Basics The Meiosis and Mitosis Somatic cell: Any cell of a multicellular organism that composes the body of that organism but does not produce gametes. Gamete: A mature reproductive cell which is capable of fusing with a cell of similar origin but of opposite sex to form a zygote from which a new organism can develop. Gametes have a haploid chromosome content. In animals, a gamete is a sperm or egg; in plants, it is pollen, spermatic nucleous, or ovum. Chromatin: material found in the eukaryotic nucleus consists of DNA and proteins Chromosomes: A nucleosome and an H1 histone protein. Chromatids: each of the two daughter strands comprising a duplicated chromosome. The term remains in use while the two chromatids are still joined at the centromere. As soon as the centromere divides, setting the two chromatids adrift (during anaphase of mitosis; and during anaphase II of meiosis), they are called chromosomes. Autosomes vs. sex chromosomes: chromosome that is the same in males and females; nonsex chromosome vs. chromosomes that differ morphologically or in number in males and females. Haploid: possessing a single set of chromosome (one genome). Diploid: possessing two sets of chromosomes (two genome) Homologous: From the same source, or having the same evolutionary function or structure. Non-homologous: from the different source Centromere: constricted region on a chromosome that stains less strongly than the rest of the chromosome; region where spindle microtubules attach to a chromosome. C value: Haploid amount of DNA found in a cell of an organism. Prophase: Stage of mitosis. In prophase, the chromosome contract and become visible, the cytoskeleton breaks down, and the mitotic spindle begins to form. Metaphase: Stage of mitosis. In metaphase, align chromosomes in the center of the cell. Anaphase: Stage of mitosis in which chromatids separate and move toward the spindle poles. Telophase: Stage of mitosis. In telophase, the chromosomes arrive at the spindle poles, the nuclear membrane re-forms, and the chromosomes relax and lengthen. Cytokinesis: process by which the cytoplasm of a cell divides. Stages of prophase I: Stage of meiosis I. In prophase I, chromosomes condense and pair, crossing over takes place, the nuclear membrane breaks down, and the spindle forms. Reductional division: Phase of meiosis in which the maternal and paternal chromosomes of the bivalent separate. Equational division: Mitotic-type division that is usually the second division in the meiotic sequence; somatic mitosis and the non-reductional division of meiosis. A chromosome division in which the two chromatids of each duplicated chromosome separate longitudinally, prior to being incorporated into two daughter nuclei. Chiasma: point of attachment between homologous chromosomes at which crossing over took place. Synapsis: close pairing of homologous chromosomes. Mendel's law of independent assortment: Independent separation of chromosome pairs in anaphase I of meiosis contributes to genetic variation. Mendel's lab of segregation: The separation of the two members of a chromosome pair from each other at meiosis; the result is seen as the separation of alleles from each other in the gametes of heterozygotes; the occurrence of different phenotypes among offspring, resulting from chromosome or allele separation in their heterozygous parents. Mendel's first principle of inheritance (the Law of Segregation) predicts that heterozygotes will produce equal numbers of gametes containing each allele.