You can find the non-pathogenic bacterium Micrococcus luteus in the soil and on human skin. The genome of this bacterium consists of double-stranded DNA with a base composition of 15% adenine. Predict which one of the following values represents the percentage of guanine.

Question

abb0t · Answer

Remember that the composition of each must equal 1. Also, remember that A pairs with T, and G with C, by Chargaff's rule, meaning that it should be a 1:1 ratio. 

So, if you have 15% A, you must also have 15% G, yes? Together, that equals 30%. Now, if you have 30% A-G pairs, how much of G-C, base pairs do you have together?

abb0t · Answer

Sorry, I mean 15% A and 15% $\sf \color{red}{T}$

abb0t · Answer

30% $\sf \color{red}{A-T}$ pair*

anonymous · Answer

So you would have 70% G-C
 pairs? since it must add up to be 100%

abb0t · Answer

Yes, 70% C-G bonds, but you're trying to find $\sf \color{red}{ONLY}$ guanine. So, 70% is not the correct answer. Instead, divide it by 2. Since remember, by chargaff's rule, it should be a 1:1 ratio of each.

anonymous · Answer

AHA! 35%

abb0t · Answer

Yes, 35% of guanine. 
And as you can see, you already checked yourself above, 35+35+15+15 = 100