Chargaff's rule

It is proven that DNA is a right-handed double helix structure by Watson and Crick. Another biochemist Erwin Chargaff observed a relationship between the molarity of the four different bases. A bracket [ ] will be used to denote the molarity of the bases. Firstly, he noticed that the concentration of adenine is always equal to thymine, [A] = [T]. Secondly, the concentration of guanine is always equal to cytosine [G]=[C]. Thirdly, the concentration of purine (A and G) are always equal to pyrimidine (T and C). Hence, [A] + [G] = [C] + [T]. These are known as Chargaff's rules.