A C G T M R W S Y K B D H V N | | | | | | | | | | | | | | | T G C A K Y W S R M V H D B NFor example, some degenerated IUPAC symbols are complemented to themselves (e.g., W–W and S–S), while others are seemingly "hard" to apprehend (e.g., B–V and D–H).
After thinking it for a bit, things begin to become clear. They are based on the complementarity of Watson-Crick base-pairs (A–T and G–C) and the meaning of each degenerated IUPAC nucleotide symbol. For example,
- W represents A/T, meaning weak (with only two hydrogen-bonds). The complements of A/T are T/A respectively, which is W again.
- B (not A) represents C/G/T, and their complements are G/C/A respectively, which is V (not U/T).
It is easy to verify that all other complementary pairs follow exactly the same basic principle.