What hydrogen bonds join cytosine to guanine?

There are three between cytosine and guanine.

1. One between the NH₂ group of cytosine and the C=O group in guanine
2. One between the pyridine N atom in cytosine and the N-H group in guanine
3. One between the C=O group of cytosine and the NH₂ group of guanine

Note: The G-C connections in DNA are stronger than A=T connections which consist of just two hydrogen bonds. Ultraviolet radiation can break A=T connections and neighboring Thymine bases can sometimes form Thymin-Thymine dimers which can lead to mutations in future rounds of DNA replication.