The Genetic Code
The genetic code is a messenger RNA (mRNA)
code. Each of the 64 triplet codons in the table is
read 5' 
3'. The table is
organized such that the first letter in the codon is
read in the four blocks at the left, the second letter
in the four columns across the top, and the third letter
as a line in each block of four. The code tables above
and below uses the IUPAC three- and single-letter
abbreviations for amino acids; Stop codons are indicated
as Stars (*).
3'. The table is
organized such that the first letter in the codon is
read in the four blocks at the left, the second letter
in the four columns across the top, and the third letter
as a line in each block of four. The code tables above
and below uses the IUPAC three- and single-letter
abbreviations for amino acids; Stop codons are indicated
as Stars (*). 
Top figure © 2012 TA Brown, Introduction
to Genetics (1st ed.); Bottom figure
and additional text © 2014 by Steven
M. Carr