A fingertip full of instructions

BIOLOGY · 6 MIN

Every cell in your fingertip holds 2 metres of code. Could it hold your photos too?

Inside each of your 30 trillion cells is a thread written in just four letters, the instructions for building you. Let's zoom from a fingertip all the way down to those letters, then see how scientists are using them to store movies and books.

9 steps · 10 quick challenges

Step 1 of 9 · You, up close

Look at your fingertip. It feels like one solid thing, but it is really a crowd. And every member of that crowd carries the same secret instruction book.

Your fingertip, like the rest of you, is built and run by instructions written in a molecule called DNA.

Built from a recipe. Your finger grew from one single cell, the fertilised egg you started as. That cell carried a full set of instructions saying how to build skin, nails, bones and everything else. Those instructions are written in a molecule called DNA, and you are still using them right now. Like a LEGO set's instruction booklet, except the booklet also builds the hands that follow it.
A molecule is a group of atoms stuck together, the way letters stick together into a word. DNA is a very long molecule, made of billions of atoms in a precise order. That order is the information.
The instructions never stop working. Your skin replaces its outer layer about every month, and every new skin cell is built by reading the same DNA. If the instructions were lost, the body couldn't repair itself.
DNA is short for deoxyribonucleic acid. 'Nucleic' because it lives in the nucleus of the cell (we'll get there soon), 'acid' because of the chemistry of its backbone.
Ridges are a clue. Your fingerprint ridges help you grip and feel textures. Their general shape (loops, arches, whorls) is guided by your DNA, but the exact pattern is set by tiny bumps and pressures in the womb. That's why even identical twins, with identical DNA, have different fingerprints. Like two bakers with the same recipe: the cakes look alike but never crack the same way.
Fingerprint ridges form between about the 10th and 16th week of pregnancy. The skin layers grow at slightly different speeds, buckle and fold, and the folds set into ridges. Genes decide how the skin grows; chance decides the final swirls.
This is a good lesson about DNA in general: it isn't a blueprint that fixes every detail. It is more like a set of rules for how cells grow and react, and the result also depends on what happens along the way.
Made of cells. Zoom in on that skin and it's made of cells: tiny living bags, each far too small to see. Your whole body has about 30 trillion of them. Almost every one carries its own full copy of your DNA. Let's dive into one. Like a city where every house keeps a full copy of the city's rule book.
A cell is the smallest thing that's alive on its own: a bag of watery gel wrapped in a thin skin (the cell membrane), packed with tiny machines. Most of your cells are 10 to 30 micrometres across. About 20 skin cells in a row would cross the full stop at the end of this sentence.
The 30 trillion figure comes from a careful 2016 count by scientists in Israel. About 84% of those are red blood cells, which are odd: they throw out their DNA as they mature, to make more room for carrying oxygen.

Cells in you: ≈30 trillion
You started as: 1 cell

Did you know? Koalas have fingerprints so similar to ours that, under a microscope, even experts struggle to tell them apart.

DNA is the instruction molecule of every living thing on Earth: you, your dog, the mould on old bread and the bacteria in your gut. The same four-letter alphabet is used by all of them, which is one of the strongest clues that all life shares a common ancestor.

Instructions for what, exactly? Mostly for building proteins. Proteins are the workers of the body: they make skin tough (keratin), carry oxygen (haemoglobin), digest food (enzymes) and copy DNA itself. A gene is a stretch of DNA holding the recipe for one protein.

In this story we go down one layer at a time: the fingertip, a single skin cell, its nucleus, a chromosome, the coiled thread, the ladder of DNA and the letters on it. Then we watch a cell read those letters, and finally see how humans have started writing their own messages in them.

If you laid every DNA thread from all your cells end to end, it would stretch from the Earth to the Sun and back about 30 times.

Step 2 of 9 · You, up close

One skin cell, cut open

So your finger is a crowd of cells, each carrying its own instruction book. Let's shrink 40 times and step into that crowd to find where the book is kept.

Each skin cell is a tiny living bag of machines, and its DNA is locked away in a ball in the middle called the nucleus.

A sheet of cells. Your skin is a tightly packed sheet of cells, glued edge to edge so germs and water can't slip between them. Each one is about 25 micrometres wide, a quarter the width of a hair. New cells form underneath and slowly move up to the surface. Like paving slabs: each one separate, but packed so tightly they form one solid floor.
The cells are held together by protein 'rivets' (desmosomes) and seals (tight junctions). That's what makes skin waterproof and germ-proof even though each cell is mostly water.
Skin cells are born at the bottom of the outer layer and pushed upward over about four weeks. On the way they fill with the tough protein keratin, flatten and die. The dust in your room is partly those old cells flaking off.
Machines inside. Cut one open and it's packed with tiny parts, each with a job. The bean-shaped ones are mitochondria: they burn sugar to make the energy the cell runs on. Thousands of smaller machines build proteins, but none of them knows what to build by itself. Like a factory floor full of busy workers, all waiting for orders from head office.
Mitochondria are so important they have their own tiny loop of DNA, about 16,600 letters long, inherited only from your mother. Scientists think they were once free-living bacteria that moved into our ancestors' cells about 2 billion years ago.
The protein-building machines are called ribosomes. A single cell has millions of them, and they are the same kind we'll meet later in this story, reading DNA's message.
The nucleus. That purple ball is the nucleus, the head office. It keeps the cell's DNA safely wrapped inside its own skin, with tiny doors that let messages out. Only about 6 micrometres across, it holds all of your DNA. Let's go inside. Like the safe in a library, where the one original copy of every book is kept.
The nucleus has a double wall (the nuclear envelope) dotted with thousands of pores. Each pore is a gate made of about 1,000 protein pieces that decides what goes in and out. DNA stays in; working copies of its recipes go out.
Why lock DNA away? The rest of the cell is a busy, rough place full of chemicals that could damage it. Keeping the master copy apart, and sending out only disposable copies, protects it.

Skin cell width: ≈25 µm
Nucleus width: ≈6 µm

Did you know? You shed about 30,000 to 40,000 skin cells every minute, and each one carried a complete copy of your DNA.

Cells were first seen in 1665, when Robert Hooke looked at a slice of cork through a home-made microscope and saw tiny boxes that reminded him of monks' rooms, or 'cells'. It took nearly 200 more years to realise that every living thing is made of them.

A typical human cell is about 70% water. The rest is proteins, fats, sugars and nucleic acids (DNA and RNA). It's surrounded by a membrane only two molecules thick, a soapy film that keeps the inside in and the outside out.

Almost every cell has the same DNA, yet a skin cell and a brain cell look and act totally differently. The difference is which genes each one switches on. Skin cells read the keratin recipe a lot; brain cells read the recipes for signalling instead.

In the 3D view the cell is cut open so you can see inside. The real thing is fully closed, and so thin and clear that biologists need a coloured stain to see it at all, like the photo below.

Your body makes about 3.8 million new cells every second, and every one gets a fresh copy of all your DNA.

Step 3 of 9 · Packing 2 metres

46 chromosomes in a tiny ball

So the DNA lives in the nucleus. But that ball is about 15 times narrower than a hair. How do you fit a whole instruction book in there? Let's step inside.

Inside the nucleus, your DNA is split into 46 pieces called chromosomes, arranged as 23 matching pairs.

2 metres inside. Pull all the DNA out of this one nucleus and line it up, and it would be about 2 metres long, taller than most adults. Yet it fits in a ball just 6 micrometres wide. The thread is very thin, only 2 nanometres (millionths of a millimetre) wide, and it's packed brilliantly. Like stuffing 20 km of hair-thin thread into a tennis ball, without a single tangle.
Your DNA is about 3.1 billion letters long, and you have two copies (one from each parent), so 6.2 billion letters per cell. Each letter step is 0.34 nanometres long, which adds up to roughly 2 metres.
The packing isn't random. Each chromosome has its own patch of the nucleus (a 'chromosome territory'), and busy genes tend to sit near the middle, close to the pores where their messages leave.
A chromosome. The DNA isn't one long thread. It's cut into 46 pieces called chromosomes, each one a single, unbroken DNA molecule. Most of the time they're loose, like cooked spaghetti. Right before a cell splits, each copies itself and squeezes into this X shape. Like an encyclopedia split into 46 volumes, so it's easier to copy and hand out.
The X shape is two identical copies joined at a pinch called the centromere. When the cell divides, little ropes pull the two halves apart so each new cell gets one full copy.
Chromosomes are numbered roughly by size, from chromosome 1 (about 249 million letters) down to chromosome 21 (about 46 million). Number 23 is the sex chromosomes: XX or XY.
Matching pairs. Chromosomes come in 23 pairs. In each pair, one came from your mother and one from your father. Both carry the same genes in the same order, but they can hold different versions, like a recipe for brown eyes on one and blue eyes on the other. Like getting two editions of the same cookbook, one from each parent, with a few recipes tweaked.
Different versions of a gene are called alleles. Which one shows depends on the gene: some versions are dominant (one copy is enough), others only show up if both copies have them.
Having two copies is also a backup. If one copy of a gene is broken, the other can often do the job, which is why many genetic conditions only appear when someone inherits a faulty copy from both parents.
When you make egg or sperm cells, each pair swaps chunks and then splits, so every child gets a unique mix. That's why siblings look alike but not identical.

DNA per cell: ≈2 m
Chromosomes: 46 (23 pairs)

Did you know? A potato has 48 chromosomes and some ferns have over 1,000. The number of chromosomes says nothing about how complex a living thing is.

A full set of a living thing's DNA is called its genome. The human genome is about 3.1 billion letters long. It was first read from start to (almost) finish by the Human Genome Project, a 13-year international effort that finished in 2003 and cost about 3 billion dollars. Today a genome can be read in a day for a few hundred dollars.

Surprisingly, only about 1 to 2% of your genome is genes that code for proteins: roughly 20,000 of them. The rest includes switches that turn genes on and off, structural parts like the chromosome ends (telomeres), and long stretches whose job we're still working out.

Any two humans share about 99.9% of their DNA letters. The remaining 0.1%, still millions of letters, is behind much of what makes each of us different, from eye colour to blood type.

The picture below is a karyotype: a photo of all 46 chromosomes from one cell, cut out and lined up in pairs. Doctors use them to spot a missing or extra chromosome.

If you typed out your genome, one letter per keystroke, at 60 words a minute for 8 hours a day, it would take about 60 years.

Step 4 of 9 · Packing 2 metres

Thread wound on tiny spools

So 2 metres of DNA is split into 46 chromosomes. But how does each one fold so small? Let's zoom into one arm of that X and start unwinding it.

Each chromosome is DNA wound around millions of protein spools, which then coil up tightly on themselves.

Tight coil. The chromosome arm is a thick rope, and that rope is a coil. Look closely: it's made of beads stacked in a tight spiral. When the cell needs to read a gene, it loosens just that part of the coil and leaves the rest packed away. Like a coiled phone cord: stretch one section to use it while the rest stays bunched up.
Tightly packed DNA is mostly 'off': the cell's reading machines can't reach it. Loosely packed DNA is 'on'. So packing isn't just about saving space; it's also how a cell decides which genes to use.
Cells tag the spools with tiny chemical labels (like methyl and acetyl groups) that tell them to pack tighter or loosen up. These tags can change with diet, stress and age, a field called epigenetics.
Histone spools. Each bead is a spool made of 8 proteins called histones. The DNA wraps around it almost twice (about 147 letters' worth), then runs to the next spool. Histones are positively charged and DNA is negatively charged, so they cling together like magnets. Like winding a garden hose onto a reel, again and again, so it never tangles.
A spool with its DNA is called a nucleosome. Each cell has about 30 million of them. Their exact shape was first seen in 1997 using X-rays, which showed the DNA making 1.65 turns around the histone core.
Charge is a basic property of tiny particles: opposite charges attract, like charges push apart. DNA's backbone carries a negative charge on every letter, and histones are rich in positive building blocks, so they stick firmly but can be loosened.
Bare thread. Between and beyond the spools, the DNA runs bare. This is the actual molecule, the 2-nanometre thread itself. Even here it's not a simple string: zoom in, and it turns out to be a twisted ladder. Like a rope that looks smooth from far away, until you see it's two strands twisted together.
The short bare stretches between spools are called linker DNA, usually 20 to 80 letters long. They're where many reading machines first grab on.
Altogether the packing shrinks DNA about 10,000 times in length for the X-shaped chromosome. It's one of the most impressive bits of packaging in nature.

Spools per cell: ≈30 million
DNA per spool: 147 letters

Did you know? Histone proteins are almost identical in peas and cows. They've hardly changed in over a billion years, because almost any change breaks them.

The packing works in levels. First, the 2 nm DNA thread wraps around histone spools, giving 'beads on a string' about 11 nm wide. Those beads coil and fold into a thicker fibre, which loops onto a protein scaffold, and finally the loops fold into the dense chromosome you can see under a light microscope.

Every level can be undone locally. When a skin cell needs keratin, special proteins slide or unwind the spools over the keratin gene, so the reading machines can get to it, while the rest stays packed.

When the cell copies its DNA, the spools are taken off just ahead of the copying machine and put back on both new copies right behind it, so the packing information is copied too.

If a nucleus were the size of a football stadium, its DNA thread would be about as thick as a pencil and 65,000 km long, enough to wrap around the Earth one and a half times.

Step 5 of 9 · The code

A twisted ladder

So the bare thread is the real molecule. Let's zoom 12 times further into it. What is a 'twisted ladder', and where is the information kept?

DNA is a ladder twisted into a spiral: two long rails with rungs between them, and each rung is a pair of chemical letters.

Backbone. The two rails are the backbone: a chain of sugar and phosphate, repeating again and again. It's strong and stays the same all the way along, so it carries no message. Its job is to hold the letters in order, like the spine of a book. Like the string of a bead necklace: it holds the beads in order but isn't the pattern.
Each unit on a rail is a nucleotide: one sugar (deoxyribose, the 'deoxyribo' in DNA), one phosphate group and one letter, called a base. The phosphate gives each unit a negative charge, which is what clings to the histones.
The two rails run in opposite directions, like the two lanes of a road. Copying machines can only move one way along a rail, which is why copying the two sides works slightly differently.
Base pairs. Each rung is two bases, one from each rail, meeting in the middle. There are only 4 kinds of base: A, T, G and C. The rungs are all exactly the same length, which is why the ladder is perfectly even. The order of the bases along the rail is the message. Like a zip: two halves with teeth that meet in the middle and lock together.
The letters stand for adenine (A), thymine (T), guanine (G) and cytosine (C). A and G are bigger, two-ring molecules; T and C are smaller, one-ring ones. Every rung is always one big plus one small, so every rung is the same width.
The two bases on a rung hold on with hydrogen bonds: weak attractions between a slightly positive hydrogen atom and a slightly negative atom opposite. Each one is weak, but millions in a row make the ladder strong, yet easy to unzip a bit at a time.
The twist. The ladder twists into a spiral called a double helix, making one full turn every 10.5 rungs. The twist packs the bases tightly in the middle, away from water, and helps keep them safe. It was discovered in 1953, partly thanks to an X-ray photo by Rosalind Franklin. Like a spiral staircase, with the steps as the base pairs.
The twist happens because the flat bases are oily and avoid water, so they stack on top of each other like a pile of coins. The stack only fits the backbone if it twists.
James Watson and Francis Crick built their famous model in 1953. A key clue was 'Photo 51', an X-ray image of DNA made in Rosalind Franklin's lab, whose X-shaped pattern only a helix could make. Franklin died in 1958, before the 1962 Nobel Prize was awarded.

Helix width: 2 nm
One full turn: 10.5 rungs

Did you know? Your DNA is so thin that 50,000 strands side by side would only be as wide as one human hair.

Chemically, DNA is a polymer: a long chain of repeating units, like a train of carriages. Each carriage (nucleotide) is a sugar, a phosphate and one of four bases. The sugars and phosphates link carriage to carriage to make the rail; the bases stick out sideways to form half a rung.

Why four letters and not two? Nobody knows for sure. Four is enough to store a lot (each letter can be one of four options, so it carries 2 bits of information), and the chemistry of these four pairs up neatly. Scientists have even built DNA with 8 letters in the lab, and it still works.

The double helix is right-handed: curl the fingers of your right hand and your thumb points along the direction it climbs. Almost all DNA in living things twists this way.

DNA is so stable that scientists read DNA from a mammoth tooth that had been frozen in Siberia for over a million years.

Step 6 of 9 · The code

A always meets T, G always meets C

So each rung is a pair of letters. But which letters can pair up? The answer is the secret of how life copies itself.

A only pairs with T and G only pairs with C, so each side of DNA is a perfect template for the other.

A meets T. Adenine (A, red) only fits thymine (T, yellow). Their shapes match, and they hold on with two hydrogen bonds, the two small links you see in the middle of the rung. A can't hold on to C or G properly: the bumps and gaps don't line up. Like puzzle pieces: only the right tab fits the right slot.
A hydrogen bond forms when a hydrogen atom, already attached to one atom, is also attracted to a nitrogen or oxygen atom across the gap. In an A–T pair, two of these line up perfectly; try A with C and the hydrogens end up facing each other instead.
With only two bonds, A–T rungs are slightly easier to pull apart. Stretches rich in A and T are often where the cell starts unzipping DNA.
G meets C. Guanine (G, blue) only fits cytosine (C, green), and they hold on with three hydrogen bonds, so a G–C rung is a bit stronger. That's all it takes: two pairing rules, and every rung on the ladder follows them. Like USB-A and USB-C plugs: each fits only its own socket.
The extra bond means G–C-rich DNA needs more heat to come apart. Microbes living in hot springs at 80 °C tend to have extra G and C in the RNA of their ribosomes, which must hold its shape in the heat.
These rules were spotted before anyone knew DNA's shape. In 1950 Erwin Chargaff noticed that in every species, the amount of A equals the amount of T, and G equals C. The double helix explained why.
Two copies in one. Because of the pairing rules, one side of the ladder fully decides the other. If one side reads ATGC, the other must read TACG. So the cell can unzip DNA, use each half as a guide, and build two perfect copies. That's how every new cell gets your full DNA. Like a photo and its negative: from either one you can print the other.
Copying is done by a machine called DNA polymerase. It walks along one unzipped side, reads each letter and adds the matching one, about 50 letters per second in human cells. Thousands of polymerases work at once, so a cell copies all 6 billion letters in about 8 hours.
Polymerase also proofreads: if it adds the wrong letter, it backs up and fixes it. Together with extra repair crews, the error rate is about 1 wrong letter per billion copied.
What if a mistake slips through? That's a mutation. Most do nothing at all, some cause disease, and a very few turn out useful. Over millions of years, those rare useful ones drive evolution.

Pairing rules: A–T · G–C
Copy errors: ≈1 in 10⁹

Did you know? Your cells copy DNA so carefully that it's like typing out the whole Harry Potter series 150 times and making only one typo.

The pairing rules are called complementary base pairing. They're the reason DNA can be copied, repaired and read. If one strand gets damaged, the cell can cut out the bad letters and rebuild them using the other strand as a guide.

They're also how DNA tests work. To find a particular stretch of DNA, scientists make a short probe with the matching letters. It only sticks where the letters pair up perfectly, like a key finding its lock among billions of others.

Watson and Crick ended their 1953 paper with a famous understatement: the pairing they'd found 'immediately suggests a possible copying mechanism for the genetic material'. They were right.

Your body copies hundreds of billions of metres of DNA every day as cells divide: roughly the distance from Earth to Jupiter.

Step 7 of 9 · The code

Reading the recipe

So DNA is a string of letters that copies itself. But a string of letters can't build a fingertip. How does a cell turn letters into the things it's made of?

The cell copies a gene into a short message, carries it out of the nucleus, and a builder reads it 3 letters at a time to make a protein.

Copying machine. First, a machine called RNA polymerase opens a small bubble in the ladder, right at the start of a gene. It reads one side and builds a matching copy, letter by letter. The DNA zips back up behind it, untouched. Like photocopying one recipe from a precious cookbook instead of tearing the page out.
Polymerase knows where to start because genes begin with special 'start here' sequences called promoters. Other proteins, switched on by signals like hormones, help it land there, which is how cells decide which genes to read.
It moves at about 20 to 80 letters per second. A typical gene takes a few minutes to copy, and a busy gene can have many polymerases following each other along it, like cars on a road.
Message copy. The copy is called messenger RNA (mRNA). It's like DNA but single-sided, and it uses U instead of T. It slips out of the nucleus through a pore to where the builders are. It's disposable: it gets broken down after a while. Like a text message: quick to send, easy to delete, and the original stays on your phone.
RNA's sugar has one extra oxygen atom (ribose instead of deoxyribose), which makes it less stable than DNA. That's a feature: messages that fade on their own let the cell stop making a protein quickly when it's no longer needed.
mRNA vaccines use this exact system. They deliver a short message recipe for a harmless piece of a virus; your ribosomes build it, your immune system learns to recognise it, and the message breaks down within days.
Protein builder. A ribosome clamps onto the message and reads it in 3-letter words called codons. Each codon means one building block (an amino acid), so the ribosome adds them one by one into a chain. The chain folds into a protein, like keratin for your fingertip. Like a bead machine reading a pattern card: 3 dots tell it which bead to thread next.
Four letters in groups of three give 64 possible codons, but there are only 20 amino acids. So most amino acids have several codons, and three codons mean 'stop'. The codon AUG means 'start here' (and the amino acid methionine).
Ribosomes add about 5 to 6 amino acids per second in human cells. A protein is usually a few hundred long, so it takes about a minute. The finished chain folds itself into a precise 3D shape, and that shape decides its job.
This code is almost the same in every living thing. That's why bacteria can be given the human insulin gene and make real human insulin for diabetes patients, which they've done since 1982.

Codon length: 3 letters
Codons → amino acids: 64 → 20

Did you know? A single skin cell can make thousands of new protein molecules every second, all from short-lived message copies of its DNA.

Biologists call this flow of information the central dogma: DNA → RNA → protein. DNA is the archive, RNA is the working copy, and proteins do the jobs. Copying DNA into RNA is called transcription; reading RNA into protein is called translation.

Why the middle step? It protects the master copy and makes the system flexible. A cell can make hundreds of copies of a busy message, or none of a gene it doesn't need, without ever touching the DNA itself.

The genetic code was cracked between 1961 and 1966. The first codon solved was UUU, which means the amino acid phenylalanine. Scientists worked it out by feeding artificial RNA made only of U to a test-tube mix of cell parts and seeing what came out.

Your body makes roughly 2 million red blood cells every second, and each one is stuffed with about 270 million copies of the haemoglobin protein.

Step 8 of 9 · Your data in DNA

Writing a file in A, C, G, T

So cells read a 4-letter code to build everything. Computers use a 2-symbol code, 0 and 1. If both are just codes, could we write our own files into DNA?

Any computer file is a string of 0s and 1s; turn every two bits into one DNA letter, build those strands, and the file is stored in DNA.

Bits to letters. Every photo, song or message on a computer is stored as bits: 0s and 1s. DNA has 4 letters, so one letter can stand for 2 bits: 00 = A, 01 = C, 10 = G, 11 = T. The word "Hi" is 01001000 01101001 in bits, which becomes CAGA CGGC. Like Morse code: a fixed rule for swapping one alphabet for another.
Real systems are cleverer than a straight swap. Long runs of the same letter (like AAAAAA) are hard to build and read, and too much or too little G and C makes strands misbehave. So the bits are scrambled and re-coded to avoid runs and keep G and C near 50%.
Those rules cost a bit of space: the best methods store about 1.6 bits per letter instead of the perfect 2. In 2017 the 'DNA Fountain' method stored a whole computer operating system, a film and more this way and got every bit back.
Building strands. A machine called a DNA synthesiser builds the strand one letter at a time, by chemistry, not by cells. It adds a letter, washes, adds the next, about every few minutes. Strands are kept short, around 200 letters, so a file is split into millions of tiny pieces. Like typing a book one letter at a time, then cutting it into numbered strips.
The standard method (phosphoramidite chemistry, from the early 1980s) adds each letter with a protective cap, then removes the cap so the next one can attach. Each step is about 99% reliable, so after 200 letters many strands have small errors. That's why strands are kept short.
Each piece gets an 'address' tag: a short index saying where in the file it belongs. Extra error-correcting pieces are added too, so the file can be rebuilt even if some strands are lost or misread, the same trick a scratched DVD uses.
Writing is still the slow, expensive part. In 2019 a fully automated machine stored the word 'HELLO' and read it back, which took 21 hours. Faster methods using enzymes (the cell's own tools) are being developed.
Glass armour. Dry DNA lasts a long time, and wrapped in a tiny glass bead it lasts even longer. Glass keeps out water and air, the two things that break DNA down. Tests suggest DNA stored in glass and kept cold could stay readable for thousands of years. Like a fossil in amber: sealed away from the air, so it barely changes.
In 2015, researchers in Zürich sealed data-carrying DNA in silica (glass) beads, then heated them to 70 °C for a week to fake centuries of ageing. The data came back without errors. They estimated it would last over 2,000 years at 10 °C.
Compare that with today's storage: hard drives and tape last about 5 to 30 years before data needs copying to new media. And DNA never becomes obsolete: as long as life uses DNA, we'll have tools to read it.

1 DNA letter: 2 bits
Typical strand: ≈200 letters

Did you know? In 2013, scientists stored all 154 of Shakespeare's sonnets, a photo and a clip of Martin Luther King's 'I have a dream' speech in DNA, then read them all back.

DNA data storage works in four steps: encode (turn the file's bits into A, C, G, T), synthesise (build the strands), store (dry, cold, often in glass), and read (sequence the strands and turn the letters back into bits).

The idea goes back decades, but the first big demonstration was in 2012, when George Church's team at Harvard stored a 53,000-word book in DNA. In 2013 a team at the European Bioinformatics Institute stored 739 kilobytes, and by 2018 Microsoft and the University of Washington stored 200 megabytes, including a music video, and could pull out any single file without reading everything.

That 'random access' trick uses the address tags. Each file's strands start with the same short tag, so a lab technique called PCR can make millions of copies of just those strands, like searching a library by the colour of the book's spine.

The catch is cost and speed. Writing DNA still costs far more per megabyte than a hard drive, and it takes hours. So for now DNA suits archives: precious data you want to keep for centuries but rarely open.

In theory, one gram of DNA can hold about 215 petabytes: roughly 50 million HD films in less DNA than the volume of a sugar cube.

Step 9 of 9 · Your data in DNA

A hard drive in a tube

So a file becomes millions of tiny glass-wrapped strands. Let's pull back out to our own scale and see what a DNA hard drive actually looks like.

Millions of data strands dry into a speck of white powder in a tube, and a pocket-sized reader can turn them back into your file.

A speck of white. All those strands, dried down, are just this: a tiny white smudge at the bottom of a tube. It looks like nothing, but it can hold more than a stack of hard drives. DNA packs data tens of thousands of times more tightly than the best memory cards. Like a grain of salt that holds a whole library.
Each strand is billions of times smaller than a grain of salt, and there are many copies of each one (redundancy helps with reading). Even so, a visible speck holds billions of strands.
Scientists estimate that all the world's data, roughly 100+ zettabytes, would fit in a few hundred kilograms of DNA. Today the same data fills warehouses of hard drives the size of football pitches.
Reading it back. To read the file, you add water and run the DNA through a sequencer. This pocket one pulls each strand through a tiny hole and measures how each letter changes an electric current. Software stitches the pieces back together using their address tags. Like reading Braille with a fingertip: feel each bump as it passes, one by one.
This reader is a 'nanopore' sequencer. A membrane has hundreds of holes, each only a few nanometres wide, with a current flowing through. As a DNA strand slides through, each letter blocks the current by a slightly different amount, and software turns those wiggles into letters.
Other sequencers work by copying the strands with glowing letters and photographing each one as it's added. Either way, every strand is read many times, and the error-correcting pieces fix the mistakes.
Built to last. Hard drives and tapes need copying every few decades. DNA, kept cool, dry and dark, can last thousands of years. Scientists have read DNA from mammoths frozen for over a million years. Nature has been testing this storage for 3.5 billion years. Like carving a message in stone instead of writing it in pencil.
DNA breaks down mainly through water (which snips the backbone) and oxygen (which damages letters). Take both away and lower the temperature, and the reactions slow almost to a stop.
The world's oldest readable DNA so far came from mammoth teeth in Siberian permafrost, about 1.2 million years old (published 2021). Ancient DNA has also revealed that most people outside Africa carry a little Neanderthal DNA.

1 gram of DNA: ≈215 PB
Lifespan in glass: 1,000s of years

Did you know? In 2019, the full text of English Wikipedia (about 16 GB) was written into DNA, and the strands fitted in a tiny vial.

A DNA archive is surprisingly simple to look after. No electricity, no spinning parts, no file format that goes out of date. Keep it cool, dry and dark, and come back in a century.

Reading it is getting cheap and portable. The pocket-sized sequencer in the photo plugs into a laptop's USB port and has been used in rainforests, in the Antarctic and even on the International Space Station.

The hard part is still writing. Building DNA letter by letter costs much more per megabyte than any hard drive, and it's slow. Big tech companies and start-ups are racing to speed it up with enzymes and chips that build millions of strands at once.

And that's the full journey: from a fingertip, into a cell, its nucleus, a chromosome, the spools, the twisted ladder and its four letters, the machines that read them, and finally to humans writing their own messages in the same alphabet life has used for billions of years.

One exabyte, a billion gigabytes, would need about 5 grams of DNA. In hard drives, it would fill a large building.

How DNA Stores Life (and Your Files): From a fingertip down to the four letters of life, and how we write data into them