DNA contains the genetic information needed to build life.

DNA contains the genetic information needed to build life.

The discovery of its molecular structure by Francis Crick and James Watson is one of the greatest breakthroughs in the history of biology.

At last, biologists could understand how physical characteristics are passed from generation to generation.

In 1857, almost a century before Crick and Watson’s discovery, an Austrian monk and botanist, Gregor Mendel (1822–1884), began a remarkable experiment.

He carefully bred pea plants with different characteristics – such as the colour or shape of the seeds – and followed how these traits passed through several generations.

Mendel worked out that his plants were receiving pairs of ‘units of inheritance’ – one from each parent – and determined the principles by which that works.

Mendel’s results were published in his local natural history society’s magazine, instead of an international journal, so they were not widely known.

They only came to the attention of international scientists around 1900 when other researchers were making similar discoveries.

Mendel’s units of inheritance were named ‘genes’ and the science of heredity became known as ‘genetics’.

One of the early pioneers was American zoologist Thomas Hunt Morgan (1866–1945).

In 1907 he realised that certain characteristics seemed to be ‘linked’, as if genes somehow occur in groups.

By 1911 he was able to show that genes are carried on chromosomes – long thin structures that could be seen inside cells (in humans there are 46 per cell).

Chromosomes had first been observed in 1878, but no one had seen their significance.

Chromosomes are made of two types of compound: proteins and nucleic acids.

At first, scientists guessed it must be the proteins that carry genes.

But each chromosome also carries a strand of DNA (deoxyribonucleic acid), divided into regions, which are Mendel’s units of inheritance: genes.

In 1944, Canadian bacteriologist Oswald Avery (1877–1955) showed that DNA could pass on inherited characteristics without any protein present.

So scientists turned their attention to the structure of the DNA molecule.

In 1951 American chemist Linus Pauling (1901–1994) worked out the structures of other long biological molecules and found them twisted like a corkscrew – a shape called a helix.

He began working on DNA.

Meanwhile, at Cambridge University, biologists Crick (1916–2004), an Englishman, and Watson (b.1928), an American, thought that DNA could be a helical molecule too.

Detail of Crick and Watson's DNA model

But the exact structure they proposed did not agree with the results of experiments by New Zealand biophysicist Maurice Wilkins (b.1916) and English chemist Rosalind Franklin (1920–1958).

Wilkins and Franklin had been studying DNA at King’s College, London, using a technique called Xray diffraction.

Bouncing X-rays off a material produces dots on photographic paper, the pattern of which depends on the arrangement of atoms inside the material.

Using Wilkins and Franklin’s results, Watson and Crick worked out that certain elements of DNA, called bases, seemed to join up in pairs; DNA had to be a double helix.

They further showed that each strand of DNA in this double helix was a template for the other.

That meant that DNA was capable of ‘copying’ itself without altering its structure.