Question

If a double-stranded DNA has 15% of adenine, find out the percent of cytosine in the DNA?

• A. 15%
• B. 30%
• C. 35%
• D. 85%

Hint:

In double-stranded DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine (Chargaff's rule).

Answer 1

The Correct Option is C

In double-stranded DNA, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C). This is known as Chargaff's rule.
Step 1: Understanding the base pairing.
- If 15% of the DNA is adenine (A), then 15% is thymine (T).
- The total percentage of adenine and thymine is \( 15% + 15% = 30% \).
Step 2: Calculating the remaining percentage.
Since the total percentage must add up to 100%, the remaining 70% must be equally divided between guanine (G) and cytosine (C), as they are complementary.
- \( 70% \div 2 = 35% \) for cytosine (C) and guanine (G).
Step 3: Conclusion.
The percentage of cytosine (C) in the DNA is 35%.
Final Answer: \[ \boxed{\text{The correct answer is (3) 35%.}} \]