In double stranded DNA, the total amount of purines always equals the total amount of pyrimidines because
- A. the sugars bond only to purines
- B. phosphate groups pair with pyrimidines
- C. hydrogen bonds form only between bases of equal size
- D. each base pair contains one purine and one pyrimidine
Explanation
Adenine and guanine are the purines, thymine and cytosine the pyrimidines, and every pair across the helix matches one of each. This is why Chargaff found A plus G equals T plus C, and it keeps the two strands a constant distance apart.
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