The noncovalent hydrogen bonds between the nucleotide base pairs are weak.
The two complementary strands can be separated (denaturation) by means of relatively weak chemical reagents (e.g., alkali, formamide, or urea) or by careful heating. The resulting single-stranded molecules are relatively stable.
With cooling, complementary single strands can reunite to
form double-stranded molecules (renaturation). Noncomplementary single strands do not unite.