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This textbook states

Proteins determined by a single gene may divide to form different proteins with various physiological actions.

First how do proteins divide? Second if it's just fragmentation then what is this process called? Third since my textbook doesn't explain it could anyone explain with an example of how different parts of original protein( considering that protein has divided by fragmentation) perform different physiological actions?

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This is not a very helpful passage, I agree. Some proteins are cleaved by other proteins (a protein that cuts another protein is called a protease). Other proteins cleave themselves. Otherwise, proteins are quite stable and don't just fragment wily-nily.

In the case of the first group (proteases acting on other proteins) the coagulation cascade may be a good place to start for you. Precursor proteins in this pathway are converted to active enzymes by a chain reaction of protein 'division' and other protein modification. This page has other interesting cases of this process, such as in food digestion, which may be what you're looking for.

Some of the second group are self-cleaving such that an intervening sequence is removed, and the ends are pasted back together. (This is analogous to how introns are spliced from messenger RNA and the exons are spliced together.) Such intervening protein sequences are called 'inteins'; the removal of one intein can generate one functional protein, so this isn't protein division. However, some viruses encode self-cleaving regions that will split the mother protein into two or more pieces. A famous example of this process, now used widely in biotechnology, is the 2A peptide, which is used to 'split' a single transgenenically-expressed protein into 2 (For instance if you want to express a protein and a marker such as GFP simultaneously, you could employ the 2A peptide to link these together.)

The first group may be of more interest to you because of the known role of these proteases in human health. Also, in your question it sounds like you also may be referring to random 'fragmentation' of proteins, which is not currently known to be an important contributor to protein breakdown in the human body, even in the stomach. Enzymes such as pepsin are responsible for digesting proteins.

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  • $\begingroup$ Good mind reading! The use of the word ‘divide’ here makes it clear the author of the book knows little or nothing about this subject. Not too suprising in a book on physiology. The questioner should be warned. $\endgroup$ – David Jan 8 '17 at 21:33
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I don't have access to the textbook, but my first thought is that the author is talking about gene duplication during evolution. If a gene duplicates, the two copies may end up diverging, creating two similar but slightly specialized proteins that fulfill different functions.Wikipedia

I agree that the sentence is not well written, whatever the intended meaning. Some context may be helpful.

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  • $\begingroup$ I don't think that's the case. It mentions "proteins determined by a single gene", as in different isoforms produced by alternative splicing. In addition, it could be referring to certain polypeptides such as PTH, which is cleaved into two smaller fragments. $\endgroup$ – Jagoe Jan 31 at 18:43

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