I thought this question was a nice example of what Ari said about reading every word in the question. It’s not a lot of words. But there’s a few key words that completely change the math. Let me know what you think.
Nucleotide has 3 different subunits.
28 nucleotides have the base adenine. Thus by Chargaff’s rule there’ll be 28 nucleotides with base thymine.
subunits in adenine containing nucleotide = 3 x 28 = subunits in thymine containing nucleotide = 84
Nucleotides remaining = 600 - 168 = 432
let’s say remaining are C cytosine containing nucleotides and G guanine containing nucleotide. By chargaffs rule again they both would be same in number.
subunit in cytosine containing = subunits in guanine containing = 3x
3x + 3x = 432
x= 432/6 = 72
Thus there are 72 guanine subunits out of 600 in this section.
The first thing I thought was interesting was that a nucleotide consists of three different “sub-units.” I figured that meant the ribose, phosphate, and nitrogenous base. But, 600 sub-units means only 200 are nitrogenous bases! I kept assuming 600 nitrogenous bases. That was my first mistake.
Then, after you do the base-paring for 200 bases, you get 28A-28T, and 72G-72C. Great. But we are asked what percentage of the sub-units are G. And there are 600 sub-units. That was my second mistake! I divided 72G by 200 bases, when I should have divided 72G by 600 sub-units.
So basically I managed to miss the same question twice. : )
Anyway, I remembered what Ari said about terms. That sub-unit term popped up twice in this question and it got me both times.
Just wanted to share.
Great question, I will show it to the class today