It is this excitation, or ionization, that produces damage to the cell structure of living tissue. The amount, and type, of damage is a function of the energy of the incident radiation and the interaction cross section of the material encountered. The reactions of living tissue to ionizing radiation is complex, and as yet rather poorly understood. A human cell consists of a membrane, an aqueous material called the cytoplasm, and the tiny nucleus that contains chromosomes and nucleic acids. Hereditary information will be found in the genes, the components of the long thread-like assemblies of DNA called chromosomes.
Most human cells reproduce by dividing, a process controlled by components of the nucleus.
Any alteration of the structure of these components, by radiation or other agents, can have serious consequences. Massive exposure results in serious injury, or death, to the organism. Some human cells do not reproduce by division. The blood cells are continuously produced by special organs, principally in the bone marrow. Brain, sensory, and motor nerve cells develop only in the womb.