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34

La Lettre

Jons Jacob Berzélius

(1779-1848)

© Georgios Kollidas - Fotolia

© Alamy

© Leigh Prather - Fotolia

Tridimensional structure of DNA by X-ray diffraction

Francis Crick

(1916-2004)

Fifty years later, the Swedish Berzelius built on Lavoisier’s approach

and proposed a theory according to which any chemical reaction

resulted from the combination of groups of atoms that he called

"chemical radicals

"

. He even invented the term "catalysis

"

to

describe the specific acceleration of a chemical reaction and

called "polymers

"

the “organic” compounds that living beings

produce from these very chemical radicals. A most animated

debate then shook the scientific community. Could one believe

in the vitalist theories that, invoking mysterious forces, claimed a

difference between organic compounds – produced by living beings – and

inorganic substances? The chemical synthesis of urea by Whöler, in 1825, brought a first demonstration

that vitalism had failed; the many organic syntheses performed since then have confirmed such failure.

The repertoire of the molecules that constitute living beings comprises high-weight polymers, or

"macromolecules

"

, including in particular proteins – another term Berzelius coined – which result from

the linear combination of 20 amino acids. Other macromolecules are formed from nucleotides – such as

desoxyribo- and ribo-nucleic acids (DNA and RNA) – , sugars – for polysaccharides – and so forth. In 1833,

it was observed that the enzymes catalyzing the chemical reactions that constitute living organisms were

proteins. Finally, in 1897, Buchner demonstrated that the whole complex processes involved in alcohol

fermentation could be obtained in vitro from an extract of yeast, in the very absence of any living yeast.

The relevant set of enzymes and their substrates were enough. From then on, living beings – including

human beings – would be understood as vast and complex chemical systems in which macromolecules

would play a central role.

Throughout the 20

th

Century, and even today, research has been dealing with the fine structures, at the

atomic level, of these macromolecules. Various physical techniques have been used, including X-ray

diffraction. In 1953, it was the double strandedDNAmodel described byWatson, Crick andRosalind Franklin

that revolutionized our understanding of heredity. It would be followed, more recently, by the elucidation of