GATE 2021 BT – Question 63
DNA at 50 μg/mL has A260=1.0. A 20 μL aliquot from 50 μL purified plasmid is diluted to 1000 μL and gives A260=0.550. Purified plasmid concentration (two decimal places μg/μL) is _______.
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Correct answer: 1.37 to 1.39
Explanation
**Step 1: concentration of the diluted sample.** A double-stranded DNA solution with concentration 50 μg/mL has $A_{260}=1.0$. The diluted sample has $A_{260}=0.550$:
$$C_{diluted}=0.550\times50=27.5\ \mu\text{g/mL}.$$
**Step 2: undo the dilution.** 20 μL was diluted to 1000 μL, a dilution factor of $1000/20=50$:
$$C_{plasmid}=27.5\times50=1375\ \mu\text{g/mL}.$$
**Step 3: convert units.**
$$1375\ \mu\text{g/mL}=1.375\ \mu\text{g/}\mu\text{L}\approx\mathbf{1.38\ \mu g/\mu L}.$$
(The original volume of 50 μL is not needed for the concentration; it would give the total yield: $1.375\times50=68.75$ μg.)