Which base-pairing pattern is correct across the center of the DNA helix?

Which base-pairing pattern is correct across the center of the DNA helix?

• A. Purines pair with purines
• B. Pyrimidines pair with purines
• C. Purines pair with purines and pyrimidines pair with pyrimidines
• D. Pyrimidines pair with pyrimidines

Answer: (C)

Base pairing across the DNA helix keeps the width of the molecule constant by always pairing a purine with a pyrimidine. Purines (two-ring structures) are larger, while pyrimidines (one-ring) are smaller, so pairing one of each maintains the uniform diameter of the double helix. This is why A pairs with T and G pairs with C in DNA, with two and three hydrogen bonds respectively. In short, the pattern across the center is purines paired with pyrimidines.

Base pairing across the DNA helix keeps the width of the molecule constant by always pairing a purine with a pyrimidine. Purines (two-ring structures) are larger, while pyrimidines (one-ring) are smaller, so pairing one of each maintains the uniform diameter of the double helix. This is why A pairs with T and G pairs with C in DNA, with two and three hydrogen bonds respectively. In short, the pattern across the center is purines paired with pyrimidines.