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As I understand it, any gene on an exposed/unpacked region of a chromosome is continuously being expressed. Regulatory genes may increase or decrease the amount of protein synthesised due to its expression, but its expression is still fairly continuous.

Assuming I am correct, is there any regulatory framework by which genes are sequentially expressed. That is where the expression of one gene causes it to be turned off and another turned on?

For example, say some chromosome has genes A, B, and C. Is there a situation where gene A is expressed for some period, then gene B, and then gene C, but no more than one is ever expressed at once and they only are expressed in that order?

EDIT: I'll clarify that I don't expect genes which are suppressed to somehow still be accessible. Making them inaccessible is (as far as I know) how they are shut down. My point in mentioning how accessible genes are expressed is to highlight the uncontrolled nature of gene expression in relation to sequential gene expression and therefore to ask how the genetic machinery can get around this problem.

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  • $\begingroup$ Herpesviruses are a good example of this. Upon entry the nucelocapsid, which is surrounded by this proteinaceous tegument, is translocated to the nucleus along with a tegument protein called VP16. VP16 induces transcription of immediate early phase α genes. These α genes tell host machinery to make ß genes that result in viral genome replication (in concert with α genes), after which you get robust transcription of late gamma genes. Described in part here, gamma genes code for structural elements that assemble before budding. $\endgroup$ – CKM Nov 27 '15 at 23:02
  • $\begingroup$ Although I'll add that for each phase, the genes aren't completely shut off, and still maintain some level of transcription. $\endgroup$ – CKM Nov 27 '15 at 23:08
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    $\begingroup$ I would say exposed/unpacked regions are ready to be expressed. bioscience.org/1998/v3/d/stein/d849-864.htm $\endgroup$ – 243 Nov 28 '15 at 0:13
  • $\begingroup$ Sequential expressions could be explained by expressions of transcriptional factors, which could bind to the enhancer regions of genes ready to be expressed. $\endgroup$ – 243 Nov 28 '15 at 0:22
  • $\begingroup$ @243 Wouldn't depending on the expression of transcriptional factors be pushing the problem back one level? Solving the question of how one set of genes are expressed sequentially by depending on the sequential expression of a different set? $\endgroup$ – Inquiz Nov 28 '15 at 1:55
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Simple answer: yes, this happens all the time.

A couple of notes:

Firstly, pretty much everything in biology is continuous. If a gene is expressed at a level of 1 protein per million cells, everybody would say it's not expressed, even though I guess you could argue the point. And don't underestimate how much regulation can affect expression -- you can easily have thousand-fold or million-fold differences between tissues and conditions.

Now, regarding the expression cascade: This is exactly what happens during the cell cycle. As the cell grows, there are genes that can sense the availability of nutrients and all the other signals that control initiation of cell division. Those then can turn on the genes first phase of the cell division cycle. Other regulators can sense whether that phase completed, and turn on the next set of genes, and maybe degradation signals for the genes from the first phase. And this then continues through the cell cycle.

Here's a figure from a paper that looked at gene expression in synchronyzed yeast cells (http://www.pnas.org/content/104/43/16892.full):

enter image description here

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