Abstract
1. Secondary root meristems of Vicia faba were treated with (a) 0.2% aqueous solution of ethyl-methane-sulfonate, for two hours, (b) aqueous solution of hydroxylamine (1 mg/ml) for 2 hours and (c) a mixture of these two agents in the proportion of 1∶1, for 1 hour; and allowed to revive. Root tips were fixed at various revival periods, from 12 hrs. to 84 hrs. These two agents are known to attack guanine and cytosine of the DNA molecule respectively. 2. Metaphase and anaphase breaks were screened and the breakage points were determined for the genome of Vicia faba. These breakage points were found to be localized and these regions were found to coincide with some of the known heterochromatic regions, of M and Sb chromosomes. Both the agents induced breaks in chromosomes exclusively on these points. 3. Centromeric regions were found to be affected by treatment with EMS as well as HA, as revealed by centric breaks as well as inactivation of centromeric activity. 4. The mechanism of action of these agents and the possible reasons for these localized effects are discussed. 5. From the data presented as well as those available in the literature on localized chromosome breaks with chemical mutagens, the possibility of mapping regions of chromosomes rich in guanine-cytosine pairs of bases and regions rich in adenine-thymine pairs of bases, is pointed out.
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