Consider these sentances regarding to the structure and nature of...

Erwin Chargaff (1950) made quantitative analysis of DNA and showed that the composition of DNA molecules varies in complex ways depending upon the source of DNA. He proposed Base Equivalence rule or Chargaff's rule, which states that, for double-stranded DNA, the proportion of adenine always equals to that of thymine and proportion of guanine always equals to that of cytosine, i.e. $\mathrm{A}=\mathrm{T}$ and $\mathrm{G}=\mathrm{C}$. It follows that, an equal proportion of purines and pyrimidines is always seen in a double-stranded DNA, i.e. $A+G=T+C$ or $\frac{A+G}{T+C}=1$