DNA STRAND BREAKAGE BY I-125 DECAY IN A SYNTHETIC OLIGODEOXYNUCLEOTIDE .2. QUANTITATIVE-ANALYSIS OF FRAGMENT DISTRIBUTION

The DNA breakage produced by decay of I-125 in a double-stranded 41 bp oligodeoxynucleotide was investigated by DNA sequencing gel analysis, Use of both 5'- and 3'-end P-32 labelling of the I-125 containing str and provided the single-stranded breakage pattern at either side of th e I-125-dC. The asymmetric pattern of breakage relative to the I-125-l abelled nucleotide enabled deconvolution of two components of breakage , One of them declines very quickly with nucleotide number from I-125- dC and dominates within 4-5 nucleotides, We assume this component to b e associated with neutralisation of charged Te atom resulted from deca y, or/and with radiation damage to an initial target other than the de oxyribosyl moiety-probably the bases. The second component depends on the geometrical distance between the I-125 atom and deoxyribosyl atoms . These two components are responsible for breakage under conditions l imiting radical mediated damage, namely in the presence of 2M dimethyl sulphoxide. The third component, associated with radical-mediated dama ge, contributes to the total breakage during incubation in 20mM phosph ate buffer alone, and under these incubation conditions dominates the breakage beyond 8-9 nucleotides from I-125-dC. The estimated average n umbers of single-stranded breaks produced in the I-125-containing stra nd by each mechanism in 41-mer oligodeoxynucleotide with I-125-dC at 2 1st position are respectively 2.33, 0.98 and 0.63.