AUG (occasionally GUG) initiates the synthesize of a protein chain. The natural question would be which codon specifies or signals the end of termination of the synthesis of the protein chain.
It has been found out that three of the 64 codons do not specify any tRNA and as such they cannot assemble any amino acid. As such these codons whenever they are found in mRNA would signal the end of the chain.
They are called termination codons as they terminate the synthesis of the polypeptide chain. They are also called nonsense codons as they do not specify any amino acid. The nonsense codons are UAG (amber) UAA (ochre) and UGA (opal).
Image Source: proteomesci.com
ADVERTISEMENTS:
Of the three termination codons UAG was the first termination codon to be discovered. It was named amber after a graduate student in Germany named Bernstein (means amber in German).
This graduate student helped in the discovery of the class of mutations. The other two termination codons were obviously given the name of the colors only to maintain uniformity.
Termination codons found in mRNA will not specify any amino acid and as such stop the process of translation and bring about the release of the peptide chain from ribosomes.
ADVERTISEMENTS:
No tRNA molecules have anticodons complementary to the termination codons found in mRNA.
Termination codons seem to have some affinity to proteins called release factors which bring about the release of the protein chain. In prokaryotes there are three release factors RF-1, RF-2 and RF-3.
RF-1 recognizes the amber and ochre codons while RF-2 recognizes ochre and opal codons, RF-3 stimulates RF-1 and RF-2 ultimately releasing the newly synthesized peptide chain. In eukaryotes there is a single factor (RF protein) which is capable of recognizing all the three termination codons.