| Cells reproduce by duplicating their contents and then dividing in two; this is a closely regulated and complex process that is termed the cell cycle. In recent years a number of key control points in the cell cycle have been elucidated. In mammals, the duration of the cell cycle varies greatly from one cell type to another, ranging from minutes to years. However, cultured immortalized mammalian cells, which are studied in experimental systems, are, typically, relatively rapidly dividing cells with a cell cycle division time of approximately 24 hours.
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| Figure 41.1 The phases of the cell cycle. The mitosis (M) phase is the stage of cell division, and takes about an hour in most cells. The G1, S, and G2 phases comprise the remaining interphase, during which cell growth and DNA synthesis occur. The restriction point is of significance in mitogenesis (see later in text). S, the synthesis phase of interphase, during which replication of nuclear DNA occurs; G1, the interval between M- and S-phases; G2, the interval between S- and M-phases. |
| Traditionally, the cell cycle is divided into several phases (Fig. 41.1). Mitosis (M-phase) is the stage of cell division; in most cells this takes only about an hour. The remainder of the cell cycle, during which cell growth and deoxyribonucleic acid (DNA) synthesis occur, is known as interphase. Replication of the nuclear DNA occurs during the synthesis (S) phase of interphase. The interval between the M- and S-phases is called the G1-phase, and the interval between the S- and M-phases is called G2. The G1- and G2-phases afford time, not only for cell growth, but also to provide checkpoints at which the progress of the cell cycle may be controlled, and
the incorporation of DNA damage prevented, before cell division. As mentioned above, individual cell cycle times vary enormously and it is during G1-phase that most of this variability occurs. This is because cells in G1-phase, which are not committed to DNA replication, can enter a specialized resting state, called G0-phase, until they receive appropriate signals allowing them to progress further through the cell cycle. In mammals, the time required for a cell to progress from the beginning of S-phase through mitosis is typically 12-24 hours irrespective of the duration of the G1-phase. Thus, almost all the variation in proliferation rates between cell types is attributable to the amount of time spent in this G0/G1 state. Furthermore, in conditions that favor cell growth, the total ribonucleic acid (RNA) and protein content of the cell increases continuously, except in M-phase, when the chromosomes are too condensed to allow transcription.
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