Regulation of Gene Expression and Cancer Development

Regulation of Gene Expression

Overview: Differential Expression of Genes

Gene expression is the process by which information from a gene is used to synthesize functional gene products, such as proteins. Both prokaryotes and eukaryotes alter gene expression in response to environmental changes, and multicellular eukaryotes use gene regulation to develop and maintain specialized cell types.

• Prokaryotes and eukaryotes adjust gene expression to adapt to their environment.

• Multicellular eukaryotes regulate gene expression for cell specialization.

• Gene expression is often regulated at the transcription stage, but other stages are also important.

Gene Regulation in Bacteria

Bacteria regulate gene expression to produce only the necessary gene products. This regulation occurs via feedback inhibition or gene regulation mechanisms such as operons.

• Operons: Groups of functionally related genes controlled by a single "on-off" switch.

• Operator: DNA segment within the promoter that acts as the regulatory switch.

• Operon: Includes the operator, promoter, and the genes they control.

Table: Operon Components

Component	Description
Operator	DNA segment acting as a switch
Promoter	Site where RNA polymerase binds
Structural Genes	Genes controlled by the operon

Operons: Repressible and Inducible

• Repressor: Protein that binds to the operator to block transcription.

• Regulatory gene: Encodes the repressor protein.

• Corepressor: Molecule that cooperates with a repressor to switch an operon off.

• Two types of operons: Repressible (e.g., trp operon) and Inducible (e.g., lac operon).

trp Operon

• By default, the trp operon is on; genes for tryptophan synthesis are transcribed.

• When tryptophan is present, it binds to the trp repressor, activating it and turning the operon off.

• Repressor is active only in the presence of its corepressor (tryptophan).

lac Operon

• Inducible operon; contains genes for enzymes that hydrolyze and metabolize lactose.

• By itself, the lac repressor is active and switches the operon off.

• An inducer (allolactose) inactivates the repressor, turning the operon on.

Regulation by Glucose and CAP

• CAP (catabolite activator protein) acts as a transcription activator when glucose is scarce.

• CAP is activated by binding with cyclic AMP (cAMP).

• When glucose levels increase, CAP detaches and transcription slows.

Eukaryotic Gene Expression

Cell Specialization and Differential Gene Expression

Regulation of gene expression in multicellular organisms is essential for cell specialization. Differences between cell types result from differential gene expression, where different genes are expressed by cells with the same genome.

• Gene expression is regulated at many stages.

DNA Packaging

• Bacterial chromosomes: Double-stranded, circular DNA with some protein, found in the nucleoid.

• Eukaryotic chromosomes: Linear DNA with large amounts of protein, found in the nucleus.

• Chromatin: DNA-protein complex; undergoes changes in packing during the cell cycle.

• Heterochromatin: Condensed, not expressed.

• Eu-chromatin: Less condensed, more active in transcription.

Regulation of Chromatin Structure

• Chromatin structure affects gene expression.

• Genes in highly condensed heterochromatin are usually not expressed.

• Chemical modifications to histones and DNA can influence chromatin structure and gene expression.

Table: Chromatin Modifications

Modification	Effect
Histone acetylation	Loosens chromatin, promotes transcription
DNA methylation	Condenses chromatin, reduces transcription

Epigenetic Inheritance

• Inheritance of traits by mechanisms not involving DNA sequence changes.

• Epigenetic modifications can be reversed.

Control of Eukaryotic Gene Expression

• Control elements: Noncoding DNA segments that bind transcription factors.

• Transcription factors: Proteins required for eukaryotic RNA polymerase to initiate transcription.

• Gene transcription depends on interactions between control elements and transcription factors.

Enhancers and Specific Transcription Factors

• Enhancers: Distal control elements that can be far from the gene they regulate.

• Activator: Protein that binds to an enhancer and stimulates transcription.

• Activators have two domains: one binds DNA, the other activates transcription.

Control of Gene Activation

• Specific combinations of control elements and activator proteins enable transcription of co-expressed genes.

• Genes can be scattered across chromosomes but share control elements for simultaneous transcription.

Post-Transcriptional Regulation

• Alternative RNA splicing: Produces different mRNA molecules from the same transcript.

• mRNA degradation: Regulatory proteins can block translation of selected mRNAs.

• Protein processing and chemical modification affect protein function and degradation.

Cancer and Gene Regulation

How Are Cancer Cells Different from Healthy Cells?

• Cancer cells exhibit abnormal regulation of genes affecting the cell cycle.

Proto-oncogenes and Oncogenes

• Proto-oncogenes: Code for proteins that stimulate normal cell growth and division.

• Oncogenes: Result from genetic changes that increase the amount or activity of the protein product.

Cancer Development

• Proto-oncogenes can become oncogenes by:

  • Movement near an active promoter

  • Gene amplification

  • Point mutations in control elements

• Tumor-suppressor genes: Encode proteins that prevent uncontrolled cell growth.

• Mutations in tumor-suppressor genes can contribute to cancer.

Table: Cancer-Related Genes

Gene Type	Function
Proto-oncogene	Stimulates cell growth
Oncogene	Promotes excessive cell division
Tumor-suppressor gene	Inhibits cell division, repairs DNA

Mutations and Cancer Progression

• Mutations in ras proto-oncogene and p53 tumor-suppressor gene are common in cancers.

• Multiple somatic mutations are needed for full-fledged cancer; incidence increases with age.

• About six changes at the DNA level are required for a cell to become cancerous.

Inheritance and Viral Involvement

• Individuals can inherit oncogenes or mutant alleles of tumor-suppressor genes.

• DNA breakage can contribute to cancer.

• Viruses can also play a role by disrupting tumor-suppressor genes or converting proto-oncogenes to oncogenes.

Additional info: These notes cover topics from chapters on gene expression regulation, operons, chromatin structure, and cancer development, relevant to General Biology.