Influence of Dna Synthesis on the Production of Chromatid Aberrations by X Rays and Maleic Hydrazide in Vicia Faba.

is well established that the irradiation of cells at any stage in development 'Tduring interphase results in the formation of chromosome aberrations, but that two basically different types of chromosome changes may be produced. Aberrations induced in roots of Vicia faba exposed to 50 rads of X rays are first seen in metaphase cells at about 2 hours after irradiation. In those cells which reach metaphase between 2 and 14 hours after irradiation, the chromosome structural changes found are exclusively of the chromatid-type in which the unit of rearrangement is the single chromatid (Figure 2). In cells observed at metaphase at later fixation times, most of the aberrations observed are of the chromosome-type, in which the unit of rearrangement is the two chromatids of a single chromosome which are broken or exchanged at identical loci (Figure 1). In studies on chemically induced chromosome aberrations, particularly in cases where alkylating agents have been employed, there have been few, if any, reports of unambiguous chromosome-type changes observed in cells seen in their first posttreatment mitosis, all the aberrations reported upon being of the chromatid-type. Moreover, with many of the chemicals it has also been found that the first appearance at metaphase of the induced chromatid aberrations occurs at about 8 to 10 hr after the initiation of chemical treatment, i.e. there is a considerable delay relative to the time of appearance of similar aberrations after X-ray treatment. This delay in the appearance of aberrations at mitosis has been reported for a large number of chemical compounds tested on Vicia roots; these include nitrogen mustard (FORD 1949; REVELL 1953), beta-propiolactone (SMITH and SRB 1951; SWANSON and MERZ 1959), a diepoxide (REVELL 1953), maleic hydrazide (MCLEISH 1953; KIHLMAN 1956; MERZ, SWANSON and HEMALATHA 1961 ) , triethylenemelamine ( OCKEY 1960), ethyl alcohol, ethyl methane sulphonate and myleran (RIEGER and MICHAELIS 1960a and b) and N-nitrosoN-methylurethan (KIHLMAN 1960). It must be pointed out that in the experiments on aberration induction by the chemicals referred to above, no information on the mitotic delay induced by these chemicals was obtained, and in many instances no detailed information on mitotic frequency and aberration yields found in the early hours after treatment has been presented. Nevertheless, the difference in time scale between the X-ray and chemical-induced effects has been interpreted (FORD 1949; REVELL 1953) as indicating that following chemical treatment, aberrations are induced only in