Three Codons Terminate Translation

KEY CONCEPTS
- The codons UAA (ochre), UAG (amber), and UGA (opal) terminate translation.
- In bacteria, they are used most often with relative frequencies of UAA > UGA > UAG.
 Only 61 of the 64 possible nucleotide triplets specify amino acids. The other three triplets are termination codons (also known as nonsense codons or stop codons), which end translation. They have casual names from the history of their discovery. The UAG triplet is called the amber codon, UAA is the ochre codon, and UGA is the opal codon.
The nature of these triplets was originally shown by a genetic test that distinguished two types of point mutations:
- A point mutation that changes a codon to represent a different amino acid is called a missense mutation. One amino acid replaces the other in the polypeptide; the effect on protein function depends on the site of mutation and the nature of the amino acid replacement.
- A point mutation that changes a codon to one of the three termination codons is called a nonsense mutation. It causes premature termination of translation at the mutant codon. Only the first part of the polypeptide is made in the mutant cell. This is likely to abolish protein function (depending, of course, on how far along the polypeptide the mutant site is located).
In every gene that has been sequenced, one of the termination codons lies immediately downstream from the codon representing the C-terminal amino acid of the wild-type sequence. Nonsense mutations show that any one of the three codons is sufficient to terminate translation within a gene. The UAG, UAA, and UGA triplet sequences are therefore necessary and sufficient to end translation, whether they occur naturally at the end of an open reading frame (ORF) or are created by nonsense mutations within coding sequences. (Sometimes the term nonsense codon is used to describe the termination triplets. Nonsense is really a term that describes the effect of a mutation in a gene rather than the meaning of the codon for translation. Stop codon is a better term.)
In bacterial genes, UAA is the most commonly used termination codon. UGA is used more frequently than UAG, although there appear to be more errors reading UGA. (An error in reading a termination codon—when an aminoacyl-tRNA improperly recognizes it—results in the continuation of translation until another termination codon is encountered or the ribosome reaches the 3′ end of the mRNA, which may result in other problems. For this circumstance, bacteria have a special RNA.)