DNA and Genes

Every cell in your body carries a complete set of instructions for making you, written in a chemical alphabet of four letters. Those instructions came half from your mother and half from your father, and they have been copied, with small changes, through an unbroken chain of ancestors going back nearly four billion years. The molecule that carries them is DNA, and the discovery of how it works is one of the great achievements of science.

A twisted ladder

DNA, deoxyribonucleic acid, is shaped like a ladder twisted into a spiral: the famous double helix, worked out in 1953 by James Watson and Francis Crick using X-ray images made by Rosalind Franklin and Maurice Wilkins. The two uprights of the ladder are chains of sugar and phosphate. The rungs are pairs of chemical units called bases, of which there are four: adenine, thymine, guanine and cytosine, abbreviated A, T, G and C.

The bases pair strictly. A always joins with T, and G always with C. That rule is the secret of inheritance. Pull the two strands apart and each one carries all the information needed to rebuild its partner. When a cell divides, enzymes unzip the helix and build a new complementary strand along each half, producing two identical copies. The copying is astonishingly accurate, making about one mistake in a billion letters.

The code

The order of the bases along the strand is the message. A gene is a stretch of DNA, typically a few thousand letters long, that spells out the recipe for one protein. The cell reads it three letters at a time; each triplet stands for one of twenty amino acids, and the sequence of triplets tells the cell in what order to string those amino acids together. The resulting chain folds up into a protein, and proteins are what cells are made of and what they use to do everything, from digesting food to carrying oxygen to detecting light in your eye.

The code is universal. The same triplets mean the same amino acids in a bacterium, a mushroom, a whale and you. That is the strongest evidence that all life on Earth descends from a single ancestor.

Your genome

The complete set of DNA in an organism is its genome. The human genome is about 3.2 billion letters long, divided among 46 chromosomes, 23 inherited from each parent. Stretched out, the DNA in one cell would be about two metres long, yet it is packed into a nucleus a few millionths of a metre across, wound around proteins like thread on spools.

Surprisingly, only about one to two percent of it codes for proteins. Humans have about 20,000 genes, roughly the same as a mouse and fewer than a rice plant. Much of the rest is control: switches and dials that decide which genes are turned on in which cells, when, and how strongly. A liver cell and a brain cell carry identical DNA; they differ because different genes are active. Some of the remainder is the remains of ancient viruses and defunct genes, and the function of much of it is still debated.

Why you are not a copy of your parents

Each of your parents gave you one of each pair of chromosomes, but which one was decided at random, and before that the pairs swapped segments with each other, shuffling the deck. The chance of two siblings getting the same combination is essentially zero, which is why they resemble each other and their parents without matching any of them. Identical twins are the exception, formed when one fertilised egg splits.

Copying errors add a little novelty too. Each child carries around 60 new mutations not present in either parent. Nearly all land in parts of the genome where they make no difference. Occasionally one damages a gene and causes disease. Very occasionally one is an improvement, and over millions of years such changes, sorted by natural selection, are what evolution is made of.

Reading and writing DNA

The first human genome took thirteen years and nearly three billion dollars to read, finished in 2003. Today a genome can be sequenced in a day for a few hundred dollars. The ability to read DNA has transformed medicine, forensics, archaeology and our understanding of the past: we can now read genomes from Neanderthal bones 100,000 years old. Since 2012 a tool called CRISPR, borrowed from the immune systems of bacteria, has made it possible to edit DNA precisely, cutting a chosen sequence and replacing it. The first cures for inherited diseases such as sickle cell anaemia using this method were approved in 2023.

Why it matters

DNA is the thread connecting everything alive. Your genome records your ancestry, your relatedness to every other species, and your individual risks of disease. Its study has told us that humans are one species with trivial differences, that we are close cousins of the apes and distant cousins of oak trees, and that the same four letters, arranged differently, are enough to write a bacterium or a brain.