|MadSci Network: General Biology|
Mitotic cells are most sensitive to radiation killing. There are many reports comparing the radiation sensitivities of cells during different phases of cell cycle. According to these studies, cells in late S phase (synthesis phase) are most radio resistant and sensitivity increases in the order of late S, early S, G1, G2 and Mitotic phase. For cells with long G1 phase, the sensitivity was found to be more at the end of the G1 phase. Apart from the increased sensitivity, change in the shape of the survival curves are also reported. Mitotic phase cells possess no shoulder, which is a typical character of repair profecient cells. On the other hand, late S phase cells exhibit very broad shoulder indicating good repair capability. For dividing cells or in particular, for stem cells, the mechanism of radiation killing is prominantly due to mitotic death. The problems associated with segregation of chromosomes with aberrations and also many other reasons trigger the cell death at metaphase. If cells are irradiated at any other phase, there is a good chance for the cell to repair them by its enzymatic pathways. This chance is scarce at metaphase. Hence, cells undergo mitotic death easily. The increased sensitivity of the cells at G2 and metaphase plays very important role in radiation therapy. When irradiated, cells have tendency to accumulate at G2 phase before proceeding to metaphase. Both G2 and metaphase cells are very sensitive. Hence, when the radiation is suitably fractionated, cells tend to accumulate at sensitive phase and leads to increased sensitivity for killing.
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