Nucleic acids absorb UV light of 250 to 270nm wavelength, with a maximum at
260nm. The reading at 260nm allows calculation of the concentration of
nucleic acid in the sample. An OD of 1
corresponds to approximately 50 
g/ml for
double-stranded DNA and 40 g/ml for single-stranded DNA and RNA.
The ratio between the readings at 260nm and 280nm (A
ratio) provides an estimate for the purity of the nucleic acid. Pure
preparations of DNA and RNA have ratios of 1.8 to 1.95, respectively.
DNA producing a ratio below 1.6 should be
re-purified
.