“It has been found experimentally...”

This sentence refers to the work of Erwin Chargaff, a biochemist at Columbia University. In the late 1940s, Chargaff analyzed the proportions of the four different types of base molecules in DNA, and found that DNA always contains equal amounts of guanine and cytosine, and equal amounts of adenine and thymine.

The significance of this discovery remained unclear until February 1953. That’s when Watson figured out how DNA’s four bases paired with one another. By fiddling with cardboard cutout versions of the base molecules, he discovered that adenine always pairs with thymine, and guanine always pairs with cytosine. Now Chargaff’s finding made perfect sense to Watson and Crick: If every adenine and thymine are paired in DNA, there must be an equal number of these two molecules. The same goes for guanine and cytosine.

© exploratorium