ABSTRACT
Figure 21.1 illustrates the four successive phases of the cell cycle. Normal, proliferating somatic cells spend the majority of their existence in interphase. Cells also perform most of their maintenance functions in interphase. As a cell prepares to divide, it enters the G1 phase, the period between mitosis (M phase) and DNA synthesis (S phase). The G1 phase lasts from several hours to days and is characterized by synthesis of cell organelles and centriole replication (the G0 phase represents a resting stage or a subphase of G1). The S phase begins when DNA synthesis starts and the chromosomes have replicated. The G2 phase starts when DNA replication is complete and the content of the nucleus has doubled. The G2 phase ends when mitosis starts. The extended part of interphase then continues in the M phase with the stages of cell division-namely, prophase, metaphase, anaphase, and telophase. During this time the M phase begins with mitosis (nuclear division) and ends with cytokinesis (cytoplasmic) division. Cell division terminates with the completion of cytokinesis.
