Some cytological aspects of antagonism in synthesis of nucleic acid

Summary

The effects of solutions of benzimidazole and 5-aminouracil onAllium cepa root meristems immersed in them have been studied cytologically. Benzimidazole and 5-aminouracil, analogues of purines and pyrimidines respectively, reduce the number of nuclear divisions being initiated. A marked decline in frequency of cells in mitosis occurs at the beginning of treatment with benzimidazole (300 p.p.m.) but not until 6 or more hours later with 5-aminouracil (125 p.p.m.).

After treatment with 5-aminouracil, small interstitial portions of chromosomes are Feulgen-negative and become extended during anaphase into thin strands. The distal portion of these chromosomes may lag during anaphase and contribute to malformations of the surface of the sister nuclei and to micronuclei-like bodies lying along their adjacent surfaces. However, stickiness resulting in chromosome bridges was not observed. After treatments with benzimidazole, chromosomes are more contracted at metaphase and anaphase than in the control. At the concentrations used, neither component was observed to affect the spindle mechanism of the divisions in progress at the beginning of treatment nor of the divisions which escape inhibition.

Reversal of these effects by the introduction of metabolites which these analogues resemble structurally, was investigated. Adenine sulphate (800 p.p.m.) acts antagonistically to benzimidazole (300 p.p.m.) by delaying the marked decline in frequency of mitoses by more than 6 hrs. By 12 hrs. the frequency of cells in mitoses was very low; thus, reversal is not permanent. Half the inhibition of growth caused by 5-aminouracil (50 p.p.m.) is reversed by thymine (50 p.p.m.) for the 24 hrs. of the test period. When 5 p.p.m. folic acid was added to this mixture, root growth exceeded that of the controls. Feulgen-negative gaps, however, are still present in chromosomes at anaphase.

Estimations of Feulgen content of nuclei to determine the effect of these treatments on DNA synthesis were made by means of a microphotometer. In control roots there are two classes of nuclei at interphase according to the amount of DNA per nucleus. The amount doubles during interphase before the next division of the nuclei in the root meristem. In roots treated with 5-aminouracil this doubling is inhibited, so that interphase nuclei with the lower amount of DNA accumulate. In roots treated with benzimidazole the relative frequencies of the two classes of nuclei is unaltered; however, mitotic activity is inhibited. This compound apparently affects not only processes concerned with doubling the amount of DNA per nucleus but also processes necessary to the division itself. There was, however, no unquestionable evidence of a nucleus being reconstituted after it had started a division.