On the non-requirement for deoxyribonucleic acid synthesis in the production of chromosome aberrations by 8-ethoxycaffeine

Abstract A combined treatment with [ 3 H]thymidine and 8-ethoxycaffeine (EOC) was used on Vicia roots to investigate the sensitivity of the various phases of the cell cycle to aberration induction and mitotic delay. The main conclusions may be summarised as follows: 1. 1. In contrast to most other radiomimetic chemicals, which induce chromatid aberrations at the time of DNA synthesis, EOC is most effective in inducing aberrations in cells exposed whilst in the post-DNA-synthesis, or G 2 , phase. The action of EOC is therefore independent of the normal processes of chromosome duplication, but may be related to other facets of chromosomal synthesis. 2. 2. EOC produces a pronounced stickiness and a reduction in spiralisation of the chromosomes in cells which are undergoing mitosis at the time of treatment. Subchromatid aberrations are induced at prophase and chromatid-type aberrations in G 2 and S (DNA-synthesising) cells. Although the evidence is not unequivocal, the results suggest that EOC, like X-rays, can also induce chromosome-type aberrations in pre-synthesis, or G 1 , cells. 3. 3. There is a tendency for aberrations to be localised to the nucleolar organising regions of the pair of metacentric chromosomes, but, in contrast to the findings of other workers, no evidence for a localisation of aberrations to the centromere regions of the chromosomes was found. 4. 4. Mitotic delay during the first post-treatment cycle occurs mainly during the G 2 phase which is increased to approximately twice its normal duration.