por We explain what DNA is, its characteristics, functions and structure
What is DNA?
DNA (deoxyribonucleic acid) is the biomolecule that makes up the genome of all living beings and some types of viruses. It contains the necessary information for the origin of an organism and for it to function.
Scientists use the term genome to refer to the complete set of all DNA molecules found in the cells of any living thing: in the eukaryotic cell nucleus (and a small amount in mitochondria and chloroplasts) and in the nucleoid region of the cytosol. of prokaryotic cells.
DNA is conceived as the set of blueprints or genetic instructions of each living organism, since the information contained in the genes (defined segments of the genome) provides the molecular bases for the manufacture of proteins, which are the main structural blocks of DNA. cells.
Proteins not only form the enzymes capable of catalyzing cellular chemical reactions, but are also the primary entities that enable a cell to regulate the activity of its genes, move, communicate with, respond to, and multiply with its environment. complete its life cycle.
Each species has a unique genome that defines it, which is faithfully transmitted from one generation to the next, and this is true for both unicellular and multicellular organisms.
Since before its formal discovery in 1869 by the Swiss Friedrich Miescher, DNA has been extensively studied and is the basis of much of the research in many of the scientific disciplines that have to do with living beings.
DNA characteristics
– DNA is found in cells, mostly in the nucleus (nuclear DNA), and a smaller amount in the mitochondria (mitochondrial DNA).
– It is a macromolecule formed essentially by carbon, hydrogen, oxygen, phosphorus and nitrogen.
– It consists of a double chain of chemical units called nitrogenous bases that complement each other (adenine, thymine, guanine and cytosine) and that are ‘armed’ in a scaffold or skeleton made up of sugars (deoxyribose) and phosphate groups.
– Forms the genome of all living beings on earth (and some viruses).
– In eukaryotic cells, DNA consists of linear molecules associated with proteins (histones) that contribute to their compaction to form chromatin, the ‘substance’ that forms the chromosomes contained in the nucleus.
– In prokaryotic cells it consists of (1) a circular molecule that is located in the nucleoid region of the cytosol, also interacting with some proteins, and (2) some ‘extrachromosomal’ molecules known as plasmids.
– The order of the nitrogenous bases that make up the DNA that forms each gene determines the peptide sequence of the protein for which it codes and this ‘code’ is ‘read and deciphered’ during the transcription and translation processes.
– It is the main cellular hereditary molecule, since the information it carries can be faithfully passed from one generation to the next, which occurs in each species on the biosphere and is essential for maintaining its identity.
DNA functions
Deoxyribonucleic acid has crucial functions for all organisms we know of, and some of these functions were inferred before its structure and physicochemical characteristics were fully understood.
This macromolecule functions fundamentally as:
Hereditary material: all cells in an organism (unicellular or multicellular) have essentially the same set of genes (the same genome), made possible only by the faithful transmission of DNA from one generation to the next through replication .
It encodes and stores information: DNA contains all the information necessary for cells and organisms to be what they are and perform their particular functions. Such information contains the code that, when read, is ‘decoded’ in the proteins that make up the cells, without which life would not be possible.
It admits changes: although constant, the hereditary information contained in DNA can change on certain occasions as a result of mutations, which not only gives living beings a certain plasticity, but also makes possible the development of new species (it is the material on which that ‘natural selection acts’).
DNA structure
DNA (2′-deoxy-5′-ribonucleic acid) is a macromolecule made up of different chemical units that are repeated in different orders, that is, it is a biopolymer and is made up of two linearly linked monomer chains, which we call nucleotides.
double helix
Its main structure consists of a double helix, which is formed by two complementary strands of nucleotides that are linked to each other linearly and transversely by different types of chemical bonds.
The complementary nitrogenous bases are arranged in such a way that the double helix resembles a spiral staircase.
The structure of DNA was elucidated in the 1950s, thanks to the work of great researchers, including J. Watson, F. Crick and R. Franklin.
monomers
Nucleotides -which are the monomers of the DNA polymer- are made up of three chemical molecules:
A sugar.
a phosphate group.
A nitrogenous base.
The sugar is the same for all the monomers: deoxyribose. Deoxyribose is a five-carbon sugar whose cyclic structure contains 4 carbon atoms and one oxygen atom, with a hydroxyl group (-OH) attached to the third carbon of the ring and a carbon atom attached to the fourth.
The phosphate group is also common to all nucleotides and is made up of four oxygen atoms attached to a phosphorous atom.
There are 4 nitrogenous bases: adenine (A), thymine (T), guanine (G) and cytosine (C). These molecules bind to the sugar deoxyribose phosphate to form a nucleotide.
They are known as ‘nitrogenous bases’ because they have more than one nitrogen atom in their structure, and are relatively complex molecules.
5′-3′ connection
One nucleotide is linked to another linearly by a type of covalent bond known as a phosphodiester bond, which connects the 5′ carbon atom of one nucleotide to the 3′ carbon atom of the next.
The 5′ and 3′ numbering refers to the carbon atoms belonging to the sugar of each nucleotide since, by convention, the carbons of deoxyribose are numbered clockwise, with the 1′ atom being the numbering. It is located to the right of the oxygen atom in the ring and the 5′ one is the one that protrudes from the structure.
Base complementarity
As we have already said, DNA has a double helix structure, made up of two complementary nucleotide chains.
While the linear sequence of nucleotides is mediated by covalent bonds such as phosphodiester bonds, the complementary bases that join the facing strands are held by noncovalent bonds called hydrogen bonds.
The complementarity of bases in DNA is such that an adenine always ‘pairs’ with a thymine and a guanine always does so with a cytosine. The bases adenine and guanine belong to the group of purines, while thymine and cytosine belong to the group of pyrimidines.
DNA Importance
Since DNA contains the information necessary to make the proteins that make up cells, this macromolecule is essential for life and its continuity.
DNA depends on the growth, metabolism, reproduction and health of all living beings. This importance becomes even more evident, for example, when DNA suffers damage or mutations, which generally have serious consequences for our health (speaking in the human context).
Although not all of the information contained in DNA functions in the production of proteins, the coding sequences are just as important as the non-coding sequences. Some authors make an interesting analogy with the written text:
Genes that code for proteins are the words in a text, and genes or segments of DNA that do not ‘read’ as proteins provide the spaces and punctuation that make the information in the text ‘readable’.
On the other hand, DNA is fundamental for the maintenance over time of the species, since its perpetuity depends on the information contained in this macromolecule being faithfully passed on to the following generations, containing the least possible number of changes.