DNA: what it all means

What is DNA?

All life on earth, from daffodils to blue whales, owes its form and substance to the genetic information carried within DNA. DNA is the most important molecule in the living world, the chemical messenger bearing the message of heredity down the generations. The letters stand for deoxyribonucleic acid.

Can you actually see DNA and what does it look like?

Although DNA is the longest molecule, you cannot see it with the naked eye. Scientists have recently taken blurred photographs of it with extremely powerful microscopes. Its double helix is just about visible. If all the DNA could be pulled out of each cell and unfurled it would be about two metres long. Human DNA contains enough information to fill 1,000 paperback books.

Who discovered it?

The German biochemist Friedrich Miescher in 1869; he found it in pus cells taken from discarded bandages. For the next 75 years, scientists thought DNA was a boring molecule. Then in 1945, Oswald Avery at the Rockefeller Institute in New York showed that DNA carried genetic material from one cell to another. Even then, biochemists found it hard to believe that DNA was interesting and Avery never won the Nobel Prize he merited.

Watson and Crick discovered the structure of DNA, from which scientists understood for the first time how genes are passed on. The discovery opened up a new and immensely powerful scientific discipline, now known as molecular biology.

So what is the structure?

DNA consists of two long chains entwined in the form of a double helix. The message of inheritance is written along the double helix, in the form of chemicals known as 'base pairs' which join the two strands rather like the steps of a spiral staircase. When a living cell divides to form two 'daughter cells', the double helix unzips and the base pairs separate, so that the two separate helices can carry genetic information to the 'daughter' cells. This was the insight denied to all previous scientists.

How was the double helix found?

Watson and Crick had access to X-ray pictures of DNA fibres taken by Maurice Wilkins and Rosalind Franklin at King's College London. The pictures were formidably difficult to interpret and personal antagonismbetween Wilkins and Franklin was hindering their research. Rather than working it out by sophisticated mathematical analysis as Franklin was trying to do, Watson and Crick decided it would be quicker to try building scale models of the molecule until they got a structure consistent with the X-ray data. It was a matter of inspired guesswork rather than logical deduction.

What does it mean for Darwinism and evolution?

Discovering the structure of DNA was the final vindication of Darwinism. The idea that living creatures were descended from common ancestors arose initially from observations of the anatomical similarity of their bodies. Analysis of DNA has since demonstrated this similarity at the level of genetic anatomy. For example, a human being's DNA differs from that of a chimpanzee by only about 2 per cent.

Can you recreate dinosaurs with it?

The idea that it is possible to recreate extinct animals from their preserved DNA is a myth. Scientists have been able to isolate preserved DNA from very old organisms, such as a termite 30 million years old fossilised in amber. But such DNA is only partially preserved, and the small fractions that can be analysed could only be used to gauge how evolution has changed the molecule over that period of time.

Have there been any practical uses of the discovery?

The technology of manipulating, splicing and amplifying DNA has spawned a multi-billion-pound biotechnology industry producing everthing from pharmaceutical products and non-squashy tomotoes to washing powders. Medicine is about to experience one of its greatest revolutions as a result. Doctors are now able to diagnose an increasing number of genetic diseases. Transplanting healthy genes into children with genetic disorders is under way. DNA fingerprinting, used by the police to identify murderers and rapists, was made possible by it.

Does every living thing contain DNA?

The only known lifeform not to have DNA is a group of viruses that use a close molecular cousin, RNA, to carry genetic instructions. The Aids virus, for instance, is an RNA virus. Like DNA, RNA can replicate itself with the help of certain enzymes. Some biologists believe that RNA evolved before DNA but it is probably too unstable to have been able to allow even organisms as simple as bacteria to exist and was supplanted by the more stable DNA.

Was it a Faustian discovery?

Crick's autobiography says:'The double helix is indeed a remarkable molecule. Modern man is perhaps 50,000 years old, civilisation has existed for scarcely 10,000 years, the United States for only just over 200; but DNA and RNA have been around for at least several billion years. All that time, the double helix has been there, and active, and yet we are the first creatures on Earth to become aware of its existence.'

The knowledge is one of the pinnacles of achievement of our civilisation. But all knowledge can be used for good or for ill. Developments stemming from Watson's and Crick's discovery have given humanity the power to alter its own genetic composition. Is this a power with which we can be trusted?