  A simplified model for the closed circular DNA (ccDNA) is proposed to
describe some specific features of the helix-coil transition in such molecule.
The Hamiltonian of ccDNA is related to the one introduced earlier for the
linear DNA. The basic assumption is that the reduced energy of the hydrogen
bond is not constant through the transition process but depends effectively on
the fraction of already broken bonds. A transformation formula is obtained
which relates the temperature of ccDNA at a given degree of helicity during the
transition to the temperature of the corresponding linear chain at the same
degree of helicity. The formula provides a simple method to calculate the
melting curve for the ccDNA from the experimental melting curve of the linear
DNA with the same nucleotide sequence.
