Question

In a DNA molecule, if the percentage of adenine (A) is 30\%, what is the percentage of cytosine (C)?

• A. 20\%
• B. 30\%
• C. 40\%
• D. 50\%

Hint:

To find base percentages in DNA, use Chargaff’s rules: \( A = T \) and \( G = C \). If given the percentage of one base, calculate its pair’s percentage, subtract from 100%, and split the remainder equally between the other pair.

Answer 1

The Correct Option is A

In a DNA molecule, base pairing follows Chargaff’s rules: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Therefore, the percentage of adenine equals the percentage of thymine (\( A = T \)), and the percentage of guanine equals the percentage of cytosine (\( G = C \)). The total percentage of all bases sums to 100%:
\[ A + T + G + C = 100\% \] Given that the percentage of adenine is 30%:
\[ A = 30\%, \quad T = 30\% \quad (\text{since } A = T) \] \[ A + T = 30\% + 30\% = 60\% \] The remaining percentage is for guanine and cytosine:
\[ G + C = 100\% - 60\% = 40\% \] Since \( G = C \):
\[ G = C = \frac{40\%}{2} = 20\% \] Thus, the percentage of cytosine (C) is 20%.