Numeral systems other than unary used in nature or in animal and human behaviours
There is at least one example where a string of symbols from an alphabet is also used: proteins.

Proteins consist of chains of 20 different amino acids (usually, in some cases, it's 21 or 22) and the sequence of amino acids determines what given protein does.

This example is closely related to the DNA example you gave, because each amino acid in a protein is encoded by a triplet of bases in DNA.

For more information, see the article Genetic code on Wikipedia.