In-depth Notes on Gene Regulation and Cell Cycle Control

Prokaryote: The Lac Operon

• Lac Operon: An inducible system in bacteria that allows the metabolism of lactose.
  • Components:
    • Inducer Molecule: Lactose, which is converted to allolactose.
    • Structural Genes: lacZ, lacY, and lacA, which encode for proteins that facilitate lactose metabolism.
  • Lac Repressor Protein: Constantly expressed, binds to the lac operator site in absence of lactose, blocking RNA polymerase from transcribing the structural genes.
  • When lactose is present, it converts to allolactose, which binds to the repressor and changes its shape. This prevents the repressor from binding to the operator, allowing transcription to occur.

Gene Regulation

• Every cell has the same DNA, but gene expression varies.
• Gene Regulation: Methods for turning genes on or off to respond to environmental changes.
  • Transcription Regulation: Proteins can enhance or inhibit transcription rates.
  • Operon Model: A series of genes regulated by a single promoter.
    • Promoter: Region on DNA where RNA polymerase binds.
    • Operator: Region where the repressor binds, blocking transcription.
  • Lac Operon Function:
    • Enzymes produced facilitate the breakdown of lactose in the bacterium.
    • In the absence of lactose, the lac repressor is bound to the operator, preventing gene expression.
    • In the presence of lactose, genes are transcribed, allowing lactose metabolism.

Tryptophan Operon

• Trp Operon: A repressible operon responsible for producing tryptophan.
  • Structural Genes: trpE, trpD, trpC, trpB, trpA
  • At low tryptophan levels, the trp repressor is inactive, allowing gene transcription.
  • At high levels, tryptophan acts as a corepressor, activating the repressor and stopping transcription.

Feedback Inhibition

• Direct inhibition of enzyme activity by end products (e.g., tryptophan inhibiting its own production).

Lac and Trp Operon Regulation

• Positive Regulation (Catabolite Repression): In absence of glucose, cAMP levels rise, activating the CAP, which enhances transcription of the lac operon.
  • Transcription Activation: In presence of glucose, CAP does not bind, reducing transcription.

Eukaryotic Gene Regulation

• Gene regulation mechanisms differ greatly from prokaryotes: involves chromatin remodeling, transcription factors, enhancers, and silencers.
• Chromatin Structure: DNA is wrapped around histones to form nucleosomes. This structure affects gene expression (euchromatin vs heterochromatin).
  • Histone Modification: Acetylation (promotes relaxation) versus methylation (promotes compaction).

Epigenetics

• Epigenetics: The regulation of gene expression without altering the underlying DNA sequence.
  • Includes mechanisms like DNA methylation and histone modification.
  • X-Inactivation in females results in one X chromosome being inactivated, leading to mosaic traits (e.g., calico cats).

RNA Processing and Regulation

• Eukaryotic mRNA undergoes processing, including splicing and the removal of introns.
• Alternative Splicing: Produces multiple mRNA variants from a single gene.
• RNA Interference (RNAi): MicroRNAs and small interfering RNAs regulate gene expression post-transcriptionally by degrading mRNA or inhibiting translation.

Cell Cycle Control

• The cell cycle consists of growth phases (G1, S, and G2) and mitosis (M phase) regulated by cyclins and cyclin-dependent kinases (CDKs).
• Checkpoints: Essential for detecting errors before progressing to the next phase, ensuring proper cell division.
  • G1 Checkpoint: Checks cell size, nutrients, and DNA damage.
  • G2 Checkpoint: Checks DNA replication completeness and halts if damage is present.

Cancer and the Cell Cycle

• Cancer arises from uncontrolled cell division due to mutations affecting cyclins, CDKs, and tumor suppressor genes (e.g., p53).
• Apoptosis: Programmed cell death is essential for removing damaged cells, and failure to undergo apoptosis can lead to cancer progression.