Note that this table may be thought of as being composed of 16 cells, each containing 4 entries defined by the first 2 nucleotides in a codon. Further note that 8 of those 16 cells specify only one amino acid, basically ignoring the 3rd nucleotide in the codon.

One might imagine that the genetic code began with a 2-letter code that was modified along the way to provide a more detailed selection of chemical constituents. For example, one could imagine that the GA* box (where "*" represents any nucleotide) originally coded for aspartic acid (or some other unknown AA with acidic properties), but that the addition of a 3rd nucleotide allowed specification of both aspartic and glutamic acids.

In such a case perhaps UA* could have coded for "Stop", and UG* for "Start", so that Phenylalanine would have represented all the "Large, nonpolar aromatics." Is there any way to tell?

		Second nucleotide
		U	C	A	G
F i r s t       n u c l e o t i d e	U	UUU Phenylalanine (Phe)	UCU Serine (Ser)	UAU Tyrosine (Tyr)	UGU Cysteine (Cys)	U
		UUC Phe	UCC Ser	UAC Tyr	UGC Cys	C
		UUA Leucine (Leu)	UCA Ser	UAA Stop	UGA Stop	A
		UUG Leu	UCG Ser	UAG Stop	UGG Tryptophan (Trp)	G
	C	CUU Leucine (Leu)	CCU Proline (Pro)	CAU Histidine (His)	CGU Arginine (Arg)	U
		CUC Leu	CCC Pro	CAC His	CGC Arg	C
		CUA Leu	CCA Pro	CAA Glutamine (Gln)	CGA Arg	A
		CUG Leu	CCG Pro	CAG Gln	CGG Arg	G
	A	AUU Isoleucine (Ile)	ACU Threonine (Thr)	AAU Asparagine (Asn)	AGU Serine (Ser)	U
		AUC Ile	ACC Thr	AAC Asn	AGC Ser	C
		AUA Ile	ACA Thr	AAA Lysine (Lys)	AGA Arginine (Arg)	A
		AUG Methionine (Met) or Start	ACG Thr	AAG Lys	AGG Arg	G
	G	GUU Valine (Val)	GCU Alanine (Ala)	GAU Aspartic acid (Asp)	GGU Glycine (Gly)	U
		GUC Val	GCC Ala	GAC Asp	GGC Gly	C
		GUA Val	GCA Ala	GAA Glutamic acid (Glu)	GGA Gly	A
		GUG Val	GCG Ala	GAG Glu	GGG Gly	G

Note that E. coli employ only a limited subset of the mammalian marker set: Secondary (lysine and arginine) and Type 2 Primary (phenylalanine, leucine, tryptophan and tyrosine) N-terminal markers.

Credits

• Chemical groupings in the first table were provided by Gerald Lushington.
  
• The list of essential amino acids was taken from Albert, et al., The Molecular Biology of the Cell.
  
• Destabilizing AA list taken from Varshavsky, A, The N-end rule: Functions, mysteries, uses, PNAS, October 1996.
  
• Original table from the Kimball web site.
  
• Colors, legend, and commentary added by Michael Grobe (without charge).
  June 2004