Question

If adenine constitutes 30% of the bases in a DNA molecule, what percentage of the bases is guanine?

• A. 30%.
• B. 40%.
• C. 20%.
• D. 25%.

Hint:

Chargaff's rule states that in a double-stranded DNA molecule, \( A = T \) and \( G = C \). Use this to calculate the percentages of bases when given the percentage of one base.

Answer 1

The Correct Option is C

Step 1: Understanding Chargaff's rule. 
According to Chargaff's rule, in a double-stranded DNA molecule: \[ \text{The amount of adenine (A)} = \text{The amount of thymine (T)}, \] and \[ \text{The amount of guanine (G)} = \text{The amount of cytosine (C)}. \] Thus, the sum of adenine and thymine is equal to the sum of guanine and cytosine: \[ A + T + G + C = 100\%. \] Step 2: Calculating the percentage of guanine. 
Given that adenine constitutes 30% of the bases, thymine must also constitute 30%: \[ A + T = 30% + 30% = 60%. \] This leaves 40% for guanine and cytosine: \[ G + C = 100% - 60% = 40%. \] Since the amount of guanine equals the amount of cytosine: \[ G = C = \frac{40\%}{2} = 20%. \] Step 3: Conclusion. 
The percentage of guanine in the DNA molecule is 20%. \[ \therefore \text{The correct answer is: 20\%}. \]