Question

dna molecules are made of units called nucleotides. the nucleotides combine to form the structure shown.
dna molecule
the part of the dna molecule indicated by the arrow consists of —
a adenine and guanine
b hydrogen bonds
c sugars and phosphates
d nitrogenous bases

Explanation:

To solve this, we recall the structure of DNA. The DNA double helix has a sugar - phosphate backbone (the outer strands) and nitrogenous bases (the rungs of the ladder - like structure in the middle). The arrow in the diagram points to the rungs of the DNA “ladder”.

• Option A: Adenine and guanine are types of nitrogenous bases, but the rungs are made of pairs of nitrogenous bases (e.g., A - T, G - C), not just two specific bases like adenine and guanine in isolation as the structure at the arrow.
• Option B: Hydrogen bonds are what hold the nitrogenous base pairs together, not the structure of the rungs themselves.
• Option C: Sugars and phosphates make up the backbone (the sides of the ladder), not the rungs.
• Option D: The rungs of the DNA molecule (the part indicated by the arrow) are made of nitrogenous bases (adenine, thymine, cytosine, guanine) that pair up.

Answer:

D. nitrogenous bases