Which one contains the Start Codon

[csx]

If each of the following mRNA nucleotide sequencescontains three codons, which one contains a start codon?

3'-GUACCGAAC-5'
5'-AAUGCGGAC-3'

Anyone wanna take a stab at this and explain their rationale? I selected wrong and do not understand why I am wrong.

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[Chrisz]

The first sequence contain the start codon. It states in the question stem that each sequence contains three codons, which implies the reading frame for each of the sequence should start from the first letter. Starting with the first letter, only the AUG in the first sequence will be translated into met.

 

[Cawolf]

@Chrisz covered the question perfectly but I think it is also important to note that you are reading the RNA in the proper direction, 5' to 3'. If you chose the second one (which does have an "AUG" in it) you would have read it backwards, and out of the reading frame, which is not the way that transcription occurs.

 

[Czarcasm]

@Chrisz covered the question perfectly but I think it is also important to note that you are reading the RNA in the proper direction, 5' to 3'. If you chose the second one (which does have an "AUG" in it) you would have read it backwards, and out of the reading frame, which is not the way that transcription occurs.

I'm a little confused what you mean by reading it backwords. In either transcript, you can only divide the mRNA sequence into 3 bp codons beginning from the very first nucleotide at the 5' end.

 

[Concrete Jungle]

We ignore the fact that the reading frame could start anywhere because they state there are three codons in the question and 9 bases in the answer right?

 

[Czarcasm]

We ignore the fact that the reading frame could start anywhere because they state there are three codons in the question and 9 bases in the answer right?

Right, although I think this is a poor question overall because technically, AUG by default is the first codon which in turn determines our reading frame. I suppose we're left to assume we're given a select region of an mRNA strand (that excludes the original AUG start codon). Otherwise, if this is indeed the mRNA strand, to say the 3bp sequences: CAA and GCC (both which appear before AUG) in the 3'-GUACCGAAC-5' mRNA strand are 'codons' would be wrong, since everything including AUG and after (in the 5' to 3' direction) should be considered codons instead. In this scenario though, given the two choices and given the assumption, only the first strand above has 9 bp that can be divided exactly into 3 bp.