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Question: We've discovered that various types of radioactive particles produced during radioactive decay affect molecules in the cells/tissues of our bodies. How are those molecules affected and what does that do to the cell/tissue?
Naturally occurring radioactivity consists primarily of (a) alpha particles, each like a helium-4 nucleus, with electric charge +2; (b) beta particles, each identical to the ordinary electron, with charge -1; and (c) gamma rays, which resemble X rays. The smallest gamma-ray energy packet is called a photon (as with light), and is uncharged.
Cellular injury results from interaction between electrically charged subatomic particles and molecules. The orbital electrons (negative) are knocked out of molecules by alpha particles (positive) by attraction between the opposite charges, and by beta particles (negative) by repulsion of the like charges. By different mechanisms, a gamma-ray photon also knocks out an orbital electron. All these ejected electrons zoom away, and each can knock many additional electrons out of other molecules. In these electric interactions, the kinetic energy (energy of motion) of each alpha, beta, or ejected electron is incrementally "cashed in" to knock orbital electrons out of molecules, and give these ejected electrons their own kinetic-energy send-offs. Missing some orbital electrons, an affected molecule can undergo abnormal biochemical reactions. Also, a charged particle may boost an orbital electron to a higher energy level (still within the molecule), thereby also enabling the molecule to undergo abnormal reactions. The most important damage is in DNA: (a) misprints in "genetic letters" (A, T, G, C); (b) breaks in either or both strands, or abnormal cross links between the two strands, of the double helix; and (c) abnormal bonds between DNA and nearby molecules. Damage in replicating DNA may be a somatic mutation leading to cancer, or a germ-line mutation leading to a genetic defect in offspring.
Also, chromosomes (larger structures containing DNA and other kinds of molecules) are broken, and the broken fragments may be lost during subsequent cell divisions. The fragments may also rejoin to make abnormal rearrangements. Thus cells are formed that must "play without a complete deck" of genes. In addition, inhibition of cell divisions prevents normal development of tissues and organs.
Life on Earth has always been exposed to natural radioactivity (and to cosmic rays, which have similar actions), and all cells evolved multiple and redundant mechanisms to repair most damaged DNA. Multicellular organisms evolved a second line of defense: Many a dangerously damaged cell can be activated by healthy cells to "commit suicide" in a process called apoptosis. Both DNA repair and apoptosis also play important roles in many other situations in biology.
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