In every cell lives a tireless scribe — an enzyme that copies the long thread of DNA. It moves along it like a read head over a tape, building a new strand alongside. It picks letters by strict rule: opposite A it always places T, opposite G it places C. But sometimes, very rarely, its hand trembles, and a different letter slips in. Such an error is called a mutation.
Scientists took this simple mechanism and ran it on a computer. They started with any random sequence of letters, as if typing without looking at the keyboard. Then they let copying proceed many times in a row, each time allowing the scribe to occasionally err. It turned out that after thousands of repetitions, a balance spontaneously emerges in each strand: the number of A's becomes nearly equal to T's, and G's nearly equal to C's. This matched a long-standing observation by biologist Erwin Chargaff, made back in the mid-20th century while peering at real DNA.
Why does this matter? It turns out that no special conditions are needed for this order — no particular shape of the strand, no help from other molecules. The logic of copying alone suffices. This explains why Chargaff's rule holds in nearly all living things, from bacteria to humans. And why deviations sometimes occur: if the scribe errs more often or copies one strand more diligently than the other, the balance shifts slightly.
Now, when scientists read an organism's genome and see slightly more A's than T's, they understand: it's not chance but a trace of how the copying machine works. Life itself, like a careful scribe, strives for symmetry.
Chargaff's rule states that in DNA, the amount of A equals T, and G equals C. Erwin Chargaff discovered it in 1950 by simply counting the letters in DNA from various organisms. This insight hinted to Watson and Crick that DNA letters pair up.