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 Problem:
In a double-stranded DNA molecule, nitrogenous bases always pair specifically. According to Chargaff's rules, Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C).
Step 2: Approach and Formula:
Apply Chargaff's formulas:
\( A = T \) and \( G = C \)
Total percentage: \( A + T + G + C = 100% \)
Step 3: Detailed Explanation:
Given that the percentage of Cytosine (\( C \)) = 28%.
According to the rule, the percentage of Guanine (\( G \)) must be equal to Cytosine.
\[ G = 28% \] The combined total percentage of Guanine and Cytosine is:
\[ G + C = 28% + 28% = 56% \] The remaining percentage of the entire DNA molecule consists of Adenine and Thymine:
\[ A + T = 100% - 56% = 44% \] Since the amounts of Adenine and Thymine are always equal (\( A = T \)), we divide the remainder by 2:
\[ 2A = 44% \implies A = \frac{44%}{2} = 22% \] Therefore, the percentage of Adenine is 22%.
Step 4: Final Answer:
The percentage of Adenine is 22%.