WHAT IS A CODON? (GENETICS) - BIOLOGY - 2025

A codon is each of the 64 possible combinations of three nucleotides, based on the four that make up nucleic acids. That is, from combinations of the four nucleotides, blocks of three "letters", or triplets, are built.

These are the deoxyribonucleotides with the nitrogenous bases adenine, guanine, thymine, and cytosine in DNA. In RNA, they are ribonucleotides with the nitrogenous bases adenine, guanine, uracil, and cytosine.

The codon concept applies only to genes that code for proteins. The DNA-encoded message will be read in three-letter blocks once the information from your messenger is processed. The codon, in short, is the basic coding unit for the genes that are translated.

Codons and amino acids

If for each position in three-letter words we have four possibilities, the product 4 X 4 X 4 gives us 64 possible combinations. Each of these codons corresponds to a particular amino acid - except for three that function as end-of-reading codons.

The conversion of a message encoded with nitrogenous bases in a nucleic acid to one with amino acids in a peptide is called translation. The molecule that mobilizes the message from DNA to the translation site is called messenger RNA.

A triplet of a messenger RNA is a codon whose translation will take place in ribosomes. The small adapter molecules that change the language of nucleotides to amino acids in ribosomes are transfer RNAs.

Message, messengers and translation

A protein-encoding message consists of a linear array of nucleotides that is a multiple of three. The message is carried by an RNA that we call a messenger (mRNA).

In cellular organisms all mRNAs arise by transcription of the encoded gene in their respective DNA. That is, the genes that code for proteins are written on DNA in the language of DNA.

However, this does not mean that this rule of three is strictly adhered to in DNA. Being transcribed from DNA, the message is now written in RNA language.

The mRNA consists of a molecule with the gene message, flanked on both sides by non-coding regions. Certain post-transcriptional modifications, such as splicing for example, allow generating a message that complies with the rule of three. If this rule of three did not seem to be fulfilled in the DNA, splicing restores it.

The mRNA is transported to the site where ribosomes reside, and here the messenger directs the translation of the message into protein language.

In the simplest case, the protein (or peptide) will have a number of amino acids equal to one third of the letters in the message without three of them. That is, equal to the number of messenger codons minus one of the termination.

Genetic message

A genetic message from a gene that codes for proteins generally begins with a codon that is translated as the amino acid methionine (codon AUG, in RNA).

A characteristic number of codons then continue at a specific linear length and sequence, and terminate at a stop codon. The stop codon can be one of the opal (UGA), amber (UAG) or ocher (UAA) codons.

These have no equivalent in amino acid language, and therefore no corresponding transfer RNA. However, in some organisms, the codon UGA allows the incorporation of the modified amino acid selenocysteine. In others, the codon UAG allows the incorporation of the amino acid pyrrolysine.

Messenger RNA complexes with ribosomes, and the initiation of translation allows the incorporation of an initial methionine. If the process is successful, the protein will elongate (lengthen) as each tRNA donates the corresponding amino acid guided by the messenger.

Upon reaching the stop codon, amino acid incorporation is stopped, translation is complete, and the synthesized peptide is released.

Codons and anticodons

Although it is a simplification of a much more complex process, the codon-anticodon interaction supports the hypothesis of translation by complementarity.

According to this, for each codon in a messenger, the interaction with a particular tRNA will be dictated by the complementarity with the bases of the anticodon.

The anticodon is the sequence of three nucleotides (triplet) present in the circular base of a typical tRNA. Each specific tRNA can be loaded with a particular amino acid, which will always be the same.

In this way, when an anticodon is recognized, the messenger is telling the ribosome that it must accept the amino acid that carries the tRNA for which it is complementary in that fragment.

The tRNA therefore acts as an adapter that allows the translation carried out by the ribosome to be verified. This adapter, in three-letter codon reading steps, allows the linear incorporation of amino acids that ultimately constitutes the translated message.

The degeneration of the genetic code

The codon: amino acid correspondence is known in biology as the genetic code. This code also includes the three translation stop codons.

There are 20 essential amino acids; but there are, in turn, 64 codons available for their conversion. If we remove the three stop codons, we still have 61 left to code for the amino acids.

Methionine is encoded only by the AUG- codon which is the start codon, but also by this particular amino acid in any other part of the message (gene).

This leads us to 19 amino acids being encoded by the remaining 60 codons. Many amino acids are encoded by a single codon. However, there are other amino acids that are encoded by more than one codon. This lack of relationship between codon and amino acid is what we call the degeneracy of the genetic code.

Organelles

Finally, the genetic code is partially universal. In eukaryotes there are other organelles (evolutionarily derived from bacteria) where a different translation is verified than that which is verified in the cytoplasm.

These organelles with their own genome (and translation) are chloroplasts and mitochondria. The genetic codes of chloroplasts, mitochondria, eukaryotic nuclei, and bacterial nucleoids are not exactly identical.

However, within each group it is universal. For example, a plant gene that is cloned and translated in an animal cell will give rise to a peptide with the same linear amino acid sequence that it would have had if it had been translated in the plant of origin.

References

1. Alberts, B., Johnson, AD, Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014) Molecular Biology of the Cell (6 ^th Edition). WW Norton & Company, New York, NY, USA.
2. Brooker, RJ (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
3. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Philadelphia, PA, USA.
4. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 ^th ed.). New York: WH Freeman, New York, NY, USA.
5. Koonin, EV, Novozhilov, AS (2017) Origin and evolution of the universal genetic code. Annual Review of Genetics, 7; 51: 45-62.
6. Manickam, N., Joshi, K., Bhatt, MJ, Farabaugh, PJ (2016) Effects of tRNA modification on translational accuracy depend on intrinsic codon-anticodon strength. Nucleic Acids Research, 44: 1871-81.