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.
Chargaff's rule states that in double-stranded DNA, the quantity of adenine (A) equals the quantity of thymine (T), and the quantity of guanine (G) equals the quantity of cytosine (C). This implies that the sum of adenine and thymine is equivalent to the sum of guanine and cytosine, with the total composition being 100%: \[ A + T + G + C = 100\%. \] Step 2: Calculating Guanine Percentage. 
If adenine is 30% of the bases, then thymine must also be 30%: \[ A + T = 30\% + 30\% = 60\%. \] Consequently, the remaining 40% is allocated to guanine and cytosine: \[ G + C = 100\% - 60\% = 40\%. \] Given that guanine and cytosine amounts are equal: \[ G = C = \frac{40\%}{2} = 20\%. \] Step 3: Conclusion. 
The DNA molecule contains 20% guanine. \[ \therefore \text{The correct answer is: 20\%}. \]