Relationship of Cell Cycle and Ionizing Radiation

The cell cycle (Figure 21-2) has four phases that are distinctly sequenced and describe the life cycle of cells: (1) Synthesis (S): DNA synthesis, (2) Gap (G2): a period of apparent cellular inactivity, (3) Mitosis (M): cellular division into two daughter cells, (4) Gap (G1): a period of apparent cellular inactivity. Cells vary in their sensitivity to ionizing radiation and this is dependent on their position within the cell cycle at the time of the exposure to the radiation (Figure 21-3). The most sensitive phases are G2 and M. The medium sensitivity phases include G1 and early S. The least sensitive phase is late S (Table 21-2).

In a given volume of tissue, its cells are asynchronously distributed within the various phases of the cell cycle. Each cell will progress through the cycle at its own individual rate. When exposed to ionizing radiation, the surviving cells will undergo partial synchronization. This occurs as a consequence of G2 arrest which delays cells in a more radiosensitive phase. Also, other surviving cells may progress into the next phase which may be more radiosensitive.

(Mitosis)

(Mitosis)

S (DNA synthetic phase)

Figure 21-2. Cell cycle showing the mitotic stages for actively growing cells. M is mitosis, G1 and G2 are gaps (periods of inactivity), S is DNA synthesis.

S (DNA synthetic phase)

Figure 21-2. Cell cycle showing the mitotic stages for actively growing cells. M is mitosis, G1 and G2 are gaps (periods of inactivity), S is DNA synthesis.

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