During translation ribosomes decode the genetic information present in the mRNA and protein synthesis takes place. During this process which of those two does move, the ribosome or the mRNA?
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1$\begingroup$ center of mass says - depends on the size of the RNA. Also at least with prokaryotes, there are multiple ribosomes per RNA, which probably means less movement as they are probably not synchronous movements. $\endgroup$– shigetaCommented Apr 10, 2012 at 17:10
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$\begingroup$ Shigeta, I think you are right about what you are saying but can you please be more specific about the answer... $\endgroup$– balakaCommented Apr 10, 2012 at 20:01
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$\begingroup$ @Shigeta. I love the argument but conservation of momentum may not hold in these low-reynolds number systems. $\endgroup$– bobthejoeCommented Apr 10, 2012 at 20:12
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1$\begingroup$ With respect to movement, it really is a question of your frame of reference. The rough ER has a number of fixed ribsomes for which concomitant protein translation and translocation occurs. In this case, the mRNA is certainly extruded from the ribosome. With respect to polyribosomes and free-floating cytoplasmic ribosomes, there is no proof that either the mRNA nor the ribosomes have any tethering, therefore we may assume the whole complex moves with respect to some fixed external point. $\endgroup$– user560Commented Apr 11, 2012 at 22:41
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$\begingroup$ @shigeta low Reynold's numbers say F = ma --> F = bv: there is no mass in the equations of motion inside a cell. It's probably misleading to talk about the center of mass at all. $\endgroup$– ShepCommented May 5, 2012 at 16:10
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5 Answers
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Here's an example in which the ribosome is fixed:
During co-translational translocation, the ribosome is essentially anchored onto the ER membrane through the Sec61 complex. It certainly cannot move along the mRNA. The mRNA is fed through the ribosome and the nascent peptide traverses into the ER lumen.
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$\begingroup$ this is the eukaryotic case, the bacterial case is more like the other descriptions, though i'd bet in that case the viscosity of the cytosol makes it so that the ribosome not move very much at all during translation. $\endgroup$– shigetaCommented Aug 14, 2013 at 15:23
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The ribosome moves relative to the mRNA by, in effect, pulling itself along it. If both the ribosome and the mRNA are freely floating and not attached to anything else (as in jp89's answer), the relative amount of movement should depend on their relative masses.
(Actually, it also depends on how much drag each of them experiences with respect to the surrounding liquid medium, but since I have no idea how much that is, and since it's probably highly conformation-dependent anyway, I'm going to just ignore that and just assume that the drag is also more or less proportional to mass, at least to first order.)
As it happens, a quick Google search and some back of the envelope calculation suggests that the mass of a ribosome and the average mass of an mRNA are both around a megadalton. Of course, the length (and thus the mass) of an mRNA varies quite a lot, so it would seem likely that sometimes it's the ribosome that moves mostly, sometimes it's the mRNA, and sometimes it's both.
Also, as shigeta and others have pointed out, there can be more than one ribosome attached to the same mRNA strand. That's going to make the mRNA move more (and, correspondingly, the ribosomes move less), since there are more ribosomes pulling it along. Then there's also the protein being transcribed, which is attached to the ribosome but also being moved with respect to it. And I really have no idea how negligible the interactions with the tRNAs and so on are. It's a mess, but my guess would be that, usually, it's mostly the mRNA that moves, but that the ribosomes aren't completely stationary either (unless they're attached to something, of course).
Ps. Here's an exercise for you, which you may try out if you happen to have a friend who works at a public swimming pool. Otherwise consider it a gedankenexperiment. You know those floating ropes that separate the lanes in the pool? Try getting your friend to let you into the pool when it's not in use and to release one of the ropes from the walls. Then get in, grab the rope with your arms and legs and try pulling yourself along it. While doing so, try to decide whether it's you or the rope who moves more. (Also, to more closely approximate the Reynolds numbers involved inside a cell, imagine doing this in treacle instead of water.)
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$\begingroup$ Your swimming pool experiment is right on. I'm not so sure about the talk of "center of mass" though. A low Reynolds numbers, it's far more important to consider the linear drag coefficient than the mass of anything involved. To imagine the inside of a cell, you need to forget newton's laws: objects in motion will stop moving unless acted on by an outside force (at low Reynolds numbers). $\endgroup$– ShepCommented May 5, 2012 at 16:07
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$\begingroup$ You may well be right. As I noted in the parenthetical remark, I based my crude back-of-the-envelope calculations on the relative masses because that was something I could get some data on. If my guessumption about the drag coefficients being roughly similar is way off, then so may be my conclusion as well. $\endgroup$ Commented May 5, 2012 at 16:12
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Movement is relative. The real events happening in translation are the conformational changes of ribosome makes itself continuous reading the base sequentially. Please refer the biochemistry textbook or cell biology textbook.
Indeed, if the ribosome is anchored, you may say the mRNA is moving.
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$\begingroup$ Well, thanks for your response but bit of a problem here. Certainly movement is relative and depends on perspective or reference frame. What I meant by movement here is relative to more stationary structures, for example cytoskeletons, which one of the two moves. precise answer in not clear from the first response. And I did already consult the text books and obviously didn't find the answer either! $\endgroup$– balakaCommented Apr 10, 2012 at 17:19
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$\begingroup$ Regarding the second response, ribosomes are certainly not 'anchored' to any part of the mRNA, cos had that be the case, how do other parts get translated then? $\endgroup$– balakaCommented Apr 10, 2012 at 19:19
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$\begingroup$ @balaka: it does not say ribosomes are anchored to mRNA, just that they are anchored. $\endgroup$– nicoCommented Apr 10, 2012 at 19:59
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$\begingroup$ True, but with which substratum it is anchored is also not mentioned. mRNA was just one plausible candidate for me for their obvious proximity. $\endgroup$– balakaCommented Apr 10, 2012 at 20:06
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$\begingroup$ @balaka: not really a plausible candidate because, as you said, it moves! I'm not an expert on the matter but I would not be surprised if they were anchored to cytoskeleton proteins. $\endgroup$– nicoCommented Apr 11, 2012 at 6:54
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The mRNA moves during translation. It is essentially threaded through the ribosome. This has been known ever since polyribosomes were discovered; see paper here. Polyribosomes are a cluster of ribosomes that read a series of mRNA molecules. Often, the ribosomes in a polyribosome will be translating the same mRNA.
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1$\begingroup$ Of course it all depends on your reference system. If an observer were on the mRNA then he would see the ribosome moving :) $\endgroup$– nicoCommented Apr 10, 2012 at 16:09
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$\begingroup$ If you think of the cell as a universal 3D space, the relevant reference point would be a fixed coordinate in this space outside of the mRNA and the ribosome. Why would a cell biologist use either the ribosome or the mRNA as the reference point!? If you adopt your attitude, there would be no motivation to identify motile features. Does the Earth revolve around the Sun, or the Sun around the Earth? By your reasoning it doesn't matter, after all it's just relative. $\endgroup$ Commented Apr 10, 2012 at 17:40
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3$\begingroup$ I think any anyone who has seen relativity understands Nico's point. As for the paper, Warner et al. fix the ribosomes onto the surface. It's an artificial surface that forces the mRNA to be threaded through the polyribosomes. One can't make a conclusion from this study. $\endgroup$ Commented Apr 10, 2012 at 20:10
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$\begingroup$ We have established that frame of reference dictates which part, if any, moves relative to the observer. My question to you @balaka is, does it matter? $\endgroup$– user560Commented Apr 11, 2012 at 23:31
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The fact that ribosomes can collide with each other on the polysome during the translation process shows clearly that the ribosome is moving along on the mRNA.
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2$\begingroup$ Welcome to SE Biology. We are looking for comprehensive answers supported by evidence on this site. It is not enough to state something is a “fact”, you need to document it with reference(s). So please provide evidence that ribosomes collide with each other. You may well be right, be I am not aware of this and can assure you that 99% of list members will be in a similar position. $\endgroup$– DavidCommented Nov 13, 2021 at 19:06