Which of the following statements about base pairing is incorrect?

• A. Base pairing is the hydrogen-bonded pairing of two complementary heterocyclic bases in the double helix of DNA.
• B. Base pairing in DNA occurs during replication, transcription, and translation.
• C. In base pairing, adenosine pairs with thymine in DNA or uracil in RNA, while guanine pairs with cytosine. 
• D. In base pairing, a purine base can be paired with another purine base.

Answer: (D)