Which purine nitrogenous base is found in nucleic acids and pairs with cytosine?

The purine nitrogenous base that pairs with cytosine in nucleic acids is guanine. In the structure of DNA and RNA, cytosine (a pyrimidine) forms three hydrogen bonds with guanine (a purine), ensuring proper base pairing. This specific pairing is critical for the stability of the nucleic acid structure and plays a vital role in the processes of DNA replication and RNA transcription.

In the context of nucleic acids, adenine pairs with thymine in DNA or with uracil in RNA, and thymine is only found in DNA. Uracil, another nitrogenous base, replaces thymine in RNA, pairing with adenine. Since guanine directly pairs with cytosine, this fundamental base pairing is essential for maintaining the integrity and functionality of genetic information.