Adenine pairs with thymine because

Analyze the question: It asks why adenine (A) pairs with thymine (T) in DNA.

Concept: DNA nitrogenous bases fall into two groups:

• Purines (double-ring): Adenine (A) and Guanine (G)
• Pyrimidines (single-ring): Thymine (T), Cytosine (C), and Uracil (U, in RNA)

In the DNA double helix, each "rung" of the ladder must have a constant width. This happens when a double-ring purine pairs with a single-ring pyrimidine.

• A pairs with T through 2 hydrogen bonds
• G pairs with C through 3 hydrogen bonds

Checking the options:

• A. Both are in the same nucleic acid. This is true, but it is not the reason for pairing, since all four bases occur in DNA. ❌
• B. Strong vs. weak base is not the basis of base pairing. ❌
• C. Two purines pairing would make the rung too wide and distort the helix. Also, A and T are not both purines. ❌
• D. One is a purine (adenine) and the other a pyrimidine (thymine). This gives the correct width and allows hydrogen bonding. ✅

Answer: The correct option is D.

Exam tip: Remember the phrase "Pure As Gold": Purines = Adenine and Guanine. Also, "CUT the Py": Cytosine, Uracil, Thymine are Pyrimidines. Then remember that a purine always pairs with a pyrimidine: A-T and G-C.