The Human Genome and DNA

The Code Inside You

Inside almost every one of the trillions of tiny cells that make up your body, there is a hidden message. It is a set of instructions so detailed that it explains the colour of your eyes, the shape of your nose, how tall you might grow, and a thousand other things about you. If you could read it, you would be reading the recipe for a human being.

That message is written in a molecule called DNA, and the full set of instructions is called your genome. It is one of the great wonders of nature — a code that life has been using for billions of years, passed faithfully from parent to child, generation after generation.

This little book unlocks the code. We will see what DNA is, how it works, how it is passed on, and how scientists finally learned to read the entire human genome.

Chapter 1: A Ladder Made of Letters

DNA stands for a long chemical name, but the molecule itself is easier to picture than the name suggests. Imagine a ladder, then twist it into a gentle spiral. Scientists call this twisted-ladder shape the double helix.

The two long sides of the ladder are like a backbone holding everything together. The interesting part is the rungs. Each rung is made of two chemicals that fit together in the middle, and there are only four such chemicals in all of DNA. Scientists call them by their first letters: A, T, C and G.

These four letters always pair in the same way: A joins with T, and C joins with G. The genius of DNA is in the order of the letters along the ladder. Just as the 26 letters of the alphabet can spell every word in a library, the four letters of DNA, in the right order, can spell out the instructions for building any living thing.

Chapter 2: From Letters to Genes

A string of DNA letters by itself does nothing. The instructions come from the order in which the letters are arranged. A meaningful section of DNA — one that carries the instructions for a particular job — is called a gene.

You can think of a gene as a single recipe in a giant cookbook. Most genes are recipes for making proteins, the tiny machines and building materials that do almost all the work in your body. One gene might hold the recipe for a protein that gives blood its red colour; another for a protein that helps you digest food; another that affects the colour of your eyes.

Humans have around 20,000 genes. Together they form the master cookbook that builds and runs a person. Different living things have different genes, which is why a sunflower grows into a sunflower and a tiger grows into a tiger. The same four letters spell completely different instructions in each.

Chapter 3: Packing It All In

A single cell holds about two metres of DNA, all coiled up. How does so much fit into something far too small to see? The answer is brilliant packing.

The DNA is wound up tightly into bundles called chromosomes. Most human cells contain 46 chromosomes, arranged in 23 pairs. You inherited one chromosome of each pair from your mother and one from your father, which is why you have features from both parents.

The chromosomes are stored in the nucleus, the control centre at the heart of the cell. When a cell needs to make a particular protein, it does not unpack all its DNA. Instead, it carefully reads just the right gene, copies it, and uses the copy as instructions. In this way, the same complete cookbook sits inside every cell, but each cell follows only the recipes it needs. To learn more about the cell itself, read The Living Cell.

Chapter 4: Copying the Code

For life to continue, DNA must be copied. Every time a cell divides to make new cells — to heal a cut, grow taller, or replace worn-out cells — it must give each new cell a complete set of instructions.

The double-helix design makes this possible in an elegant way. The ladder "unzips" down the middle, splitting the paired rungs apart. Because A always pairs with T, and C always pairs with G, each half can be used as a template to rebuild its missing partner. The result is two identical copies of the original DNA, one for each new cell.

This copying is astonishingly accurate, but not perfect. Now and then a letter is copied wrongly, creating a small change called a mutation. Most mutations are harmless. Some cause problems. And a rare few, over long stretches of time, lead to helpful changes — which is the raw material of evolution. You can read that bigger story in The Story of Evolution.

Chapter 5: Passing It On

DNA is the reason you resemble your family. When parents have a child, each passes on half of their DNA, mixed together in a brand-new combination. That is why you might have your father's eyes but your mother's smile.

Some features are decided by a single gene, but most are shaped by many genes working together, sometimes along with the world around you. Your final height, for example, depends both on the genes you inherited and on the food and care you received while growing.

This mixing is also why no two people — except identical twins — have exactly the same DNA. Even brothers and sisters with the same parents inherit different combinations. Out of just four letters, nature creates billions of unique individuals. You are, quite literally, one of a kind.

Chapter 6: Reading the Whole Book

For most of history, DNA was a complete mystery. The double-helix shape was only worked out in 1953. Then scientists set an even bigger goal: to read the entire human genome — every single letter, in order.

This enormous effort was called the Human Genome Project. Thousands of scientists across many countries worked together for over a decade. In 2003 they finished, having read the roughly three billion letters of human DNA for the first time. Printed out, the sequence would fill a stack of books taller than a person.

Reading the genome did not answer every question, but it opened countless doors. Scientists could now hunt for the genes behind diseases, study how humans are related to other living things, and explore why each of us is unique. It was one of the greatest achievements in the history of science.

Chapter 7: What the Genome Means for Us

Understanding DNA has already changed our world. Doctors use it to diagnose and treat inherited illnesses. Detectives use DNA fingerprinting to identify people from a single hair or drop of blood. Farmers and conservationists use it to protect crops and endangered animals.

One of the most surprising lessons is how alike we all are. Any two humans, anywhere on Earth, share about 99.9% of their DNA. All our differences — in appearance, background and ability — come from the tiny remaining fraction. The code reminds us that, deep down, every person is overwhelmingly the same.

Reading further still, we share huge stretches of DNA with other living things too: with mice, with fish, even with bananas. It is powerful proof that all life on Earth is connected, written in the same four-letter language.

Your genome is the oldest story you carry — a message handed down through countless generations, now living inside you. To keep exploring the machinery of life, read How Our Body Works: A First Guide, or meet the scientists behind these discoveries in Great Scientists and Their Discoveries.