Laser UV Microirradiation of Interphase Nuclei and Post-Treatment with Caffeine A New Approach to Establish the Arrangement

Summary. Laser UV microirradiation of Chinese hamster interphase cells combined with caffeine post-treatment produced ifferent patterns of chromo-some damage in mitosis following irradiation of a small area of the nucleus that may be classified in three categories: I) intact metaphase figures, II) chromo-some damage confined to a small area of the metaphase spread, III) mitotic figures with damage on all chromosomes. Category III might be the con-sequence of a non-localized istortion of nuclear metabolism. By contrast, category II may reflect localized DNA damage induced by microirradiation, which could not be efficiently repaired due to the effect of caffeine. If this interpretation is right, in metaphase figures of category II chromosome damage should occur only at the irradiation site. The effect might then be used to investigate neighbourhood relationships of individual chromosomes in the interphase nucleus