What is DNA?

This molecule is now recognised as being the single most important component of all living organisms, says James Randerson

From bacteria to giraffes, jellyfish to fleas, DNA (or deoxyribonucleic acid) is the substance that contains the master code for life. It is the instruction book that each organism uses to run its body and govern its behaviour, a book that each creature hands on to its offspring, either in full or in part.

Popular descriptions of DNA's action often refer to it as a "blueprint". This is misleading. The molecule does not form a miniature map of the creature it codes for. A better analogy would be a recipe book. The DNA in your genome includes around 20,000 to 25,000 genes, each an instruction to make an individual ingredient in the right place at the right time in your body's cells, which go together to make you. But more of them later. What of the DNA molecule itself?

Until the 1950s, DNA was regarded as a rather uninspiring molecule. It does not have the complexity and variety of proteins so most scientists thought it was unlikely to be the source of heredity - the molecule that passes genetic instructions from one generation to the next. Some of the key experiments to overturn this view were done by Alfred Hershey and Martha Chase in 1952. They studied viruses called phages, which infect bacteria. Phages make their living by attaching to the surface of bacteria and injecting their DNA (or in some cases a related molecule called RNA). Their DNA then hijacks the bacteria's cellular machinery so that it replicates the phage's own DNA and ultimately lots of new copies of the phage.

Hershey and Chase didn't know any of these details and were in the dark about DNA's role as the information carrier. In two experiments they labelled phages with either radioactive phosphorus (common in DNA but not proteins) or sulphur (common in proteins but not DNA) and used them to infect bacteria. After separation from water in a centrifuge they found that, with the phosphorus-labelled phage, it was the bacteria that were radioactive, but in the second experiment the water mixture was radioactive. The pair concluded that in phages it is therefore the DNA that transmits the genetic information inside when bacteria are infected.

Thanks to James Watson and Francis Crick's classic study in 1953 at Cambridge University and subsequent work, we now know what the DNA molecule looks like and how it encodes genetic instructions - a code that is virtually identical in all living things. It was Watson and Crick who discovered the famous double helix shape by the way x-rays bounced off DNA crystals.

The term double helix refers to two DNA strands that coil around each other. Each strand is made up of a backbone consisting of alternating sugars (deoxyribose) and phosphate groups. Attached to each sugar is one of four chemical bases: adenine (A), thymine (T), guanine (G) and cytosine C. It is the sequence of As, Ts, Gs and Cs running along the DNA molecule that forms the genetic cipher.

One more important thing about the double helix is that the two DNA strands are bonded together by their bases and are complementary. A is always bound opposite T, and G is always attached to C. So if you know the sequence of one strand - say ATTCGC - then you can work out the sequence of its opposite, in this case TAAGCG. When a cell wants to copy its DNA it simply unzips the two strands and fills in a new complementary strand for each.