DNA Structure (Meaning and Explanation)

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 it is biochemically composed, that is, It is the specific form of organization of the proteins and biomolecules that make up the DNA molecule.

To begin, let's remember that DNA is the acronym for DeoxyriboNucleic Acid. DNA is a biopolymer of nucleotides that is, a long molecular structure composed of segments (nucleotides) composed in turn of a sugar (ribose) and a nitrogenous base.

The nitrogenous bases of DNA can be of four types: adenine (A), cytosine (C), thymine (T) or guanine (G), along with a phosphate group. In the sequence of this compound stores 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 genetic characters.

In prokaryotic living beings, DNA is usually linear and circular. But in eukaryotes, the DNA structure 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 antiparallel (pointing in opposite directions): their nitrogenous bases face each other.

Between these two chains there are hydrogen bonds that keep them together and in the form of a double helix. Traditionally, three levels of this structure are distinguished:

Primary structure It is made up of the sequence of chained nucleotides, whose specific and specific sequence encodes the genetic information of each individual that exists.
Secondary structure The aforementioned double helix of complementary chains, in which the nitrogenous bases are joined in 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 fold and arrange themselves 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 molecule. DNA, which requires the intervention of other proteins.
Quaternary structure It refers to the chromatin present in the nucleus of eukaryotic cells, from which chromosomes are formed during cell division.

Discovery of the structure of DNA

discovery of DNA - Watson and crick — James Watson (left) and Francis Crick (right)

The specific molecular form of DNA was discovered in 1950 despite the fact that the existence of this type of biological compounds had already been known since 1869. Its discovery is mainly attributed to the scientists James Watson, American, and Francis Crick, British, who proposed the double helix model of the structure of the DNA.

However, they were not the only ones investigating this topic. Their 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 molecules.

Thanks to a particularly clear 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.

Types of DNA

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:

B-DNA 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 pair of bases has the same size, although leaving grooves (larger and smaller successively) 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 low humidity and lower temperature, such as those found in many laboratories. It presents, like B, recurring 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 the horizontal and more symmetrically in the center.
Z-DNA It is distinguished from the previous ones in that it is a double helix with a left-handed turn (left-handed) in a zigzag skeleton, and it is common in DNA sequences that alternate purines and pyrimidines (GCGCGC), which is why it requires a concentration of cations greater than that of B-DNA. It is a double helix that is narrower and longer than the previous ones.

RNA structure

structure of dna rna — RNA has a single chain of nucleotides.

Unlike DNA, RNA (Ribonucleic Acid) does not usually appear in the form of a double helix. On the contrary, The structure of RNA is a simple, 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 phosphodiester bonds. Sometimes they can generate folds in the RNA chain by attracting each other, thus forming certain types of loops, helices or hairpins during short regions.