What is DNA? The molecule that carries life's instructions

DNA, short for deoxyribonucleic acid, is the molecule that carries the instructions for building and running every known living thing, and many viruses. It is a long chain written in four chemical letters, A, T, G and C, and two chains wind around each other in the famous double helix. Cells read stretches of it, called genes, to make proteins, and copy the whole thing each time they divide.

In short

• DNA is a code of four letters; the order of the letters is the information, as the order of letters makes words.
• A gene is read in three-letter words, each naming one building block of a protein.
• Because the letters pair up, A with T and G with C, each strand is a template for the other, which is how DNA copies itself.

What is DNA made of?

DNA is built from units called nucleotides. Each has three parts: a sugar, a phosphate group and one of four bases, adenine (A), thymine (T), guanine (G) or cytosine (C). The sugars and phosphates link into a long backbone, with the bases sticking out from it. Two backbones run side by side, and their bases meet in the middle in pairs: A always pairs with T, and G always pairs with C. The paired strands twist into a double helix, like a ladder twisted along its length.

The human genome, the full set of DNA in a cell, holds about 3 billion base pairs and about 20,000 genes, packed into 23 pairs of chromosomes, according to the National Human Genome Research Institute. Stretched out, the DNA in one cell would be about 2 meters long; it fits into a nucleus only about 6 micrometers (thousandths of a millimeter) across by wrapping around proteins and coiling on itself. More than 99% of the bases are the same in all people, the NIH's MedlinePlus notes. The small remainder accounts for much of what makes each person genetically distinct.

How does DNA become a protein?

A gene is a stretch of DNA that holds the recipe for a protein, or for a working RNA molecule. Proteins do most of the work in a cell: they form muscle and hair, carry oxygen and speed up chemical reactions. Making one takes two steps, transcription and translation.

1. Transcription: the cell copies the gene into a messenger molecule, mRNA, a single strand that uses the letter U (uracil) in place of T.
2. Translation: a ribosome reads the mRNA three letters at a time. Each three-letter word, called a codon, names one of 20 amino acids, the building blocks of proteins, or tells the ribosome to stop.

Here is a tiny gene, worked through:

Step	Sequence
DNA (the coding strand)	ATG TTT GGC TGA
mRNA copy	AUG UUU GGC UGA
Amino acids	Methionine, Phenylalanine, Glycine, stop

AUG means "start here" and also adds methionine, and UGA means stop. Real genes run to hundreds or thousands of codons, and the chain of amino acids folds into a working shape.

A single wrong letter can matter. In the gene for part of hemoglobin, the protein that carries oxygen in red blood cells, one codon is normally GAG, which adds the amino acid glutamic acid. Change the middle letter to T, giving GTG, and the cell adds valine instead. That one-letter change causes sickle cell disease. Most letter changes do far less, and many do nothing at all, because only about 1% to 2% of the human genome codes for proteins. Changes like these, passed down and sorted by survival over generations, are the raw material of evolution.

How does DNA copy itself?

Before a cell divides, it copies its entire genome. Enzymes unzip the double helix, and each strand serves as a template: wherever a strand has A, the new strand gets T; wherever it has G, the new strand gets C. The result is two identical double helices, each made of one old strand and one new one. James Watson and Francis Crick saw this at once. Their one-page 1953 paper in Nature ends: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." Experiments by Matthew Meselson and Franklin Stahl confirmed in 1958 that copying works this way.

Copying is accurate but not perfect. The enzymes check their own work and fix most slips, and further repair systems catch most of the rest, leaving roughly one mistake for every billion letters copied. A human cell carries about 6 billion letters, two copies of each chromosome, so each division can still introduce a handful of new changes. Over a lifetime, some of them can contribute to cancer. In sperm and egg cells, they become the new variations passed to children.

Who discovered DNA and its structure?

DNA was known long before anyone knew what it did.

Year	Discovery
1869	Friedrich Miescher isolates a substance he calls nuclein from white blood cells
1944	Oswald Avery and colleagues show that DNA, not protein, carries genetic information in bacteria
1950	Erwin Chargaff finds that DNA always has as much A as T and as much G as C
1952	At King's College London, Rosalind Franklin and Raymond Gosling record X-ray images of DNA, among them the famous Photograph 51
1953	Watson and Crick publish the double helix in Nature, alongside papers by Maurice Wilkins's group and by Franklin and Gosling
1958	Franklin dies of ovarian cancer at 37
1962	Watson, Crick and Wilkins share the Nobel Prize; Franklin, who had died four years earlier, could not be nominated
2003	The Human Genome Project completes its reference sequence
2022	Researchers publish the first complete, gap-free sequence of a human genome, lacking only the Y chromosome, which followed in 2023

Still argued

How to credit Franklin is still debated. The popular story says Watson and Crick won the race after seeing her data without her knowledge, including Photograph 51 and a research report she contributed to, and that she failed to see what her images showed. In a 2023 article in Nature, the historians Matthew Cobb and Nathaniel Comfort drew on an unpublished 1953 news article and an overlooked letter to argue she "was an equal contributor to solving it," working toward the structure herself. Others stress that her data reached the Cambridge pair through colleagues without her consent. Both sides agree her measurements were essential.

What do DNA tests actually read?

Most home ancestry and wellness tests don't read your whole genome. They use a chip that checks a few hundred thousand chosen positions where people commonly differ, well under 0.1% of the 3 billion letters. Matching those positions against reference groups gives the ancestry estimate, and links found in research give trait and risk predictions. Whole-genome sequencing, which reads nearly every letter, is a different and more complete test.

That difference explains the limits. A chip can miss rare changes that matter for disease, and ancestry estimates shift as companies' reference groups change. MedlinePlus advises that results from direct-to-consumer tests generally can't diagnose a condition, and that clinical testing through a healthcare provider is usually needed to confirm one. This note explains how the tests work; it isn't medical advice.

Can DNA be edited?

Yes, and increasingly precisely. CRISPR-Cas9, a tool adapted from a defense system that bacteria use against viruses, cuts DNA at a chosen sequence. Emmanuelle Charpentier and Jennifer Doudna won the 2020 Nobel Prize in Chemistry for developing it. On 8 December 2023, the U.S. Food and Drug Administration approved Casgevy, the first FDA-approved treatment to use genome editing, for sickle cell disease in patients 12 and older. Editing cells that are passed to children remains widely restricted.

DNA is also why living things differ in their toughness, from tardigrades that carry genes protecting their DNA from radiation to viruses, which carry DNA or RNA but need a living cell to copy it, as viruses versus bacteria explains. Reading and editing it has become one of the central tools of modern science.

Check yourself

• Without looking back, explain why base pairing lets DNA copy itself.
• Turn the DNA word GGC into mRNA, and say what the ribosome does with it.
• What does a typical home DNA test read, and what does it miss?

Sources

• Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid — James Watson and Francis Crick, Nature, 1953
• Deoxyribonucleic acid (DNA) fact sheet — National Human Genome Research Institute
• What is DNA? — MedlinePlus, U.S. National Library of Medicine
• What is direct-to-consumer genetic testing? — MedlinePlus, U.S. National Library of Medicine
• What Rosalind Franklin truly contributed to the discovery of DNA's structure — Matthew Cobb and Nathaniel Comfort, Nature, 2023
• FDA approves first gene therapies to treat patients with sickle cell disease — U.S. Food and Drug Administration, 2023