5.2 : Il DNA come modello genetico

Two structural features of the DNA molecule provide a basis for the mechanisms of heredity: the four nucleotide bases and its double-stranded nature. The Watson-Crick model of double-helical DNA structure, proposed in 1952, drew heavily upon the X-ray crystallography work of researchers Rosalind Franklin and Maurice Wilkins. Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine for their work in 1962. Franklin was, controversially, excluded from the prize for reasons that are still debated.

The Watson-Crick model of DNA, very simply, proposed that DNA is made up of two strands of nucleotides that twist around each other to form a right-handed helix and that nucleotide pairing takes place between a purine and a pyrimidine.

The two DNA strands are antiparallel, meaning that the 3’ end of one strand faces the 5’ end of the other. This allows each strand to act as a template for its partner during DNA replication, producing two new strands of DNA that are exactly complementary to each other. However, whether or not DNA replication occurred in this fashion was not clear.

The Meselson and Stahl Experiment

Meselson and Stahl grew E. coli for several generations in a medium containing a “heavy” isotope of Nitrogen, ¹⁵N. Over time, the heavy Nitrogen was incorporated into the nitrogenous nucleotide bases and, thus, into the DNA. After this, the E. coli was placed into a medium containing a different isotope of Nitrogen, ¹⁴N, and grown for several more generations. After each generation, a DNA sample was isolated from some of the cells, loaded into a gradient, and centrifuged at high speeds. In a gradient, the DNA will separate according to its buoyant density (i.e. the density within the gradient where the DNA will float).

In the ¹⁵N medium, a single band of high density was observed in the lower portion of the centrifuge tube. Immediately after the bacteria were transferred to the “lighter” media, this single band shifted upwards in the column, indicating a lower density. However, subsequent generations resulted in two bands: one corresponding to the ¹⁴N density and another in an intermediate location. This result could only be explained by a semi-conservative mode of replication, thus validating the Watson-Crick model.