Explanation

Out of A-T-, G-C pairing, bases of DNA may exist in alternate valency state owing to arrangement called tautomerisational mutation. It involves presence of tautomeric forms of nitrogen bases, $e.g.,$ imino tautomer instead of amino group ($i.e.,$ cytosine-adenine) or enol group instead of keto group ($i.e.,$ thymine-guanine).