DNA is the molecule that carries the instructions for building and running every living thing, from a bacterium to a blue whale to you. If your body were a vast construction project, DNA would be the complete set of plans, stored safely and copied into every cell.
What DNA is
DNA, short for deoxyribonucleic acid, is a long molecule that stores the genetic information an organism needs to develop, grow, function and reproduce. It is found in nearly every cell of your body, and it carries the coded instructions for making proteins — the molecules that do most of the actual work in living things.
In short: DNA holds the recipes, and proteins are what those recipes build.
The famous double helix
DNA's structure is one of the most recognisable shapes in science: the double helix, like a twisted ladder. Its discovery in 1953, by James Watson and Francis Crick using crucial experimental data from Rosalind Franklin and Maurice Wilkins, was a turning point in biology.
The structure has a few key parts:
- Two long strands, twisted around each other.
- The "sides" of the ladder are made of alternating sugar and phosphate groups, forming a strong backbone.
- The "rungs" are made of pairs of chemical units called bases.
There are four bases, usually written as their initials: A (adenine), T (thymine), C (cytosine) and G (guanine).
Base pairing: the elegant rule at the heart of DNA
The four bases do not pair up randomly. They follow a strict rule:
A always pairs with T, and C always pairs with G.
This is called complementary base pairing, and it is the single most important feature of DNA's design. Because each base has only one partner, the two strands are mirror images of each other. If you know the sequence of one strand, you automatically know the other.
This rule is what makes DNA so good at being copied. When a cell divides, the double helix unzips down the middle, and each separate strand acts as a template to build a new partner strand. The result is two identical copies of the original — one for each new cell. Accurate copying like this is essential to life, and errors in it relate directly to the theory of evolution, since rare copying mistakes are a source of the variation that natural selection acts on.
From DNA to genes to proteins
A gene is a section of DNA that contains the instructions to make a particular protein (or sometimes another functional molecule). Your DNA contains many thousands of genes, each effectively a single recipe within the larger book.
The flow of information runs roughly like this:
- The instructions in a gene are first copied into a related molecule called RNA.
- That RNA is then read by the cell's machinery to assemble a protein, building it from smaller units called amino acids in the order the gene specifies.
Proteins then carry out the tasks of life: they form structures such as muscle and skin, act as enzymes that drive chemical reactions, carry oxygen, fight infection and much more. So the precise sequence of bases in your DNA shapes which proteins you make, and therefore how your body is built and behaves. The chemistry that links the bases and builds proteins is part of the same molecular world described by the periodic table.
How DNA is packaged
A single human cell contains about two metres of DNA, far too much to sit loose inside a microscopic cell. To fit, it is wound up tightly and organised into structures called chromosomes.
- Humans typically have 46 chromosomes, arranged in 23 pairs, with one of each pair inherited from each parent.
- Almost every cell carries a complete copy of this set.
- A small, separate piece of DNA also lives in the mitochondria, the parts of the cell that release energy.
The complete set of an organism's DNA is called its genome.
When DNA changes: mutations
Copying two metres of code into every new cell is astonishingly accurate, but not perfect. Occasionally a base is changed, added or lost. A change in the DNA sequence is called a mutation.
Most mutations are harmless or have no noticeable effect. Some can be useful, and a few can be harmful, for example by disrupting an important gene. Mutations can happen by chance during copying, or be caused by factors such as certain chemicals or ultraviolet light from the sun.
Importantly, only mutations in egg or sperm cells can be passed to children; changes in ordinary body cells are not inherited. Over very long timescales, the gradual build-up of inherited changes is the raw material that drives the diversity of life.
Why DNA matters
Understanding DNA has transformed science and medicine:
- Inheritance. DNA explains how characteristics are passed from parents to children, and why family members resemble one another.
- Health. Many conditions have a genetic component; reading DNA helps diagnose disease and tailor treatments, an approach sometimes called personalised medicine.
- Forensics and identity. Because each person's DNA sequence is almost unique, it is used in criminal investigations and to establish family relationships.
- Biotechnology. Tools that read and edit DNA underpin advances from new medicines to improved crops, and even shape some modern approaches to how vaccines work.
This is general scientific information rather than medical advice. For questions about genetic testing or inherited conditions, speak to a qualified healthcare professional.
The bottom line
DNA is the molecule of life: a double helix whose two strands are held together by paired bases following the simple rule that A joins T and C joins G. That pairing lets DNA be copied faithfully every time a cell divides. Stretches of DNA called genes carry the instructions to build proteins, which do the real work in the body, and the whole genome is packaged into chromosomes inside nearly every cell. From inheritance to medicine to forensics, DNA is the thread that runs through all of biology.