Question

A DNA strand has a length with 12 full turns. Then identify the number of Nitrogen bases in total

• A. 120
• B. 200
• C. 240
• D. 480

Hint:

Always distinguish between "base pairs" and "individual nitrogen bases".
Total nitrogen bases = \( 2 \times \text{number of base pairs} \).

Answer 1

The Correct Option is C

Step 1: Recall the basic DNA helix numbers.
In the standard Watson and Crick B-DNA model, one full turn of the helix always contains 10 base pairs, this is a fixed structural constant you can rely on directly.
Step 2: Scale it up to 12 turns.
$12 \times 10 = 120$ base pairs are present across the whole stretch of DNA described in the question.
Step 3: Convert base pairs into individual bases.
Every base pair is made of two bases, one on each strand, so the total number of nitrogenous bases is $120 \times 2 = 240$.
Final answer: Option 3, 240 nitrogenous bases in total.