Chapter 16

REGULATION OF GENE EXPRESSION

Definition of Gene Regulation

  • On/off: Gene regulation refers to the ability of a cell to control the timing, amount, and location of gene expression.

  • Focus Areas:

    • Location

    • Timing

    • Amount

Importance of Gene Regulation

  • Response to Environmental Conditions:

    • Both prokaryotic (bacteria) and eukaryotic (plants, animals) cells can respond to changes in environmental conditions.

  • Efficiency:

    • Regulation maintains cellular efficiency.

  • Energy Conservation:

    • Helps conserve energy by expressing only the necessary genes.

  • Cell Type Specificity:

    • Ensures genes are expressed in the appropriate cell types.

  • Temporal Regulation:

    • Genes are expressed as needed during correct developmental stages.

  • Protein Amount Regulation:

    • Control over the amount of proteins expressed.

Differential Gene Expression

  • Concept:

    • Different cell types show differential gene expression, meaning while all somatic cells have the same genetic content, they do not express the same genes.

  • Examples of Cell Types:

    • An eye cell vs. a liver cell.

  • Results of Differential Gene Expression:

    • Leads to the formation of a complete organism through specific gene expression patterns in different cells.

Mechanisms of Gene Regulation

Prokaryotic Regulation

  • Main Stage: Regulation occurs primarily at the transcriptional level.

  • Operon Model:

    • Cluster of genes regulated together, transcribed into a single mRNA (polycistronic mRNA).

    • Operons contain genes involved in the same metabolic pathways, regulated by a common promoter.

  • Arrangement:

    • Genes are organized in succession along the DNA strand.

Eukaryotic Regulation

  • Increased Complexity:

    • Gene expression regulation is more complex than in prokaryotes as it can occur at multiple stages:

    1. Epigenetic Regulation:

    • Occurs before transcription and involves temporary changes to nuclear proteins and DNA that do not alter the nucleotide sequence but change gene expression.

    1. Transcription-Level Regulation:

    • Involves transcription factors (TFs) and regulatory events at transcription process.

    1. Post-Transcriptional Modifications:

    • Modifications that occur after transcription prior to translation.

    1. Translation Control:

    • Regulatory proteins can inhibit or promote translation regulation.

    1. Post-Translational Modifications:

    • Chemical changes to proteins after translation that affect function.

Epigenetic Regulation

  • Influence of Nucleosome Spacing:

    • When nucleosomes are closely spaced, transcription factors cannot bind, effectively turning off gene expression.

    • If nucleosomes are spaced far apart, DNA is exposed for transcription factor binding, allowing gene expression.

Transcription Regulation

  • mRNA Types:

    • Monogenic mRNA: Carries information for one gene.

    • Polycistronic mRNA: Carries information for multiple genes, common in prokaryotes.

  • General and Specific Transcription Factors:

    • General TFs: Required for all protein-coding gene transcription initiation.

    • Specific TFs: Facilitate transcription of certain genes under specific conditions.

    • Role of Activators and Repressors:

    • Activators enhance transcription while repressors inhibit it.

Control Elements in Gene Regulation

  • Enhancer Elements:

    • Distal control elements that can enhance transcription.

    • Work with specific transcription factors available within specific cell types.

  • Promoter Elements:

    • DNA sequences where general transcription factors bind to initiate transcription.

    • Includes TATA box and other proximal control elements.

  • RNA Polymerase II:

    • An enzyme that synthesizes mRNA from a DNA template.

Regulation Post-Transcription

RNA Stability Control

  • Role of RNA-Binding Proteins:

    • Found at untranslated regions (UTRs) regulating RNA stability and localization.

  • Preventing Degradation:

    • 5' cap with methylated guanosine triphosphate (GTP) and poly-A tail prevent decay of the RNA transcript.

Translational Regulation

  • Inhibition of Translation Initiation:

    • Regulatory proteins can block the initiation stage by attaching to the 5' UTR, preventing ribosomal subunit binding.

    • At the appropriate time, factors enhance ribosomal binding to initiate translation.

Post-Translational Modifications

  • Protein Activation and Inactivation:

    • Regulatory proteins undergo phosphorylation or dephosphorylation reactions for activation or inactivation.

    • Modification examples include glycosylation (sugar addition).

    • Polypeptide chains can be cleaved into functional units.

  • Proteins Lifespan and Regulation:

    • The lifespan of proteins varies; some persist for long periods while others, especially those regulating transcription, exist for shorter durations.

    • Proteins marked with ubiquitin for degradation enter proteasomes for recycling, termed as "doom tags".

Comparative Gene Expression Regulation

Prokaryotic vs. Eukaryotic Regulation

  • Organizational Structures:

    • Prokaryotic organisms lack a nucleus, with DNA found within the cytoplasm, and have simultaneous transcription and translation.

    • Eukaryotic organisms contain a nucleus, confining DNA; transcription occurs in the nucleus followed by translation in the cytoplasm.

  • Regulatory Levels:

    • Prokaryotic gene expression is primarily at the transcription level, while eukaryotic gene expression regulation occurs at multiple levels including:

    • Epigenetic

    • Transcriptional

    • Nuclear shuttling

    • Post-transcriptional

    • Translational

    • Post-translational.