START codons: The codonAUGis called the START codon as it the first codon in the transcribed mRNA that undergoes translation. AUG is the most common START codon and it codes for the amino acid methionine (Met) in eukaryotes and formyl methionine (fMet) in prokaryotes. During protein synthesis, the tRNA recognizes the START codon AUG with the help of some initiation factors and starts translation of mRNA.

STOP codons : There are 3 STOP codons in the genetic code – UAG UAA, and UGA. These codons signal the end of the polypeptide chain during translation. These codons are also known as nonsense codons or termination codons as they do not code for an amino acid. 

The three STOP codons have been named as amber (UAG), opal or umber (UGA) and ochre (UAA). “Amber” or UAG was discovered by Charles Steinberg and Richard Epstein and they named it amber after the German meaning of the last name of their friend Harris Bernstein. The remaining two STOP codons were then named “ochre” and “opal” so as to maintain the “color names” theme.