Cells and DNA: Inside a Human Cell, the Double Helix in 3D

The smallest living units

The picture starts with a crowd. Every living thing is made of cells, and the human body has roughly 37 trillion of them, according to one well-known estimate. There are about 200 different kinds, from long thin nerve cells and round red blood cells to flat skin cells, each shaped for its job. The word cell was coined in 1665 by Robert Hooke, who saw a honeycomb of little boxes in a slice of cork. A typical human cell is only about 10 to 30 micrometres wide, so you would need a microscope to see it.

A city inside a membrane

Now the model cuts one cell in half. The outer boundary is the cell membrane, a thin double layer of fat that controls what goes in and out. Inside is the nucleus, a sphere holding the DNA. Around it float mitochondria, the cell's power plants, which turn food and oxygen into the energy molecule ATP. Ribosomes, countless tiny machines, build proteins. Other parts, such as the endoplasmic reticulum and the Golgi apparatus, fold, sort and ship them. Pick a part in the sandbox to see what it does.

The double helix

The camera enters the nucleus and finds DNA, short for deoxyribonucleic acid. It looks like a twisted ladder, a double helix. The sides are a sugar-phosphate backbone, and each rung is a pair of bases: A always pairs with T, and C always pairs with G. In 1953 James Watson and Francis Crick published this structure in the journal Nature, building on the X-ray image known as Photo 51, taken by Rosalind Franklin and Raymond Gosling. The human genome has about 3 billion base pairs, and a single cell's DNA would stretch around 2 metres if pulled straight.

A code of four letters

The order of the letters is the message. A gene is a stretch of DNA that carries the instructions for one product, usually a protein, and humans have roughly 20,000 genes that code for proteins. Two unrelated people share about 99.9 percent of their DNA, so the small differences make each of us unique. The DNA is packed into 23 pairs of chromosomes, 46 in total, which are the tightly coiled form you see just before a cell divides. Now type a strand in the sandbox and the matching partner strand appears, pairing each letter by the A–T and C–G rule.

Reading the instructions

DNA never leaves the nucleus, so the cell makes a working copy. This step, transcription, builds a strand of messenger RNA, or mRNA. RNA uses the letter U instead of T. The mRNA leaves the nucleus and meets a ribosome, which reads it three letters at a time. Each three-letter codon names one amino acid, and a helper molecule brings each one to the ribosome. The amino acids are joined into a chain, and the chain folds into a protein with a specific shape. The sandbox shows the RNA message built from your own strand.

One cell becomes two

The last moment shows cell division. Before dividing, a cell copies all of its DNA. In mitosis the chromosomes coil tight, line up in the middle, and each is pulled apart so that every new cell gets a full set. The cell then pinches in two, producing two identical daughter cells. This is how you grew from a single fertilised egg, and how skin, blood and the lining of the gut are renewed all the time. Estimates suggest hundreds of billions of cells are replaced every day. When this control fails, cells can divide without stopping, which is how a tumour begins.

Cells and DNA