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There's this little paragraph in my textbook, parts of which I find redundant. My teacher is absolutely fine with this ("Aaron, you've got to write down what's in the textbook if you want to score in exams, nothing extra...."), but all the same.....

It's regarding 'Why does transcription occur at one DNA strand at a time?'

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According to their first point, forming two complementary RNA strands would result in the production of two different proteins for the same character.

Now according to their second point, formation of two complementary strands of RNA side-by-side would just result in them pairing to form dsRNA, which would be transcriptionally inactive and therefore wouldn't take part in the translation process.

The thing is, they've mentioned the second point after the first point, which I find confusing. For if the second point were correct, the first point wouldn't even exist.

Is there a sound reason for the points being stated this way? Am I missing something here?

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  • $\begingroup$ This should better be posted in biology.SE $\endgroup$ – Nilay Ghosh Nov 23 '16 at 16:14
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This textbook would never have made it into my classroom. The second "answer" is an arguably correct concept but is specious as a reason. The first "point" has a silly justification.

The real reason is an evolutionary one. In polypeptides, particular sequences of amino acids fold in ways that provide function. Tiny variations in this sequence can render the resultant polypeptide non-functional. With this sort of sensitivity what are the odds that the non-coding strand, whose sequence is dictated by the coding strand, could actually possess a viable gene? Furthermore, genes evolve over time. Any changes in the sequence of one would necessitate a change in the other. What are the odds of that being efficatious?

That said, I believe there are some cases where a gene on one strand has some overlap with a gene on the sister strand. I doubt these are lengthy or common, possibly occuring with the help of introns, portions of the DNA message that are "spliced out" of the final RNA transcript.

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