6.4 Regulation of Gene Expression

Unit 3: Molecular Genetics

Lesson 7: Regulation of Gene Expression

Gene Regulation

• Definition: Control and modification of gene expression based on cellular or environmental conditions.

• Involves activating or inactivating genes depending on the requirement for gene products.

Housekeeping Genes

• Housekeeping (constitutive) genes: Constantly expressed genes (transcribed and translated).

  • Regulation: Not typically regulated; their constant expression is vital for survival.

  • Notes: Most genes do not fall into this category.

Stages of Gene Expression

1. Transcription

   • Location: Nucleus

   • Product: mRNA

2. Translation

   • Location: Cytoplasm

   • Product: Protein

Regulation of Gene Expression in Prokaryotes

• Three Levels of Control:

  1. Transcriptional

  2. Translational

  3. Post-translational

Operons

• Definition: Cluster of genes controlled by a single promoter (specific to prokaryotes).

• Key Components:

  • Operator: DNA sequence where the repressor binds to block transcription.

  • Promoter: Binding site for RNA polymerase.

Regulators

• Types of Regulators:

  • Repressor: Protein that binds to the operator to inhibit transcription.

  • Activator: Protein that binds to increase the transcription rate.

  • Examples: LacI repressor, trp repressor.

Effectors

• Definition: Substances that bind to regulatory proteins affecting transcription.

• Types:

  • Inducers: Example: Allolactose

  • Co-repressors: Example: Tryptophan

Examples of Transcriptional Regulation

• Lac Operon

• Trp Operon

Lac Operon

• Composition: Lactose is a disaccharide made of glucose + galactose; important in E. coli metabolism.

• Key Features:

  • Enzyme: β-galactosidase catalyzes lactose cleavage.

  • Regulation: Genes (LacZ, LacY, LacA) controlled under one promoter region.

  • Wastes resources if β-galactosidase is produced in the absence of lactose.

Mechanism of Lac Operon

• Absence of Lactose:

  • LacI repressor binds to lac operator, blocking RNA polymerase from binding.

  • No transcription occurs (no mRNA or protein produced).

• Presence of Lactose:

  • Allolactose binds LacI, altering its shape and freeing the operator for RNA polymerase binding.

  • Transcription and protein synthesis occur.

Trp Operon

• Composition: Tryptophan is an amino acid; genes (TrpE, TrpD, TrpC, TrpB, TrpA) encode enzymes for its synthesis.

• Regulation: Controlled by a single promoter.

  • Inactive Repressor: When tryptophan is absent, the repressor does not bind, allowing transcription.

  • Active Repressor: When tryptophan reaches threshold, it binds the repressor, enabling it to bind the operator and block transcription.

Gene Expression Regulation in Eukaryotes

• Four Levels of Control:

  1. Pre-transcriptional

  2. Transcriptional

  3. Post-transcriptional

  4. Translational

Pre-transcriptional Regulation

• Chromatin Remodelling: Complexes loosen DNA for transcription machinery access.

Transcriptional Regulation

• Each gene has its own promoter; Transcription Factors are required to initiate transcription and enable RNA polymerase to bind.

• Activators bind to enhancer regions to elevate transcription rates.

Post-transcriptional and Translational Regulation

• Importance of 5' cap and Poly-A tail addition to primary transcript; lack leads to mRNA degradation.

• RNA Interference: Small RNA molecules (miRNA, siRNA) degrade mRNA or inhibit translation.

Post-translational Regulation

• Involves modifications to the polypeptide chain to activate proteins.

• Ubiquitination: Signals for protein degradation.