During the process of DNA isolation, the DNA sample most of the time get contaminated with RNA and protein. To avoid such contamination proteinases and Rnases should be used during isolation process to degrade proteins and RNAs respectively.
After isolation the amount of DNA obtained is estimated by Spectrophotometer. Readings are taken at wavelengths 260nm and 280nm. The ratio between the readings at 260nm and 280nm provide the purity of DNA sample. If the ratio is approximately 1.8 then the DNA sample is pure. But if the ratio is more than 1.8 then there might be contamination of RNA and if the ratio is less than 1.8 then there might be contamination of protein.