What is a codon? Features and functions

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

That is why it is common for the average reader to feel frightened when dealing with certain issues in relation to the genetics: “DNA”, “RNA”, “polymerase”, “metagenomics” and many other terms seem to escape knowledge general. Nothing is further from reality.

Like everything in this life, the science of the processes encoded by the genetics of organisms can be explained simply and easily. In this space you will find a summary explanation about what a codon is, and how without this functional unity, life as we know it would not be possible.

• Related article: "Differences between DNA and RNA"

Codon: the triplet of life

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

About the ARN and its organization

The acronym for RNA corresponds to the term “ribonucleic acid”. It is a polymer 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, among 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 for us to clarify what exactly messenger RNA is. To do this, we first need to go to the types of RNA. These are the following:

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

As we have seen, each type of RNA plays an essential role in protein synthesis: one translates and transports the DNA information, another carries the assembly "blocks" to the ribosomes where proteins are synthesized and another is part of the synthesizing “machinery” itself same. It seems incredible that such a seemingly simple molecule can do such complex jobs, right?

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

Now that you understand all the major types of RNA, it's time to figure out why the term codon is so important.

The importance of the genetic code

The genetic code is a term that responds to the set of instructions that tell the cell how to make a specific protein. That is, the letters that we have seen before, both DNA and RNA. In DNA, the code for each gene combines the four letters (A, G, C, and T) in different ways to make form three-letter “words” that 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 information is found. genetics and transport the instructions for the synthesis of that protein to the ribosomes (located in the cytoplasm).

Each one of the "three-letter words" of DNA translated and contained in the mRNA is, as you may have already 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 most living beings there are 20 different amino acids., and it should be noted that each of them (not in all cases but in almost all) are encoded 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 made up of three ribonucleotides).

• You may be interested in: "DNA translation: what it is and what are its phases"

The role of the codon in translation

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

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

For it, tRNA has a sequence of three nucleotides that match those of the codon: the anticodon. This allows this ribonucleic acid to recognize the order of amino acids in the protein, based on the instructions given 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. On the other hand, mutations in the first and second letters can be harmful, since they tend to give rise to a different amino acid than the one sought, thus breaking the assembly chain so elaborate.

Beyond genetics

As we have seen, this association of three nucleotides 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 the living being, the expression of the genes can.. Epigenetics is responsible for exploring these mechanisms.

Various genes can be silenced in the DNA of living beings, resulting in the inhibition of some processes of transcription and translation of certain proteins at the cellular level. If the genetic information is not transcribed into the mRNA, each of the codons will not occur, and therefore, they will not be able to 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, therefore also the tissues, which allows the formation of the living being in question.

Therefore, we do not exaggerate when saying that without this nucleotide triplet, life as we know it today would not be possible.

Bibliographic references:

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