# Gene expression-This problem is impossible!

1. Oct 25, 2009

### stanton

I was interpreting a question, and it does not fit with my knowledge and common sense.
This is the problem with mystery...

Part of the sequence of a gene along the template strand for transcription was shown to be 5'-AGCTCA-3'
Using that partial DNA sequence, determine the corresponding sequence of the transcript encoded by that region of the gene.
A)5'-UGCAGU
B)5'-UGAGCU
C)5'-UGACGU
D)5'-UCGAGU

I thought the answer was D. Because in the given sequence in the question and option D fits according to rules [A always matches with T(or U), C always matches with G]
Now, my answer: A fits with U. G fits with C in option D. C fits with G. T fits with A. And so on. If you are suspicious, I can show you.
5'-AGCTCA-3'
5'-UCGAGU
It all fits and make correct strand. I think...

:surprised

How could this possible? I gathered all my knowledge, contemplated for almost half and hour, thinking how B can be the answer. B does not fit with any of the given strand. Is there something wrong with the answerbook or there is something I did not learned about it?
My prof told students to report any mistakes in the problem-answer sheet he gave to us. However, I feel not willing to go to him and talk about this. What if I am wrong?

Last edited: Oct 25, 2009
2. Oct 25, 2009

### Moonbear

Staff Emeritus
Look at the direction of the two strands.

3. Oct 25, 2009

### stanton

Ah! I get it. The directions are the same.
5'-AGCTCA-3'
5'-UCGAGU
They cannot have same 5' at one side. So the my strand must be reversed.
So, instead of UCGAGU, UGAGCU is the answer(this is reversed form of my answer), which is B.

4. Nov 15, 2009

### AyazM

Remember that examiners use this trick many times to confuse students and distinguish the bright from the average. ANY sequence that is given without any labels is ALWAYS READ 5' to 3' no matter what! You cannot violate this law without using appropriate labels.

Another example you might want to work on is deciphering the anticodon on t-RNA for recognition of the mRNA sequence. In a double stranded DNA, both strands are anti-parallel. Remember this also applies to the tRNA vs mRNA base pairing because this essentially is ALSO anti-parallel! Just thought about telling you, because these tricks are commonly used in exams.

5. Nov 15, 2009

### stanton

Thank you so much for your tip. :) How wicked those examiners are!