Microbiology, 2021

11.4 • Protein Synthesis (Translation) 423
number of amino acids and a given amino acid is encoded by more than one codon (Figure 11.12). This
redundancy in the genetic code is called degeneracy. Typically, whereas the first two positions in a codon are
important for determining which amino acid will be incorporated into a growing polypeptide, the third
position, called the wobble position, is less critical. In some cases, if the nucleotide in the third position is
changed, the same amino acid is still incorporated.
Whereas 61 of the 64 possible triplets code for amino acids, three of the 64 codons do not code for an amino
acid; they terminate protein synthesis, releasing the polypeptide from the translation machinery. These are
called stop codons or nonsense codons. Another codon, AUG, also has a special function. In addition to
specifying the amino acid methionine, it also typically serves as the start codon to initiate translation. The
reading frame, the way nucleotides in mRNA are grouped into codons, for translation is set by the AUG start
codon near the 5’ end of the mRNA. Each set of three nucleotides following this start codon is a codon in the
mRNA message.
The genetic code is nearly universal. With a few exceptions, virtually all species use the same genetic code for
protein synthesis, which is powerful evidence that all extant life on earth shares a common origin. However,
unusual amino acids such as selenocysteine and pyrrolysine have been observed in archaea and bacteria. In
the case of selenocysteine, the codon used is UGA (normally a stop codon). However, UGA can encode for
selenocysteine using a stem-loop structure (known as the selenocysteine insertion sequence, or SECIS
element), which is found at the 3’ untranslated region of the mRNA. Pyrrolysine uses a different stop codon,
UAG. The incorporation of pyrrolysine requires the pylS gene and a unique transfer RNA (tRNA) with a CUA
anticodon.
Figure 11.12
This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a termination signal
in a nascent protein. The first letter of a codon is shown vertically on the left, the second letter of a codon is shown horizontally across the
top, and the third letter of a codon is shown vertically on the right. (credit: modification of work by National Institutes of Health)
CHECK YOUR UNDERSTANDING
• How many bases are in each codon?
• What amino acid is coded for by the codon AAU?
• What happens when a stop codon is reached?
The Protein Synthesis Machinery
In addition to the mRNA template, many molecules and macromolecules contribute to the process of
translation. The composition of each component varies across taxa; for instance, ribosomes may consist of