Repair at Chromosome Exchange Sites in Tradescantia paludosa microspores

An X-ray dose-response curve for chromosome exchanged has been obtained at the time when interaction with lesions produced by a priming dose of 300 rad was known to be minimal. A comparison of the parameters of this curve with those of a single-exposure dose-response curve obtained at the same time, indicated that there is a marked reduction in the number of effective chromosome exchange sites available to a second dose. Consequently, the observed yield of exchanges at this time is less than that expected on the basis of simple additivity of die yields of the two doses given Independently (the ^base line⁾. On die assumption that die original unirradiated population had a limited number of sites for chromosome exchange, the data were consistent with die hypothesis of the repair of partially damaged chromosome exchange sites to pre-Irradiation sensitivity of sites with only one potential exchange lesion. However, the possibility of dose-dependent inactivation of sites cannot be entirely excluded, and if a significant amount of this is occurring, then it is possible to interpret the fall in exchange yield with fractionated doses, and the apparent loss of interaction between lesions without Invoking repair. (author)