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Three codons terminate protein synthesis


KEY TERMS:
  • The amber codon is the triplet UAG, one of the three termination codons that end protein synthesis.
  • The ochre codon is the triplet UAA, one of the three termination codons that end protein synthesis.
  • The opal codon is the triplet UGA, one of the three termination codons that end protein synthesis. It has evolved to code for an amino acid in a small number of organisms or organelles.
  • Missense mutations change a single codon so as to cause the replacement of one amino acid by another in a protein sequence.
  • Premature termination describes the termination of protein or of RNA synthesis before the chain has been completed. In protein synthesis it can be caused by mutations that create termination codons within the coding region. In RNA synthesis it is caused by various events that act on RNA polymerase.
  • A nonsense mutation is any change in DNA that replaces a codon specifying an amino acid with a translation-termination codon (UAG, UGA, or UAA).
  • A stop codon (Termination codon) is one of three triplets (UAG, UAA, UGA) that causes protein synthesis to terminate. They are also known historically as nonsense codons. The UAA codon is called ochre, and the UAA codon is called amber, after the names of the nonsense mutations by which they were originally identified.
KEY CONCEPTS:
  • The codons UAA (ochre), UAG (amber) and UGA terminate protein synthesis.
  • In bacteria they are used most often with relative frequencies UAA>UGA>UAG. 

Only 61 triplets are assigned to amino acids. The other three triplets are termination codons (or stop codons) that end protein synthesis. 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 sometimes called the opal codon.
The nature of these triplets was originally shown by a genetic test that distinguished two types of point mutation:
  • 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 protein; the effect on protein function depends on the site of mutation and the nature of the amino acid replacement.
  • When a point mutation creates one of the three termination codons, it causes premature termination of protein synthesis at the mutant codon. Only the first part of the protein is made in the mutant cell. This is likely to abolish protein function (depending, of course, on how far along the protein the mutant site is located). A change of this sort is called a nonsense mutation.
(Sometimes the term nonsense codon is used to describe the termination triplets. "Nonsense" is really a misnomer, since the codons do have meaning, albeit a disruptive one in a mutant gene. A better term is stop codon.)
In every gene that has been sequenced, one of the termination codons lies immediately after 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 protein synthesis within a gene. The UAG, UAA, and UGA triplet sequences are therefore necessary and sufficient to end protein synthesis, whether occurring naturally at the end of a gene or created by mutation within a coding sequence.
In bacterial genes, UAA is the most commonly used termination codon. UGA is used more heavily than UAG, although there appear to be more errors reading UGA. (An error in reading a termination codon, when an aminoacyl-tRNA improperly responds to it, results in the continuation of protein synthesis until another termination codon is encountered.)