Q.30 A single base pair of DNA weighs
1.1 × 10−21 grams. How many picomoles of a plasmid vector of
length 2750 bp are contained in 1 μg of purified DNA?
Calculation of Picomoles of Plasmid DNA from DNA Mass
Calculating the number of picomoles of plasmid DNA from a given DNA mass
is an essential concept in molecular biology and
genetic engineering. This calculation is widely used in cloning,
ligation reactions, and competitive exams such as GATE and
CSIR-NET.
Given Data
- Weight of one DNA base pair = 1.1 × 10-21 g
- Length of plasmid vector = 2750 bp
- Total DNA mass = 1 µg = 1 × 10-6 g
Step-by-Step Calculation
Step 1: Weight of one plasmid molecule
Weight = 2750 × 1.1 × 10-21 g
= 3.025 × 10-18 g
Step 2: Number of plasmid molecules in 1 µg DNA
Number of molecules =
(1 × 10-6) / (3.025 × 10-18)
= 3.31 × 1011 molecules
Step 3: Convert molecules to moles
Using Avogadro’s number (6.022 × 1023)
Moles = (3.31 × 1011) / (6.022 × 1023)
= 5.5 × 10-13 moles
Step 4: Convert moles to picomoles
1 pmol = 10-12 moles
= 0.55 pmol
Correct Answer
Option (B): 0.55
Key Concept Summary
- DNA mass can be converted to molar quantity using base-pair weight
- Picomoles are commonly used in cloning and ligation calculations
- Accurate unit conversion is essential in molecular biology
Conclusion
The number of picomoles of a 2750 bp plasmid vector present in
1 µg of purified DNA is 0.55 pmol. Therefore,
Option (B) is the correct answer.
