DNA incubated above 90o C, raised to a pH of 10.5 or greater, or mixed with formamide, urea, or other organic compounds will denature, since the hydrogen bonds between complementary base pairs break.  This separates the duplex (double-stranded) molecule into two single strands.

The two strands will renature given the proper temperature, pH, and salt conditions.  This is called DNA hybridization.

If separate DNA nucleosides from different sources are combined, they will hybridize under "high stringency" conditions only if all of the nucleotide sequences on each are complementary.  Under "low stringency" conditions, partial hybridization can occur between single strands having less complementarity.