biology
...s occurs most notably among the fungi and slime moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development. In a diploid eukaryotic cell, there are two versions of each chromosome, one from the mother and another from the father. The two corresponding chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical; they have the same genes, but may have different alleles. For example, a gene for eye color at one locus (location) on the father chromosome may code for green eyes, while the same locus on the mother chromosome may code for brown. When DNA is replicated, each chromosome will make an identical copy of itself. The copies are called sister chromatids, and together they are considered one chromosome. After separation, however, each sister chromatid is considered a full-fledged chromosome by itself. The two copies of the original chromosome are then called sister chromosomes. Mitosis allocates one copy, and only one copy, of each sister chromosome to a daughter cell. Consider the diagram above, which traces the distribution of chromosomes during mitosis. The blue and red chromosomes are homologous chromosomes. After DNA replication during S phase, each homologous chromosome contains two sister chromatids. After mitosis, the sister chromatids become sister chromosomes and part ways, going to separate daughter cells. Homologous chromosomes are therefore kept together, resulting in the complete transfer of the parent's genome. "Sister chromosomes", "sister chromatids", "mother cells", "daughter cells", and "parent cells" have no actual gender. The use of feminine or masculine terminology is by scientific convention only. Errors in mitosis Although errors in mitosis are rare, the process may go wrong, especially during early cellular divisions in the zygote. Mitotic errors can be especially dangerous to the organism because future offspring from this parent cell will carry the same disorder. In non-disjunction, a chromosome may fail to separate during anaphase. One daughter cell will receive both sister chromosomes and the other will receive none. This results in the former cell having three chromosomes coding for the same thing (two sisters and a homologous), a condition known as trisomy, and the latter cell having only one chromosome (the homologous chromosome), a condition known as monosomy. These cells are considered aneuploidic cells. Aneuploidy can cause cancer. Mitosis is a traumatic process. The cell goes through dramatic changes in ultrastructure, its organelles disintegrate and reform in a matter of hours, and chromosomes are jostled constantly by probing microtubules. Occasionally, chromosomes may become damaged. An arm of the chromosome may be broken and the fragment lost, causing deletion. The fragment may incorrectly reattach to another, non-homologous chromosome, causing translocation. It may reattach to the original chromosome, but in reverse orientation, causing inversion. Or, it may be treated erroneously as a separate chromosome, causing chromosomal duplication. The effect of these genetic abnormalities depend on the specific nature of the error. It may range from no noticeable effect at all to organism death. In biology, meiosis is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploid cell's genome. Meiosis forms the basis of sexual reproduction and can only occur in eukaryotes. In meiosis, the diploid cell's genome, which is composed of ordered structures of coiled DNA called chromosomes, is replicated once and separated twice, producing four sets of haploid cells each containing half of the original cell's chromosomes. These resultant haploid cells will fertilize with other haploid cells of the opposite gender to form a diploid cell again. The cyclical process of separation by meiosis and genetic recombination through fertilization is called the life cycle. The result is that the offspring produced during germination after meiosis will have a slightly different blueprint which has instructions for the cells to work, contained in the DNA. This allows sexual reproduction to occur. Biochemically, meiosis uses many similar processes ...