In the DNA double helix, what components form the “rungs” of the molecular ladder?

• A. Alternating deoxyribose sugars and phosphate groups linked by phosphodiester bonds
• B. Paired phosphate groups held together by ionic bonds
• C. Stacks of amino acids connected by peptide bonds
• D. Paired nitrogenous bases (A with T, and G with C) held together by hydrogen bonds

Answer: (D)

Verified step by step guidance

1. Recall the structure of the DNA double helix, which resembles a twisted ladder with two main components: the sugar-phosphate backbone and the rungs connecting the two strands.
2. Identify that the sugar-phosphate backbone consists of alternating deoxyribose sugars and phosphate groups linked by phosphodiester bonds, forming the sides of the ladder, not the rungs.
3. Understand that the rungs of the ladder are formed by pairs of nitrogenous bases, specifically adenine (A) pairing with thymine (T), and guanine (G) pairing with cytosine (C).
4. Recognize that these base pairs are held together by hydrogen bonds, which provide the necessary stability and specificity for base pairing in the DNA double helix.
5. Conclude that the correct components forming the rungs of the DNA ladder are the paired nitrogenous bases connected by hydrogen bonds, not the sugar-phosphate backbone or amino acids.