This is the best Genetic Code chart ever! For novices, that is. Those 20 proteinogenic amino acids are real characters! Each has its own charming personality. They do fall into four nice categories of side-chain chemical quality, indicated by colored backgrounds. The idea for drawing side-chain atoms as kissing circles, and the representation of the invariant amino acid backbone as a simple chain-link, came from Mahlon Hoagland and/or Bert Dodson. Page 121 of their wonderful 1995 book The Way