Structure of DNA – Concept, discovery, types and RNA

We explain what is the structure of DNA, what are the types of structure, and how it was discovered. We also talk about the structure of RNA.

What is the structure of DNA like?

The molecular structure of DNA (or the structure of DNA) is the biochemical composition of DNA, the specific form of organization of the proteins and biomolecules that make up the DNA molecules.

To begin, let’s remind ourselves that DNA is an acronym for deoxyribonucleic Acid. DNA is a nucleotide biopolymer, a long molecular structure made of segments (nucleotides) composed in turn by 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). In this sequence of compound are stored all the genetic information of a living being, essential for protein synthesis and for reproductive inheritance. Without DNA there would be no transmission of genetic material.

In prokaryotic living things, DNA is usually linear and circular. But in eukaryotes, the structure of DNA is in the form of a double helix. In both cases, we deal with a double-stranded biomolecule, composed of two long chains in an antiparallel arrangement (pointing in opposite directions), while 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 four levels of this structure:

• Primary level is made up of the sequence of chained nucleotides, whose specific and punctual sequence encodes the genetic information of each individual.
• Secondary level. in a double helix of complementary chains, 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 level refers to the way DNA is stored inside the cells, within structures called chromosomes. These molecules must be folded and arranged in a finite space, so in the case of prokaryotic organisms they usually do it in the form of a superhelix. On the other hand, in the case of eukaryotes, given the larger size of the DNA, which requires the intervention of other proteins, we deal with a more complex compaction.
• Quaternary level refers to the chromatin present inside the nucleus of eukaryotic cells, from which chromosomes are formed during cell division.

Discovery of the structure of DNA

James Watson (left) and Francis Crick (right)

Although the existence of this type of biological compounds had already been known since 1869, the specific molecular shape of DNA was discovered in 1950. Its discovery is attributed mainly to scientists James Watson, from the United States, and Francis Crick, from the Great Britain, who proposed the double helix model of the structure of the DNA.

However, they weren’t the only ones investigating this topic. To determine the structure of the molecules, their work was in fact based on information previously obtained by the British Rosalind Franklin, an expert in X-ray crystallography.

Thanks to a particularly sharp image that Franklin obtained using this technique on her 51st X-ray diffraction pattern (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 – 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 (successively larger, and smaller) with a variation of 35° with respect to the previous one, allowing access to the nitrogenous bases from the outside.
• DNA-A – appears in conditions of low humidity and lower temperature, such as those found inside laboratories. Like the B type above, it presents recurrent grooves although of different proportions (wider and shallower for the minor groove), in addition to a more open structure. The nitrogenous bases are further away from the axis of the double helix, more horizontally inclined and more symmetrical in the center.
• Z-DNA – differs from the previous ones by having a double helix with a left turn (left-handed) in a zigzag skeleton. It is common in DNA sequences that alternate purines and pyrimidines (GCGCGC), so it requires a concentration of cations greater than that of a DNA-B. It is a narrower and longer double helix than the previous ones.

RNA structure

RNA has a single strand of nucleotides.

Unlike DNA, RNA (ribonucleic acid) does not usually appear as a double helix. Conversely, the structure of RNA is a simple, with a single-stranded sequence of nucleotides. Its nitrogenous bases are identical to those of DNA, except for thymine (T), replaced in RNA by uracil (U).

These nucleotides are linked together by phosphodiester bonds. When they attract each other they can generate folds in the RNA chain, thus forming on short areas certain types of loops, helices or hairpins.