Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Translation
Properties of the Genetic Code

METABOLISM/28.html">The Genetic Code is characterized by A number of properties, which we will examine below.

Triplet Nature of the Code

As noted above, a codon consists of a sequence of three NUCLEOTIDES, from which stems the most crucial property of the code: its triplet nature. A codon cannot be represented by a sequence of two nucleotides, since the number of distinct dinucleotides composed of four different nucleotides is 4²=16. This is insufficient to encode the 20 standard Amino Acids. Meanwhile, the number of possible trinucleotides (triplets) made up of four different nucleotides is 4³=64. Of these 64 triplets, 61 are sense codons—meaning they encode amino acids—while 3 are non-coding, or nonsense codons, responsible for terminating Protein Synthesis.

Degeneracy of the Code

Each of the 61 sense codons encodes the incorporation of one of the 20 standard amino acids into a protein. Because the number of codons exceeds the number of encoded amino acids by approximately a factor of 3, a given amino acid is typically specified by multiple codons. This feature of the code is known as code degeneracy. Only Two amino acids are encoded by a single codon each: Methionine and Tryptophan. Leucine, Serine, and Arginine are encoded by six codons each, whereas valine, Alanine, Glycine, Proline, and Threonine are encoded by four codons each (Fig. 6.2). Triplets that encode the same amino acid most frequently differ from one another at the third nucleotide position (Fig. 6.2).

Obviously, knowing the genetic code allows one to determine The sequence of amino acid residues in a polypeptide chain from the Introduction/19.html">Primary Structure of its mRNA. At the same time, due to the degeneracy of the code, it is impossible to unambiguously translate a protein's Amino Acid Sequence back into The nucleotide sequence of mRNA.

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Fig. 6.2. The genetic code

Unambiguity of the Code

This property means that each codon corresponds to one strictly defined amino acid. For instance, the triplet GGU encodes only glycine and no other amino acid.

Non-overlapping Nature of the Code

In accordance with this property, the codons following the initiation triplet are read consecutively, without overlapping and without any gaps, all the way to the nonsense codon (Fig. 6.1).

Universality of the Code

The universality of the code means that it is identical across All living organisms. Nevertheless, exceptions do exist. In mitochondrial mRNA, certain triplets have a different meaning than they do in nuclear-derived mRNA. For example, in mitochondrial mRNA, the triplet UGA encodes tryptophan, AUA encodes methionine, and AGA and AGG are read as stop codons.