system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5; Why are some genes expressed in some cells and not others?

ELI5 ; 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.
gpt
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.