According to Chargaff's rules, which statement is true about the base composition of DNA?

According to Chargaff's rules, which statement is true about the base composition of DNA?

• A. Adenine pairs with cytosine
• B. The amounts of adenine and thymine are equal, and the amounts of cytosine and guanine are equal
• C. All four bases are present in equal amounts
• D. Thymine equals cytosine

Answer: (B)

Chargaff’s rules come from the way DNA strands pair up. In double-stranded DNA, adenine always pairs with thymine and cytosine always pairs with guanine, so the numbers of A and T are equal and the numbers of C and G are equal. This complementary pairing fixes the composition on each strand and across the molecule, even though the total amounts of A+T versus C+G can vary between species. So the statement that adenine and thymine are present in equal amounts, and cytosine and guanine are present in equal amounts, exactly reflects this pairing pattern. The other ideas—A pairing with C, all four bases in equal amounts, or thymine equaling cytosine—don’t align with how base pairing works in DNA.

Chargaff’s rules come from the way DNA strands pair up. In double-stranded DNA, adenine always pairs with thymine and cytosine always pairs with guanine, so the numbers of A and T are equal and the numbers of C and G are equal. This complementary pairing fixes the composition on each strand and across the molecule, even though the total amounts of A+T versus C+G can vary between species. So the statement that adenine and thymine are present in equal amounts, and cytosine and guanine are present in equal amounts, exactly reflects this pairing pattern. The other ideas—A pairing with C, all four bases in equal amounts, or thymine equaling cytosine—don’t align with how base pairing works in DNA.