The Human Genome Project, launched in 1990 with a budget of $ 3 billion, has set itself the overall objective of mapping the chemical bases that produce our DNA and of 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 around 3,000 million base pairs and around 20,000 to 25,000 genes. However, there is still a lot to describe, as the functions of each of the sections of genetic information that we have encoded in each of our cells are not known.
As scientists study, the general population is becoming more and more aware of what genetics is, of the science that studies this alphabet of molecules that organize and encode inheritance, and of each of our vital functions. We are nothing without our genes, and even if they are not visible to the naked eye, all living material “is” thanks to them. Since we cannot acquire knowledge without starting from the beginning, in this article we present to you the basic structure that codes our existence: DNA nucleotides.
What is a nucleotide
A nucleotide is defined as an organic molecule formed by the covalent bond of a nucleoside (pentose + nitrogenous base) and a phosphate group.
A nucleotide sequence is a genetic word in itself, because its order encodes the synthesis of proteins by the cellular machinery and therefore the metabolism of living beings. But let’s not go any further: we will first focus on each of the parts that give rise to this unique molecule.
Pentoses are monosaccharides, simple carbohydrates (sugars), formed by a chain of 5 carbon atoms units that perform a clear structural function. Pentose can be ribose, giving rise to a ribonucleoside, the basic structure of RNA. On the other hand, if ribose loses an oxygen atom, deoxyribose appears, the pentose which is part of deoxyribonucleoside, the main structure of DNA.
2. Nitrogen base
As we said before, pentose and a nitrogenous base give rise to a ribonucleoside or a deoxyribonucleoside, but what is a base? Nitrogenous bases are cyclic organic compounds which contain at least two nitrogen atoms. in them is the key to the genetic code, because they give a specific name to each of the nucleotides of which they are part. There are 3 types of these heterocyclic compounds:
Pure nitrogen bases: adenine (A) and guanine (G). Both are part of both DNA and RNA. Pyrimidine nitrogen bases: cytosine (C), thymine (T) and uracil (U). Thymine is unique in DNA, while uracil is unique in RNA.
Nitrogenous bases isoaloxacin: flavin (F). It is neither DNA nor RNA, but it performs other processes.
So, if a nucleotide contains a thymine base, it is directly called (T). Nitrogenous bases are what give these sequences a name that we have all seen on a blackboard or popular scientific material at some point in our lives. For example, Gattaca is an example of a DNA sequence with 7 nucleotides, each with a base that gives it its name..
3. Phosphate group
We already have the entire nucleoside, as we described in the pentose, which is linked by a glycosidic bond to one of the bases A, G, C and T. in full: the phosphate group.
A phosphate group is a polyatomic ion composed of a central phosphorus (P) atom 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 of this chemical information is good, but how do you put it into practice? Well, first of all we have to keep in mind that each of the three coding nucleotides forms a different sentence to provide information about each of the couplings 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 DNA nucleus of the cell contain coupling instructions for 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 all three sequences are needed to couple isoleucine, but that all three are interchangeable because they all encode this amino acid (redundancy).
Through a process that is not too much for us here, the cellular machinery performs a procedure called transcription, whereby these triplets of DNA nucleotides are translated into RNA. Since the nitrogenous base of thymine is not part of RNA, each (T) must be replaced by a (U). So these nucleotide triplets would look like this:
- IN THEM
If the cell requires isoleucine, an RNA transcribed with one of these three triplets (now called codons) will travel from the cell nucleus to the ribosomes in the cell’s cytosol, where they will be ordered to integrate the amino acid. isoleucine in the protein being built.
Using this nitrogen-based nucleotide language, a total of 64 codons can be generated., Which encode the 20 amino acids necessary to build any protein in living things. It should be noted that, except on rare occasions, each amino acid can be encoded by 2, 3, 4 or 6 different codons. In the case we saw before isoleucine, for example, three possible nucleotide combinations are valid.
Proteins are generally made up of 100 to 300 amino acids. So a protein composed of 100 of them, by calculation, will be encoded by 300 codons (each base triplet corresponds to an amino acid, remember), which will be the product of the translation of 300 DNA nucleotides present. in the genome of the cell.
A brief explanation
We understand that this whole explanation can suddenly be somewhat dizzying, but we assure you that with the comparisons we present below, the function of DNA nucleotides will be clearer to you than water.
We need to 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. So all three of these nucleotide letters form a short sentence.
A tireless librarian, in this case the RNA polymerase enzyme in the cell seeks to turn the words in one of the books into tangible material. Well, then it will search for the specific book, the specific phrase, and since words cannot be extracted from the pages (DNA cannot be moved from the nucleus), it will copy the relevant information in its form into its own notebook. .
“Copied sentences” are nothing more than DNA nucleotides converted to RNA nucleotides, ie codons. Once this information has been transcribed (transcribed), a machine is ready to assemble the information contained in each of the words in a coherent manner. They are ribosomes, places where proteins are synthesized from a sequence of amino acids in a specific order. Easier like that, right?
As you may have noticed, explaining the complex processes encoded by DNA is almost as complex as understanding them. However, if we want you to stick with one concrete idea of all this terminology conglomerate, it’s this one: the order of nucleotides in the DNA of living things codes for the correct synthesis of proteins, Which results in various metabolic processes and in each of the parts of our bodies that define us, as these make up 50% of the dry weight of almost all tissues.
Thus, the expression of DNA (genotype) through cellular mechanisms gives rise to our external traits (phenotype), characteristics that make us what we are, both at the individual and species level. Sometimes the explanation for huge phenomena lies in understanding 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 and Rotenberg, RG (2004). Genomic medicine: applications of nucleotide polymorphism and DNA micromatrices. Medicine (Buenos Aires), 64 (6): p. 533-542.