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BIO NMDCAT Structure of DNA
nmdcat.online July 3, 2026

21. The primary reason why a purine base cannot pair with another purine base inside a stable DNA double helix is that such an orientation would

A. Break the phosphodiester bonds
B. Cause a local distortion that widens the helix beyond 2 nanometers
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C. Prevent the formation of any hydrogen bonds
D. Force the DNA to become a single-stranded RNA

📝 Explanation

Purines are double-ringed; pairing two purines would exceed the 2 nm diameter, while pairing two pyrimidines would make the helix too narrow.

📖 Additional Information

  • Break the phosphodiester bonds
  • Cause a local distortion that widens the helix beyond 2 nanometers
  • Prevent the formation of any hydrogen bonds
  • Force the DNA to become a single-stranded RNA

Purines are double-ringed; pairing two purines would exceed the 2 nm diameter, while pairing two pyrimidines would make the helix too narrow.

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