The rings of the bases are made up of alternating single and double bonds. Such systems absorb in the u.v., or if the system is large enough, in the visible spectrum. Each of the four nucleotide bases has a slightly different absorption spectrum, and the spectrum of DNA is the average of them. A pure DNA solution appears transparent to the eye, and absorption doesn't become measurable until 320 nm. Moving further into the u.v. region, there is a peak at about 260 nm, followed by a dip between 220 and 230, and then the solution becomes essentially opaque in the far u.v. A solution of double stranded, native DNA, with a concentration of 0.04 mg/mL has an absorbance (or optical density, OD) of about 1.0 at the 260 nm peak. The absorbance of DNA in the u.v. is the reason u.v. radiation can be used to sterilize: the absorbed energy destroys the DNA and kills the organism. It is also the reason u.v. causes skin cancer: the absorbed radiation causes mutations which inactivate regulatory mechanisms needed to control cell division.     When a DNA helix is denatured to become single strands, e.g. by heating, the absorbance is increased about 30 percent. This increase, called the hyperchromic effect, reveals the interaction between the electronic dipoles in the stacked bases of the native helix.