A student compares the nitrogenous bases in DNA and RNA. In DNA, adenine pairs with thymine; in RNA, adenine pairs with uracil. Both pairings use hydrogen bonds between specific functional groups on the bases. Which statement best explains why uracil can replace thymine in RNA base pairing with adenine?
- Uracil and thymine present similar hydrogen-bonding patterns to adenine, enabling complementary pairing. (correct answer)
- Uracil is a purine like adenine, so purine–purine pairing preserves uniform helix width in RNA.
- Uracil forms three hydrogen bonds with adenine, compensating for RNA being single-stranded.
- Thymine lacks hydrogen-bonding groups, so RNA must use uracil to allow any base pairing.
- Uracil covalently bonds to adenine, allowing RNA duplexes to remain intact during heating.
Explanation: This question tests the skill of nucleic acid structure–function analysis. Uracil and thymine both have similar hydrogen-bonding patterns, with two hydrogen bonds forming with adenine via specific keto and amino groups, allowing uracil to functionally replace thymine in RNA without disrupting pairing specificity. This similarity arises from their pyrimidine structures, where thymine's extra methyl group in DNA provides stability against deamination but is not essential for bonding in RNA. Thus, the base pairing in RNA maintains the same complementary logic as DNA, supporting processes like transcription and translation. A tempting distractor is choice C, which claims uracil forms three hydrogen bonds with adenine, but this misconceives the bond count (two for A-U, three for G-C) and overestimates pairing strength. When analyzing base substitutions across nucleic acids, compare functional groups involved in hydrogen bonding to predict pairing compatibility.