All 4 of the DNA bases absorb ultraviolet light, with an average maximum absorption near 260 nm. The decrease in absorbance observed upon forming the duplex is called the hypochromic shift and can be used to monitor denaturation. A denaturation, or melting, profile of a DNA solution is illustrated below. The temperature at which half the DNA has become single-stranded is called its melting temperature, or T m , and this is a measure of the stability of the duplex under the specific conditions of the experiment. Once the strands are separated, if the conditions are returned to ones that favor duplex formation, the strands can reassociate.
Incorrect collisions are unstable and rapidly fall apart.
Renaturation is the basis of nucleic acid hybridization, which we will use later in several techniques. The basic idea is that we can isolate specific DNA sequences and mark them with radioactive or specific chemical labels, generating what is called a probe. When the strands of the probe are separated, they will be able to bind to complementary sequences, either the original binding partners or the same sequences found in any other DNA molecules whose source interests us. Using procedures that locate the label on the probe, we can detect those specific sequences in the DNA sample of interest. This capability is at the heart of all DNA diagnostic methods.