Cell Division: the fission (splitting) of a cell. (MeSH) Basic cell process to create tissues and organs. (Brooker, 203)

Involves the production of separate “daughter cells” from a “progenitor cell” by division of the “nucleus” (“mitosis”) and division of the “cytoplasm” (“cytokinesis”). (NCIt) During cell division, "DNA" winds tightly around “histones.” (Norman, 6/17/09) Each time a cell divides, it must replicate all its DNA. (Watson, 204) Abnormalities in "chromosome" number invariably cause profound dysfunction. They arise spontaneously through accidents in the cell divisions leading to the generation of "sperm" and "egg cells." (Watson, 326)

Crossing-Over: the breaking during meiosis of one maternal and one paternal chromosome, the exchange of corresponding sections of DNA, and the rejoining of the chromosomes. (HGPIA) All four "chromatids" that comprise each... chromosome pair are pressed together and parts exchanged. After crossing over, each “homolog” bears some "genes" from each parent. (Lewis, 47) This process can result in an exchange of "alleles" between chromosomes. Allows for increased variation in the genetic information each parent may pass to the offspring. (Brooker, G-9) The swapping of genetic material. During the formation of egg and sperm cells, also known as "meiosis," paired chromosomes from each parent align so that similar "DNA sequences" from the paired chromosomes cross over one another. Crossing over results in a shuffling of genetic material and is an important cause of the "genetic variation" seen among offspring. (NHGRI) Involves a physical exchange between chromosome pieces. (Brooker, 314)

Cytokinesis: the phase of the cell cycle during which the cytoplasm actually divides. It follows “nuclear” division and involves the splitting of the cell into two cells. (Brooker Online, C15 Animation) The division of one mother cell into two daughter cells. (Brooker, 306) A "cleavage furrow" separates the 2 cells. "Microfilaments" cinch the replicated cell in half. Results in the complete formation of two genetically identical daughter cells. Each daughter cell has same number of chromosomes as the parent cell. (Norman, 7/6/09)

Cleavage Furrow: constricting band of “actin filaments.” (Brooker Online, C15 Animation) Constricts like a drawstring to separate the cells. (Brooker, 312) The furrow deepens until it eventually slices all the way into the center of the cell. At this point, the cell divides in two. (Brooker Online, C15 Animation)

Daughter Cells: mitosis and cytokinesis ultimately produce two daughter cells having the same number of chromosomes as the mother cell. The two daughter cells are genetically identical to each other and to the mother cell from which they were derived. (Brooker, 312) When a cell divides, each daughter cell gets one copy of each chromosome. When the cell finally finishes splitting in two, each new daughter cell has the original number of chromosomes. In this way, each cell within a living thing has identical genetic information, except for cells that are eggs or sperm. (Batiza, 9, 68)

Kinetochore: a group of proteins necessary for sorting each chromosome that binds to the "centromere". (Brooker, G-20) Large multi-protein complexes that "bind" the centromeres of the chromosomes to the “microtubules” of the mitotic “spindle” during “metaphase” in the “cell cycle.” (MeSH)

Meiotic Cell Division: a type of cell division that produces four daughter cells, each having half the number of chromosomes of the original cell. (OxfordMed) Two successive (“nucleus”) divisions following a single chromosome duplication. (MeSH) Goal is to generate genetically unique “gametes.” Results in 4 genetically unique daughter cells, each with half the chromosome number of the parent cell. For example, produces 4 human sperm cells, each with 23 chromosomes. Includes one replication of the "genome", two complete cellular divisions, and nine phases as following: one "interphase," two "prophases," two "metaphases," two "anaphases," and two "telophases." (Norman, 7/8/09)

Meiosis I: two cells are produced, each with 23 pairs of "sister chromatids" or 46 total. It is a 'reduction' division. The original “diploid cell” had its chromosomes in “homologous” pairs, while the two cells produced at the end of Meiosis I are considered “haploid” - they do not have pairs of homologous chromosomes. (Brooker, 316) Editor's note - includes interphase, prophase 1, metaphase 1,  anaphase 1, telophase 1, and cytokinesis.

Prophase I: the first phase of cell nucleus division, in which the chromosomes become visible, the cell nucleus starts to lose its identity, the mitotic spindle apparatus appears, and the “centrioles” migrate toward opposite poles. (MeSH) The "nuclear envelope" disintegrates. "Nucleoli" disappear, "chromatin" coils into chromosomes. Each replicated chromosome appears as a sister chromatid. Chromosomes condense. Each pair of sister chromatids has a centromere with a specialized region called a kinetochore. "Centrosomes" migrate toward poles. (Brooker, 314)

Bivalent: pair of sister chromatids associated with each each other, lying side by side. (Brooker, G-5)

Chiasma: the connection at a crossover site. (Brooker, G-7) An anatomy term for an x-shaped crossing (for example, of nerves or tendons). (NCIt) After crossing over occurs, the arms of the chromosomes tend to separate but remain adhered at the crossover site. This connection is called a 'chiasma' because it physically resembles the greek letter chi. (Brooker, 314) Plural - 'chiasmata.'

Meiosis II: four haploid cells are produced, each with 23 chromosomes, so 23 chromosomes total. (Brooker, 316) Editor's note - includes prophase II, metaphase II, anaphase II, telophase II, and cytokinesis.

Mitotic Cell Division: the division of the nucleus into two nuclei. (Brooker, 306) Divides one cell nucleus into two nuclei, distributing the duplicated chromosomes so that each daughter cell will receive the same complement of chromosomes. (Brooker, 185) The sorting process that ensures that each daughter cell will obtain the correct number and types of chromosomes. (Brooker, 308) The "microtubule" apparatus assembles and binds to the chromosomes. The sister chromatids are drawn apart. (Brooker Online, C15 Animation) Also referred to as 'mitosis.' Editor's note - cell division phases follow listed in order of appearance.

Interphase: the interval between two successive cell divisions during which the chromosomes are not individually distinguishable. (MeSH) (Includes) the "G1," "G2," and "Synthesis" phases. The portion of the "cell cycle" between divisions. (Brooker Online, C15 Animation) Interphase is considered the resting stage of the cell. (NCIt)

G1 Phase: phase of the cell cycle preceding DNA synthesis. (NCIt) The cell carries out “metabolic” activities and increases in size. (Brooker Online, C15 Animation) Typically the longest phase of the cell cycle. (Brooker, 309) Serves as a "cell regulation" checkpoint. Synthesis can now occur. (Norman 7/6/09) Also referred to as ‘G1' and ‘Gap 1 Phase.’

Synthesis Phase: “DNA replication” occurs. (Brooker Online, C15 Animation) Includes replication of the cytoplasm and replication of “organelles.” DNA Replication forms two identical copies of the original "double helix" called sister chromatids. (Norman 7/6/09) Also referred to as ’S phase.’

G2 Phase: a second growth phase in which preparations are made for chromosomal separation. (Brooker Online, C15 Animation) Replication of the cytoplasm. Replication of organelles including the centrosome. Cell regulation checkpoint: if 'MPF protein' reaches the 'threshold level,' mitosis can proceed. (Norman, 7/6/09) Also referred to as ‘G2' and ‘Gap 2 phase.’

Prophase: the first phase of mitosis, in which the chromosomes become visible, the nucleus starts to lose its identity, the spindle appears, and the centrioles migrate toward opposite poles. (NCIt) The nuclear envelope starts to break down. Chromosomes are visible by "light microscopy." Each consists of a pair of sister chromatids. Sister chromatids condense. Sister chromatids are held together by a centromere on each chromatid. Kinetochore proteins bind to each centromere. Kinetochore proteins bind to each other. Each of the two centrosomes defines a pole of the spindle. The spindle is formed by a centrosome, from protein fibers called microtubules. Three types of microtubules are formed: 'astral,'  'polar,' and kinetochore. Astral microtubules (or 'asters') are important for positioning the spindle apparatus within the cell. Polar microtubules overlap each other in the cell middle and play a role in the separation of the two poles. Kinetochore microtubules bind to kinetochores, which are bound to the centromere of each chromosome. (Norman, 7/6/09)

Metaphase: the second phase of mitosis, in which the chromosomes line up across the equatorial plane of the spindle prior to separation. (NCIt) The nuclear membrane has completely dissociated into "vesicles." Chromosomes become arranged across the 'equator' of the cell in a single row. Also referred to as the 'metaphase plate,' it is aligned at a point halfway between the poles. Kinetochore microtubules growing toward the center of cell make contact with and become firmly attached to the kinetochore. The two kinetochores on each pair of sister chromatids are now bound to kinetochore microtubules from opposite poles. The spindle is now fully formed. (Norman, 7/6/09)

Anaphase: the third phase of mitosis during which the two groups of daughter chromosomes separate and move along the fibers of the central spindle. (NCIt) Sister chromatids are pulled apart by kinetochore microtubules. One chromatid from each pair goes to each of the poles. Each chromatid has now become a chromosome. Kinetochore microtubules shorten, pulling the chromosomes toward the pole to which they are attached. Polar microtubules lengthen and push the poles apart. (Norman, 7/6/09)

Telophase: the final phase of cell division, in which two daughter nuclei are formed, the cytoplasm divides, and the chromosomes lose their distinctness and are transformed into “chromatin” networks. (NCIt) A new nuclear envelope forms around each group of chromosomes, one at each pole. The chromosomes gradually lose their form and can no longer be recognized as distinct structures. Uncoiling forms diffuse chromatin. Non-kinetochore microtubules elongate cell in preparation for division. (Norman, 7/6/09)

Segregate: to separate, as in chromosomes during mitosis. (Brooker, G-33) Separation of the two chromatids of a duplicated mitotic chromosome and the allocation of each to a different daughter nucleus. (Lawrence)

Senescence: loss of the power of cell division and growth. (Oxford) Cells that have doubled many times and have reached a point where they have lost the capacity to divide any further. (Brooker, 227) Adjective - ‘senescent.’