So DNA is modified by other proteins/molecules such as Methlyation, Acetylation, Phosphorylation, Ubiquitination and Sumylation.

These groups and other specific protiens (for example HP1) attach to DNA-itself and/or histone protiens (DNA wraps itself around histone proteins). These molecules function like traffic signs… Like Red Lights or green lights.. that tell polymerases if the should transcribe genes ‘marked’ by these "traffic-signs".

So essentially what happens is these molecules differentially decorate the promoter (regulatory regions) of genes in different tissues. Therefore certain genes expressed by pancreatic cells wil not be expressed by liver cells.. and vice versa based on the "traffic signs".

…. of course this is an oversimplified explanation because gene/regulation can also be controlled at the translational level by siRNA/miRNA …. (but this is probably way beyond the scope of your question)

Anyways, please let me know if you need furhter clarification or additional explanations!

Take care!
– KIM

The receive signals that make them differentiate.

Also note that not all cells have a full complement of genes. Red blood cells have no nucleus and gametes (sperm and egg) only have half the complement.

Go google and seach for Eukaryoti transcriptin,
Mediators,
Promoters.

Your answer is behind these keywords.

well.. how than these specific proteins know which gene should be expressed in which cell.. how they know that in the case of liver cell they should turn off different genes than in the cheek cells? who tell them:)

Same for you too.. 😛

Mediators …. 😮