The DNA structurethe molecule that carries genetic information, was discovered exactly 70 years ago Watson e Crick. The story of DNA is fascinating and says many things about scientific research, its progress and its doubts. DNA has been known since the dawn of modern biology: it is the substance that makes the liquid obtained by breaking cells viscous and sticky. However, it seemed a rather inert molecule, devoid of obvious functions and trivial compared to the much more dynamic proteins, capable of enzymatic and structural activities. The history of the discovery of the functions of DNA begins with an experiment conducted in 1928 by Frederick Griffith in London. Griffith studied bacteria called streptococci of which he had two variants: one capable of producing a protective cell coat, the capsule, the other genetically incapable of doing so.
A mouse injected with capsulated streptococci invariably develops, or nearly so, a‘fatal infection; in contrast, a mouse injected with non-encapsulated streptococci develops a mild infection from which it recovers. Griffith tried inoculating mice with a mixture of live unenveloped streptococci and heat-killed encapsulated streptococci and observed that the mice died and that at the autopsy their organs were full of capsulated streptococci: there was a “transforming factor” which could be transmitted from dead to live bacteria and modified their genetic properties.
Griffith’s experiment remained an isolated observation until 1944, when Oswald T. Avery in New York he decided to identify the transforming factor. Avery inoculated mice with live unenveloped streptococci and each of the purified components that could be obtained by lysing killed encapsulated streptococci: polysaccharides, proteins, RNA, and DNA. Only mice inoculated with capsulated streptococcal DNA died: the transforming factor was DNA. After this experiment, DNA suddenly became the most important molecule in biology and many scientists rushed to study it.
In Cambridge, England, the method of X-ray crystallography had just been developed to study the structure of biological macromolecules. Crystals are ordered structures in which molecules occupy defined positions and have repetitive orientations. If a crystal is illuminated with a thin beam of X-rays, the electrons of the molecules scatter the beam producing on the film a map made up of distinct speckles, one for each of the diffraction beams. From the diffraction map it is possible to trace the position of the atoms in the molecule and reconstruct its three-dimensional structure.
In Cambridge they had already been resolved three-dimensional structures of the proteins myoglobin (by John Kendrew) and hemoglobin (by Max Perutz). Perutz entrusted the study of DNA to Rosalind Franklin, an extremely gifted young researcher, who managed to crystallize the DNA and obtain the diffraction maps. Unfortunately the maps were difficult to interpret and the calculators of the time were rudimentary; moreover, Perutz knew that Linus Pauling in California was working on the same problem and had obtained diffraction maps of the DNA.
Then Perutz decided to involve DNA in the project James Watson e Francis Crick. Crick in particular was a physicist with a considerable background in mathematics and managed to solve the maps by determining the structure of DNA: two elongated molecules, like filaments wrapped around each other to form the double spiral (which we mistranslate from English as double helix). Watson and Crick announced their discovery on February 28, 1953, at a lecture given in the Eagle pub where they used to go for lunch. In the pub there is still the Watson and Crick table and on the wall behind it two brass plaques remember the first Watson and Crick and the second Rosalind Franklin.
Watson and Crick got the Nobel Prize in 1962 together with Maurice Wilkins, an associate of Rosalind Franklin who had participated in the determination of crystallographic maps. Unfortunately Rosalind Franklin died in 1958, before she was 38, and the Nobel Prize is not given in her memory. In the subsequent scientific publication, Watson and Crick in a masterpiece of British understatement pointed out that the structure of the double helix implicitly suggested the replication mode of DNA and therefore the transmission of genetic information: each strand can be used as a “template” for the biosynthesis of the complementary strand.
Until 1962 many Nobel prizes revolved around DNA and others would be given later: Perutz, Pauling and precisely Watson, Crick, Wilkins. Three top-level scientists who certainly would have deserved the Nobel, but didn’t get it: Griffith, Avery e Franklin – demonstrating the fact that genius is only one of the ingredients of recognition and that luck and politics also have a weight.