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A Note on Radiation Damage "There is no safe level of radiation exposure. So the question is not: What is a safe level? The question is: How great is the risk?" Karl Z. Morgan There have been three major theories as to how radiation damages living tissue, all set by physicians. All are approximations, and based on broad assumptions. (1) The threshold hypothesis: asserts that there exists a safe level of radiation. The idea behind this thinking is that if the does is low, then the cell repair rate is of the order of the damage rate. Hence you get no resultant damage. (2) The linear hypothesis: under this theory, you would expect 1 malignant cancer for 1000 person-rems. For example, you would find one cancerous patient if you exposed 500 people to 2 rems, or 10000 people to 0.1 rems. (3) The supralinear hypothesis: the main result here is that for low doses you get more cancers/person-rem than at high doses. Here they not saying you get more radiation; instead, you get more damaged surviving cells. Some Facts There are 4 types of ionising radiation. These are alphas (fast moving helium nuclei), betas (electrons), gammas (high energy EM radiation), and neutrons (highly penetrating). How does damage occur? In other words, how does radiation cause cancer? A typical cell is around 0.02mm across, a cell nucleus is about 0.001mm. When radiation, say a gamma, enters your body, there is a chance it will intersect with one of your cells. Inside any cell is a nucleus, which contains chromosomes. These are essentially DNA helixes. DNA looks like two entwined strings of nucleotides - the amino acids A, T, C, and G. Across strands they are paired A-T and C-G. A portion of DNA (a series of these acids) is called a gene. Genes exist along chromosomes, and they contain the data for proteins. If the radiation happens to pass into the cell nucleus (which is a relatively large entity compared to the rest of the cell), one of 4 things can happen. All exposure subjects cells to risk. In order of decreasing probability: (1) radiation goes right thru, no interaction. (2) radiation does irrepairable destruction, and cell dies. (3) radiation does damage to nucleus. Cell survives in this damaged state. After it repetitively divides, it grows into a solid tumour after 30 odd years - cancer. (4) radiation does repairable damage, and cell returns to normal state. (Very low probability). Possibility (3) is the one to watch out for. During division, the DNA strands stretch out, and it is during this time which your cells are most susceptible to damage. It is also possible for the radiation to ionise the water in the cell cytoplasm, leading to the formation of free radicals, which can travel some distance. They can react chemically with the DNA in the nucleus, interfering with the chemical bonding along the helix. Two types of damaging interaction can occur with the amino acids. (a) point mutations - deletion - substitution - inversion - addition (b) large scale mutations (chromosome aberrations) - deletion e.g. retinoblastoma - amplification - translocation It is also possible to have compound breaks along the DNA, which is not easy for the cell to repair, unlike single strand or double strand breaks. The cell and nuclear membranes are also susceptible to damage. This could be due to alterations in permeability/osmosis in the membrane due to the radiation-induced imbalance of ionised particles. Once a certain threshold is exceeded, you will start saturating the cells. This lethal threshold serves to define two categories of radiation. Effects EFFECT NATURE THRESHOLD? DOSE DEPENDENCE Stochastic Non-lethal mutations No Probability of (somatic or affecting single cells effect increases genetic) with dose Deterministic Lethal mutations Yes Severity of affecting large number effect increases of cells with dose Cancer Stem cells are ones which are able to undergo mitosis when the human body has reached full maturity.
Approximate Word count = 2520
Approximate Pages = 10.1
(250 words per page double spaced)
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