  We consider a new model which consists of a DNA together with a RNA. Here we
assume that DNA is from a mammal or bird but RNA comes from a virus. To study
thermodynamic properties of this model we use methods of statistical mechanics,
namely, the theory of Gibbs measures. We use these measures to describe phases
(states) of the DNA-RNA system. Using a Markov chain (corresponding to Gibbs
measure) we give conditions (on temperature) of DNA-RNA renaturation.
