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I was reading this article: "Overview of vector design for mammalian gene expression." for an explanation of why mammalian cell lines are used for expressing cloned genes, and one of the reasons given is that "DNA cloned from higher eukaryotic cells is readily expressed since the signals for transcription, mRNA processing, and translation are conserved in higher eukaryotic systems." What does it mean for the signals to be "conserved," and why does this make mammalian cells a better host than other types of cells?

Follow-up question, in case the answer to that question doesn't make it clear: the article also says the machinery for protein folding and assembly are "conserved" - what does that mean (if not the same thing as in the first question) - and again, why does this make mammalian cells a superior choice of host for expressing cloned genes?

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    $\begingroup$ It means the signalling pathways or the mechanisms used for a particular process is the same across a number of species. The reason an eukaryotic cell was used for the expression of a gene is that the particular gene in question required a certain machinery and processes which are found in eukaryotes so for example expressing that gene in an E. coli might not have been suitable/worked. In research its always the best idea to express a gene from the species it came from to ensure its expression. $\endgroup$ – Bez Nov 7 '14 at 0:15
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I concur with @Bez but wish to elaborate on the meaning of 'conserved'. It is generally used in the context of evolution. A conserved characteristic or gene or protein means that it has 'survived' a long time without being altered. As @Bez mentions, certain parts of the protein machinery in eukaryotes is very different from prokaryotes, the latter being considered to have arisen much earlier in evolution. For example, some post-transcriptional processes (e.g. splicing and many more differences) only occur in eukaryotes. But there are many similarities as well, such as the basic use of mRNA etc (see book section). The latter are examples of conserved mechanisms. In the context of your question, it means that when you take eukaryotic DNA to express eukaryotic proteins it doesn't really matter which cell or host organism you take as it will be transcribed and folded correctly. For example, yeast is often used to express DNA from higher organisms as its protein synthesis is much the same across various eukaryotic species. However, expressing eukaryotic DNA in prokaryotes likely ends up in misfolded proteins without proper post-transcriptional modification and should be avoided.

As to your follow-up question: Folding of proteins happens in the endoplasmatic reticulum (ER) in eukaryotes (wiki), a cell organelle absent in prokaryotes as they lack a nucleus and associated organelles. Again, expression of eukaryotic genes in a prokaryotic expression system will end up in mis-folded proteins.

To get back to your overall question - the 'signals': DNA transcription, splicing, protein synthesis, post translational modification and folding are all regulated by signals, such as specific DNA codons to start and stop transcription, RNA codes for splicing and start translation and protein tags that are placed on specific amino acid combinations. These signals are well conserved across eukaryotic species.

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  • $\begingroup$ I only made an edit to the post - nothing more. $\endgroup$ – Chris Nov 7 '14 at 11:53
  • $\begingroup$ auch - it's Bez thanks @Chris. Must be your name ;) $\endgroup$ – AliceD Nov 7 '14 at 12:00
  • $\begingroup$ I think you agree that it's a special name :-) $\endgroup$ – Chris Nov 7 '14 at 12:02

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