Meiosis211 - Meiosis and Sexual Life Cycles 1 The process...

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Meiosis and Sexual Life Cycles - 1 The process of mitosis, just discussed, ensures that each cell of an organism has the same DNA as the original cell from which it originated (absent mutations). Transmitting the DNA of our chromosomes from generation to generation (parents to offspring) is equally important. A critical role of heredity is to maintain and obtain genetic variation among members of a species through time. These variations are the result of the specific genes each generation inherits from its parents. Although asexual reproduction, which uses mitosis to make new individuals (genetically the same as the parent) is common in protists, plants, fungi and some animals, most organisms produce offspring by a process of sexual reproduction, in which a gamete from one parent joins a gamete from a second parent to form a zygote (or fertilized egg) at some time in their life history. This process results in offspring that have a combination of parental chromosomes (and the genes on those chromosomes). Throughout most of our human history, we didn't know how genetic information was transmitted from parent to offspring. Gregor Mendel proposed the mechanism for transmitting genetic information in the mid-1800's, but Mendel's work was not widely recognized until the early 1900's when Mendel's papers were "discovered" about the time other researchers were drawing the same conclusions based on similar research. Soon after, Walter Sutton showed that Mendel's principles of inheritance applied to chromosomes and that chromosomes are the units of heredity. We shall discuss Mendel's principles and inheritance patterns soon, but first we'll look at how chromosomes are transmitted from generation to generation by meiosis and sexual reproduction, events that provide for genetic variation within populations while maintaining a constant species chromosome number from generation to generation. Two important genetic characteristics provide for this: Sexually reproducing organisms have, at least at some stage in their life cycle, two sets of genetic information in their cells, so that chromosomes can be arranged in homologous (or matching) pairs. Each chromosome of the homologous chromosome pair has the same genes, although the specifics of the genes can differ. (See later.) Organisms that have two sets of genetic information are called diploid. Meiosis is a process of cell division that reduces the chromosome number by half in the meiotic product cells by first pairing up and then separating the homologous chromosome pairs. The meiotic product cells will have half the total number of chromosomes as the original cell that did meiosis, but each product cell, called haploid, will have one set of genetic information: one of each homologous chromosome pair but not both of them.
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Meiosis and Sexual Life Cycles - 2 Meiosis occurs at just one stage in an organism's life cycle: to form gametes in animals, to initiate the gamete producing stage in plants, or, for some organisms,
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Meiosis211 - Meiosis and Sexual Life Cycles 1 The process...

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