ANALYSIS OF DELETIONS INDUCED IN THE GENOME OF MAMMALIAN-CELLS BY IONIZING-RADIATION

A theory is presented for the distribution in size of deletions induce d by ionizing radiation, based on three assumptions: (1) deletions tha t are observed delete part or all of a gene to make a mutation, but no t adjacent DNA sequences essential for survival of the mutant; (2) del etions are distributed at random along the DNA; (3) the probability of formation is proportional to the rate at which the two endpoints, whi ch must meet to form the deletion, collide with each other. Experiment al data for radiation-induced deletions in human and hamster hprt gene s are in good agreement with calculations that assume the inducing les ion does not break the intracellular chromatin fiber; calculations ass uming the inducing lesion is a break are not a good fit to the data. T he low frequency of deletions observed in the hamster aprt gene is sho wn to be a consequence of the small gene size and the presence of a ne arby essential DNA sequence, ensuring that most deletions affecting th e gene also delete the essential sequence and are thus not observed.