9.5 How Genes Are Regulated

Gene regulation is all about cells being smart with their resources. It's how they make sure they're only producing the proteins they need, when they need them. This saves energy and helps cells adapt to their environment.

For simple organisms like bacteria, gene regulation is pretty straightforward. But in more complex organisms like us, it's a whole different ball game. There are multiple levels of control, from how DNA is packaged to how proteins are modified after they're made.

Gene Regulation

Gene expression selectivity

• Cellular specialization enables cells to differentiate into specific types with distinct functions (neurons, muscle cells, epithelial cells)
  • Each cell type requires a unique set of proteins to perform its specialized functions
• Conservation of energy and resources is crucial for cellular efficiency
  • Producing all proteins constantly would be inefficient and wasteful of cellular resources (amino acids, ATP)
  • Selective gene expression allows cells to produce only the proteins they need for their specific functions
• Response to environmental changes allows cells to adjust gene expression based on external stimuli (nutrient availability, temperature, pH)
  • Enables organisms to adapt to changing conditions and maintain homeostasis

Prokaryotic vs eukaryotic regulation

• Prokaryotic gene regulation primarily occurs through operons, which are groups of genes under the control of a single promoter
  • Lac operon is regulated by the presence or absence of lactose
  • Trp operon is regulated by the presence or absence of tryptophan
• Prokaryotic regulation mainly involves transcriptional control, which is the regulation of RNA synthesis
• Eukaryotic gene regulation is more complex and occurs at multiple levels
  • Chromatin remodeling involves changes in DNA packaging that affect gene accessibility (histone modifications, DNA methylation)
  • Transcriptional control involves regulation by transcription factors (activators, repressors) and enhancers
  • Post-transcriptional control involves regulation of mRNA processing (alternative splicing, RNA editing) and stability
  • Translational control involves regulation of protein synthesis at the ribosome level
  • Post-translational control involves regulation of protein activity (modifications, localization) and degradation

Levels of eukaryotic expression control

• Chromatin remodeling affects gene accessibility and transcription
  • Histone modifications include acetylation, methylation, and phosphorylation, which alter chromatin structure
  • DNA methylation involves the addition of methyl groups to cytosine bases, generally associated with gene silencing
  • Chromatin condensation results in tightly packed DNA that is less accessible for transcription
• Transcriptional control involves the regulation of gene expression by transcription factors and enhancers
  1. Transcription factors are proteins that bind to specific DNA sequences and can act as activators to enhance transcription or repressors to inhibit transcription
  2. Enhancers are distant DNA sequences that increase transcription of target genes
  • Promoter regions are DNA sequences located near the transcription start site that help initiate gene expression
• Post-transcriptional control involves the regulation of mRNA processing and stability
  • Alternative splicing creates multiple mRNA variants by combining different exons (cassette exons, mutually exclusive exons)
  • RNA editing modifies the mRNA sequence, potentially altering the encoded protein
  • RNA stability is regulated by controlling the mRNA degradation rate
• Translational control involves the regulation of protein synthesis at the ribosome level
  • Regulation of ribosome binding to mRNA affects translation initiation
  • Control of translation elongation and termination can also modulate protein synthesis
• Post-translational control involves the regulation of protein activity and degradation
  • Protein modifications, such as phosphorylation, glycosylation, and ubiquitination, can alter protein function and stability
  • Protein localization involves the transport of proteins to specific cellular compartments (nucleus, mitochondria, endoplasmic reticulum)
  • Protein degradation is the targeted breakdown of proteins by proteases, regulating their abundance and activity

Regulatory mechanisms in gene expression

• Epigenetics involves heritable changes in gene expression without alterations to the DNA sequence
• Regulatory sequences are specific DNA regions that control gene expression by interacting with various regulatory proteins
• Feedback loops are mechanisms where the output of a process influences its input, helping maintain homeostasis in gene expression
• Signal transduction pathways transmit external signals to the cell's interior, often resulting in changes in gene expression