What Is DNA?

DNA (short for deoxyribonucleic acid) is the molecule that holds the instructions for building and running every living thing on Earth. From the smallest bacterium to a giant blue whale, every cell carries a complete set of DNA written in the same four-letter chemical code. Your own DNA contains around 3 billion letters: a complete recipe for making you. It is packed inside almost every one of your cells, in a tiny package called the nucleus.

  • Full nameDeoxyribonucleic acidA very long name for a very long molecule
  • ShapeDouble helixTwo strands twisted into a spiral ladder
  • Letters4A, T, G and C
  • Human DNA lengthapprox. 2 metres per cellPacked into a 6 micrometre nucleus
  • Discovered structure1953Watson, Crick, Franklin and Wilkins
  • Length if stretched outSun and back, 200 timesFor all the DNA in your body

What does DNA look like?

DNA is shaped like a long, slightly twisted ladder, a structure called a double helix. The two long sides of the ladder are made of sugar and phosphate molecules. The rungs of the ladder are made of pairs of chemicals called bases, joined together in the middle.

There are four different bases:

  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)

The bases always pair up the same way: A always pairs with T, and G always pairs with C. The order of those letters along the strands is the genetic code that tells your cells what to make.

How DNA stores information

DNA stores information in the order of its four letters, the same way written language stores information in the order of its letters. A short stretch of DNA might read AGCTTGGAA..., and that exact sequence of letters specifies a particular protein the cell needs to make. Different sequences mean different proteins; different proteins do different jobs.

The human genome is around 3 billion base pairs long. If you printed it out at 60 letters per line and 50 lines per page, it would fill over a million pages, roughly 2,000 large books. Yet inside each of your cells, that whole library fits into a nucleus only about 6 micrometres wide.

How DNA was discovered

People had known about heredity (the passing of features from parent to child) for thousands of years, and Gregor Mendel had worked out the basic rules in 1865 with his pea plant experiments. But nobody knew what the actual chemical was that carried the inheritance information.

The double helix structure of DNA was worked out in 1953 by four scientists at Cambridge and King's College London:

  • Rosalind Franklin: an X-ray crystallographer at King's. Her famous "Photograph 51" of DNA crystals (taken with Raymond Gosling in 1952) was the most important single piece of evidence.
  • Maurice Wilkins: also at King's, working alongside Franklin on the X-ray work.
  • James Watson and Francis Crick: two researchers at Cambridge who built the model of the double helix based on the X-ray data.

Watson, Crick and Wilkins shared the 1962 Nobel Prize in Physiology or Medicine. Rosalind Franklin had died of cancer in 1958 and could not be awarded the Nobel posthumously, but her work was essential and is now widely recognised.

Fact Every living thing on Earth, from a bacterium to a blue whale, uses the same four-letter DNA code. The same DNA letter sequence in any creature codes for the same amino acid. This is one of the strongest pieces of evidence that all life shares a single common ancestor: it would be an incredible coincidence if life had evolved this exact same code multiple times independently. Scientists think the common ancestor (nicknamed LUCA, the Last Universal Common Ancestor) lived around 4 billion years ago.

How DNA makes you

Your DNA is not the same as the proteins that build you. DNA is more like a master recipe book. When the cell needs to build a particular protein, a short stretch of DNA is copied into a related molecule called messenger RNA (or mRNA), which is then carried to a tiny machine called a ribosome. The ribosome reads the mRNA three letters at a time and builds a chain of amino acids in the order specified. That chain folds up into the finished protein, ready to do its job.

Each set of three DNA (or mRNA) letters codes for one specific amino acid. There are 20 different amino acids; chains of them fold into proteins of every shape and size. Almost everything in your body (your muscles, hair, hormones, enzymes, antibodies, eye colour) is made of proteins built from these recipes.

Did you know? Humans share roughly 99% of their DNA with chimpanzees, our closest living relatives. We share around 85% with mice, 60% with bananas, and even about 25% with daffodils. Most basic biological machinery (like how cells divide or how energy is released from food) is the same across all life, so the DNA for it has been preserved almost unchanged for hundreds of millions of years.
Deeper dive: how DNA copies itself

One of the most elegant features of DNA is that it can copy itself. Before a cell divides, it needs to make a complete second copy of its DNA so that each daughter cell gets a full set. The double helix structure makes this incredibly easy.

To copy itself, the two strands of the DNA double helix simply unzip down the middle, breaking the bonds between paired bases. Each separated strand then acts as a template: special enzymes (helicase and polymerase) read along it and snap in new matching bases (A with T, G with C) to build a complementary strand. By the time the unzipping is finished, you have two complete double helices where there used to be one, each identical to the original.

The whole process happens at extraordinary speed: a human cell can copy its entire 3-billion-base-pair genome in about 8 hours. The accuracy is also breathtaking: the cell makes only about one mistake per billion bases, thanks to built-in proofreading enzymes that fix copying errors as they go. The few mistakes that do slip through are the source of new genetic variation, the raw material of evolution.

For genes and chromosomes, see genes and chromosomes. For how DNA mistakes happen, see mutations.