Genetic code.

The 3 bases of a codon are read in the 5' - 3' direction along the mRNA.

The 64 codons specify 20 different amino acids.

The codon AUG not only stands for the amino acid methionine (Met but also functions as a "start" signal to begin translating the mRNA.

Three codons are "stop" signals, marking the end of a polypeptide.