In our textbook it says that translation occurs more in a cell than transcription but I couldn't find anything that explains why it happens
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8$\begingroup$ Suppose you had a book that said how to make a candle mold, and you wanted some candles. Would you make molds from the instructions as often as you would make candles from the molds? $\endgroup$– mgkrebbsCommented Apr 16, 2020 at 20:55
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3 Answers
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The simple answer
Under the assumption that each mRNA molecule is translated at least once, by necessity translation will happen more often than transcription. This is because the only way to get a protein is to translate an mRNA.
In other words, as long as there are more protein molecules (translation products) than mRNA molecules in a cell, then the process of translation must have occurred more than transcription.
One estimate of the ratio of protein molecules to mRNA molecules given here is on the order of 100-1000 for E. coli. ("Under exponential growth at medium growth rate E. coli is known to contain about 3 million proteins and 3000 mRNA (BNID 100088, 100064). These constants imply that the protein to mRNA ratio is ≈1000, in line with the estimate given above. [MP note: the numbers quoted are actually closer to ~400K and ~1.4K, so this is not completely accurate; the source paper itself estimates between $10^2-10^4$ for the same ratio of protein to mRNA. Note though that the yeast cell estimates are even higher.]) So there are likely between 100 and 10,000 times as many translation as transcription events.
The speculative answer
From the perspective of "why" this is the case in terms of why cells might "choose" to have this ratio of proteins to mRNA (accepting that it must be true based on the ratio of protein to mRNA), we might say that proteins are useful and mRNA is not. Therefore, it's better to have more proteins (implied but not stated by @mgkrebbs comment). Or rather, mRNA is only useful because it gets you proteins, which are useful.
So it "makes sense" for cells to minimize the amount of energy put into generating mRNA and maximize energy put into making proteins. But that's human logic projected onto the cell, we can't know that that is the logic for why cells do what they do (or evolved to do, or whatever).
answered Apr 16, 2020 at 23:11
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1$\begingroup$ I accept that a comment is not an answer, but I would have left the poster to work things out for herself, given that she has not even finished reading the Tour. I find your contorted additional stuff about usefulness and energy unhelpful. It’s your logic, not “ours”. And please don’t talk about logic in relation to cells. The best way to express this is “why this happens” or “why it has developed in this manner”. $\endgroup$– DavidCommented Apr 17, 2020 at 19:14
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$\begingroup$ @David thanks for your input. $\endgroup$ Commented Apr 17, 2020 at 19:26
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$\begingroup$ Your answer and link is unclear. See the original article supplementary materials : a way to estimate the ratio is to synthesize DNA encoding for yellow fluorescent protein, electroporation into bacterias, synthesize many small fluorescent probes complementary to a piece of the mRNA for the YFB, add to the culture with RNase inhibitor, 9h later select some cells expressing YFP and observe to estimate how many mRNA there are and how many YFP have been synthesized. $\endgroup$– reunsCommented Apr 18, 2020 at 0:49
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$\begingroup$ Hi @reuns, thanks fo checking. I looked into it and edited the answer to reflect more closely what the bionumbers actually are and to add more context. $\endgroup$ Commented Apr 18, 2020 at 2:35
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$\begingroup$ Albert Lehninger had no problem in speaking about the 'molecular logic of living organisms' or about the 'molecular logic of the living state' (Biochemistry, 2nd Edn) $\endgroup$ Commented Nov 7, 2021 at 9:18
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Short answer: multiple Ribosomes can stack on the same mRNA making multiple proteins on a single mRNA strand before it gets recycled. A single mRNA strand may be translated dozens of times or more.
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It would be very wasteful (and hence evolutionary unfavorable) for cells to have the process other way round (more mRNA than protein). Remember it takes a lot of energy to synthesize mRNA, so it must be used as efficiently as possible (make many protein copies from a single mRNA).
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1$\begingroup$ I fail to see that your answer adds anything to that from @MaximilianPress except most dubious and unnecessary remarks about energy utilization and efficiency. If your argument is correct, eukaryotic introns in pre-mRNA are a waste of energy. Let it rest with the obvious. The comment about candle moulds should have been enough for the poster and anyone else. $\endgroup$– DavidCommented Nov 6, 2021 at 21:18