Hi Appu, Adding the extinction coefficients of the nucleotides is a reasonable approximation; a somewhat more rigorous approximation takes into account nearest neighbor frequencies; see, for example, some commercial web sites such as http://www.operon.com/products/custom_oligos/calculations.aspx.
However, these calculation only apply to unstructured ssDNA. Base stacking in dsDNA causes a decrease of 25-35% in UV absorbance (hypochromicity), an effect that was used in the old days to monitor DNA denaturation. So you should measure your oligos as ssDNA before annealing them, or denature the DNA by boiling or diluting into NaOH solution. Finally, beware of UV-absorbing contaminants in your DNA solution: nucleotides, protein, oxidized DTT... Opher
