Gene Expression and Regulation

Gene Expression

• Constitutive: Genes expressed constantly, encoding essential proteins.
• Regulated: Genes expressed in response to environmental changes.
• Operons: Sets of genes transcribed together, producing polycistronic mRNA for multiple proteins.
  • Promoter: RNA polymerase binding site.
  • Operator: Repressor protein binding site.
  • Inducible Operons: Usually off, activated by inducers (like lactose)
  • Repressible Operons: Usually on, inhibited by corepressors (like tryptophan).

Regulatory Genes

• Encode proteins that control gene expression. (Trans-acting)
• Repressors: Bind the operator to block transcription.
  • Inducers: Promote transcription by disabling repressors.
  • Corepressors: Inhibit transcription by enabling repressors..
• Activators: Enhance transcription by promoting RNA polymerase binding.
  • Inhibitors: Inhibit transcription by disabling activators.

Lac Operon (Inducible)

• No Lactose: Repressor blocks transcription.
• Lactose Present: Allolactose (inducer) binds repressor, allowing transcription.
  • Cis-acting elements: Physically attached to the gene (e.g., lacO - operator, lacP - promoter).
  • Trans-acting genes: Control genes located elsewhere (e.g., lacI - encodes repressor).
  • Structural genes: Encode proteins involved in lactose metabolism (e.g., lacZ encodes β-galactosidase, lacY encodes permease, lacA encodes transacetylase).
  • CAP-cAMP: Low glucose stimulates cAMP (high levels), enhancing transcription. High glucose reduces cAMP, reducing transcription.

Trp Operon (Repressible)

• No Tryptophan: Transcription active, synthesizing enzymes.
• Tryptophan Present: Tryptophan acts as a corepressor, enabling the repressor, blocking transcription.

Chromatin Remodeling

• Histone Acetylation: Loosens chromatin, allowing transcription.
• DNA Methylation: Inhibits activator binding, compacts chromatin, decreases transcription, often at CpG islands.
• Histone Methylation: Activators bind enhancers, increasing transcription; repressors bind silencers, decreasing transcription.
• Mediator Complex: Coordinates regulatory proteins and RNA polymerase; regulates transcription initiation/elongation.

Promoters

• DNA sequences where transcription begins.
• Core Promoter: Contains the TATA box (critical for binding).
• Proximal Promoter: Closer to the start site, contains CCAAT box, GC boxes (enhance transcription).
• Distal Promoter: Farther from the start site, includes enhancers and silencers.

Cis-Acting Elements

• DNA sequences regulating nearby genes.
• Enhancers: Increase transcription.
• Silencers: Decrease transcription.

Trans-Acting Elements (Proteins)

• Proteins interacting with cis-elements to regulate transcription.
• Zinc Finger, Helix-Turn-Helix, Leucine Zipper, Helix-Loop-Helix: Different protein motifs for binding specific DNA sequences.

Post-Transcriptional Regulation

• Alternative Splicing: Different protein products from a single gene.
• RNA Editing: Changing mRNA sequence after transcription.
• RNA Interference (RNAi): Small RNA molecules degrade mRNA, or block translation.
  • MicroRNAs (miRNAs): Endogenous; partial complementarity inhibits translation.
  • Small Interfering RNA (siRNA): Exogenous; perfect complementarity degrades mRNA.

Translational Regulation

• RNA Binding Proteins: Regulate translation efficiency/stability by binding mRNA 5' UTR.

Applications (Examples)

• Glucocorticoid Hormones: Regulate genes via hormone response elements.
• GloFish: Fluorescent genes (GFP/RFP) with constitutive promoters for monitoring/decoration.
• Metallothionein IIA: Protein for heavy metal binding, responsive to heavy metal response elements (MREs).

Cancer Development

• Definition: Uncontrolled cell growth.
• Causes: Oncogene activation, tumor suppressor inactivation.
• Characteristics: Altered shape, lack of contact inhibition, metastasis.
• Cell Cycle Regulation: Controlled by cyclins and CDKs.
  • Knudson's "Two-Hit" Model: Cancer requires multiple mutations (both alleles of a tumor suppressor must be inactivated).
• Proto-Oncogenes and Oncogenes: Proto-oncogenes promote normal cell division, oncogenes cause uncontrolled growth; Ras gene is an example.
  • Dominant mutations; only one allele needs to be mutated.
• Tumor Suppressor Genes: Regulate cell cycle and/or repair; loss leads to uncontrolled growth.
  • Examples include Rb gene (prevents cell cycle progression), and p53 gene (halts cell cycle or apoptosis).

Explore the intricacies of gene expression, focusing on constitutive and regulated genes, as well as operons. This quiz covers pivotal concepts like promoters, operators, and the regulation mechanisms involving repressors and activators, including detailed discussions on the Lac operon. Test your knowledge of these essential genetic processes.