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Post transcription, introns are removed and exons are rearranged randomly. If that's the case (which it is according to the book EDEXCEL SNAB A2 topic 6) then shouldn't the protein produced be different every time? Shouldn't the order of amino acids be different? And if it is, doesn't that mean that the protein produced is different?

To quote : "When mRNA is produced, its introns are removed by spliceosomes and a different number of exons is put together at different orders which changes the sequence of amino acids in proteins."

Quote from the book:

  1. Genes contain sections that don't code for amino acids.
    1. These sections of DNA are called introns. All the bits that do code for amino acids are called exons.
    2. During transcription both introns and exons are copied into mRNA.
    3. The introns are then removed by a process called splicing -- introns are removed and exons joined forming mRNA strands. This takes place in the nucleus.
    4. The exons can be joined together in different orders to form different mRNA strands.

If this process happens,how can a gene code for a specific protein? This process means that exons are arranged differently, and this means that amino acids are arranged differently, and doesn't that mean that different proteins are going to be produced than the one coded for?

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marked as duplicate by Bryan Krause, mgkrebbs, David, De Novo, John Dec 13 '18 at 22:04

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    $\begingroup$ Can you provide a quote or something from the book? It's hard to evaluate the statement otherwise, but "exons rearranged randomly" is certainly not the usual way of business, if it happens ever (I can't think of a situation where it would). Maybe somewhere in the immune system which I am not fully familiar with. $\endgroup$ – Bryan Krause Dec 11 '18 at 0:19
  • $\begingroup$ See also biology.stackexchange.com/questions/5/… $\endgroup$ – Bryan Krause Dec 11 '18 at 0:21
  • $\begingroup$ Alternative splicing of mRNA transcripts can certainly give rise to protein products differing in molecular weight ('isoforms'). A very good example is microtubule-associated protein tau, the main component of the paired helical filament of Alzheimer's disease, which appears as four (bovine) or six (human) closely spaced bands on an SDS gel, due to differential mRNA splicing. See here for the bovine case. $\endgroup$ – user1136 Dec 11 '18 at 0:33
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    $\begingroup$ @user1136 Differential splicing isn't the same thing as "rearranging exons randomly" $\endgroup$ – Bryan Krause Dec 11 '18 at 4:27
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    $\begingroup$ @user1136 But OP doesn't seem to, and that makes your statement misleading and OP might misinterpret it as an answer to their question. $\endgroup$ – Bryan Krause Dec 11 '18 at 16:22
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"then shouldn't the protein produced be different every time? Shouldn't the order of amino acids be different? And if it is, doesn't that mean that the protein produced is different?"

It is not different every single time, but it is different depending on what kind of post transcriptional modification takes place. The proteome is larger than the genome thanks to this, genes can code for many different proteins thanks to post transcriptional and post translational modification as well.

So as I understand it, gene segment = same order of nucleotides = same primary mrna

post transcriptional modification to primary mrna = different mature mrna = different order of amino acids = different polypeptide chain (then different protein later on)

Hope this helps

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