Ch. 8 Gene Expression

Chapter 8: Gene Expression

Introduction to Gene Expression

• Importance of knowledge beyond DNA sequence:

  • Understanding how genes are selectively used is crucial.

  • Cell differentiation results from different genes being expressed in various cells.

Experimental Support for Gene Expression

• Differentiated cells possess all genetic instructions needed for organismal formation.

  • Example: Adult frog skin cells placed in a culture dish.

  • Nucleus is extracted using a pipette and injected into an unfertilized frog egg where:

    • The original nucleus is destroyed by UV light.

    • Results in the development of a normal embryo, which can grow into a tadpole.

The Concept of Gene Expression

• Definition of Gene Expression:

  • A complex process guiding selective synthesis of proteins and RNA.

Central Dogma of Molecular Biology

• Draw It: Create a diagram of the processes involved in the Central Dogma:

  • Processes involved:

  • DNA → Pre-mRNA → mRNA → Protein (primary structure) → Functional protein.

  • Key processes illustrated in the sketch:

  • Transcription

  • RNA processing

  • Translation

  • Folding & modification

  • Key molecules involved:

  • RNA polymerase

  • Promoter site

  • Transcription factors

  • 5' cap

  • Poly-A tail

  • Splicing

  • Ribosomes

  • tRNA

  • tRNA synthetase enzymes

Gene Regulation and Cell Differentiation

• Cells contain the same genes but express only about 50% at any given time.

• Cell differentiation results from this selective gene expression.

Steps of Gene Expression Regulation

• Gene expression can be regulated at multiple steps:

  1. Transcriptional Control (in the nucleus)

  2. RNA Processing and Transport

  3. Translational Control (in cytosol)

  4. mRNA Degradation Control

  5. Protein Degradation Control

  6. Protein Activity Control

Influence of External Signals on Gene Expression

• Gene expression can change in response to external signals:

  • Example Components:

  • Receptor proteins

  • Growth factors -> involved in driving growth and division.

  • Translation and transcription factors impacting cell division and growth-promoting gene expression.

Transcription Regulation

• Transcription regulators (transcription factors):

  • Proteins that bind to specific DNA sequences, controlling the transcription process by turning genes on or off.

  • Activator: Facilitates RNA polymerase binding to the promoter.

  • Repressor: Inhibits RNA polymerase from transcription.

  • Regulatory DNA Sequences: Sites where these proteins bind to switch gene activity.

Example: Tryptophan Synthesis in Bacteria

• Genes cluster together in an operon:

  • Tryptophan (Trp) operator, promoter, and required enzymes for tryptophan biosynthesis.

• Regulation by a repressor protein influenced by tryptophan levels:

  • Low Tryptophan concentration → Increased operon transcription.

  • High Tryptophan concentration → Decreased operon transcription.

Enhancing Promoter Functionality

• Ineffective promoters can be enhanced by:

  • Binding of activator proteins that enable better RNA polymerase attachment.

The Lac Operon in E. coli

• Controlled by both an activator and a repressor:

  • Condition for transcription:

  • No glucose -> CAP binds.

  • Presence of lactose -> Repressor not bound.

• Functions of the Lac Operon:

  • Codes for proteins necessary for importing and digesting lactose.

  • Key components include:

  • Lac Repressor: Binds to the operator when lactose is absent.

  • CAP Activator: Binds when glucose is absent, enhancing transcription.

• Various conditions leading to transcription:

  • No glucose, no lactose → Minimal transcription.

  • No glucose, lactose present → Transcription occurs.

Eukaryotic Gene Expression Regulation

• Eukaryotic transcription regulators control gene expression even from afar:

  • Transcription initiation complex: includes enhancer sites, mediator proteins, general transcription factors, and RNA polymerase.

Chromatin Structure Influence on Transcription

• DNA looping mechanisms keep enhancers effectively organized:

  • Enhancer sites can interact distantly with transcription sites.

  • Enhancer and target gene coordination typically requires protein bridges.

• Chromatin Remodeling Complex:

  • Increases transcription efficiency by loosening DNA-histone interaction.

  • Distinction between heterochromatin (tightly packed) and euchromatin (loosely packed).

Bacterial mRNA Regulation

• Regulatory mechanisms can obstruct ribosome binding to mRNA:

  • Start codon binding site is obstructed.

  • Mechanisms to control access include:

  • Blocking the site.

  • Hiding it from translational machinery.