In 1953 Watson and Crick discovered the double helix. They were both curious about the structure of the DNA since it was the main component (molecule) of life. There were four nitrogen bases which were: T, A, C, G, and these were paired with the corresponding one, this was proposed by Levene. However, his tetranucleotide theory was incorrect, he believed that the DNA had a sequence that was repetitive. Erwin Chargaff later discovered that the T and A belonged together and then the C and the G, this was because their number came close to one another. Watson and Crick, later discovered through an X-ray pattern that the structure of the DNA was shaped like an “X” and hinted the shape of a double helix. They wondered how they could pair the bases