Molecular Biology: Codon Structure and Translation

Study Notes

Twenty different amino acids are incorporated into polypeptides during translation, requiring at least 20 different codons with the four bases available in mRNA.
Francis Crick and colleagues published evidence in 1961 supporting a triplet code, using genetic analysis of mutations induced by the chemical proflavin.

The genetic code is ordered, with multiple codons for a given amino acid and codons for amino acids with similar chemical properties being closely related, usually differing by a single nucleotide.
The genetic code contains start and stop codons, with specific codons used to initiate and terminate polypeptide chains.
The genetic code is nonoverlapping, with each nucleotide in mRNA belonging to just one codon, except in rare cases where genes overlap.
The genetic code is comma-free, with no commas or other forms of punctuation within the coding regions of mRNA molecules.
The genetic code is degenerate, with all but two of the amino acids specified by more than one codon.

The occurrence of more than one codon per amino acid is called degeneracy.
All amino acids except methionine and tryptophan are specified by more than one codon.
Three amino acids—leucine, serine, and arginine—are each specified by six different codons.
Isoleucine has three codons, and the other amino acids each have either two or four codons.
The degeneracy in the genetic code is not at random, but is highly ordered, with multiple codons specifying a given amino acid differing by only one base, the third or 3’ base of the codon.

A single base-pair addition or deletion will alter the reading frame of the gene and mRNA for that portion of the gene distal to the mutation.
The codon structure can be read in different ways, such as AAA, GGG, CCC, TTT, or AA, AGG, GCC, CTT, T.
Codons can be grouped into two categories, plus (+) and minus (-), based on whether they result from additions or deletions.

In both prokaryotes and eukaryotes, the codon AUG is used to initiate polypeptide chains.
In rare instances, GUG is used as an initiation codon.
Specific codons are used to terminate polypeptide chains.