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Safety_Series_025_1968 - gnssn - International Atomic Energy ...

Safety_Series_025_1968 - gnssn - International Atomic Energy ...

This publication is no

This publication is no longer validPlease see http://www.ns-iaea.org/standards/radiation the chrom osom es stick together and are afterwards unevenlydistributed during subsequent cell division. The quantitativestudy of chromosomal aberrations in man, using new techniques fortheir identification, is of considerable importance for studies ofdose-effect relationships.In many cases the nucleus and the cytoplasm increase in volume,vacuoles are formed, and, after large doses of radiation (severalhundred rads), collapse of the entire cell structure ensues. Of thevariety of particular structures, the mitochondria are very importantfor cellular respiration which releases a substantial part of energyused in biochemical reactions. Mitochondria are also connectedwith lipid metabolism. After irradiation the mitochondria increasein volume and show an alteration in staining qualities.1.2. 1.4.4. Functional changes in the cell (division). A sensitiveindicator of post-irradiation change is a delay in cell division. Thisis particularly the case when the cells are irradiated before divisioncommences. There is a critical point during cell division when thechromosomes become visible and the nuclear membrane and nucleolusdisappear. Mitosis is stopped when the cells are irradiated beforethis critical point, but is relatively less affected after this stage haspassed. The final outcome depends on the dose delivered. Somevariations of this scheme are observed when different types of cellsare studied.The delay of cell division may be due to the inhibition ofdesoxyribonucleic acid synthesis, but interference with other factorsmust also be taken into consideration. In some cases cell division vmay be more sensitive than DNA synthesis.The growth rate of cell cultures is affected under chronic irradiation.Giant cells appear with increased volume and ploidy. Thegrowth rate of Phycomyces sporangiophores is slowed down by extremelylow doses (0.001 rad).In conclusion it may be said that rapidly dividing cells are moresensitive to radiation than are non-dividing cells (with the exceptionof non-dividing lymphocytes).1 .2 .1 .4 . 5. Occurrence of mutations . Irradiation of the nuclearstructure of sexual cells induces mutations which manifest themselvesin subsequent generations and are generally deleterious.It is now a generally accepted hypothesis that genetic informationis carried in the chains of the macro-molecules of desoxyribonucleic14

This publication is no longer validPlease see http://www.ns-iaea.org/standards/acid which are integrated into the complicated structure of thechromosomes. According to this hypothesis, genes, the determ i­nants of inheritable characters, are more or less identical with asequence of nucleotide pairs in the chain of desoxyribonucleic acid.A change in the sequence of nucleotide pairs constitutes a point mutation.On the other hand, when a whole part of a chromosome ischanged through breakage of chromosomes or abnormal reunions, achromosome mutation takes place.To a very small degree mutations occur spontaneously, i.e .from unknown reasons. The overall frequency of mutations is increasedby irradiation of the germ cells. Many factors influencethe final result.. Of primary importance is the absorbed dose ofradiation and the rate of dose delivery. The stage in the developmentof the germ cells is also decisive. The increase of the fr e ­quency of mutations for any site (locus) of a chrom osom e pair isalways small, even when the highest sub-lethal doses in experimentalanimals are used.Some studies with Drosophila indicated that the frequency ofmutations is directly proportional to the dose absorbed by the gonads.In later studies it was observed that the proportionality factor varieswith the stage of the irradiated germ cells and with the delivery rateof radiation. When the same total dose is given at low dose rate,fewer mutations are induced than when a high delivery rate is used.These facts, which were substantiated by irradiation of large populationsof mice, point to the presence of a repair process. On thebasis of experiments with unicellular organisms, it is believed thatafter the primary physical event there is some time lapse duringwhich the process of mutation is not definitively established and maybe interfered with by repair action. When fully established, themutation can be reversed only by back mutation (reverse mutation).This may occur spontaneously or by irradiation of the mutant. Inany case it is a rare event and is not a practical recovery process.The present evidence suggests that ionizing radiation may producehereditary damage even at the lowest doses and dose rateswhich have been investigated. As regards man it should be bornein mind that up to the present there is little data on hereditarydamage after irradiation. On the other hand, all the experimentalevidence with other organisms proves a dose dependency. It is difficultto believe that man is an exception to this. On the other hand,animals of different species show variation in their sensitivity to themutagenic action of radiation. Further careful studies are necessaryto establish the sensitivity of man in this respect.15

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