ELI5; Why are some genes expressed in some cells and not others?

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; What makes some genes not to be expressed in some cells and be expressed in others. Like cells of the skin have the same genome as the pancreatic islet cells yet only the pancreas can produce insulin and every cell in the body has the gene for insulin.

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I can answer this! It’s related to what are known as “molecular switches” which are a way in which cells can regulate gene expression. Certain parts of your dna can be blocked, temporarily or permanently, by cellular machinery which means that transcription machinery (the way in which mRNA molecules are made that allow peptides to be produced) can only access and process certain stretches of DNA. That’s a very simplistic way of looking at it, expression can be turned up or down too when needed. Think of it as like having a traffic controller that can block off a road and then reopen it when needed.

This is an active area of research. If you figure out the complete answer to this, you’d probably win a Nobel prize.

That’s the whole field of epigenetics – there are many, many mechanisms by which certain regulatory regions of the genome are turned on and off

For example, you can wind DNA up into a tight ball so you can’t access that area to make read the DNA. You can also unwind that ball to allow transcription to occur there.

Remember, DNA/chromatin is a physical structure, so how that structure is oriented in 3D space will effect how it is interacted with. Promoters and enhancers (which help regulate the writing of RNA [expressing a gene]) for example, can be brought much closer to the body of a gene by making kinks (called loops) in the DNA

Imagine you had a string laid out flat with a gene on one end and an enhancer at the other end. If you push the two ends close to each other, you create a loop which can bring transcriptional machinery from the enhancer to the gene. Those loops also can help concentrate different proteins and factors at certain regions of the genome to drive the expression of a gene

In your example, in pancreatic beta cells, the insulin locus is accessible and primed for expression, while in nearby alpha cells, the insulin locus is blocked off and the glucagon locus is now primed