On the infleunce of 5-amino uracil on the cell cycle of root tip meristems

Premature onset of mitosis and potentiation of chromosome damage induced by poly-D-lysine in plant cells: evidence for G2 repair

Poly-D-lysine has been reported to induce a triggering of mitosis in plant cells due to a selective stimulatory effect on cells arrested in G2. Root-tip cells of Allium cepa L. were first exposed to maleic hydrazide (MH) early in the cell cycle and posttreated with different concentrations of the polycationic agent while in G2. The result was a dose-dependent potentiation of chromosome damage observed at metaphase without any apparent effect induced by poly-D-lysine itself. The enhancement of the yield of chromosomal aberrations was concomitant with an increase in the frequency of mitosis. In order to test further the stimulatory effect of poly-D-lysine on mitosis, as well as the consequences of a shortening of the time available for repair, cells synchronized by protracted treatment with 5-aminouracil (5-AU), which also induces chromosome damage, were allowed to recover in the presence of the polycationic compound. Our data show that a premature arrival at mitosis resulted in an increase in the frequency of damaged cells observed.

Cytological demonstration of DNA synthesis inhibition at late S by 5-amino-uracil

Meristematic cells of Allium cepa were treated with 5-amino-uracil (5-AU) while incorporating 5-bromodeoxyuridine (BrdU) into DNA until complete inhibition of mitosis was obtained. The pattern of BrdU substitution in interphase nuclei detected by FPG technique in the cells so treated was that known as corresponding to BrdU incorporation in only early and middle replicating DNA. In addition, when the cells were allowed to reach mitosis in the absence of BrdU after the synchronization induced by 5-AU all the metaphase chromosomes showed, likewise, the typical late-replicating pattern. These results are a cytological demonstration of a preferential inhibition of late-replicating DNA synthesis by 5-AU proposed by several authors.

Sister chromatid exchanges and chromosomal aberrations in 5-aminouracil-synchronized cells

5-AU-synchronized cells of Allium cepa L.meristems show increased yields of chromosomal aberrations as well as SCE's. However, although a caffeine post-treatment increases the frequency of 5-AU-induced abnormal ana-telophases, it has a negligible influence on the 5-AU-induced SCE's. These results strongly suggest that the mechanisms through which the 5-AU-induced DNA damage is transformed into chromosomal aberrations and SCE's are at least partly different.

Further in vitro studies on the biochemistry of the inhibition of nucleic acid and protein synthesis induced by arsenic

Human lymphocytes, stimulated by PHA, and exposed to increasing Na2HAsO4 concentrations, show an identical incorporation rate for 14C thymidine and 14C-TTP into the DNA. 14C uridine is incorporated 3--4 per cent less at an As concentration of 1.0 mug/ml medium, above this however approximately 15 per cent less into the RNA as compared to 14C-UTP. In free pyrimidine bases, the incorporation of labelled triphosphates into the DNA and RNA is significantly reduced above 1 mug and 10 mug Na2HAsO4. Corresponding to its approximately uniform distribution into DNA and RNA, the incorporation rate of 14C-ATP above 10 mug Na2HAsO4/ml culture medium lies between that of 14C-UTP and 14C-TTP. The incorporation of 14C alanine and 14C leucine into cellular protein is not reduced below 10 mug Na2HAsO4/ml. The incorporation rate is 41 per cent at a concentration of 100 mug Na2HAsO4/ml medium. Compared to 14C-UTP, 14C-ATP and 14C-TTP it is increased by the factor of 1.8; 2.5 and 6.8 respectively. The inhibition of enzymes of the dark repair mechanisms and the synthesis of biopolymers together with their altered sequence and the involvement of long-lived messenger RNA serve as an explanation of the observed alterations of the lymphocyte metabolism, caused by arsenic.