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This question already has an answer here:

Ribosomes are protein factories of cell and are composed of two subunits; one smaller and other larger unit I want to know that why ,ribosomes of eukaryotes have a 40s and 60s subunit and prokaryotic ribosomes have 30s and50s subunits ?If we combine 30s and 50s subunits shouldn't it be 80s ribosome instead of 70s ribosome .

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marked as duplicate by canadianer, another 'Homo sapien', Bryan Krause, kmm, David Apr 15 '18 at 21:25

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    $\begingroup$ What exactly is your question? Are you asking why there is difference between prokaryotic and eukaryotic ribosomal subunit masses? Or do you simply wanna know why these numbers dont add up? $\endgroup$ – another 'Homo sapien' Apr 10 '18 at 5:51
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    $\begingroup$ I want to know why these numbers donot add up?I know that difference in their masses is due to difference of masses of their proteins.(of subunits). $\endgroup$ – Rabik John Apr 10 '18 at 6:17
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    $\begingroup$ Mass and sedimentation-coefficients are not the same thing, although related. $\endgroup$ – Always Confused Apr 10 '18 at 6:40
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I think I can get the point after your second comment.

Not only protein composition, but rRNA composition and sequence length of Bacterial and Eukaryotic ribosome is very different. And the difference of sequence lengths (and hence mass) in both take part in the difference.

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The S units are an artifact of the historical way these RNAs were characterized, based on the rate at which they sediment in an ultracentrifuge. S stands for Svedberg, which is a non-metric unit that measures sedimentation rate, based on size and shape. Larger molecules sediment slower. A better way to refer to ribosomal subunits is by calling them large subunit (LSU), and small subunit (SSU), because ALL ribosomes consist of one SSU and one LSU. Even the 5.8S is simply a part of eukaryotic LSU. Put subunits together and the shape changes, so it they would not sediment at a rate equal to the sum of each subunit.

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  • $\begingroup$ You mean sedimentation depends on shape of subunits as they combine they slightly change their shape and will now sediment at 70s instead of 80s? $\endgroup$ – Rabik John Apr 11 '18 at 5:38

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