Tuesday, 21 June 2016

The discovery of DNA's double helix, 1953

25th April 1953 is a date that should be commemorated worldwide, because it was on this day that the journal Nature published an article that would have far-reaching consequences.

The article was by two scientists from Cambridge University, James Watson and Francis Crick, and it announced their discovery of the molecular structure of deoxyribonucleic acid, which is better known by its initials as DNA. The discovery solved a problem that had puzzled scientists ever since Charles Darwin had published “The Origin of Species”, namely what the mechanism was by which living things were able to pass on their characteristics to the next generation, and what could cause changes to happen that would lead to the evolution of new species.

Crick and Watson

Francis Crick (1916-2004) and James Watson (born 1928) met at the Cavendish Laboratory of Cambridge University in 1951, where they became close friends as well as professional colleagues. Their work depended to a large extent on work done by others, notably Rosalind Franklin, Raymond Gosling and Maurice Wilkins. Of these, Franklin would have had most cause to be aggrieved at the lack of recognition given to her work, especially as it proved to be essential to the work of Crick and Watson.

The Double Helix

What Crick and Watson did was not to discover DNA as such – its existence was already well established – but to visualise how the molecule was constructed.  They were able to build a model that demonstrated the molecule as a “double helix”, or a long ladder-like structure in which the rungs comprise pairs of four possible bases – guanine, adenine, cytosine and thymine.

The order in which the pairs of bases are arranged forms a chemical code that instructs the cell to make a particular amino acid. Because the basic units are so simple, and because every strand of DNA contains the code for the entire organism, the full DNA strand is immensely long. If fully unravelled, the DNA in each cell would extend to more than two metres. All the DNA in a human body would stretch to 200 billion kilometres!

The double helix provides the clue to how DNA works. The “ladder” is able to split apart so that portions of DNA can act as templates for the assembly of new DNA, or strands of RNA (ribonucleic acid) can act as messengers in the building of new proteins.

DNA and evolution

Crick and Watson’s discovery paved the way for understanding how evolution works by providing the mechanism by which “errors” can be introduced into new generations of an organism. The splitting and re-assembly of DNA strands is not always perfect, which means that the chemical code can be distorted and new characteristics introduced that may or not be beneficial to the new individual.  When beneficial changes happen, these are likely to be passed on to future generations, which will therefore differ in some respect from what went before. Given enough time, and enough such DNA errors, new species can evolve.

Amazingly enough, there are still people in the world who refuse to believe the evidence that has been presented for all to see, and prefer to ascribe the variety of life on Earth to divine creation. These people would probably claim that black was white if the Bible said so!

© John Welford