Regulation of Gene Expression

Rucete ✏ Biology In a Nutshell

1. Gene Regulation in Prokaryotes (e.g., E. coli)

a) Operon Model

• Operon: A cluster of functionally related genes regulated together under a single promoter.
• Key components:
  • Promoter: DNA sequence where RNA polymerase binds.
  • Operator: Region where regulatory proteins bind.
  • Structural genes: Code for proteins.
  • Regulatory gene: Produces a repressor or activator protein.

b) lac Operon (Inducible)

• Controls genes needed to digest lactose.
• Normally off: repressor binds operator → blocks transcription.
• Presence of lactose: lactose binds repressor → inactivates it → transcription proceeds.

c) trp Operon (Repressible)

• Controls genes for making tryptophan.
• Normally on: repressor inactive → transcription occurs.
• Excess tryptophan: acts as a corepressor → activates repressor → blocks transcription.

2. Gene Regulation in Eukaryotes

a) Regulatory Proteins

• Repressors: inhibit transcription.
• Activators: promote transcription.
• Bind to promoter or enhancer regions and influence RNA polymerase binding.

b) Chromatin Structure

• Nucleosome packing affects gene accessibility.
• Tightly packed (heterochromatin) = gene off
• Loosely packed (euchromatin) = gene on

c) RNA Interference (RNAi)

• siRNA (small interfering RNA): binds to complementary mRNA and prevents translation.
• Leads to gene silencing by degrading target mRNA or blocking its translation.