Expert: Dana Krempels, Ph.D. - 1/27/2010

Question
An anticodon nucleotide sequence of five successive tRNA’s involved in protein synthesis was
analysed, yielding the following content:
 A     G      C     T     U
40% 27% 13% 0% 20%

What was the corresponding anti-sense strand of original DNA?
     A     G      C      T     U
a. 20% 13% 27% 40% 0%
b. 40% 27% 13% 20% 0%
c. 60% 27% 13% 0% 0%
d. 20% 13% 27% 0% 40%
e. 40% 27% 13% 0% 20%

The answer is supposed to be B, but I'm not really sure why.. Could you please explain? :) Thanks!

Answer
Dear Arimie

Since this is tRNA, you're twice removed from the DNA.  If the tRNA was 40% A, then the corresponding mRNA must have been 40% U.  Similarly, if tRNA was 27% G, the mRNA was 27%, and so on.

Once you know the composition of the mRNA (40% U, 27% C, 13% G, 20% A), you can bind that to the DNA to find that the original template strand was 40% A (which binds to U), 27% G (binds to C), 13%C (binds to G) and 20% T (binds to A).

Hope that helps explain why B is the correct answer.  (Hey, that's a good question!  I might have to use that on my genetics exam next time!)  :)

Dana