Transcriptional regulation
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Transcriptional regulation is the mechanism that coordinates the expression of DNA with the needs of various life processes such as development, gestation and metabolism. The same DNA exists in every cell and it codes for proteins. So how does it know to produce digestive enzymes in the stomach and neurotransmitters in the brain?
In an old folktale, the Sorcerer's Apprentice, the young apprentice uses a magic spell to command a broom to fetch buckets of water to his shop. Trouble is he forgets the spell to stop the brooms and the shop is soon flooded. He needs a pair of spells. Activators and repressors provide this magic for DNA.
The details are an area of ongoing investigation. Each gene has a promoter region where activators and repressors can attach. Prokaryotes so far appear to rely on a single activator-repressor pair for each gene. Eukaryotes may require cascades of activators and repressors. This allows a more sophisticated response to multiple conditions in the environment. Eukaryotes also make use of enhancers, distant regions of DNA that can loop back to the promoter.