Chargaff's rule also implies that the amounts of guanine and cytosine are equal in DNA. Which other pair is equal?

Chargaff's rule also implies that the amounts of guanine and cytosine are equal in DNA. Which other pair is equal?

• A. Adenine and Thymine
• B. Adenine and Cytosine
• C. Thymine and Guanine
• D. Adenine and Guanine

Answer: (A)

Bases in DNA pair in a specific way, and that pairing keeps each pair’s amounts balanced across the molecule. Adenine always pairs with thymine, and cytosine always pairs with guanine. Because of this complementary pairing, the total number of adenine bases equals the number of thymine bases, just as cytosine equals guanine. So the other equal pair is adenine and thymine. If you imagine a DNA sample with a certain percentage of A, it must have the same percentage of T, while C and G balance each other. The pairing A–T, not A–C or T–G, drives this equality.

Bases in DNA pair in a specific way, and that pairing keeps each pair’s amounts balanced across the molecule. Adenine always pairs with thymine, and cytosine always pairs with guanine. Because of this complementary pairing, the total number of adenine bases equals the number of thymine bases, just as cytosine equals guanine. So the other equal pair is adenine and thymine.

If you imagine a DNA sample with a certain percentage of A, it must have the same percentage of T, while C and G balance each other. The pairing A–T, not A–C or T–G, drives this equality.