In recombinant plasmids with insulin genes, how does it know when to express the gene / make the protein? ?

Question

In a broad sense, I am wondering what it is that causes a cell to choose to express a particular gene at a particular time? Where is the instructions for this?

And as a more specific example, in the creation of a bacterium with a recombinant plasmid containing the gene for insulin production, how would the bacteria 'know' when to express the gene?

Thanks to all the helpful answerers out there

Answer 1

I'm not exactly sure but i do know that when the plasmid has been taken up by the bacterium it is heat shocked by placing them in ice cold calcium chloride and then heating it up probably in an autoclave this makes them express the gene but it doesn't always work.  

Answer 2

The insulin gene that you insert into the plasmid will have a promotor attached that determines when the gene is expressed. Depending on the promotor that you used the gene might be constitutive (always on) or can be turned on and off by the presence of a specific chemical.