Until 1828, chemists believed that compounds were divided into two distinct categories or classes: inorganic compounds, from the mineral world, such as table salt, and organic compounds, from living organisms both animals and vegetables, such as sugar.

In 1828, a German chemist, Friedrich Wohler, succeeded in obtaining urea from inorganic compounds. Urea is a purely organic compound found in animal urine, and he obtained it by heating ammonium cyanate (NH₄ CNO), a purely inorganic reagent. The two classes, hitherto distinct, are from this point on linked together: organic chemistry can be created and synthesized from inorganic chemistry.

In the 19th century, chemists began synthesizing more and more new molecules, effectively starting modern organic chemistry, the chemistry of carbon compounds.

Small parenthesis: chemists not only began to synthesize organic elements, but also colours, which until then had only been natural, and so the synthetic dye industry began. The first was Perkin’s mauve colour in 1857, which changed the history of fashion and is one of the symbols of industrialization and modernity.

In organic chemistry, biomolecules began to be classified into large groups including proteins, lipids, and carbohydrates. But surely the most sensational moment in the history of 20th-century biochemistry was the discovery of the structure of the DNA double helix in 1953 by James Watson and Francis Crick. Their understanding of DNA and the genetic code provided the fundamental link to link chemistry and biology forever.

DNA is a polymer, i.e. a large molecule made up of the union of many smaller, similar molecules called monomers. The monomers that form DNA are called nucleotides. Each nucleotide consists of three distinct parts: a pentose, i.e. a sugar with 5 carbon atoms (deoxyribose), a nitrogenous base, and a phosphate group.

The nitrogenous bases are of two classes, the purines, double-ring structures formed of nine atoms, and the pyrimidines, single-ring structures formed of six atoms. There are two purines: adenine

(A) and guanine (G), and two pyrimidines: thymine (T) and cytosine (C), both formed of carbon and nitrogen atoms.

In summary, DNA contains four nitrogenous bases: adenine, guanine, cytosine, and thymine;

composed of carbon (C) and nitrogen (N).

The DNA molecule, made up of these four nitrogenous bases, has the shape of a double helix; we can imagine it as a rung ladder in which the risers are formed by alternating phosphate and sugar (pentose) groups and each step corresponds to a nitrogenous base pair. The nitrogen base pairs are planar (lying horizontally) and in the centre of the molecule are stabilized by hydrophobic interactions, which contribute to the overall stability of the double helix.

Since the AT (adenine – thymine) and GC (guanine – cytosine) pairs have the same length, and therefore fit easily between the two rungs like the rungs of a ladder, the helix has a constant diameter. In addition, each rung is rotated about the previous rung by approximately 36°. The propeller, therefore, makes one complete turn every 10 pairs of rungs. The helix is right-handed: when viewed from above, it appears to wind clockwise.

Every living organism has cells inside it that have a nucleus in which there are chromosomes, structures that contain a person’s genes. There are 23 pairs of chromosomes present in the nuclei of our cells.

Genes are a nucleotide sequence, i.e. a portion of DNA. What characterizes a gene is ‘having a function. Genes are said to ‘code for a protein’, i.e. they express instructions for building the basic building blocks of our body, the proteins, the functional units of our body that perform a wide variety of tasks, from metabolic functions to transporting molecules from one place to another.

Nucleotides are the ‘letters’ that form the instructions to make our organism, which has more than 3 billion nucleotide base pairs, function. In DNA is written all the information that parents have passed on to their children.

More than other disciplines, chemistry has given human beings not only the ability to understand the elements of nature, but also the knowledge that they can transform them.