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The base sequences of codons 57-58 in the cytochrome b5 reductase gene is CAGCGC. The mRNA produced upon transcription of this gene will contain the sequence :


Question from Kaplan Lecture Notes 2010 ...
Dumb explanation given to it in the notes. Cant understand it..



In this question we are simply given a sequence with no indication of whether it is the coding strand or template strand, and no indication of 5' or 3'. In a case such as this I think that this is traditionally the coding strand given from 5'-->3'.

Remember we are given the coding strand in 5'-->3', which is complementary to the template strand. The template strand is the one actually being transcribed. The mRNA is synthesized 5'-->3' while reading the template strand 3'-->5'.

Coding Strand: 5'-CAGCGC-3'
Template Strand: 3'-GTCGCG-5'
mRNA: 5'-CUGCGC-3'

So, answer is B. CUGCGC.

On Aug 11, 2010 - 9:10 PM, indsha008 responded:
I didnt know that we are supp to assume that it is coding strand...

Anyways, you made a little mistake in transcription. your mRNA should be 5' CAGCGC 3'.

Hence, the right answer would be D...


Edited by djroac01 on Aug 11, 2010 - 10:03 AM


djroac1 - good explanation...


Good catch on my answer, silly mistake on my part.

I think that if you are ever given a DNA sequence like this and it is not explicitly stated as the coding/template strand or 5'-->3'/3'-->5', then assume that it is the coding strand given 5'-->3'.


yes in the old dvd's of Dr. lionel ..he said that if nothing is mentioned about starnd ..we should assume that its coding strand and the direction is 5' --->3' and coding stand and mRNA will be identical with only one exception, that T will replaced by U


The correct answer for this question is D, not B. The explanation is correct however, since the mRNA has to be identical to the coding strand in this case since there is no T in the coding strand there should be no U in the mRNA.
Coding Strand: 5'-CAGCGC-3'
Template strand: 3'-GTCGCG-5'
mRNA: 5'-CAGCGC-3'

You would only have a U in the mRNA if the coding strand actually had a T in the sequence.




At first I could not make it, thought it was too difficult to read then got help from
and did it the answers given also played a good role in making me understand


Very well explained buddy, you have solved the difficult one I appreciate it and would like to share this site where similar kind of problems are solved

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