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 millimetrically in each of our cells as if it were a perfect chain of work, we need imagination and above all knowledge.

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

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

    Codó: the triplet of life

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

    About the ARN and its organization

    The acronym RNA stands for “Ribonucleic Acid”. It is a polymer chain made up 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 other things, in that the latter has the nitrogenous base of thymine (T) instead of uracil (U). In general, nucleotides are named after the nitrogen 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 messenger RNA is. To do this, you must first go to types of RNA. These are:

    • Messenger RNA (mRNA): DNA has the information necessary for the synthesis of proteins. MRNA is responsible for its translation and transport to ribosomes.
    • RNA transfer (tRNA): transports specific amino acids to the site of protein growth.
    • Ribosomal RNA (rRNA): is combined with several 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 carries information on DNA, another carries the coupling “blocks” to ribosomes where proteins are synthesized, and another is part of the synthetic “machinery” itself. It seems incredible that such a seemingly simple molecule can do such a complex job, doesn’t it?

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

    Once you understand all the major types of RNA, it’s time to find out why the term codon is so important.

    The importance of the genetic code

    The genetic code is a term that corresponds to the set of instructions telling the cell how to synthesize a specific protein. In other words, 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 form three-letter “words”, which each specify the amino acids that make up a protein.

    These “words” encoded in DNA are transcribed by a process called transcription, whereby a segment (gene) of DNA gives rise to the messenger RNA explained above. This RNA is mobile, so it can leave the cell nucleus where the genetic information is located and transport the instructions for synthesizing this protein to the ribosomes (located in the cytoplasm).

    Each of the “three letter words” in DNA translated and contained in mRNA is, as you may have guessed, the codon that belongs to 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 things, 61 of which encode amino acids. For most living things there are 20 different amino acids, And it should be noted that each of them (not in all cases but in almost all) is encoded by 2, 3, 4 or 6 different codons. Therefore, and applying basic math, an amino acid made up of 6 codons would be encoded by 18 translated nucleotides (remember that each codon is made up of three ribonucleotides).

      The role of the codon in translation

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

      Translation is defined as the process of translate (worth the redundancy) a messenger RNA molecule into an amino acid sequence that will give rise to a specific protein. As we have argued above, transfer RNA (tRNA) is responsible for transferring amino acids to the construction zone (the ribosome), but not only, as it is also responsible for sorting it along. the messenger RNA molecule.

      For that, TRNA has a sequence of three nucleotides that correspond to those of the codon: The anticodon. This allows this ribonucleic acid to recognize the order of amino acids in the protein, according to the instructions provided by the mRNA codons.

      Codons and mutations

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

      For example, leucine responds to the CUU, CUC, CUA codons. Thus, the third letter mutations are considered to be silent, because the same amino acid is synthesized and the protein can pair without problem. On the other hand, mutations in the first and second letter can be harmful, as they usually lead to an amino acid other than the one sought, thus breaking the assembly line 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 genetic code of the individual. Although genetic information itself does not change throughout a living being’s life, gene expression can.. Epigenetics is responsible for exploring these mechanisms.

      In the DNA of living beings, various genes can be silenced, which results in the inhibition of certain processes of transcription and translation of certain proteins at the cellular level. If the genetic information is not transcribed into mRNA, it will not result in each of the codons, and therefore cannot be translated into amino acids and the protein in question will not be coupled.

      conclusions

      In these lines we have tried to convey what the codon is a form of organization of genetic information essential for the synthesis of proteins at the cellular level in living organisms. These proteins constitute the cells, and therefore also the tissues, which allows the formation of the living in question.

      Therefore, we will not exaggerate by saying that without this triplet of nucleotides, life as we know it today would not be possible.

      Bibliographical references:

      • Crick, FHC (1966). Codon-anticodon pairing: the swing hypothesis.
      • Bennetzen, JL and Hall, BD (1982). Selection of codons in yeast. Journal of Biological Chemistry, 257 (6), 3026-3031.
      • Déctor, MA, and Arias, CF (2004). RNA interference: a primitive defense system. Science, 55, 25-36.
      • Neissa, JI and Guerrero, C. (2004). From genetic code to epigenetic code: new therapeutic strategies. Journal of the Faculty of Medicine, 52 (4), 287-303.

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