Once a ribosome has “read” a piece of mRNA to add an amino acid to a protein that is being constructed, what happens to the mRNA? Is the readout a destructive process, or does the mRNA just sort of “spool out” of the ribosome again in such a way that it can be used again by another ribosome? Textbooks don’t seem very clear on this point. Obviously this has some bearing on recent Covid vaccines, but it’s really a general question.
Readout (it’s called translation) is not an inherently destructive process. mRNA degradation is carried out by specific enzymes, and the process is regulated as one aspect of the overall process of regulating gene expression - cellular control of how much of a protein is produced and when.
Here’s a review of the mechanisms, slightly out of date but it appears to be open access:
https://www.nature.com/articles/nrg3160
Here’s the pdf of that paper if you can’t see it.
Translation of mRNA isn’t a destructive process and, indeed, multiple ribosomes can simultaneously translate the same piece of mRNA. This is called a polysome or polyribosome.
Wikipedia:
Bioninja, including a scanning electromicrograph of some polysomes:
I should add that Figure A on the above Bioninja site shows polysomes forming on mRNA while it is being transcribed from DNA. This can only happen in prokaryotes (i.e., bacteria) since they don’t have a nucleus. In eukaryotes (i.e., plants, animals, fungi), transcription occurs inside the nucleus then mRNA is transported outside into the cytoplasm where translation occurs. Polysomes can still form regardless.
And to answer the original question, average lifespan of mRNA in E. coli (prokaryote) is about 5 minutes. In yeast (single-celled eukaryote) it’s 20 minutes. In humans (multicellular eukaryote) it’s 600 minutes.
Thanks, guys; useful info. I’ve bookmarked several of the links in my “science” folder!