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The suggested answer to the question is the following: "M L S C D K S D Stop". I do not understand how they get that result.

The question is as follows:

An RNA polymerase transcribes the following DNA sequence, in the direction of the arrow. Which amino acid sequence will be generated?

the genetic code DNA sequence

Thanks in advance!

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  • $\begingroup$ I get that, and the first ATG is at 5' CCA CC ATG TTA. I thought it always reads the code in triplets? Thats why that option doesn't make any sense to me. $\endgroup$
    – user23451
    Commented Apr 24, 2016 at 9:39
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    $\begingroup$ @OLi4 The polymerase certainly does NOT search for a start codon to start transcription. The polymerase produces a mRNA with a 5' untranslated region before the start codon. However this question is not about RNA polymerase, its a theoretical question about the genetic code. $\endgroup$
    – David
    Commented Apr 24, 2016 at 9:49

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This is a deceptively and badly worded trick question. Your confusion is because it starts with a mention of RNA polymerase, which transcribes DNA into mRNA, but asks about the protein, which is produced by the translation of the mRNA on ribosomes.

The first step in solving the problem is asking what is the sequence of the mRNA produced by the transcription. The question doesn't actually say which strand is copied, but the 5'-end of the mRNA is synthesized first, so you can assume the mRNA is the complement of the bottom strand, which is copied. (The convention of direction presumably tells you the same thing). Thus, the mRNA has the sequence:

CCACCAUGUUA...

Now you need to go along the sequence (as a ribosome) looking for where to start translating. However the question doesn’t tell you whether this is a prokaryote or a eukaryote (unless you omitted to tell us). In order to solve the problem we have to assume it is a eukaryote where the ribosome normally scans along from the cap to the first AUG to initiate translation (the region italicized above):

AUG (ATG) = M

UUA (TTA) = L

etc

(If it were a prokaryote you’d need to look for a Shine and Dalgarno ribosome-binding site.)

You might wish to raise my criticisms with your instructor (although I doubt that he will thank you for doing so).

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