in the body, DNA is “unzipped” by an enzyme called DNA helicase, which requires energy to function, so really energy is put into the system to allow the DNA to split. If left alone at physiological temperature it is reasonable to assume that it never would. It is also possible to denature DNA by heating, as in Polymerase Chain Reaction, or PCR, which is used in the lab to rapidly copy strands of DNA for various applications.