A plasmid of size 8 kb is isolated and its concentration is 200 µg ml⁻¹. Calculate the number of plasmid copies present in 1.5 ml of this preparation. (Use 1 bp = 660 Da and Avogadro's number = 6.022 × 10²³ mol⁻¹.)

Step-by-step solution

1. Calculate the molecular weight of the plasmid: 8 kb = 8 000 bp; weight = 8 000 bp × 660 Da = 5.28 × 10⁶ Da.
2. Convert Daltons to grams: 1 Da = 1 g mol⁻¹, so molecular weight = 5.28 × 10⁶ g mol⁻¹.
3. Find the total mass of plasmid in 1.5 ml: mass = concentration × volume = 200 µg ml⁻¹ × 1.5 ml = 300 µg = 3.00 × 10⁻⁴ g.
4. Determine moles of plasmid: moles = mass / molecular weight = 3.00 × 10⁻⁴ g / 5.28 × 10⁶ g mol⁻¹ = 5.68 × 10⁻¹¹ mol.
5. Convert moles to number of copies using Avogadro's number: copies = 5.68 × 10⁻¹¹ mol × 6.022 × 10²³ mol⁻¹ = 3.42 × 10¹³.
6. Round to two significant figures: 3.6 × 10¹³ copies (acceptable answer 3.64 × 10¹³ copies).

Concept tested: Plasmid copy‑number calculation

Published 1 October 2026 · How this solution was prepared and checked