The cell cycle is made up of four phases (Fig. 6). In the M phase (M=mitosis), the chromatin condenses into chromosomes, and the two individual chromatids, which make up the chromosome, segregate to each daughter cell. In the G1 (Gap 1) phase, the cell either progresses toward DNA synthesis or another division cycle or exits the cell cycle reversibly (G0) or irreversibly to commit to differentiation. During G1, the cell is particularly susceptible to control of cell cycle progression; this may occur at a number of restriction points, which determine whether the cell will re-enter the cycle, withdraw from it, or withdraw and differentiate. G1 is followed by the S phase (DNA synthesis), in which the DNA replicates. S in turn is followed by the G2 (Gap 2) phase in which the cell prepares for reentry into mitosis. Checkpoints, at the beginning of DNA synthesis and in G2, determine the integrity of the DNA and will halt the cell cycle to allow either DNA repair or entry into apoptosis if repair is impossible. The Phospho Histone H3 Imaging Kit (Roche) is a convenient method for fast cell cycle analysis by quantification of mitotic cells. Apoptosis, or programmed cell death, is a regulated physiological process whereby a cell can be removed from a population. Characterized by DNA fragmentation, nuclear blebbing, and cell shrinkage, apoptosis can be detected via a number of marker enzymes and kits (see Roche Applied Science products). Roche DNA Fragmentation Imaging Kit is a TUNEL assay-based method for accurate and fast quantitative fluorescence detection of apoptosis in medium to high throughput cellular workflows.
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