Question

A DNA solution of 50 \(\mu\)g mL\(^{-1}\) gives absorbance of 1.0 at 260 nm. A 20 \(\mu\)L aliquot from a 50 \(\mu\)L purified plasmid solution is diluted to 1000 \(\mu\)L. The diluted solution gives absorbance 0.550 at 260 nm. The purified plasmid concentration in \(\mu\)g \(\mu\)L\(^{-1}\) (rounded to two decimals) is \(\underline{\hspace{1cm}}\).

Hint:

Remember: 1 mL = 1000 \(\mu\)L, so converting concentrations requires dividing by 1000.

Answer 1

Correct Answer: 1.37

DNA concentration is proportional to absorbance:
\[ C_{\text{diluted}} = 50 \times 0.550 = 27.5\ \mu g\ mL^{-1} \]
Dilution factor:
\[ DF = \frac{1000}{20} = 50 \]
Thus original plasmid concentration:
\[ C_{\text{original}} = 27.5 \times 50 = 1375\ \mu g\ mL^{-1} \]
Convert to \(\mu g\ \mu L^{-1}\):
\[ 1375\ \mu g\ mL^{-1} = 1.375\ \mu g\ \mu L^{-1} \]
Rounded to two decimals: \[ 1.38\ \mu g\ \mu L^{-1} \]