Mechanisms of radiation, cells and DNA

　What does it mean for a person to be exposed to X-ray or gamma ray at 1mSv?
　When the whole body is exposed to radiation at 1mSv, it means that an average one ray of radiation passes through the nucleus of every cell in the body. In the figure, only the radiation that passes through the nucleus of the cell is depicted, but radiation also passes through other cells at this density.

　As the figure on the previous page shows, hardly anyone could survive when exposed to radiation over 6,000 to 7,000 mSv at once. It is because 6,000 to 7,000 rays of radiation pass through the nucleus of every cell and cut DNA (the blueprint of the body) into pieces, making cells unable to divide (this will be explained later). It is thought that fifty percent of people exposed to 3,000 to 4,000 mSv of radiation would die from it.

　Unlike other diseases, treatment would not cure the harm done by radiation; because the blueprint of the body, DNA, is destroyed and new cells cannot be created. Let’s think about this using human skin as an example.

　Everyday, old skin cells die and fall off as dead skin. The reason why we aren’t left peeled of skin is because new cells replacing them are created underneath the skin. Not only the skin cells, but all our cells have life spans. The cells die when they get old and the dead cells are replaced by new cells. In order to grow new cells, DNA, the blueprint of the body inside the cells, needs to be copied exactly as shown in the figure on the upper right. This happens in our stomach, intestines, blood and various other organs, so the body is kept in the same condition all the time.

　Radiation cuts the DNA, as shown on the left. If it is a small amount of radiation, the DNA can be repaired to some extent automatically and can return to normal cells.
　But if the DNA is exposed to a large amount of radiation, it is damaged beyond its capacity to repair itself. The cells can’t divide due to the occurrence of repair mistakes and the loss of information needed for cell reproduction. Hence the dead cells can’t be replaced by the new cells.
　When this happens, skin and the intestinal membranes are peeled off and bleed or ‘hemorrhage’.

THINK AND USE YOUR BRAIN

　The cells can’t divide even in the bone marrow that makes blood cells; therefore, neither white blood cells that fight against bacteria or platelets that serve to stop bleeding can be created. This causes bacterial infection and loss of enormous amount of blood and leads to death.
　Acute radiation syndrome is such changes occurring in a short time after radiation exposure.