Once DNA is isolated, it must be quantified. There are two common methods for quantifying the amount of DNA present in a solution. A spectrophotometer measures the amount of 260 nm light that shines through the solution. The absorbance is based on the ratio of transmitted light to incident light. Based on this, a concentration of DNA is calculated. Unfortunately, a spectrophotometer cannot distinguish between DNA and RNA, so the reported concentration includes all nucleic acids. Gel electrophoresis is quite accurate in telling size of DNA chains, but concentration of DNA of particular sizes in solution may also be inferred. A ladder is loaded in the first lane of the gel in order to compare the samples to. The darkness of the samples is compared to the ladder to estimate concentration. Gel electrophoresis utilizes the fact that the phosphate groups of DNA nucleotides give the molecule an overall negative charge. An applied electric field causes DNA to move down the gel towards the positively charged electrode. Smaller DNA molecules will migrate faster than bigger ones, and will appear lower on the gel. Results from these two methods can be analyzed to draw conclusions about the true concentration of DNA in solution.
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