The DNA or deoxyribonucleic acid according to abbreviationfinder, is a type of nucleic acid, one macromolecule that is part of all cells.
It contains the genetic information used in the development and functioning of known living organisms and some viruses, being responsible for their hereditary transmission.
From a chemical point of view, DNA is a nucleotide polymer, that is, a polynucleotide. A polymer is a compound made up of many simple units connected together, as if it were a long train made up of wagons. In DNA, each wagon is a nucleotide, and each nucleotide, in turn, is made up of a sugar (deoxyribose), a nitrogenous base (which can be
- adenine (A),
- thymine (T),
- cytosine (C) or
- guanine (G) and
a phosphate group that acts as a coupling of each wagon with the next.
What distinguishes one wagon (nucleotide) from another is, then, the nitrogenous base, and therefore the DNA sequence is specified by naming only the sequence of its bases. The sequential arrangement of these four bases along the chain (the ordering of the four types of wagons along the entire train) is what encodes the genetic information: for example, a DNA sequence can be ATGCTAGATCGC…
In living organisms, DNA occurs as a double chain of nucleotides, in which the two strands are linked together by connections called hydrogen bonds. In order for the information contained in DNA to be used by the cellular machinery, it must first be copied into shorter nucleotide trains with different units, called RNA. The molecules of RNA are copied exactly DNA by a process called transcription. Once processed in the cell nucleus, the RNA molecules can exit to the cytoplasm for later use.
The information contained in the RNA is interpreted using the Genetic Code, which specifies the sequence of the amino acids of the proteins, according to a correspondence of a nucleotide triplet (Codon) for each amino acid. That is, genetic information (essentially: what proteins are going to be produced at each moment in the life cycle of a cell) is encoded in the nucleotide sequences of DNA and must be translated in order to be used. Such translation is done using the genetic code as a dictionary. The dictionary “nucleotide sequence-amino acid sequence” allows the assembly of long chains of amino acids (proteins) in the cytoplasm of the cell.
For example, in the case of the DNA sequence indicated above (ATGCTAGATCGC …), the RNA polymerase would use as a template the complementary strand of said DNA sequence (which would be TAC-GAT-CTA-GCG -…) to transcribing an mRNA molecule that would read AUG-CUA-GAU-CGC -…; the resulting mRNA, using the genetic code, would be translated as the amino acid sequence Methionine – Leucine – Aspartic acid – Arginine -…
The DNA sequences that constitute the fundamental, physical and functional unit of heredity are called Genes. Each gene contains a part that is transcribed into RNA and another that is responsible for defining when and where they should be expressed. The information contained in genes (genetics) is used to generate RNA and Proteins, which are the basic components of cells, the “building blocks” that are used to build cell organelles, among other functions.
Inside cells, DNA is organized into structures called chromosomes that, during the cell cycle, are duplicated before the cell divides. The eukaryotic organisms (e.g., animals, plants, and fungi) store the vast majority of its DNA into the cell nucleus and a fraction in the cellular elements called Mitochondria and chloroplasts, and the microtubule organizing centers or Centrioles, if have them; the prokaryotes (Bacteriaand Archeas) store it in the cytoplasm of the cell, and finally, DNA viruses do it inside the protein capsid.
There are many proteins, such as Histones and transcription factors, that bind to DNA, giving it a specific three-dimensional structure and regulating its expression. Transcription factors recognize regulatory sequences in DNA and specify the transcription pattern of genes. The complete genetic material of a chromosomal endowment is called the Genome and, with minor variations, it is characteristic of each species.
DNA was first isolated during the winter of 1869 by the Swiss physician Friedrich Miescher while working at the University of Tübingen. Miescher was conducting experiments on the chemical composition of Pus from discarded surgical bandages when he noticed a precipitate of an unknown substance that he chemically characterized later. It took almost 70 years of research to identify the components and structure of nucleic acids. In 1919 Phoebus Levene identified that a nucleotide is made up of a base, a Sugar and a Phosphate. However, Levene thought that the chain was short and that the bases repeated in a fixed order. In 1937 William Astbury produced the first X-ray Diffraction pattern showing that DNA had a regular structure.
The biological function of DNA began to be elucidated in 1928, with a basic series of experiments in modern genetics carried out by Frederick Griffith, who was working with “smooth” (S) or “rough” (R) strains of the Pneumococcus bacteria (causing pneumonia), according to the presence (S) or not (R) of a sugary capsule that is the one that confers virulence (see also Griffith’s experiment).
Injecting live S pneumococci into mice kills them, and Griffith observed that if he injected mice with live R pneumococci or heat-killed S pneumococci, the mice did not die. However, if he injected both live R pneumococci and dead S pneumococci, the mice died, and live S pneumococci could be isolated from their blood.
Since the dead bacteria could not have multiplied within the mouse, Griffith reasoned that some kind of change or transformation from one bacterial type to another had to take place through a transfer of some active substance, which he called the “transforming principle.” This substance provided the ability to R pneumococci to produce a sugary capsule and thus become virulent. Over the next 15 years, these initial experiments were duplicated by mixing different types of heat-killed bacterial strains with live ones, both in mice (in vivo) and in test tubes (in vitro).
The search for the “transforming factor” that was capable of making strains that initially were not virulent continued until 1944, the year in which Oswald Avery, Colin MacLeod and Maclyn McCarty carried out a now classic experiment. These researchers extracted the active fraction (the transforming factor), and by means of chemical, enzymatic and serological analyzes, they observed that it did not contain proteins, unbound lipids, or active polysaccharides, but was mainly constituted by “a highly viscous form of deoxyribonucleic acid. polymerized, “that is, DNA.
The DNA extracted from the heat-killed S bacterial strains was mixed “in vitro” with live R strains: the result was that S bacterial colonies were formed, so it was unequivocally concluded that the transforming factor or principle was DNA.
Regarding the chemical characterization of the molecule, in 1940 Chargaff carried out some experiments that helped him to establish the proportions of the nitrogenous bases in DNA. He found that the proportions of purines were identical to those of pyrimidines; the “equimolecularity” of the bases ([A] = [T], [G] = [C]) and that the amount of G + C in a given DNA molecule is not always equal to the amount of A + T and it can vary from 36% to 70% of the total content. With this information and together with the X-ray Diffraction data provided by Rosalind Franklin; James Watson and Francis Crick proposed in 1953 the DNA double helix model to represent the three-dimensional structure of the polymer.
In 1962, after Franklin’s death, scientists Watson, Crick, and Wilkins were jointly awarded the Nobel Prize in Physiology or Medicine. The Nobel Prize in Physiology or Medicine 1962 Nobelprize .org (Revised December 22, 06). However, the debate continues as to who should receive credit for the discovery.
Physical and chemical properties
DNA is a long Polymer made up of repetitive units, the Nucleotides. A DNA double strand is 22 to 26 Angstroms (2.2 to 2.6 Nanometers) wide, and a unit (a nucleotide) is 3.3 Å (0.33 nm) long.
In the organisms alive, the DNA does not usually exist as a single molecule, but as a closely associated pair of molecules. The two strands of DNA twist around themselves forming a kind of spiral staircase, called a double helix. The double helix structure model was proposed in 1953by James Watson and Francis Crick (the article Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid was published on April 25, 1953 in Nature ). Watson JD; Crick FHC. A structure for Deoxyribose Nucleic Acid. Nature 171 (4356): 737-738.. (April, 1953) Full Text
The success of this model lay in its consistency with the physical and chemical properties of DNA. The study also showed that the complementarity of bases could be relevant in their replication, and also the importance of the sequence of bases as a carrier of genetic information.Each repeating unit, the nucleotide, contains a segment of the support structure (sugar + phosphate), which holds the strand together, and a Base, which interacts with the other strand of DNA in the helix.
In general, a base linked to a sugar is called a Nucleoside and a base linked to a sugar and to one or more phosphate groups is called a Nucleotide. When many nucleotides are united, as it happens in DNA, the resulting polymer is called a Polynucleotide. Abbreviations and Symbols for Nucleic Acids, Polynucleotides and their Constituents IUPAC-IUB Commission on Biochemical Nomenclature (CBN), accessed 3 January 2006