McKenna's Pharmacology for Nursing, 2e

78

P A R T 2  Chemotherapeutic agents

CELL CYCLE Most cells have the ability to reproduce themselves through the process of mitosis. The genetic makeup of a particular cell determines the rate at which that cell can multiply. Some cells reproduce very quickly (e.g. the cells lining the gastrointestinal tract have a generation time of 72 hours), and some reproduce very slowly (e.g. the cells found in breast tissue have a generation time of a few months). In some cases, certain factors influence cell reproduction. Erythropoietin, a hormone produced by the kidney, can stimulate the production of new red blood cells. Active leucocytes release chemicals that stimulate the production of white blood cells when the body needs new ones. Regardless of the rate of repro- duction, each cell has approximately the same life cycle. The life cycle of a cell, called the cell cycle , consists of four active phases and a resting phase (see Figure 7.6). G 0 phase During the G 0 phase, or resting phase, the cell is stable. It is not making any proteins associated with cell division and is basically dormant as far as reproduction goes. These cells are just functioning to do whatever they are supposed to do. Cells in the G 0 phase cause a problem in the treatment of some cancers. Cancer chemotherapy usually works on active, dividing cells, leaving resting cells fairly untouched. When the resting cells are stimu­ lated to become active and regenerate, the cancer can return, which is why cancer chemotherapeutic regimens are complicated and extended over time, and why a 5-year cancer-free period is usually the basic guide for considering a cancer to be cured. G 1 phase When a cell is stimulated to emerge from its resting phase, it enters what is called the G 1 phase, which lasts from the time of stimulation from the resting phase until

the formation of DNA. During this period, the cell syn- thesises substances needed for DNA formation. The cell is actively collecting materials to make these substances and producing the building blocks for DNA. S phase The next phase, called the S phase, involves the actual synthesis of DNA, which is an energy-consuming activity. The cell remains in this phase until the amount of cellular DNA has doubled. G 2 phase After the cellular DNA has doubled in preparation for replication, the G 2 phase begins. During this phase, the cell produces all of the substances required for the manufacture of the mitotic spindles. M phase After the cell has produced all of the substances nec- essary for formation of a new cell, or daughter cell, it undergoes cell division. This occurs during the M phase of the cell cycle. During this phase, the cell splits to form two identical daughter cells, a process called mitosis. ■■ All cells progress through a cell cycle, which allows them to reproduce. ■■ Each cell goes through a resting phase (G 0 ); a gathering phase (G 1 ), when the components needed for cell division are collected by the cell; a synthesising phase (S), when DNA and other components are produced; a final gathering phase (G 2 ), when the last substances needed for division are collected and produced; and an M phase, when actual cell division occurs, producing two identical daughter cells. CHAPTER SUMMARY ■■ The cell is composed of a nucleus, which contains genetic material and controls the production of proteins by the cell; a cell membrane, which separates the inside of the cell from the outside environment; and a cytoplasm, which contains various organelles important to cell function. ■■ The cell membrane functions as a fluid barrier made of lipids and proteins. The arrangement of the lipoprotein membrane controls what enters and leaves the cell. ■■ Proteins on the cell membrane surface can act either as receptor sites for specific substances or as histocompatibility markers that identify the cell as a self-cell (i.e. a cell belonging to that individual). KEY POINTS KEY POINTS

5-FU

G 2

S

M

Phases of cell cycle

G 0

G 1

FIGURE 7.6  Diagram of the cell cycle, showing G 0 , G 1

, S, G 2

and

M phases.