teins, not just enzymes. Proteins which are not enzymes are much more
passive kinds of beings; many of them, for instance, are structural
molecules, which means that they are like girders and beams and so forth in
buildings: they hold the cell's parts together. There are other kinds of
proteins, but for our purposes, the principal proteins are enzymes, and I
will henceforth not make a sharp distinction.
Amino AcidsProteins are composed of sequences of amino acids, which come in twenty
primary varieties, each with a three-letter abbreviation:
ala alanine
arg arginine
asn asparagme
asp aspartic acid
cys cysteine
gin glutamine
glu glutamic acid
gly glycine
his histidine
ile isoleucine
leu leucine
Iys lysine
met methionine
phe phenylalanine
pro proline
ser senne
thr threonine
trp tryptophan
tyr tyrosine
val valineNotice the slight numerical discrepancy with Typogenetics, where we had
only fifteen "amino acids" composing enzymes. An amino acid is a small
molecule of roughly the same complexity as a nucleotide; hence the build-
ing blocks of proteins and of nucleic acids (DNA, RNA) are roughly of the
same size. However, proteins are composed of much shorter sequences of
components: typically, about three hundred amino acids make a complete
protein, whereas a strand of DNA can consist of hundreds of thousands or
millions of nucleotides.Ribosomes and Tape RecordersNow when a strand of mRNA, after its escape into the cytoplasm, encoun-
ters a ribosome, a very intricate and beautiful process called translation takes
place. It could be said that this process of translation is at the very heart of
(^518) Self-Ref and Self-Rep