We explain what the structure of DNA is, what types exist and how it was discovered. Also, the structure of RNA.
What is the structure of DNA like?
The molecular structure of DNA (or simply the structure of DNA) is the way in which it is composed biochemically, that is, it is the specific form of organization of the protein Y biomolecules that constitute the DNA molecule.
To begin, let's remember that DNA is the acronym for DeoxyriboNucleic Acid. DNA is a nucleotide biopolymer, that is, a long molecular structure composed of segments (nucleotides) composed in turn of a sugar (ribose) and a nitrogen base.
The nitrogenous bases of DNA can be of four types: adenine (A), cytosine (C), thymine (T) or guanine (G), together with a phosphate group. In the sequence of this compound, all the genetic information of a living being, essential for protein synthesis and reproductive inheritance, that is, without DNA there would be no transmission of characters genetic.
In living beings prokaryotes, DNA is usually linear and circular. But in the eukaryotes, the structure of DNA is in the form of a double helix. In both cases, it is a double-stranded biomolecule, that is, composed of two long chains arranged in an antiparallel manner (pointing in opposite directions): their nitrogenous bases are facing each other.
Between these two chains there are hydrogen bonds that hold them together and in the form of a double helix. Traditionally, there are three levels of this structure:
- Primary structure. It is composed of the sequence of chained nucleotides, whose specific and punctual sequence encodes the Genetic information of every individual that exists.
- Secondary structure. The aforementioned double helix of complementary chains, in which the nitrogenous bases are joined following a strict order: adenine with thymine, and cytosine with guanine. This structure varies depending on the type of DNA.
- Tertiary structure. It refers to the way DNA is stored within structures called chromosomes, inside the cell. These molecules must be folded and arranged in a finite space, so in the case of prokaryotic organisms they usually do so in the form of a superhelix, while in the case of eukaryotes a more complex compaction is carried out, given the larger size of the DNA, which requires the intervention of other proteins.
- Quaternary structure. It refers to the chromatin present in the nucleus of eukaryotic cells, from where chromosomes are formed during cell division.
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Discovery of the structure of DNA
James Watson (left) and Francis Crick (right)
The specific molecular shape of DNA was discovered in 1950, despite the fact that the existence of this type of biological compound had already been known since 1869. Its discovery is attributed mainly to scientists James Watson, from the United States, and Francis Crick, from the British, who proposed the double helix model of the structure of DNA.
However, they weren't the only ones investigating this topic. His work, in fact, was based on information previously obtained by the British Rosalind Franklin, an expert in X-ray crystallography to determine the structure of the molecules.
Thanks to a particularly sharp image that Franklin obtained using this technique (the famous "Photograph 51"), Watson and Crick were able to deduce and formulate a three-dimensional model for DNA.
DNA types
By studying its structure, that is, its specific three-dimensional conformation, it is possible to identify three types of DNA observed in living beings, which are:
- DNA-B. This is the most abundant type of DNA in living beings and the only one that follows the double helix model proposed by Watson and Crick. Its structure is regular, since each base pair has the same size, although leaving grooves (successively larger and smaller) with a variation of 35 ° with respect to the previous one, to allow access to the nitrogenous bases from the outside.
- DNA-A. This type of DNA appears in conditions of scarce humidity and less temperature, like those in many laboratories. It presents, like B, recurrent grooves although of different proportions (wider and shallower for the minor groove), in addition to a more open structure, with the nitrogenous bases further away from the axis of the double helix, more inclined with respect to horizontally and more symmetrically in the center.
- Z-DNA. It differs from the previous ones in that it is a double helix with a left turn (left-handed) in a zigzag skeleton, and it is common in DNA sequences that alternate purines and pyrimidines (GCGCGC), so it requires a concentration of cations greater than that of B-DNA. It is a narrower and longer double helix than the previous ones.
RNA structure
Unlike DNA, RNA (Ribonucleic Acid) does not usually appear as a double helix. Rather, the structure of RNA is a single, single-stranded sequence of nucleotides. Its nitrogenous bases are identical to those of DNA, except in the case of thymine (T), replaced in RNA by uracil (U).
These nucleotides are linked together by links phosphodiester. Sometimes they can generate folds in the RNA chain when they attract each other, thus forming certain types of loops, helices or hairpins during short regions.