Which relationship will be found in the percentages of bases of a double-stranded DNA molecule?

• A. All four bases are present in equal percentages.
• B. The percentage of adenine equals the percentage of cytosine, and the percentage of guanine equals the percentage of thymine.
• C. The percentage of adenine equals the percentage of guanine, and the percentage of thymine equals the percentage of cytosine.
• D. The percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine.

Answer: (D)

Verified step by step guidance

1. Understand the structure of DNA: DNA is a double-stranded molecule composed of nucleotide bases. The four bases are adenine (A), thymine (T), guanine (G), and cytosine (C). These bases pair specifically due to hydrogen bonding: adenine pairs with thymine, and guanine pairs with cytosine.
2. Recall Chargaff's rule: Chargaff discovered that in a double-stranded DNA molecule, the amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine. This is because of the base-pairing rules.
3. Analyze the percentages: Since adenine pairs with thymine, their percentages in the DNA molecule will be equal. Similarly, guanine pairs with cytosine, so their percentages will also be equal. This relationship ensures the complementary nature of the DNA strands.
4. Eliminate incorrect options: The first option stating all four bases are present in equal percentages is incorrect because base pairing does not require equal amounts of all four bases. The second and third options incorrectly pair adenine with cytosine and guanine with thymine, which contradicts the base-pairing rules.
5. Conclude the correct relationship: The correct relationship is that the percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine. This is consistent with Chargaff's rule and the structure of DNA.