A consistent theory of DNA unwinding under the action of formaldehyde which makes possible calculation of all characteristics of the process based on a given nucleotide sequence has been developed. The kinetic parameters are obtained with the use of the theory of helix-coil transition in DNA. These parameters are then employed for obtaining the final characteristics of the unwinding process by formaldehyde, the kinetic curves of unwinding (and their anamorphoses) and the maps of denaturation. The calculations which were performed for heteropolynucleotides with random sequences and with different concentrations of defects revealed that a simple theory previously developed for the homopolymer case, is also valid for the heteropolymer. This result agrees with the experimental data and justifies, from the theoretical point of view, applications of the kinetic formaldehyde method to the natual DNAs. Denaturation maps have also been calculated which demonstrated that a purely random sequence proved to give only slightly noticeable singularities. However, the insertion of the AT dusters (the regions consisting of 20--40 AT pairs) into the random sequence gives rise to the clear-cut peaks on the maps. The magnitude of such a peak drops sharply if only a few GC pairs are present within the AT cluster. Electron microscopic malSping data obtained by the Inman method, have been discussed on the basis of the above theoretical results. This consideration allows one to attribute the sharp peaks on the denaturation maps to T7 phage DNA as originated from the AT dusters.