DNA Base Pairs
You pull the zipper up on a winter coat. One row of plastic teeth slides into the other row. If one tooth bends, the two sides cannot lock together. The coat stays open and the cold air gets in. A cell keeps its genetic instructions on a ladder that works like that zipper. The rungs are split down the middle. Each half only fits its matching partner across the gap.
DNA is written in four chemical letters called A, T, C and G. Across the ladder, A always pairs with T and C always pairs with G. Each side of the ladder carries all the information needed to rebuild the other. The memory line 'Apples in the tree, cars in the garage' helps you remember which letters match. But the line stops being true because it never names the chemicals: adenine, thymine, cytosine and guanine. It also changes in RNA, the working copy of DNA, where A pairs with uracil instead of T.
Therefore
When you know the letters on one strand of DNA, you can always find the letters on the other. You only need to match each letter to its partner.
Erwin Chargaff, 1950
Erwin Chargaff worked at Columbia University in New York. Before 1950, most scientists thought DNA was a dull molecule with equal parts of four chemicals repeating in order. In May 1950, Chargaff published tests on DNA from humans, cows and bacteria. He found that different species had different mixes of chemicals. But in every sample, the amount of A always matched the amount of T, and the amount of C always matched the amount of G. He shared those ratios with James Watson and Francis Crick in 1952. His rule showed them how the two strands fit together.
A small reminder, on a notebook or a water bottle, that every rung in your DNA is two chemical letters locked together.
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