Stop Codon

A stop codon is a sequence of three bases (genetic ‘letters’) present in DNA and messenger RNA (mRNA) that terminate the translation process of nucleotides into amino acid chains (polypeptides). Let’s take a step back and observe what happens within the cell so we can better understand the function of the stop codon:

A machine called an RNA polymerase wizzes down the DNA strand producing from the DNA a very similar strand called RNA. When the polymerase runs into a stop codon, it releases the growing RNA strand (called a messenger RNA transcript, denoted as ‘mRNA’). The mRNA enters another machine called the ribosome where special transfer RNA (tRNA) create an amino acid chain at the ‘instruction’ of the mRNA. When a stop codon is reached, tRNA stop adding to the amino acid chain. This chain is released to be folded into a protein which then does its job in the cell.

List of Stop Codons

There are three different types of stop codons for both DNA and RNA, and all have the same function. These are as follows (the upper-case letters refer to the nucleotide base):

TAG (amber) UAG (amber)
TAA (ochre) UAA (ochre)
TGA (opal or umber) UGA (opal)
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