meiosis - http:/en.wikipedia.org/wiki/Meiosis The word...

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http://en.wikipedia.org/wiki/Meiosis The word " meiosis " comes from the Greek meioun, meaning "to make smaller," since it results in a reduction in chromosome number in the gamete cell. Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes. In the gametic life cycle, of which humans are a part, the living organism is diploid in nature. The organism’s diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells. In the zygotic life cycle, the living organism is haploid. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa are members of the zygotic life cycle. Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism’s germ- line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism’s germ cells then combine with another haploid organism’s cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles, and indeed its diagram supports this conclusion. Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. Interphase is divided into three phases:
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This note was uploaded on 04/14/2008 for the course BIOL 115 taught by Professor Diggs during the Fall '07 term at Austin College.

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meiosis - http:/en.wikipedia.org/wiki/Meiosis The word...

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