Genetic Code: Introduction


The genetic code establishes the relationship between all 64 possible arrangements of triplets (codons) of the four nucleotide bases contained in either DNA (A, T, G and C) or RNA (A, U, G and C) and the 20 amino acids that are used to construct proteins via ‘translation’ system as well as signals of translation initiation and termination. The historical events from 1950s to 1960s that contributed to the deciphering of the genetic code led to the development of the field of molecular biology. In 1960s, the genetic code was established to be ‘universal’ for all living organisms. However, from late 1970s, variations of genetic code have been found in various genetic systems. Variations of genetic code promote studies on the origin and evolution of the genetic code.

Key Concepts:

  • Genetic code is the correlation between nucleotide triplet and the corresponding amino acid.

  • Genetic code consists of 64 triplet codons and sometimes ‘degenerate’.

  • Central dogma states that genetic information flows unidirectionally from DNA to protein via RNA as an intermediary.

  • A cell‐free protein system contains ribosome, S100 fraction, tRNA, amino acids, ATP, an energy recycling system and a template.

  • Termination codon consists of UAG (amber codon), UAA (ochre codon) and UGA (opal codon), which code for no amino acid but instead cause protein synthesis to terminate.

  • Initiation codon is AUG, which initiates protein synthesis with formyl‐methionine in bacteria and phage.

  • Universal genetic code shows the way of assignment of 64 triplet codons to each of 20 amino acids and 3 termination codons, which is common to almost all extant organisms – bacteria, yeasts, viruses, plants and animals.

Keywords: amino acids; DNA; nucleotide; codon; genes; universal genetic code

Figure 1.

Gene map of frameshift mutants of T4 phage rIIB cistron caused by proflavin. Modified from Crick et al. .

Figure 2.

Frameshift mutants of T4 phage rIIB cistron caused by proflavin. Addition and deletion on the nucleotide sequence. Redrawn from Crick et al. .

Figure 3.

Replacement of the amber codon with amino acids in the head protein of T4 phage amber mutant grown in various E. coli Su+ strains.

Figure 4.

Nucleotide sequences of the translation initiation sites in the coat protein, replicase and A protein of R17 RNA. The underlined sequences are the Shine–Dalgarno sequences. Redrawn from Steitz .

Figure 5.

Comparison between amino acid sequences of lysozyme of wild‐type T4 phage and eJ42eJ44 frameshift mutant. Determination of in vivo code. Redrawn from Terzaghi et al. .

Figure 6.

The universal genetic code. At the time of the Cold Spring Harbor Symposium (1966) the UGA opal codon was not identified and all the codons were not completely allocated. The initiation codon was uncertain.



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Watanabe, Kimitsuna, and Yokobori, Shin‐ichi(Oct 2011) Genetic Code: Introduction. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000809.pub2]