## Tuesday, September 11, 2007

### Are You as Smart as a Third Year University Student? Q2

I thought it might be fun to post some multiple choice questions from old exams to see if Sandwalk readers are as smart as my old third year molecular biology students. Here's a question from 1998.

The sequence of the coding region of an E. coli ribosomal protein mRNA consists of 21% G's and 23% C's. What do you predict would be the composition of the part of the gene (double-stranded DNA) from which this mRNA coding region is derived?

a) 56% T's
b) 44% T's
c) 28% T's
d) 23% T's

1. c) 28%

G or C is 44% of the coding strand, and so also 44% of the double-stranded construct. (Because each coding G is accompanied by a C in the complementary strand, and each coding C is accompanied by a G in the complementary strand)

This means 56% of the coding strand will be either T, or A (which requires a T in the complementary strand).

What did I win?

2. e) impossible to answer correctly*

* actually, I'd say impossible to answer at the requested level of precision.

Anyway, the coding strand is 21% G, and 23% C. Thus, the double-stranded gene is 44% G-C. That leaves 56% A-T, but for all we know the coding strand could be 56% A and zero% T.

3. but for all we know the coding strand could be 56% A and zero% T.

Uh-huh. But the question didn't specify the coding strand, but the double-stranded gene. Neener, neener.

4. Neener, neener.

Curse my overzealous brain!

This is exactly the kind of mistake that half the time I would make on tests, the other half I'd catch myself and feel all smug.

5. c) 28% (I didn't peek)

6. Well we know the rest had to be A or T, but since it's 2x stranded, it means that having one would literally equal having the other, and thus will always have an even split between A and T in the final percentage.

so 44% is C or G, making 56% either A or T. Half of 56% is 28%. C.