Meiosis160 - Meiosis and Life Cycles - 1 We have just...

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Meiosis and Life Cycles - 1 We have just finished looking at the process of mitosis, a process that produces cells genetically identical to the original cell. Mitosis ensures that each cell of an organism has the same DNA as the original fertilized egg or zygote. Passing chromosomes and genetic information from generation to generation is equally important. A critical role of heredity is to maintain and obtain variation among members of a species. These variations are the result of the specific genes we inherit from our parents through sexual reproduction. However, chromosome number doesn't double with each generation. Each generation of a species has the same chromosome number as the preceding generations. Meiosis is the process that ensures that each new generation has the same chromosome number as the preceding generation. Meiosis is a process that reduces chromosome number by half and occurs at just one stage in an organism's life cycle (to form gametes in animals, or to start the gamete producing stage in plants, or for some organisms to restore the appropriate chromosome number for the assimilative stage of its life history). Sexual reproduction or more properly, fertilization, then restores the "typical" number of chromosomes for the next generation. Meiosis reduces chromosome number by half forming haploid nuclei, which have one of each of the homologous chromosomes (discussed earlier) . Each haploid cell after meiosis has half as many chromosomes as the diploid cells of the organism. "Ploid" as a general term means a "set", so we can also say that a diploid cell has two sets of chromosomes, or two of each kind of chromosome. A haploid cell has no pairs of chromosomes, just one of each kind. (It's possible to have more than 2 chromosomes of each kind. Polyploids are quite common in agriculture as a result of plant breeding. Polyploids are less common in animals.) In sexual reproduction, cells from two individuals fuse to form a nucleus with a combination of chromosomes and DNA from each parental cell. The new cell (called a zygote) has a unique set of chromosomes, different from each parent, having obtained one of each homologous chromosome from each parental cell. Without sexual reproduction, the opportunity to express the variations found within the genes of species would be greatly reduced. However, despite the fusion of cells from two individuals, chromosome number doesn't double with each generation. Each generation of a species has the same chromosome number as the preceding generations. Meiosis is the process that ensures that each new generation of sexually reproducing organisms has the same chromosome number as the preceding generation.
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Meiosis and Life Cycles - 2 Meiosis and Genetic Variation Meiosis is necessary to ensure that each new generation has the same chromosome number as the preceding generation, but meiosis has a second function for living organisms: maintaining genetic variation. Each time meiosis occurs, followed by,
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Meiosis160 - Meiosis and Life Cycles - 1 We have just...

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