A&P Lecture 2: Cell Cycle and Cell Division

ANATOMY AND PHYSIOLOGY LECTURE 2

CELL CYCLE AND CELL DIVISION

The life of a cell is called the cell cycle. It is usually divided into five phases or gaps: G0, G1, S, G2, and M. Some cells may not have a G1 phase, and others may not have a G2 stage. However, all cells must grow, replicate their genetic material if they are to divide, and undergo the process of mitosis if they are to replicate. G0 is the stage during which 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 is starting 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 are beginning to replicate. G2 is the premitotic phase and is similar to G1 as for RNA and protein synthesis. The M phase is the phase during which cell mitosis occurs. Nondividing cells, such as mature nerve cells and cells not preparing for mitosis, are said to be in the G0 phase of the cell cycle .

Cell Division

Cell division, or mitosis, which was first described in 1875, 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. Despite the early cytologic description of the four stages of mitosis, it was not until the early 1950s that the importance of the cell cycle was realized. 

Mitosis, which is a dynamic and continuous process, usually lasts from 1 to 11⁄2 hours. It is divided into four stages: prophase, metaphase, anaphase, and telophase (Fig. 4-13). The phase during which the cell is not undergoing division is called interphase. During prophase, 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 the 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 intracellular concentrations and activity of three major groups of

intracellular proteins: (1) cyclins, (2) cyclin-dependent kinases, and (3) the anaphase-promoting complex. The central components of the cell cycle control system are the cyclin-dependent kinases, whose activity depends on association with the regulatory units called cyclins. Oscillations in the activity of the various cyclin-dependent kinases lead to initiation of the different phases of the cell cycle. For example, activation of the S-phase cyclin-dependent kinases initiates the S phase of the cell cycle, whereas activation of the M-phase cyclin-dependent kinases triggers mitosis. The anaphase-promoting complex is responsible for the breakdown of the M cyclins and other regulators of mitosis.

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. In addition, the cell cycle is regulated by several checkpoints that determine whether DNA replication has occurred with a high degree of fidelity. Two of the better understood are the DNA damage and the spindle formation checkpoints. If these biochemical checkpoints are not faithfully met, the cell may default to programmed

cell death or apoptosis.