Reversibility of partial denaturation of DNA

The hypochromicity recovery of a partially denatured DNA sample when salt concentration is suddenly increased at an intermediate stage of the transition, is studied. The results of CsC1 gradient analysis, polyethylenglycol/dextran partition analysis, and the behaviour in a new thermal transition, support the view that the process is an intramolecular double chain renaturation as that induced by cooling. The degree of denaturation irreversibility is dependent on the size of DNA segments between two nicks and on the helicity value at which salt concentration-jump is performed.