[Campbell Biology P.369] Which of the following statements accurately describes a structural... | Practice Question

Which of the following statements accurately describes a structural feature of the DNA double helix, as presented in the text?

  • A: Each full turn of the helix is 0.34 nm long and contains 10 base pairs.
  • B: The sugar-phosphate backbones form the 'rungs' of the ladder, while nitrogenous bases form the 'sides'.
  • C: The double helix has a uniform diameter because a purine always pairs with a pyrimidine.
  • D: There are 3.4 base pairs per full turn of the helix.

Explanation

The text states that 'Pairing a purine with a pyrimidine is the only combination that results in a uniform diameter for the double helix'. This is why initial 'like-with-like' pairing models e.g., A with A, C with C did not fit X-ray data. A full turn is 3.4 nm long, contains 10 base pairs, and the sugar-phosphate backbones form the sides, with nitrogenous bases forming the rungs.