Question

In a DNA molecule, if 30% of the bases are adenine, what will be the percentage of cytosine bases

• A. 30
• B. 25
• C. 60
• D. 20

Hint:

To find base percentages quickly, use the relationship:
\[ %A + %C = 50% \] Since we are given \(%A = 30%\), we can calculate %C directly:
\[ %C = 50% - 30% = 20% \]

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Erwin Chargaff formulated rules regarding the base composition of double-stranded DNA molecules.
According to Chargaff's rules, the concentration of purines equals the concentration of pyrimidines, and specific base pairing dictates equal ratios of adenine to thymine, and guanine to cytosine.

Step 2: Key Formula or Approach:
For any double-stranded DNA molecule:
\[ %A = %T \] \[ %G = %C \] Since the total percentage of all bases must equal \(100%\):
\[ %A + %T + %G + %C = 100% \]

Step 3: Detailed Explanation:
Let us calculate the percentage of cytosine bases using the given information:
1. We are given that the percentage of adenine (%A) is \(30%\).
2. According to Chargaff's rule, the percentage of thymine (%T) must also be \(30%\):
\[ %T = %A = 30% \] 3. The combined percentage of adenine and thymine is:
\[ %A + %T = 30% + 30% = 60% \] 4. The remaining percentage of the genome must consist of guanine and cytosine:
\[ %G + %C = 100% - 60% = 40% \] 5. Since the percentage of guanine equals the percentage of cytosine (%G = %C), we divide the remaining percentage equally:
\[ %C = \frac{40%}{2} = 20% \] Therefore, the percentage of cytosine bases is \(20%\).

Step 4: Final Answer:
The percentage of cytosine bases is 20, matching Option (D).