  We determine the Renyi entropies K_q of symbol sequences generated by human
chromosomes. These exhibit nontrivial behaviour as a function of the scanning
parameter q. In the thermodynamic formalism, there are phase transition-like
phenomena close to the q=1 region. We develop a theoretical model for this
based on the superposition of two multifractal sets, which can be associated
with the different statistical properties of coding and non-coding DNA
sequences. This model is in good agreement with the human chromosome data.
