ell cycle tend to have a quite dramatic impact on synchronized cell cultures. Synchronized cultures consist of cells that are all in the same cycle phase (e.g., S or early G2). In many cases, irradiation of such cultures leads to marked desynchronization. Furthermore, with increasing radiation doses, cellular synchrony has been demonstrated to decrease at faster and faster rates (15:59-74).
One important effect of ionizing radiation on the cell cycle is known as G2 delay. Following acute irradiation, cells will often be temporarily arrested in the G2 phases of the cell cycle (13:3718). This phenomenon tends to occur regardless of the individual cell's position in the cycle. For example, cell cycle studies involving particles--e.g., alpha-particles, negative pions, and ions with 300 keV/¦m--having high linear energy transfer (LET) clearly indicate that irradiation causes delayed division. As cells' progress through the cell cycle is postponed, they tend to accumulate in G2. This phenomenon is known as G2 block. Researchers have found that the duration of this block may be directly correlated with both radiation dose and quality. Thus, particles with LETs of about 100-200 keV/¦m generally create a greater cellular disturbance than, for instance, X-rays (15:59-74). Moreover, for very high LET values, many cells will become virtually stuck in, for example, S or late G2/M. This phenomenon essentially corresponds to radiation-induced cellular inactivation (15:59-74).
Heavy ions can cause cell cycle delays in a number of different cycle phases. Cells irradiated with neon or uranium particle types may exhibit delays in G1, S, late S, and late G2/M. In addition, uranium irradiation can also block the transition from the G1 to the S phases of the cell cycle (15:59-74).
As with radiosensitivity, radiation-induced cell cycle delays are additionally dependent on the particular phase that the cells are in when th...