Re: what are the effects of radiation on Mitotic Cells

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.