DNA, the double-stranded molecule, has the unique ability to replicate itself. That is, DNA can make exact copies of itself. This is important, for instance, for the function of biological reproduction and cell growth. The replication function occurs when the hydrogen bond that exists between nucleotide bases is broken, which results in the two DNA strands existing as separate single strands of DNA.
The cell then makes available two new complementary bases (the original nucleotide base having broken hydrogen bonds) that are paired with the two single DNA strands. The result is the formation of two new double-stranded DNA molecules built from the original but separated strands plus new bases.