DNA nucleotides: what they are, their characteristics and functions

Let's see what DNA nucleotides are and how they function in the cellular machinery.

The human genome project, launched in 1990 with a budget of 3 billion dollars, had the overall goal of mapping the chemical bases that produce our DNA and identifying all the genes present in the genome of the human species. Sequencing was completed in 2003, 13 years later.

Thanks to this titanic task of molecular and genetic cutting, we now know that the human genome contains approximately 3,000 million base pairs and some 20,000-25,000 genes. Even so, much remains to be described, since the functions of each and every one of the sections of genetic information encoded in each of our cells are not known.

While scientists are doing research, the general population is becoming increasingly aware of genetics, the science that studies the alphabet of molecules that organize and code heredity and each of our vital functions. We are nothing without our genes and, although they are not visible to the naked eye, all living material "is" thanks to them. Since we cannot acquire knowledge without starting at the beginning, in this article we introduce you to the basic structure that encodes our existence: DNA nucleotides..

What is a nucleotide?

A nucleotide is defined as an organic molecule formed by the covalent union of a nucleoside (pentose + nitrogenous base) and a phosphate group..

A succession of nucleotides is a genetic term of its own, since its order encodes the synthesis of proteins by the cellular machinery and, therefore, the metabolism of the living being. But let's not get ahead of ourselves: let's focus first on each of the parts that make up this unique molecule.

1. Pentose

Pentoses are monosaccharides, simple carbohydrates (sugars), made up of a chain of 5 carbon atoms linked together that have a clear structural function. atoms that have a clear structural function. The pentose can be a ribose, which gives rise to a ribonucleoside, the basic structure of RNA. On the other hand, if the ribose loses an oxygen atom, deoxyribose arises, the pentose that forms part of the deoxyribonucleoside, the main structure of DNA.

2. Nitrogenous base

As mentioned above, pentose and a nitrogenous base give rise to a ribonucleoside or deoxyribonucleoside, but what is a base? Nitrogenous bases are cyclic organic compounds that include two or more nitrogen atoms. They are the key to the genetic code. are the key to the genetic code, since they give a specific name to each of the nucleotides of which they are a part.. There are 3 types of these heterocyclic compounds:

Pubic nitrogenous bases: adenine (A) and guanine (G). Both are part of both DNA and RNA. Pyrimidine nitrogenous bases: cytosine (C), thymine (T) and uracil (U). Thymine is unique to DNA, while uracil is unique to RNA.

Isoaloxacin nitrogenous bases: flavin (F). It is not part of either DNA or RNA, but fulfills other processes.

Thus, if a nucleotide contains a thymine base, it is directly called (T). The nitrogenous bases are the ones that give name to those sequences that we have all seen on a blackboard or in scientific material at some point in our lives. For example, GATTACA is an example of a DNA sequence of 7 nucleotides, each with a base that gives it its name..

3. Phosphate group

We already have the nucleoside in its entirety, since we have described the pentose, which is linked by a glycosidic bond to one of the bases A, G, C and T. Now we only need one compound to have the nucleotide in its entirety: the phosphate group.

A phosphate group is a polyatomic ion composed of a central phosphorus atom (P) surrounded by four identical oxygen atoms in a tetrahedral arrangement.. This combination of atoms is essential for life, as it is part of the nucleotides of DNA and RNA, but also of those that transport chemical energy (ATP).

Nucleotide: Nucleoside (base + pentose) + phosphate group

Deciphering life through DNA nucleotides

All this chemical information is all very well, but how do we put it into practice? Well, first of all, we have to take into account that every three coding nucleotides form a different phrase to provide information for each of the assemblies that give rise to a protein.. Let us take an example:

• ATT: adenine, thymine and thymine.
• ACT: adenine, cytosine and thymine
• ATA: adenine, thymine and adenine

These three nucleotide sequences encoded in the cell's DNA core contain the instructions for assembling the amino acid isoleucine, which is one of the 20 amino acids used for the synthesis of functional proteins. Let us clarify the following: it is not that all three sequences are necessary to assemble isoleucine, but that all three are interchangeable because they all code for this amino acid (redundancy).

By a process that does not concern us much here, the cellular machinery performs a procedure called transcription, by which these triplets of DNA nucleotides are translated into RNA.. Since the nitrogenous base thymine is not part of the RNA, each (T) would have to be replaced by a (U). Thus, these nucleotide triplets would look like this:

• AUU
• ACU
• AUA

If the cell requires isoleucine, an RNA transcribed with any of these three triplets (now called codons) will travel from the cell nucleus to the ribosomes in the cytosol of the cell, where they will be instructed to integrate the amino acid isoleucine into the protein currently being constructed.

By means of this language of nucleotides based on the nitrogenous bases, a total of 64 codons can be produced, which code for the isoleucine amino acid isoleucine.which code for the 20 amino acids needed to build any living protein. It should be noted that, except in rare cases, each amino acid can be encoded by 2, 3, 4 or 6 different codons. In the case of isoleucine, for example, three possible nucleotide combinations are valid.

Proteins are generally composed of between 100 and 300 amino acids.. Thus, a protein composed of 100 of them, by calculation, will be encoded by 300 codons (each triplet of bases corresponds to an amino acid, remember), which will be the product of the translation of 300 nucleotides of DNA present in the genome of the cell.

A summary explanation

We understand that all this explanation at once can be a bit dizzying, but we assure you that with the similes that we present below, the function of DNA nucleotides will be clearer than water.

We must see the DNA inside the cell nucleus as a huge library full of books.. Each of the books is a gene, which contains (in the case of humans) about 150 letters, which are nucleotides ordered for a specific purpose. Thus, every three of these nucleotide letters form a small sentence.

A tireless librarian, in this case the RNA polymerase enzyme the cell's RNA polymerase enzyme, is seeking to transform the words in one of the books into tangible material.. Well, it will look for the particular book, the particular sentence and, as the words cannot be torn from the pages (DNA cannot be moved from the nucleus), it will copy the information relevant to its form into its own notebook.

The "copied phrases" are nothing more than DNA nucleotides converted into RNA nucleotides, i.e. codons. Once this information has been transcribed (transcription), a machine is ready to assemble the information contained in each of the words accordingly. These are the ribosomes, places where proteins are synthesized from a sequence of amino acids in a specific order. Easier that way, isn't it?

Summary

As you may have noticed, explaining the intricate processes encoded by DNA is almost as complex as understanding them. Even so, if we want you to have a concrete idea of all this terminological conglomerate, this is the following: the order of the nucleotides present in the DNA of living organisms encodes the correct synthesis of proteins, which translates into a variety of different processes.This translates into various metabolic processes and into each of the parts of our body that define us, since they represent 50% of the dry weight of almost any tissue.

Thus, the expression of DNA (genotype) through cellular mechanisms gives rise to our external features (phenotype), the characteristics that make us who we are, both individually and as a species. Sometimes, the explanation of enormous phenomena lies in the understanding of much, much smaller things.

Bibliographical references:

• Nucleic acids, University of Valencia.
• Genetic code, National Human Genome Research Institute (NIH).
• FOX KELLER, E. V. E. L. Y. N. (2005). From nucleotide sequences to systems biology. Science, (077).
• Spalvieri, M.P. & Rotenberg, R.G. (2004). Genomic medicine: applications of single nucleotide polymorphism and DNA microarrays. Medicine (Buenos Aires), 64(6): pp. 533 - 542.