Chapter 18

• Operon


  A DNA stretch containing a cluster of functionally related genes controlled by an operator
• Operator


  A DNA segment within the promoter where a repressor can bind to control gene transcription
• Repressor


  A protein that can bind to the operator to inhibit transcription
• Corepressor


  A molecule that cooperates with a repressor to switch an operon off
• Operon Function


  • Operons are gene clusters controlled by operators, with repressors regulating their activity
• Adaptive Advantage of Operons

  • Grouping bacterial genes into operons allows coordinated regulation
• Repressible Operons


  Typically active but can be repressed by a repressor with a corepressor (e.g., trp operon)
• Inducible Operons


  Normally inactive, activated by an inducer that inactivates the repressor (e.g., lac operon)
• Bacteria can also control enzyme production through feedback inhibition
• Repressible Operons (trp operon)


  Repressor requires corepressor (tryptophan) to switch off transcription
• Inducible Operons (lac operon)

  Repressor is inactivated by inducer (lactose), turning on transcription
• Positive Gene Regulation


  The catabolite activator protein (CAP) activates transcription when glucose levels are low by binding cAMP
• Differential Gene Expression


  • Cells with the same genome can express different genes, leading to cell specialization
• Chromatin Structure Regulation


  • Histone Acetylation: Acetyl groups on histones loosen chromatin structure, enhancing transcription
  • DNA Methylation: Methylation can inactivate genes long-term
• Epigenetic Inheritance


  Inheritance of traits via mechanisms other than direct DNA sequence changes
• Proximal Control Elements


  Located near the promoter
• Distal Control Elements (Enhancers)

  Can be far from the gene or within an intron
• Transcription Factors


  Proteins that bind to enhancers to stimulate transcription
• Post-Transcriptional Regulation


  • Alternative RNA Splicing: Different mRNA molecules can be produced from the same primary transcript
  • mRNA Degradation: Determines the life span of mRNA and affects protein synthesis
  • Initiation of Translation: RNA-binding proteins (RBPs) can affect mRNA stability and translation
• Post-Translational Modification


  Includes protein cleavage and degradation by proteasomes
• MicroRNAs (miRNAs)


  Small single-stranded RNAs that can bind to mRNA to block translation or degrade it
• RNA Interference (RNAi)


  Uses small interfering RNAs (siRNAs) to inhibit gene expression
• siRNAs and miRNAs


  Differ in their RNA precursors but both can regulate gene expression
• Chromatin Remodeling and Gene Silencing

  • siRNAs: Can play a role in heterochromatin formation and block transcription of specific genes
• Cell Division and Differentiation

  • The zygote divides and differentiates into various cell types
• Cytoplasmic Determinants


  Maternal substances in the egg that influence early development
• Induction


  Signals from nearby embryonic cells induce differentiation in target cells
• Determination

  Commits a cell to its final fate before differentiation
• Differentiation


  Production of tissue-specific proteins that mark cell specialization
• Positional Information

  Molecular cues that tell a cell its location relative to body axes and other cells
• Morphogenesis


  The process that gives an organism its shape
• Oncogenes


  Cancer-causing genes
• Proto-oncogenes

  Normal genes responsible for cell growth and division; conversion to oncogenes can lead to cancer
• Conversion Mechanisms

  • DNA movement, gene amplification, and point mutations
• Tumor-Suppressor Genes


  Help prevent uncontrolled cell growth
• Tumor-Suppressor Proteins


  Inhibit the cell cycle, repair DNA, and control cell adhesion
• p53 and ras Genes


  Common mutations in human cancers, regulating the cell cycle
• Requires multiple mutations for full-fledged cancer
• Often involves activation of oncogenes and inactivation of tumor-suppressor genes
• Some individuals can inherit oncogenes or mutant alleles of tumor-suppressor genes, leading to a predisposition for cancer