The Cell Cycle and Cell Division
The life cycle of a cell is called the cell cycle. It is usually divided into five phases:
5. M (Fig. 4.11)
G0 is the stage when the cell may leave the cell cycle and either remain in a state of inactivity or reenter the cell cycle at another time. G1 is the stage during which the cell begins to prepare for mitosis through DNA and protein synthesis and an increase in organelle and cytoskeletal elements. The S phase is the synthesis phase, during which DNA replication occurs and the centrioles begin to replicate. G2 is the premitotic phase and is similar to G1 in terms of RNA activity and protein synthesis. The M phase is the phase during which cell mitosis occurs. Tissues may be composed primarily of quiescent cells in G0, but most tissues contain a combination of cells that are continuously moving through the cell cycle and quiescent cells that occasionally enter the cell cycle. Nondividing cells, such as neurons and skeletal and cardiac muscle cells, have left the cell cycle and are not capable of mitotic division in postnatal life.
Cell division, or mitosis, is the process during which a parent cell divides and each daughter cell receives a chromosomal karyotype identical to the parent cell. Cell division gives the body a means of replacing cells that have a limited life span such as skin and blood cells, increasing tissue mass during periods of growth, and providing for tissue repair and wound healing.
Mitosis is a dynamic and continuous process. It is divided into four stages—prophase, metaphase, anaphase, and telophase (Fig. 4.12). The phase during which the cell is not undergoing division is called interphase. During pro- phase, the chromosomes become visible because of increased coiling of the DNA, the two centrioles replicate, and a pair moves to each side of the cell. Simultaneously, the microtubules of the mitotic spindle appear between the two pairs of centrioles. Later in prophase, the nuclear envelope and nucleolus disappear. Metaphase involves the organization of the chromosome pairs in the midline of the cell and the formation of a mitotic spindle composed of microtubules. Anaphase is the period during which separation of the chromosome pairs occurs, with the microtubules pulling one member of each pair of 46 chromosomes toward the opposite cell pole. Cell division or cytokinesis is completed after telophase, the stage during which the mitotic spindle vanishes and a new nuclear membrane develops and encloses each complete set of chromosomes.
Cell division is controlled by changes in the concentrations and activity of three major groups of intracellular proteins:
2. Cyclin-dependent kinases (CDKs)
3. Anaphase-promoting complex
The central components of the cell cycle control system are the CDKs, whose activity depends on their association with the regulatory units called cyclins. Oscillations in the activity of the various CDKs lead to initiation of the different phases of the cell cycle. Cell division is also controlled by several external factors, including the presence of cytokines, various growth factors, or even adhesion factors when the cell is associated with other cells in a tissue.