DNA Base Pairs
You pull a zipper up your jacket on a cold morning. The two tracks look different on each side, but their teeth fit together tooth by tooth. If one side loses three teeth, the other side will not close properly. DNA works with a matching system like that, but it is built out of chemical shapes instead of metal teeth. Your cells copy billions of these teeth every time they divide, and they match each one against its partner.
DNA is written in four chemical letters called bases: A, T, C and G. Across the twisting ladder of the molecule, A always bonds with T, and C always bonds with G. Because of this rule, one side of the ladder holds all the information needed to build the other side. People remember the pairs with the phrase, "Apples in the tree, cars in the garage." This is where the memory line stops being true, because it leaves out what the letters stand for: adenine, thymine, cytosine, and guanine. The rule also changes when a cell makes a working copy of DNA called RNA, where A pairs with a different base called uracil instead of thymine.
Therefore
When you look at one side of a DNA strand, you can write out the matching side yourself. Just swap every A for a T and every C for a G.
Erwin Chargaff, 1950
In 1950, Erwin Chargaff worked at Columbia University in New York. He measured the amounts of the four bases in DNA from humans, oxen, and bacteria. Scientists had thought all four bases appeared in equal amounts, like a repeating pattern. Chargaff showed that the amount of A always matched the amount of T, and the amount of G always matched the amount of C. In 1953, James Watson and Francis Crick in Cambridge used Chargaff's numbers, along with an X-ray photograph taken in 1952 by Raymond Gosling and Rosalind Franklin in London, to work out the shape of the double helix.
A small reminder, on a laptop sleeve or a notebook, that every cell copies its instructions by matching four chemical letters across a ladder.
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Related patterns
pairs with
Nucleus — Where the Instructions Are Kept
The nucleus holds the DNA and lets out working copies called RNA.
extends to
Chromosomes — Twenty-Three Pairs, Forty-Six in All
Millions of base pairs wind into forty-six dense packages called chromosomes.
contrasts with
Cell Membrane — What Comes In and What Goes Out
The membrane is a physical barrier, while DNA is a coded message.