The following ratio is generally constant for a given species:

The $\mathrm{C}+\mathrm{G} / \mathrm{A}+\mathrm{T}$ ratio varies from organism to organism among the prokaryotes), but within (particularly the limits of experimental error, $\mathrm{A}=\mathrm{T}$ and $\mathrm{C}=\mathrm{G}$. Related Theory Erwin Chargaff discovered that the number of nitrogenous bases in the DNA was present in equal quantities. The amount of $A$ is equal to $T$, whereas the amount of $C$ is equal to $G$, i.e., $A=T ; C=G$