Abstract
The process of thymine dimers formation in DNA under the action of ultraviolet irradiation has been analyzed theoretically taking into account the long range transfer of electronic excitation along the molecule. First, for the case of homopolymer (poly dA.dT) an exact analytical solution of the problem has been obtained. For the case of random walks of excitation as well as for the case of its directional motion the distribution functions of undamaged regions on their lengths have been calculated. These functions turned out to differ drastically from a trivial exponential distribution taking place without excitation's transfer. At the same time the transfer of excitation leads to the effect of thymine dimers clustering—the new dimers are formed mainly in the vicinity of dimers formed earlier. The degree of clustering γ (the mean number of dimers per cluster) depends quadratically on the concentration of clusters C in the homopolymer for the case of random walks of excitation. For the case of a heteropolymer having inhomogeneous distribution of potential dimers (two thymines situated one by one in the same strand) a machine algorithm for numerical solution of the problem by the Monte-Carlo method has been proposed. Calculations for the heteropolymer with a random sequence of bases have shown that in this case the curve γ( c) is S-shaped and its form agrees with the experimental one obtained by Shafranovskaya, Trifonov, Lazurkin & Frank-Kamenetskii (1972). The values of parameters providing the best agreement between theory and experiment have been obtained. Using these values of parameters the calculations of distribution function of undamaged regions and curves γ( c) for different concentrations of AT-pairs in DNA have been performed. These calculations show that the degree of clustering γ rises dramatically with increasing concentration of AT-pairs in DNA. Theoretical predictions which can be tested experimentally have been summarized.
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