Denaturation and Renaturation of DNA

The first clue that eukaryotic DNA contains several types of sequences not present in prokaryotic DNA came from studies in which double-stranded DNA was separated and then allowed to reassociate. When double-stranded DNA in solution is heated, the hydrogen bonds that hold the two nucleotide strands together are weakened, and with enough heat, the two strands separate completely, a process called denaturation or melting. The temperature at which DNA denatures, called the melting temperature (Tm), depends on the base sequence of the particular sample of DNA: G–C base pairs have three hydrogen bonds, whereas A–T base pairs have only two, so the separation of G–C pairs requires more heat (energy) than does the separation of A–T pairs. The denaturation of DNA by heating is reversible: if single-stranded DNA is slowly cooled, single strands will collide and hydrogen bonds will form again between complementary base pairs, producing double-stranded DNA. This reaction is called renaturation or reannealing.