DNA nucleotides: what are they, characteristics and functions
The human genome project, launched in 1990 with a budget of 3 billion dollars, set the global 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 work of molecular and genetic cutting, we now know that the human genome contains approximately 3 billion base pairs and 20,000-25,000 genes. Even so, much remains to be described, since the functions of each and every section of genetic information that we have encoded in each of our cells are not known.
While scientists investigate, the general population is becoming more and more aware of what genetics is, the science that studies that alphabet of molecules that organize and encode 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 basal structure that codes 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 sequence of nucleotides is its own genetic word, 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: we are going to focus first on each of the parts that give rise to this unique molecule.
Pentases are monosaccharides, simple carbohydrates (sugars), formed by a chain of 5 carbon atoms united that fulfill a clear structural function. Pentose can be a ribose, which gives rise to a ribonucleoside, the basic structure of RNA. On the other hand, if ribose loses an oxygen atom, deoxyribose arises, the pentose that is part of the deoxyribonucleoside, the main structure of DNA.
2. Nitrogen base
As we have said before, pentose and a nitrogenous base give rise to a ribonucleoside or deoxyribonucleoside, but what is a base? Nitrogen bases are cyclic organic compounds that include two or more nitrogen atoms. In them the key to the genetic code is found, as they give a specific name to each of the nucleotides of which they are part. There are 3 types of these heterocyclic compounds:
Nitrogenous purine 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.
Isoaloxacinic nitrogenous bases: flavin (F). It is not part of DNA or RNA, but it 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 some blackboard or informative scientific material at some point in our lives. For example, GATTACA is an example of a 7 nucleotide DNA sequence, each with a base that gives it its name.
3. Phosphate group
We already have the complete nucleoside, since we have described 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 with 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 carry chemical energy (ATP).
Nucleotide: Nucleoside (base + pentose) + phosphate group
Deciphering life using DNA nucleotides
All this chemical information is great, but how do we put it into practice? Well, first of all, we must bear in mind that every three coding nucleotides form a different phrase to provide information on each of the assemblages that give rise to a protein. Let’s 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 nucleus contain the instructions for assembling the amino acid isoleucine, which is one of the 20 amino acids used for the synthesis of functional proteins. We clarify the following: it is not that the three sequences are necessary to assemble isoleucine, but rather that the three are interchangeable because they all code for this amino acid (redundancy).
Through a process that does not concern us too much here, the cellular machinery performs a procedure called transcription, by which these DNA nucleotide triplets are translated into RNA. As the nitrogenous base thymine is not part of the RNA, each (T) should be replaced by a (U). Thus, these nucleotide triplets would look like this:
If the cell requires isoleucine, an RNA transcribed with any of these three triplets (now called codons) will travel from the nucleus of the cell to the ribosomes in the cytosol of the cell, where they will be ordered to integrate the amino acid isoleucine into the cell. protein that is being built at that time.
Using this nucleotide language based on nitrogenous bases, a total of 64 codons can be produced, which code for the 20 amino acids necessary to build any protein in living beings. It should be noted that, except for a few occasions, each amino acid can be encoded by 2,3,4 or 6 different codons. In the case we have seen before of isoleucine, for example, three possible nucleotide combinations are valid.
Proteins are generally made up of between 100 and 300 amino acids. Thus, a protein composed of 100 of them, making calculations, will be encoded by 300 codons (each triplet of bases responds 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 out of the blue can be somewhat 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 within the nucleus of the cell 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 phrase.
A tireless librarian, in this case the cell’s RNA polymerase enzyme, is seeking to transform the words of one of the books into tangible material. Well, it will look for the specific book, the specific phrase, and since words cannot be ripped from the pages (DNA cannot be moved from the core), it will copy the relevant information into its own form in its own notebook.
The “copied phrases” are nothing more than DNA nucleotides converted into RNA nucleotides, that is, 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 ribosomes, places where proteins are synthesized from a sequence of amino acids in a specific order. Simpler like that, right?
As you may have observed, explaining the intricate processes encoded by DNA is almost as complex as understanding them. Even so, if we want you to have a specific idea of this entire conglomerate of terminology, this is the following: the order of nucleotides present in the DNA of living beings codes for the correct synthesis of proteins, which translates into various metabolic processes and in each of the parts of our body that define us, since these represent 50% of the dry weight of almost any tissue.
Thus, the expression of DNA (genotype) through cellular mechanisms gives rise to our external traits (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 smaller things.
- Nucleic acids, University of Valencia.
- Genetic code, National Human Genome Research Institute (NIH).
- FOX KELLER, EVELYN (2005). From nucleotide sequences to Systems Biology. Sciences, (077).
- Spalvieri, MP & Rotenberg, RG (2004). Genomic medicine: Applications of nucleotide polymorphism and DNA microarrays. Medicine (Buenos Aires), 64 (6): pp. 533-542.