occurring in its familiar role in animals or the unusual forms seen in plants and fungi, it is still the mechanism used to create the haploid cells necessary for successful sexual reproduction of those species. By introducing genetic variation, meiosis ensures the survival of species while maintaining each species’ unique chromosome number.
Reference: Campbell, Neil and Reece, Jane. Biology, 7th Edition. Benjamin Cummings, San Francisco, 2005.