From Eterna Wiki

The chemical structure of adenine.
The chemical structure of adenosine.

Adenine (A) is a purine base, and forms Watson-Crick base pairs specifically with the uracil pyrimidine. Adenine is joined to ribose at the 9th position to create adenosine.

In Eterna, Adenine is color-coded yellow. When a new puzzle is opened, all nucleotides are assigned to adenine by default.