Why 21 Amino Acids?

Most textbooks say all living creatures have 20 amino acids, and neglect the 21st - which is rarely found. At least one bacterium has a 22nd called pyrrollysine, also found in some small organisms called the archaea. The Halobacterium salinarum we looked at is one of those, and this 22nd amino acid is also a variation on a stop instruction, like selenocysteine. Prokaryotes, which are cells without a distinct nucleus, have a 23rd amino acid called formylmethionine.

However, it seems more consistent to stick to what is universal for all living things: 21 amino acids and several stop commands. This stop command is a DNA base combination that gives a command to cell machinery to stop assembling amino acids into proteins at genetically prescribed places.

So, for our exercise the first 21 amino acids correspond to the first 21 letters of the 22-letter Hebrew Alphabet, and one of the stop commands corresponds to the 22nd (least used) Hebrew letter. You can check the correspondences again in the table here.