ID #2029

I had a question about multiple protein synthesis in Human mRNA. When we were talking about the exons, you told us that Exon variability leads to a synthesis of multiple Protein from mRNA. However, I was just looking at the AUG initiator slide from lecture 19 and I have the statement "Only one Protein per Cap for Monocistronic Eukaryotic mRNA" written in down. Could you please explain this apparent contradiction?

Worded the way you have worded it, there would be a direct contradiction, but the truth is the situation is slightly different than the way you've written it. Alternative splicing allows eukaryotes to create different mature mRNAs from a single gene in the DNA (via the primary mRNA which will of course be the same). If a primary mRNA found its way accidentally into the cytoplasm (but somehow got capped), every ribosome would do exactly the same thing to it. But it isn't your primary mRNAs that get exported, it's the mature mRNAs. And by the time they leave the nucleus, they have been fully processed including splicing of introns. It's the ability to use different combinations of exons that leads to different mature mRNAs. Now each of those different mature mRNAs (let's say there are three different types) gets exported to the cytoplasm. Every copy of "type 1" will be translated into the exact same polypeptide chain. Every copy of "type 2" will be translated into the exact same polypeptide chain. And every copy of "type 3" will be translated into the exact same polypeptide chain. But Protein 1 has a different primary sequence than Protein 2 or Protein 3. Each eukaryotic mRNA can only be translated into a single type of protein, but those mRNAs came from identical primary mRNAs and the same gene in the DNA.


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