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# Iodine-125 Decay in a Synthetic Oligodeoxynucleotide. I. Fragment Size Distribution and Evaluation of Breakage Probability

P. N. Lobachevsky and R. F. Martin
Vol. 153, No. 3 (Mar., 2000), pp. 263-270
Stable URL: http://www.jstor.org/stable/3580288
Page Count: 8
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## Abstract

Incorporation of ${}^{125}{\rm I}\text{-}{\rm dC}$ into a defined location of a double-stranded oligodeoxynucleotide was used to investigate DNA breaks arising from decay of the Auger electron-emitting isotope. Samples of the oligodeoxynucleotide were also labeled with 32 P at either the 5′ or 3′ end of either the ${}^{125}{\rm I}\text{-}{\rm dC}\text{-containing}$ (so-called top) or opposite (bottom) strand and incubated in 20 mM phosphate buffer or the same buffer plus 2 M dimethylsulfoxide at 4°C during 18-20 days. The 32 P-end-labeled fragments produced by 125 I decays were separated on denaturing polyacrylamide gels, and the 32 P activity in each fragment was determined by scintillation counting after elution of fragments from the gel. The relative fragment size distributions were then normalized on a per decay basis and converted to a distribution of single-strand break probabilities as a function of distance from the ${}^{125}{\rm I}\text{-}{\rm dC}$. The results of three to five experiments for each of eight possible combinations of labels and incubation conditions are presented as a table showing the relative numbers of 32 P counts in different fragments as well as graphs of normalized fragment size distributions and probabilities of breakage. The average numbers of single-strand breaks per 125 I decay are 3.3 and 3.7 in the top strand and 1.3 and 1.5 in the bottom strand with and without dimethylsulfoxide, respectively. Every 125 I decay event produces a break in the top strand, and breakage of the bottom strand occurs in 75-80% of the events. Thus a double-strand break is produced by 125 I decay with a probability of approximately 0.8.

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