Mitosis consists of four phases (Fig. 3.12):  Prophase begins as the replicated chromosomes condense and become visible. As the chromosomes continue to condense, each of the four chromosomes derived from each homologous pair can be seen to consist of two chromatids. The sister chromatids are held together by the ring of proteins called cohesins and the centromere. In late prophase or prometaphase (sometimes identified as a separate phase of mitosis), the nuclear envelope begins to disintegrate into small transport vesicles and resembles the sER. The nucleolus, which may still be present in some cells, also completely disappears in prometaphase. In addition, a highly specialized protein complex called a kinetochore appears on each chromatid opposite to the centromere (Fig. 3.13). The protein complexes that form kinetochores in the centromere region of chromatid are attached to specific repetitive DNA sequences known as satellite DNA that are similar in each chromosome. Microtubules of the developing mitotic spindle attach to the kinetochores and thus to the chromosomes.  Metaphase (Fig. 3.14) begins as the mitotic spindle, consisting of three types of microtubules, becomes organized around the microtubule-organization centers (MTOCs) located at opposite poles of the cell. The first type, the astral microtubules, are nucleated from the -tubulin rings in a starlike fashion around each MTOC (see Fig. 2.54). The second type, the polar micro-tubules, also originates from the MTOC; however, these microtubules grow away from the MTOC. The third type, the kinetochore microtubules, emanates from the MTOC to probe the cytoplasm in search of kinetochores. When a kinetochore is finally captured by a kinetochore microtubule, it is pulled toward the MTOC, where additional microtubules will attach. The kinetochore is capable of binding between 30 and 40 microtubules to each chromatid. In some species, kinetochore microtubules are formed by MTOCindependent mechanisms that involve kinetochores. Kinetochore microtubules and their associated motor proteins direct the movement of the chromosomes to a plane in the middle of the cell, the equatorial or metaphase plate.
 Anaphase (Fig. 3.15) begins at the initial separation of sister chromatids. This separation occurs when the cohesins that have been holding the chromatids together break down. The chromatids then begin to separate and are pulled to opposite poles of the cell by the molecular motors (dyneins) sliding along the kinetochore micro-tubules toward the MTOC.
 Telophase (Fig. 3.16) is marked by the reconstitution of a nuclear envelope around the chromosomes at each pole. The chromosomes uncoil and become indistinct except at regions that will remain condensed in the interphase nucleus. The nucleoli reappear, and the cytoplasm divides (cytokinesis) to form two daughter cells. Cytokinesis begins with the furrowing of the plasma membrane midway between the poles of the mitotic spindle. The separation at the cleavage furrow is achieved by a contractile ring consisting of a very thin array of actin filaments positioned around the perimeter of the cell. Within the ring, myosin II molecules are assembled into small filaments that interact with the actin filaments, causing the ring to contract. As the ring tightens, the cell is pinched into two daughter cells. Because the chromosomes in the daughter cells contain identical copies of the duplicated DNA, the daughter cells are genetically identical and contain the same kind and number of chromosomes. The daughter cells are (2d) in DNA content and (2n) in chromosome number.
Meiosis involves two sequential nuclear divisions followed by cell divisions that produce gametes containing half the number of chromosomes and half the DNA found in somatic cells.