Codon: Essential Building Blocks of the Genetic Code

In the language of genetics, a codon is a sequence of three nucleotides that together form a unit of genetic code in DNA or RNA. Codons specify which amino acid will be added next during protein synthesis, and are essential for the proper translation of genetic information into proteins.

The genetic code refers to the set of rules that defines how sequences of three nucleotides, or codons, specify which amino acid will be added to a polypeptide chain during protein synthesis. With four different nucleotide bases (adenine, uracil, guanine, cytosine) and three nucleotides per codon, there are 64 possible codons (43 = 64).

Of these 64 codons:

Importantly, the genetic code is nearly universal, shared by all known organisms. This universality suggests that the genetic code originated very early in the evolution of life.

The translation of codons into amino acids is mediated by transfer RNA (tRNA) molecules. Each tRNA molecule has an anticodon that is complementary to a specific codon. During protein synthesis, the anticodon of a tRNA pairs with its complementary codon on the messenger RNA (mRNA). This codon-anticodon pairing ensures that the correct amino acid is added to the growing protein chain.

While there are 64 possible codons, there are only 20 amino acids. This means that the genetic code is degenerate, with multiple codons specifying the same amino acid. For example, the amino acid leucine is coded for by six different codons. This degeneracy provides a layer of protection against mutations, as some mutations in the third nucleotide of a codon may not change the amino acid that is specified.

Protein synthesis begins with a start codon and ends with a stop codon. The most common start codon is AUG, which also codes for the amino acid methionine. The three stop codons are UAA, UAG, and UGA. These codons do not code for any amino acids but instead signal the termination of protein synthesis.

Although multiple codons can code for the same amino acid, not all codons are used with equal frequency. This phenomenon is known as codon bias. Different organisms show different patterns of codon usage, which can reflect mutation biases, selection for efficient and accurate translation, and even viral adaptation to host codon usage.

Understanding the genetic code and the role of codons is crucial for many applications in biotechnology. For example, when designing genes for expression in a particular host organism, researchers often optimize the codon usage to match the codon bias of the host. This codon optimization can significantly enhance protein expression levels.

In addition, researchers are exploring ways to expand the genetic code by assigning new amino acids to certain codons. This expansion could allow for the incorporation of novel amino acids with unique properties into proteins, opening up new possibilities for protein engineering and synthetic biology.

Nature Reviews Molecular Cell Biology, Codon optimality, bias and usage in translation and mRNA decay

Annual Review of Biochemistry , Synonymous but not Silent: The Codon Usage Code for Gene Expression and Protein Folding

Frontiers in Bioengineering and Biotechnology, Codon-optimization in gene therapy: promises, prospects and challenges