From body to molecule
Zoom from your whole body down to the DNA inside one cell, with the scale at every stop.
Your whole body
Start here, where you are: a body about as tall as a doorway, made of about 30 thousand billion living cells. Hold on to this size as we zoom in, because each stop on the way is drawn on its own scale, not one giant model.
An adult human stands around 1.5 to 1.9 m (about 1.7 m on average). The body holds roughly 30 to 37 trillion cells, and they are very different sizes: a white blood cell is about 12 µm across, a skeletal muscle fibre can run the length of your thigh. The skeleton and heart shown here are real Z-Anatomy scan meshes at their true relative size.
An organ: the heart
Now we zoom to one organ, about the size of your fist. It is a pump of muscle that beats around 70 times a minute. But the heart is not just a bag of muscle: it is built from millions of tiny muscle cells, packed side by side. Let us zoom onto its wall.
The heart is roughly 12 cm long, 8 to 9 cm wide and 250 to 350 g in adults, in the middle of the chest. Its wall, the myocardium, is cardiac muscle: a mesh of branching muscle cells (cardiomyocytes) that squeeze together on every beat. The four chambers and great vessels you see are scan meshes; the muscle cells inside them are not modelled at this size, so the next stop switches to a drawing.
A tissue: cardiac muscle
Zoom into the heart wall and you find rows of muscle cells joined end to end, like carriages of a train. The dark stripes are the parts that pull. This is a drawing, not a scan: it shows a small patch, not the whole wall. A real patch this size holds many more cells than the handful drawn here.
Cardiac muscle cells, or cardiomyocytes, are about 100 µm long and 15 µm wide. Each is striped (striated) by its sarcomeres, the repeating units that shorten to make the muscle pull, and each cell has one or two nuclei. Cells are joined end to end at intercalated discs, which carry desmosomes (to hold the cells together) and gap junctions (so the electrical signal passes quickly from cell to cell). That coupling is why the heart can contract as one unit. A thread of capillary (the pale tube, red cells inside) runs between the cells to bring blood. This is a schematic illustration: 12 cells are drawn, spaced out for clarity; a real 0.5 mm patch would hold on the order of a thousand.
A cell
Now one muscle cell on its own, about a tenth of a millimetre long. It has a wall, called the membrane, and a control room in the middle, the nucleus, where the DNA is kept. The orange pods are mitochondria, the cell’s power plants; heart muscle is packed with them because it works non-stop.
This is an animal cell, drawn as a generic one rather than a true muscle cell shape. A cardiac muscle cell is a long, branching cell about 100 µm long with a nucleus near its centre; the cartoon here is rounder and simpler. The pale blue sphere is the cell membrane, the purple sphere is the nucleus, the pink tangle inside it is chromatin (DNA wrapped on proteins), and the orange capsules are mitochondria. Mitochondria make the cell’s energy, and in heart muscle they fill about a third of the cell volume. Again, this is a code-built illustration at its own scale, not a scan.
Inside the nucleus
Zoom right in on the nucleus. That pink tangle is your DNA. If you pulled all the DNA out of just this one cell and stretched it straight, it would be about 2 metres long. All of that has to fit inside a space far smaller than a speck of dust.
A typical cell nucleus is about 6 µm across. Inside it, the 2 m of DNA from a cell is wound around histone proteins to make chromatin, and packed in levels until it fits. Most of the time the thread is loose like this so the cell can read it. This stop is the same code-built nucleus; the tangle is a cartoon of chromatin, not an accurate path.
A chromosome
When a cell is about to split, it coils the long DNA thread up tight into an X shape called a chromosome. Your cell has 46 of them. Coiling it up is how the cell moves it around without it tangling, like winding a garden hose onto a reel.
The X shape appears only during cell division, after the DNA has been copied. At this point a human chromosome is a few micrometres long (roughly 1 to 8 µm depending on which one). Chromosome 1, the largest, holds about 250 million base pairs. Between cell divisions the chromosomes are uncoiled and spread through the nucleus as the chromatin seen in the last stop. This X is a schematic drawing.
DNA, the molecule
And here is the DNA itself: a twisted ladder, about two millionths of a millimetre wide. The rails and rungs are atoms. The order of the letters A, T, C and G along the ladder is the instruction for building and running you. You have just travelled from a whole body to a single molecule. Press Back to climb out, one stop at a time.
The DNA double helix is about 2 nm wide, with one base pair every 0.34 nm and about 10.5 pairs per full turn. Your genome is about 3 billion base pairs, and each body cell holds two copies of it, about 6 billion pairs in all (12 billion after copying), and the letters here are the start of the real insulin gene. This is where the separate "What DNA is" lesson picks up: it reads a real gene, letter by letter, and turns it into a protein. The molecules shown are cartoons, drawn far thicker than real atoms so you can see them.