You Are Written in 4 Letters

Shows a twisted rope-ladder built from paired letters, three rungs boxed off together as one three-letter word, a music CD with a string of letters printed across its face, and a whale and a mushroom holding up the same four letters you are.

Before you start. You need to know that your body is built from trillions of tiny cells, and that each one carries its own full copy of the instructions for making you. That is the whole of what this lesson assumes.

The idea in 30 seconds

Your DNA is a twisted ladder built from four letters, and each rung locks together only one way - know one side, and you already know the other.
The whole message runs to about 775 megabytes, close to a music CD’s worth, though the exact number runs a little over.
The reading machinery reads three letters at a time (a codon); sixty-four possible three-letter words cover only about twenty jobs, with the spares parked exactly where copying slips are most likely to land.
Copying goes wrong only about once in a billion bases, not because any one step is perfect, but because three separate checks are stacked on top of each other.
When a slip gets through anyway and sticks, it’s called a mutation.
Your genetic message behaves less like a blueprint or a program and more like a recipe: it lists parts and steps, and the result depends on the kitchen reading it.

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