DNA Molarity:
From A260 to Answers
Why A260 Works
Nucleic acids absorb UV light at 260 nm because of the conjugated ring systems in the bases. This is not arbitrary. The pi-electron systems in purines and pyrimidines have electronic transitions that happen to fall in the UV range, and the peak absorption sits at approximately 260 nm for both DNA and RNA. This physical property gives us a direct, non-destructive way to estimate concentration from a spectrophotometer reading.
The relationship between absorbance and concentration is governed by the Beer-Lambert law: A = epsilon * c * l, where epsilon is the extinction coefficient, c is the concentration, and l is the path length. For double-stranded DNA, the commonly used conversion is that an A260 of 1.0 corresponds to approximately 50 ug/mL. For single-stranded DNA it is approximately 33 ug/mL, and for RNA approximately 40 ug/mL. These are approximations based on average base composition, and they hold well for most practical purposes.
Getting Molarity from Mass Concentration
The A260 reading gives you a mass concentration (ug/mL or ng/uL). To convert this to molarity, you need to know the molecular weight of your nucleic acid. For a known sequence, this is straightforward. For double-stranded DNA, the average molecular weight per base pair is approximately 649 Da, so a 1000 bp fragment has a molecular weight of roughly 649,000 Da. Divide your mass concentration by the molecular weight, and you have your molar concentration.
Where this gets practically important is in cloning. Ligation reactions require insert and vector at specific molar ratios, not mass ratios. A 500 bp insert and a 5000 bp vector at equal mass are at a 10:1 molar ratio, not 1:1. Getting this wrong is one of the most common reasons cloning fails, and it starts with not converting your A260 reading to molarity.
Purity Ratios
The A260 reading alone tells you concentration, but not purity. The A260/A280 ratio indicates protein contamination (pure DNA should be approximately 1.8, pure RNA approximately 2.0), and the A260/A230 ratio indicates contamination from salts, phenol, or carbohydrates (ideally 2.0 to 2.2). Low ratios suggest your concentration measurement is inflated by contaminants absorbing at 260 nm, which means your actual nucleic acid concentration is lower than the number suggests.
What BenchCalc Does
BenchCalc includes a DNA molarity calculator that handles the full conversion from A260 readings to both mass concentration and molar concentration, accounting for your specific nucleic acid type and length. It also includes tools for primer Tm calculation, primer resuspension, and agarose gel preparation, covering the nucleic acid workflow from quantification through to analysis.