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Summary

Gap genes are expressed in presence of the right combination and amount of transcription factors. But is there any additional mechanism of timing the expression of the gap genes to ensure that they are expressed at the right time?

Context and Detailed Question

Let's look at gap gene expression in Drosophila:

  1. Firstly, bicoid and nanos are expressed and diffused from anterior and posterior ends respectively and form gradients;

  2. Then, hunchback gets expressed, with bicoid acting as promoter and nanos acting as suppressor;

  3. Then, Krüppel gets expressed, with hunchback acting as both promoter and suppressor - meaning that there must be enough hunchback, but not too much.

enter image description here (or a video: https://www.youtube.com/watch?v=uaedzlrnBGY)

Makes perfect sense, unless you look at it from dynamic, biochemical perspective:

Firstly, bicoid and nanos slowly build up in the embryo: enter image description here

As soon as there's enough bicoid at the anterior region, hunchback immediately starts building up too: I mean, it can't just choose to wait for bicoid to establish its gradient or something - if there's a promoter for it, it should react accordingly. enter image description here

And so it "chases" the bicoid gradient, in a manner of speaking, and will continue to chase it until it stumbles into nanos protein, which suppresses it.

All this makes sense so far.

The only problem is that at this point, there's just perfect amount of it for Krüppel at the anteriormost region, so Krüppel should start being expressed there: enter image description here

Eventually, there'll be too much hunchback at the anterior region, so Krüppel should cease to be produced - but some quantity of it should be expressed there nonetheless. The final gradient should look like this: enter image description here

The only way to avoid it is if Krüppel kindly waited until the hunchback establishes its final gradient before even attempting to be expressed. But how is this possible? Is there some mechanism that prevents Krüppel from being expressed too early - and if so, then what releases it? Or maybe there's some mechanism of timing, that prevents genes from being expressed too early?

This Krüppel example is just one examples of several, when there must be some timing control in order for genes to be expressed properly.

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  • $\begingroup$ I'm voting to close this question as off-topic because this is not the kind of precise answerable question that SE Biology is for. It is basically an argument against a model of gene expression followed by an “am I right?”. $\endgroup$ – David Dec 13 '18 at 23:33
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    $\begingroup$ @David 1. It's a precise answerable question (if there is a known mechanism that prevents kruppel from expressing too early, then a person who knows about it can answer). 2. Where did you see an argument against the existing model of gene expression? 3. Where did you see "am I right?" or even me proposing anything? I explained the thing about dev. biology that I don't understand and look for help. If you know the answer, you could just answer; or if you think that I misunderstood something, you could just explain where I am wrong. $\endgroup$ – Slavus Dec 14 '18 at 0:56
  • $\begingroup$ OK, I'll withdraw my close vote and restrict myself to some constructive help and advice. 1. Your question is complex and very few people on this list are likely to be able to answer it. 2. They would have to read through text and 11 diagrams to get to the actual question to see if they could answer it. I think that few are likely to do so. The very least you should do is to start your question with a summary of what it is, before telling people what to consider. I have started this for you. You can delete it if you like, but I would advise you to improve on it instead. And your title. $\endgroup$ – David Dec 14 '18 at 13:36
  • $\begingroup$ I have absolutely now idea about drosophila embryo development, but you/your diagrams seem to assume that all these genes are - dynamically - expressed as along as their promotor is active (enough) at a given position in the embryo. I would suggest you to also look into the temporal dynamics of these genes as well - I wouldn't be surprised if these 'gap' genes only start expressing when the nanos/bicoid gradient is already fully established, maybe looking into that direction can help you find an answer. $\endgroup$ – Nicolai Dec 14 '18 at 14:00
  • $\begingroup$ @David, thanks. I've tried to make question and summary more clear. As you say yourself, the question is very specific and complex, and it took me two days just to formulate it properly - so I found no other way to make it clear but to include diagrams and stuff... $\endgroup$ – Slavus Dec 14 '18 at 19:19

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