He called the compound "nuclein." This is today called nucleic acid, the "NA" in DNA (deoxyribo-nucleic-acid) and RNA (ribo-nucleic-acid).
Two years earlier, the Czech monk Gregor Mendel, had finished a series of experiments with peas.
Nine years later, in 1962, they shared the Nobel Prize in Physiology or Medicine with Maurice Wilkins, for solving one of the most important of all biological riddles.
Half a century later, important new implications of this contribution to science are still coming to light.
His observations turned out to be closely connected to the finding of nuclein.
Mendel was able to show that certain traits in the peas, such as their shape or color, were inherited in different packages. For a long time the connection between nucleic acid and genes was not known.
In 1949 he showed that even though different organisms have different amounts of DNA, the amount of adenine always equals the amount of thymine. For example, human DNA contains about 30 percent each of adenine and thymine, and 20 percent each of guanine and cytosine.
With this information at hand Watson was able to figure out the pairing rules.
During cell division, the DNA molecule is able to "unzip" into two pieces.
One new molecule is formed from each half-ladder, and due to the specific pairing this gives rise to two identical daughter copies from each parent molecule.