What is a codon? Characteristics and functions

A summary of this concept related to DNA and Biological processes associated with genes.

In the world of genetic code, if one quality is valued, it is abstraction. To understand the processes that happen millimetrically in each of our cells as if it were a perfect chain of work, it takes imagination, and above all, knowledge.

This is why it is common for the average reader to feel frightened when dealing with certain topics in genetics: "DNA", "RNA", "polymerase", "metagenomics" and many other terms seem to escape general knowledge. Nothing could be further from the truth.

Like everything in life, the science of the processes encoded by the genetics of organisms can be explained in a simple and straightforward way. In this space you will find a summarized explanation of what a codon isand how without this functional unit, life as we know it would not be possible.

Codon: the triplet of life

A codon is a sequence of three nucleotides that is located in messenger RNA.. It is clear that to understand the functioning of this very special subunit, we must first understand the terms contained in its more general definition.

About RNA and its organization

RNA stands for "ribonucleic acid". It is a polymeric chain composed of a series of monomers, in this case nucleotides. Each nucleotide is made up of three different components:

• A five-carbon monosaccharide (pentose).
• A phosphate group.
• A nitrogenous base, which can be adenine (A), cytosine (C), guanine (G) and uracil (U).

RNA differs from DNA, in addition to many other things, in that the latter has the nitrogenous base thymine (T) instead of uracil (U). In general, nucleotides are named according to the nitrogenous base they carry.

Once we have dissected what a nucleotide is, the first conflicting term in the definition of the codon, it is time to clarify what exactly the messenger RNA is. To do this, we must first turn to the types of RNA. These are as follows:

• Messenger RNA (mRNA): the DNA holds the information for protein synthesis. The mRNA is responsible for translating it and transporting it to the ribosomes.
• Transfer RNA (tRNA): transports specific amino acids to the site of protein growth.
• Ribosomal RNA (rRNA): it is combined with various proteins to form ribosomes, where the proteins needed by the cell are synthesized.

As we have seen, each type of RNA plays an essential role in protein synthesisOne translates and transports the DNA information, another carries the assembly "building blocks" to the ribosomes where proteins are synthesized, and another is part of the synthesizing "machinery" itself. It seems incredible that such a seemingly simple molecule can do such complex work, doesn't it?

There are other types of RNA, such as RNA interference, MICRO RNAs, long non-coding RNAs...etc. We will explain them another time, as these complex ribonucleic acids are far from the term to be discussed.

Having understood all the major types of RNAs, it is time to find out why the term codon is so important.

The importance of the genetic code

The genetic code is a term that refers to the set of instructions that tell the set of instructions that tell the cell how to synthesize a specific protein.. That is, the letters we have seen previously, both in DNA and RNA. In DNA, the code of each gene combines the four letters (A,G,C and T) in different ways to form three-letter "words", which specify each of the amino acids that make up a protein.

These "words" encoded in the DNA are transcribed by a process called transcription, by which a segment (gene) of DNA gives rise to the messenger RNA explained above. This RNA is mobile, therefore, it can leave the cell nucleus where the genetic information is located and carry the instructions for the synthesis of that protein to the ribosomes (located in the cytoplasm).

Each of the translated DNA "three-letter words" contained in the mRNA is, as you may have guessed, the codon that concerns us today. We can therefore say that Each of these nucleotide triplets is the most basic functional unit of the genetic code..

There are 64 different codons common to all living beings, of which 61 code for amino acids. For the majority of living beings there are 20 different amino acidsIt should be noted that each of them (not in all cases but in almost all) is coded by 2, 3, 4 or 6 different codons. Therefore, and applying basic mathematics, an amino acid made from 6 codons would be encoded by 18 translated nucleotides (remember that each codon is three ribonucleotides).

The role of the codon in translation

We have established that transcription is the process by which DNA information is transcribed into an mRNA that will carry the protein synthesis instructions to the ribosomes, right? The codon plays an even more important role in the translation process.

Translation is defined as the process of translating (redundancy) a messenger RNA molecule into a sequence of amino acids that will give rise to a specific protein.. As we have already mentioned, the transfer RNA (tRNA) is in charge of transferring the amino acids to the construction area (the ribosome), but not only that, since it is also in charge of ordering them along the messenger RNA molecule.

For this purpose, the tRNA has a sequence of amino acids in the ribosome, the tRNA has a sequence of three nucleotides that are paired with those of the codon: the anticodon.the anticodon. This allows this ribonucleic acid to recognize the order of the amino acids of the protein, according to the instructions provided by the codons of the mRNA.

Codons and mutations

A point mutation occurs when a single base pair (nucleotide) of the genetic code is altered. In the case of codons, it is usual that the third of the letters differs for the synthesis of the same amino acid..

For example, leucine responds to the codons CUU, CUC, CUA. Thus, mutations in the third letter are considered silent, since the same amino acid is synthesized and the protein can be assembled without problems. In contrast, mutations in the first and second letters can be detrimental, as they usually result in a different amino acid than the one sought, thus breaking the elaborate assembly chain.

Beyond genetics

As we have seen, this three-nucleotide association known as a codon is one of the basic functional units of the individual's genetic code. Although the genetic information itself does not change throughout the life of a living being, the expression of the genes can.. Epigenetics is responsible for exploring these mechanisms.

In the DNA of living beings, several genes can be silenced, which results in the inhibition of some transcription and translation processes of certain proteins at the cellular level. If the genetic information is not transcribed in the mRNA, each of the codons will not be transcribed, and therefore, neither will they be translated into amino acids and the protein in question will not be assembled.

Conclusions

In these lines, we have tried to convey that the codon is a form of organization of genetic information essential for protein synthesis at the cellular level in living beings.. These proteins make up the cells, thus also the tissues, which allow the formation of the living being in question.

Therefore, it is no exaggeration to say that without this triplet of nucleotides, life as we know it today would not be possible.

Bibliographical references:

• Crick, F. H. C. (1966). Codon-anticodon pairing: the wobble hypothesis.
• Bennetzen, J. L., & Hall, B. D. (1982). Codon selection in yeast. Journal of Biological Chemistry, 257(6), 3026-3031.
• Déctor, M. A., & Arias, C. F. (2004). RNA interference: a primitive defense system. Science, 55, 25-36.
• Neissa, J. I., & Guerrero, C. (2004). From genetic code to epigenetic code: New therapeutic strategies. Journal of the Faculty of Medicine, 52(4), 287-303.