A plasmid of 6.5 kb is isolated and its concentration is 250 µg ml⁻¹. Calculate the number of plasmid copies present in 2 ml of this preparation. (Use 1 bp = 660 Da and Avogadro's number = 6.022 × 10²³ mol⁻¹.)
Answer
Approximately plasmid copies.
The correct answer is Approximately plasmid copies. — Convert moles to molecules using Avogadro's number: molecules = 1.17 × 10⁻¹⁰ mol × 6.022 × 10²³ mol⁻¹ ≈ 7.0 × 10¹³.
Step-by-step solution
- Calculate the molecular weight of the plasmid: 6.5 kb = 6500 bp; MW = 6500 × 660 Da = 4.29 × 10⁶ g mol⁻¹.
- Convert the total DNA mass in 2 ml: 250 µg ml⁻¹ × 2 ml = 500 µg = 5.0 × 10⁻⁴ g.
- Find the number of moles: moles = mass / MW = 5.0 × 10⁻⁴ g ÷ 4.29 × 10⁶ g mol⁻¹ = 1.17 × 10⁻¹⁰ mol.
- Convert moles to molecules using Avogadro's number: molecules = 1.17 × 10⁻¹⁰ mol × 6.022 × 10²³ mol⁻¹ ≈ 7.0 × 10¹³.
Common mistake: Failing to convert µg to grams before the mole calculation
Concept tested: Calculating plasmid copy number from DNA mass
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