After the acquittal of Amanda Knox and Raffaele Sollecito, we look at how physics underpinned the discovery of the structure of DNA.

Svilen Milev

Knox and Sollecito were acquitted of the murder of British student Meredith Kercher on appeal, after the DNA evidence on which they were convicted was deemed to be unreliable.

Although the processes used in DNA profiling for criminal investigations or paternity tests are based on chemistry, the structure of the DNA molecule itself was originally determined using techniques from physics – specifically, by using x-ray diffraction.

X-ray vision
The structure of materials is commonly investigated using a beam of x-rays.

The beam is diffracted by crystal structures, and the angles at which the diffracted x-rays travel, along with their intensities, can be used to work out the positions of electrons within the crystal via Bragg’s law.

A sample of the material being investigated is rotated while being bombarded with x-rays, and the two-dimensional diffraction image produced can then be converted into a three-dimensional model of the crystal’s electron density.

Similar techniques are still used to probe the structure of matter at finer detail using facilities such as the Diamond Light Source or ISIS neutron and muon source.

In 1953, biologist James Watson and biophysicist Francis Crick used x-ray diffraction, along with then-unpublished data gathered by Rosalind Franklin, to show that DNA has a double-helix structure.

Having taught himself the maths and physics behind x-ray diffraction, Crick had begun to work out the theoretical basis of diffraction by a helical structure from 1951, expecting that this would be useful in the study of proteins.

But after he and Watson were given one of Franklin’s x-ray photographs by Maurice Wilkins, King’s College’s physicist-turned-biologist, the pair developed their three-dimensional model of the molecule.

It showed a helical structure with two antiparallel strands, held together by pairs of nucleotides. The pairs are each made up of two bases from four used in total – adenine, thymine, guanine and cytosine – and the order in which they appear is what encodes genetic instructions.

Physics had also earlier played a minor role in the discovery that it was DNA and not proteins that was the part of the chromosome responsible for transmitting genetic information, by using radioactive tracers on a type of virus that infects bacteria.

The discovery of the structure of the molecule, with the inside of the double helix containing sequences of base pairs that can be pulled apart and reassembled, suggested a mechanism by which that genetic material can be copied.

The discovery of its molecular structure was one step on the path to being able to make use of DNA in applications such as profiling of criminal suspects, genetic engineering of improved or even novel organisms, and gene therapy to cure or prevent diseases.

However the use of Franklin’s x-ray crystallography data without her knowledge meant Crick and Watson receiving credit for the discovery was highly controversial – though not so much as Knox and Sollecito’s murder trial.

Other IOP websites

Your guide to physics on the web