Regulation of Gene Expression

Overview

Regulation of gene expression is essential for cells to respond to environmental changes and maintain homeostasis. Both prokaryotic and eukaryotic cells use a variety of mechanisms to control when and how genes are expressed, ensuring that proteins are produced only when needed.

• Gene expression: The process by which information from a gene is used to synthesize a functional gene product, usually a protein.

• Regulation can occur at multiple stages, including transcription, RNA processing, translation, and post-translational modification.

Regulation of Metabolic Pathways

Enzyme Activity vs. Enzyme Production

• Regulation of enzyme activity: Involves feedback inhibition, a rapid response to temporary changes in the cell's environment.

• Regulation of enzyme production: Involves controlling gene expression, which is a slower response to long-term changes.

Gene Regulation in Prokaryotes

Operons: Coordinated Gene Control

Prokaryotes often organize related genes into operons, allowing coordinated regulation of gene clusters.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: Acts as an on/off switch, controlling access of RNA polymerase to the genes.

• Operon: A stretch of DNA including the promoter, operator, and multiple genes with related functions.

Types of Gene Regulation in Prokaryotes

• Negative control: A repressor protein blocks transcription by default.

• Repressible operon (e.g., trp operon): Genes are usually on but can be turned off when the end product is abundant.

• Inducible operon (e.g., lac operon): Genes are usually off but can be turned on in the presence of a specific substrate.

• Positive control: An activator protein (e.g., CAP) enhances transcription when bound to DNA.

The trp Operon

• ON when tryptophan is absent; OFF when tryptophan is present and acts as a corepressor by binding to the repressor protein.

• Regulatory gene: Codes for the repressor protein.

• Corepressor: A small molecule (tryptophan) that activates the repressor.

The lac Operon

• OFF by default; turned ON when lactose (inducer) is present.

• Inducer (allolactose): Binds to the repressor, inactivating it and allowing transcription.

Positive Control: CAP and cAMP

When glucose is low, cAMP levels rise, activating CAP, which binds to the promoter and enhances RNA polymerase binding, boosting transcription of the lac operon.

• Activator: Protein that binds DNA and stimulates transcription (e.g., CAP).

Gene Regulation in Eukaryotes

Multiple Levels of Regulation

Eukaryotic gene expression is regulated at many stages, from chromatin structure to post-translational modification.

• Housekeeping genes: Expressed in all cells (e.g., genes for cellular respiration).

• Unique genes: Expressed only in specific cell types (e.g., hemoglobin in red blood cells).

Chromatin Modification

• Histone acetylation: Addition of acetyl groups to histone tails loosens chromatin structure, making DNA more accessible for transcription.

• DNA methylation: Addition of methyl groups to cytosines silences genes; methylation patterns can be inherited.

Epigenetic Inheritance

Epigenetic changes are heritable modifications that do not alter the DNA sequence but affect gene expression. Environmental factors such as diet, toxins, and stress can influence these changes.

Transcriptional Regulation

• Control elements: DNA sequences that bind transcription factors and mediator proteins.

• Proximal control elements: Located near the promoter (e.g., TATA box).

• Distal control elements (enhancers): Located far from the gene; DNA-bending proteins bring enhancers close to the promoter to facilitate transcription.

Regulation of mRNA

• RNA splicing: Removal of introns and joining of exons to produce mature mRNA.

• mRNA degradation: Enzymes remove the poly-A tail (deadenylation), 5’ cap (decapping), and degrade mRNA (exonucleolysis).

Non-coding RNAs and Gene Regulation

Small RNAs

• siRNA (small interfering RNA): Pairs perfectly with target mRNA, leading to its cleavage and degradation.

• miRNA (microRNA): Pairs partially with target mRNA, repressing translation and destabilizing mRNA.

RNA Interference (RNAi)

• Dicer enzyme processes double-stranded RNA into siRNA or miRNA.

• RISC (RNA-induced silencing complex) uses these small RNAs to find and silence matching mRNA.

• Used by scientists to study gene function by silencing specific genes.

Long Noncoding RNAs (lncRNAs)

• Longer than 200 nucleotides; do not code for proteins.

• Regulate gene expression by signaling, acting as decoys, guiding proteins, or serving as scaffolds for protein complexes.

• Example: XIST lncRNA inactivates one X chromosome in females.

Differential Gene Expression and Development

Embryonic Development and Cell Differentiation

• Stem cells: Undifferentiated cells with varying potency (totipotent, pluripotent, multipotent).

• Cell differentiation: Process by which cells become specialized for specific functions.

Pattern Formation and Hox Genes

• Pattern formation: Establishment of body axes and structures during development.

• Hox genes: Highly conserved genes that determine the identity of body regions along the anterior-posterior axis; gene order on the chromosome matches body region order (collinearity).

• Mutations in Hox genes can cause homeotic transformations (e.g., body parts developing in the wrong location).

Cancer and Gene Regulation

Cancer: Genetic Changes and Cell Cycle Control

• Cancer results from the accumulation of mutations that disrupt normal cell cycle regulation.

• Proto-oncogenes: Normal genes that promote cell division; mutations convert them to oncogenes, causing uncontrolled growth.

• Tumor suppressor genes: Genes that inhibit cell division or promote DNA repair (e.g., p53, BRCA1, BRCA2); loss of function increases cancer risk.

Key Genes in Cancer

• p53: Master regulator of DNA repair, cell cycle arrest, and apoptosis; mutated in about half of all cancers.

• Ras: Proto-oncogene; mutations keep Ras active, leading to continuous cell division signals.

• BRCA1/2: Tumor suppressors involved in DNA repair; mutations increase risk of breast and ovarian cancer.

HeLa Cells: A Model for Cancer Research

• HeLa cells are immortal human cancer cells derived from Henrietta Lacks in 1951.

• They have abnormal karyotypes and can divide indefinitely, making them valuable for research but genetically distinct from normal human cells.