DNA Base Pairs
A zipper on a winter coat only works because the two sides match tooth for tooth. When you pull the slider up, each tooth on the left finds one specific tooth on the right. If one side loses a few teeth, the whole track jams. The two sides don't look identical, but they fit together neatly. When you pull the slider down, the two halves come apart cleanly. Each half still keeps the exact pattern needed to rebuild the whole closed seam.
DNA works like that zipper, built from four chemical letters named A, T, C and G. Across the ladder, an A on one side always links to a T on the other, and a C always links to a G. That pairing lets a cell copy its instructions by unzipping the two strands and rebuilding the missing halves. A common memory aid is 'Apples in the tree, cars in the garage.' That rhyme helps you pair the letters, but it doesn't give their full names: adenine, thymine, cytosine and guanine. The pairing rule also shifts when a cell makes an RNA copy of a gene, because RNA replaces thymine with a letter called uracil that pairs with adenine instead.
Therefore
When you read one side of a DNA strand, you can immediately write out the other half by swapping the letters. Just remember that working RNA copies swap out the Ts for Us.
Erwin Chargaff, 1950
In 1950, Erwin Chargaff worked at Columbia University in New York. He measured the chemical letters in DNA taken from different plants and animals. In every sample, the amount of A matched the amount of T, and the amount of G matched the amount of C. In 1953, James Watson and Francis Crick used Chargaff's ratios at Cambridge to figure out the double helix. Watson shifted cardboard shapes of the four letters across his desk. He saw that an A joined to a T made the exact same shape and width as a C joined to a G, so every rung fit inside the spiral.
A small reminder, on a notebook or a laptop lid, that your cells copy genetic instructions using four chemical letters that fit together like teeth on a zipper.
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Related patterns
pairs with
Nucleus — Where the Instructions Are Kept
The nucleus holds these base pairs and protects them from damage.
extends to
Chromosomes — Twenty-Three Pairs, Forty-Six in All
A chromosome is one long strand of base pairs wound tightly.
extends to
Ribosome — Read the Recipe, Build the Protein
Ribosomes read the copied letters in sets of three to build proteins.