Nice DNA question

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.

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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.


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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.

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Great question, I will show it to the class today