Nucleic acids are giant molecules (macromolecules) formed by smaller monomeric units known as nucleotides. Each nucleotide, in turn, consists of three parts:
- a sugar of the pentose group (monosaccharides with five carbon atoms);
- a phosphate radical, derived from the orthophosphoric acid molecule (H3PO4).
- a nitrogenous organic base.
They were known to be present in cells, but the discovery of their function as cellular activity controlling substances was one of the most important steps in the history of biology.
From the nineteenth century, with the work of the Swiss doctor Miescher, began to suspect that the nucleic acids were directly responsible for everything that happened in a cell. In 1953, the American biochemist James D. Watson and the molecular biologist Francis Crick proposed a model that sought to clarify the structure and principles of operation of these substances.
The amount of knowledge accumulated thereafter characterizes the most extraordinary biological knowledge that culminated in the creation of Genetic engineering, an area of biology that deals directly with nucleic acids and their biological role.
Of its three components (sugar, phosphate radical and nitrogenous organic base) only the phosphate radical does not vary in nucleotide. Sugars and nitrogenous bases are variable.
As for sugars, two types of pentoses can be part of a nucleotide: ribose and deoxyribose (so-called because it has one less oxygen atom than ribose.
Already the nitrogenous bases belong to two groups:
- purine: adenine (A) and guanine (G);
- the pyrimidine: thymine (T), cytosine (C) and uracil (U).