by Jennifer Allsbrook

Created on: July 06, 2008   Last Updated: July 07, 2008

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

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