A scientist makes three artificial mRNA strands:

Question

(x) 5' AAAUUUAAAUUUAAAUUUAAAUUUAAA 3'
(y) 5' UUUCCCUUUCCCUUUCCCUUUCCCUUU 3'
(z) 5' AUAUAUAUAUAUAUAUAUAUAUAUAU 3'

When he analyzes the polypeptides produced, he finds that:
x produces a polypeptide that is 50% phenylalanine and 50%lysine.
y produces a polypeptide that is 50% phenylalanine and 50% proline.
z produces a polypeptide that is 50% isoleucine and 50% tyrosine.

Based on these results only, the best conclusion to make is that


A.  AUA codes for isoleucine
B.  AAA codes for phenylalanine
C.  AAA codes for lysine
D.  AAA codes for lysine and AUA codes for isoleucine
E.  AAA codes for phenylalanine and AUA codes for isoleucine


Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?

Gather Content
· What do you already know about the genetic code?

Consider Possibilities
· Consider the different answer options. Which can you rule out?

Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  AAA codes for lysine

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses the genetic code.
· What type of thinking is required?
        o This question is asking you to weigh and judge evidence, or evaluate, to choose the best of the possible answers.
· What key words does the question contain and what do they mean?
        o Artificial mRNAs are similar to mRNAs in a cell, but they were designed by a scientist and produced in vitro.
        o Polypeptides are proteins – in this case, resulting from translation of the artificial mRNAs.

Gather Content
· What do you already know about the genetic code?
        o The genetic code was elucidated by experiments similar to this one.
        o By producing artificial mRNAs with a defined sequence, Nirenberg and Khorana could infer the sequence of codons that encoded each amino acid.

Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o Is there enough information to say anything about what AUA encodes? No, because it could either be isoleucine or tyrosine. We don’t have enough information to distinguish between them.

Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Is there enough information to say what AAA encodes? Yes.
        o From experiment (x), we know that the AAA and UUU together make phenylalanine and lysine. But we don’t know which is which.
        o From experiment (y), we know that the CCC and UUU together make phenylalanine and proline. But we don’t know which is which.
        o Putting both together, we realize that UUU must encode phenylalanine, since that codon and that amino acid are in both experiments.
        o That means that AAA must make the other amino acid from experiment (x), namely lysine.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked what conclusion you could draw about what codons encode what amino acids.
        o The question required you to weigh and judge evidence, or evaluate, to choose the best of the possible answers.
        o Did you recognize that you needed to use experiment (x) and (y) together to determine what AAA encoded?