Prokaryotic Gene Regulation Quiz

Operon

• A group of genes that are transcribed together and code for functionally similar proteins
• Consists of promoter, operator, and genes for proteins

Key Players in Operon

• Promoter: a section of DNA where RNA polymerase binds
• Operator: controls activation of transcription, acting as an on/off switch between the promoter and genes for proteins
• Repressor protein: binds to the operator to block RNA polymerase and shut down transcription
• Corepressor: keeps the repressor protein on the operator
• Inducer: pulls the repressor off the operator, turning on the operon

Types of Gene Regulation

• Negative Regulation: a repressor protein binds to an operator to prevent a gene from being expressed
• Positive Regulation: a transcription factor is required to bind at the promoter to enable RNA polymerase to initiate transcription

Lac Operon Components

• Structural Genes:
• LacZ: encodes β-galactosidase, an enzyme that cleaves lactose into glucose and galactose
• LacY: codes for lactose permease, enabling lactose entry into the cell
• LacA: encodes transacetylase, which modifies lactose metabolites for excretion
• Operator (O): regulatory DNA sequence where a repressor protein binds, inhibiting transcription when lactose is absent
• Regulatory Genes:
• LacI: encodes the repressor protein, which suppresses transcription in the absence of lactose
• Inducer Molecule (Lactose): initiates operon transcription by binding to the repressor protein, rendering it inactive

The Central Principle of Gene Regulation

Cis-Acting Elements

• Definition: DNA sequences on the same chromosome as the regulated gene
• Function: Directly regulate adjacent genes, e.g., promoters, enhancers, silencers, operators
• Location: Non-coding regions: upstream (promoter), downstream (terminator), or within introns
• Effect: Modulate transcription rate, impacting overall gene expression levels

Trans-Acting Factors

• Definition: Proteins or molecules regulating genes, produced by different genes, binding to cis-acting elements
• Function: Bind to cis-acting elements, directly influencing target gene transcription
• Location: Synthesized in the same or different cells within an organism
• Effect: Presence, absence, and affinity for cis-acting elements dictate gene expression levels

Regulation of Transcription in Eukaryotes

• Eukaryotic DNA: packaged into chromatin, which limits its availability as a template for transcription
• Non-coding RNAs and Proteins: regulate transcription via modifications of chromatin structure
• Cis-Acting Sequences: many regulate expression of eukaryotic genes
• Identification of Eukaryotic Regulatory Sequences: using Gene Transfer Assays and Transfection

Promoter

• Definition: The region of DNA involved in the initiation of transcription
• Function: Provides a binding site for RNA polymerase and other proteins necessary for transcription

Enhancers

• Definition: Regulatory sequences located farther away from the start site
• Function: Bind transcription factors that then regulate RNA polymerase
• Location: Can be located at a distance from the promoter, and can even be found on a different chromosome

Repressors

• Definition: Proteins that inhibit gene expression by binding to specific DNA sequences
• Function: Turn off transcription by blocking activators from binding to enhancers

Insulators

• Definition: Specific DNA sequences that define chromatin domain boundaries
• Function: Regulate interactions between enhancers, silencers, and promoters
• Role: Maintain enhancer-promoter specificity, and organize chromosomes into looped domains

Transcriptional Activators

• Definition: Proteins that stimulate transcription by binding to regulatory DNA sequences
• Have two independent domains: one region binds DNA, the other stimulates transcription