Question

In a DNA molecule, cytosine is 28%. Calculate the percentage of adenine.

• A. 56%
• B. 36%
• C. 22%
• D. 18%

Hint:

Quick mathematical rule for base pairing percentages: Any single purine + any single pyrimidine pair (e.g., A + C or T + G) always adds up to 50%. If C = 28%, then A = 50% - 28% = 22%.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept: According to Chargaff's rules for double-stranded DNA, the amount of adenine (A) strictly equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C).
Step 2: Key Formula or Approach:
Use the mathematical formula: \[ A + T + G + C = 100% \] Since \( A = T \) and \( G = C \), we can deduce: \[ 2A + 2C = 100% \implies A + C = 50% \] Step 3: Detailed Explanation:
We are given that Cytosine (\(C\)) constitutes 28% of the bases.
According to Chargaff's rule, Guanine (\(G\)) must also be 28%.
The total combined percentage of G + C is \(28% + 28% = 56%\).
The remaining percentage must be completely made up of Adenine and Thymine.
Total percentage of A + T = \(100% - 56% = 44%\).
Since Adenine equals Thymine (\(A = T\)), the percentage of Adenine alone is exactly half of this remainder.
\[ \text{Adenine} = \frac{44%}{2} = 22% \] Alternatively, using the direct shortcut formula derived in step 2: \( A = 50% - C = 50% - 28% = 22% \).
Step 4: Final Answer: The percentage of adenine is 22%.