CENTRIC RINGS, ACENTRIC RINGS AND EXCESS ACENTRIC FRAGMENTS BASED ON A RANDOM-WALK INTERPHASE CHROMOSOME MODEL

Excess acentric fragments, consisting of acentric rings and acentric l inear fragments, are among the most frequent kinds of chromosome-type aberrations produced by radiation. The frequency of acentric rings can not be obtained directly by experiment but is estimated here from the ratio of acentric to centric rings, evaluated using a random-walk mode l for the organization of chromatin during interphase and an assumptio n that the probability of an exchange formation is proportional to the rate of collision between two DSB. This ratio is calculated to be 2.5 in low-LET irradiated human fibroblasts, significantly greater than t he ratio if proximity effects are not considered. The calculated frequ ency of acentric rings is insufficient to account for all the observed excess acentric fragments. Assuming that the rest of the excess acent ric fragments are due to incomplete exchanges, all possible recombinat ions between two DSB that result in acentric rings and acentric linear fragments have been identified. From the chromosome aberration data, the incompleteness parameter has been estimated. Intra-arm chromosome exchanges, either complete or incomplete, were estimated to account fo r more than 50% of the excess acentric fragments in human fibroblasts.