Gene Expression & Insulin Regulation Lecture Notes

Learning Objectives

• Describe DNA constituents: bases, nucleosides, nucleotides, introns, exons.
• Differentiate levels & mechanisms of gene-expression regulation (transcriptional → protein degradation).
• Identify stimuli & regulatory pathways that maintain blood-glucose homeostasis through insulin.
• Recall keywords: DNA, RNA, chromatin, promoter, transcription factor, operon, GLUT2, alternative splicing, ubiquitination…

Fast “Fun Facts” Recap

• Humans share ≈ 50 % of total DNA sequence with bananas (only ≈ 1 % of our genome encodes proteins).
• DNA per cell ≈ 6–9 ft; stretched from all cells it reaches Sun ↔ Earth > 600×.
• Any two humans are $99.9\%$ identical; the $0.1\%$ drives phenotypic diversity.
• Theoretical info density ≈ 25 GB per inch of DNA.
• Typical meal ingested: $\sim10^{12}$ cells → ≈ 6.3 × 10^{4}–9.3 × 10^{4} miles of DNA.

Molecular Architecture of DNA

• Repeating unit = nucleotide (base + deoxyribose + phosphate).
• Deoxyribose: 5-C sugar; carbon-2 lacks OH → “de-oxy”.
• Phosphate groups impart global –ve charge; join nucleotides via 3′→5′ phosphodiester bond.
• Nucleoside = base + sugar; nucleotide = nucleoside + phosphate.

Nitrogenous Bases

• Purines (double ring): adenine (A), guanine (G).
• Pyrimidines (single 6-membered ring): cytosine (C), thymine (T, DNA), uracil (U, RNA).
  • Replacement of ring carbons by N → heterocyclic.
• Base pairing rules in dsDNA: A⇄T (2 H-bonds); G⇄C (3 H-bonds). Higher G + C % → ↑ melting T°.

Strand Polarity & Representation

• 5′ end: free phosphate on C-5; 3′ end: free OH on C-3.
• Antiparallel duplex: one strand 5′→3′ aligns with complementary 3′→5′.
• Shorthand sequence written 5′→3′, e.g. “ACGTA…”.

Genes, Genome & Mutation

• Gene = ordered nucleotide stretch that encodes one peptide (multi-subunit proteins need >1 gene).
• Genome = sum of all genes.
• < \sim1.5\%$of human genome is protein-coding; remainder once deemed “junk” is now regulatory.</li> <li>Mutation: base substitution, insertion, deletion. Spontaneous rate small (evolutionary); chemical/radiation-induced rate large → cancer.<ul> <li>Repair machinery corrects ≈$10^{5} lesions per cell cycle.

Chromatin Organization

• Need for packing AND differential gene expression.
• Nucleosome: 146 bp DNA wrapped 1.7 turns around histone octamer (2× H2A/H2B/H3/H4).
  • H1 = linker histone (entry/exit).
• Euchromatin: open, transcription-competent.
• Heterochromatin: compact, transcriptionally silent. Same DNA segment can be eu- in one cell type & hetero- in another.
• Example: X-chromosome in females. One copy condensed → Barr body (dosage compensation; seen in amniocentesis).

DNA Replication Quick View

• Helicase unwinds; leading strand replicated continuously, lagging strand discontinuously (Okazaki fragments) → ligated by DNA ligase.

Prokaryotic vs Eukaryotic Regulation

• Prokaryotes: no true nucleus; operons control clusters (e.g. lac operon).
• Eukaryotes: compartmentalized; multilayer regulation.

Six Major Levels of Gene-Expression Control (Eukaryote)

1. Transcriptional (promoter accessibility, TF binding).
2. RNA processing (capping, poly-A, splicing / alternative splicing).
3. mRNA export via nuclear-pore gating.
4. mRNA translation (initiation factors, ribosomal loading).
5. mRNA degradation (half-life modulation, microRNAs, deadenylation).
6. Protein turnover (post-translational mods, ubiquitin-proteasome, lysosomal routes).

Detailed Snapshots

1 Transcriptional Control

• Promoter = upstream DNA motif (e.g. TATA box) + specific response elements (A4, A3, C1, E1 in insulin gene).
• Transcription factors (TFs):
  • Activators (e.g. MAF A for insulin) → recruit RNA-pol II.
  • Repressors (tumor-suppressor TFs locking repair genes).

2 RNA Processing / Alternative Splicing

• Introns = intervening sequences removed; exons retained/ligated.
• Alternative splice choice subjective to cell-type stimulus.
  • Example: common pre-mRNA →
  • Thyroid cell splicing ⇒ Calcitonin peptide.
  • Neuronal cell splicing ⇒ CGRP (Calcitonin Gene-Related Peptide).

3 mRNA Export

• Nuclear-pore complex diameter & selectivity regulated by phosphorylation cascades.
• Allows rapid release of pre-stocked mature insulin mRNAs for short-term glucose response.

4 Translational Control

• Ribosome recruitment controlled by translation initiation factors (eIFs) that are substrates of signaling kinases (e.g. mTOR, MAPK).

5 mRNA Degradation

• mRNA half-life tuned via 3′ UTR motifs (AU-rich elements), deadenylation, decapping, microRNA-RISC.

6 Protein Degradation

• Misfolded/aged proteins tagged by poly-ubiquitin (( \ge 4 ) moieties) → 26S proteasome → peptides.
• Mono-ubiquitination of histones instead alters chromatin status (regulatory).

Blood-Glucose & Insulin Regulation Case Study

Pancreatic β-Cell Signaling Cascade

1. High extracellular glucose enters via GLUT2.
2. Glycolysis & TCA ↑ → ATP ↑, \frac{ATP}{ADP} ratio rises.
3. ATP-sensitive K⁺ channels close → depolarization → Ca²⁺ influx.
4. Ca²⁺ triggers exocytosis of pre-formed insulin granules (≈ 5 % pool) → rapid response (minutes).
5. Parallel kinase cascade (PKA, PKC, MAPK) activates TFs (e.g. MAF A, PDX-1, NeuroD1) that bind insulin promoter elements (A4, A3, C1, E1, TATA) → transcriptional up-regulation.
6. Newly made pre-pro-insulin translated on rER. Signal peptide removed → proinsulin.
7. Proinsulin folded, disulfide-bonded, packaged in Golgi → secretory granules; C-peptide cleaved to yield mature insulin + C-peptide.
8. Reserve granules released upon sustained high-glucose when intracellular pH drops due to H⁺ generation.

Short-Term vs Long-Term Control

• Short-term: mobilize existing mRNA & granules (steps 1–4 & 6-8 minimal).
• Long-term: enlist transcription, translation & vesicle biogenesis (all 6 regulatory levels engaged).

Ethical / Practical Implications

• Mis-regulation of TFs → diabetes, cancer (oncogene activation, tumor-suppressor silencing).
• Understanding chromatin openness (epigenetics) informs CRISPR therapy & regenerative medicine.
• Amniocentesis & Barr-body staining employed for prenatal sexing, but raises sex-selective abortion ethics.
• PCR primer-design demands base-pairing, GC-content & 5′→3′ orientation awareness.

Key Numeric / Formula Reminders

• Nucleosome core DNA ≈ 146\,bp$; linker DNA variable ≈$10–80\,bp$.</li> <li>Hydrogen bonds: A–T = 2; G–C = 3 →$T_m \propto 4(G+C)+2(A+T)$(Wallace rule for short DNA).</li> <li>Spontaneous DNA damage events per cell cycle ≈$10^{5}$.</li> <li>Human chromosome count =$46$(23 pairs); females = XX, males = XY.</li> <li>Okazaki fragment size (eukaryote) ≈$100–200\,bp$$.

Concept Connections & Recap

• DNA → Chromatin state → Promoter accessibility → TF binding → pre-mRNA → splicing/export → translation → protein → post-translational mods → function/degradation.
• Each layer offers a therapeutic drug target (e.g. HDAC inhibitors open chromatin; proteasome inhibitors in myeloma).
• Insulin example illustrates how extracellular nutrient signals integrate with all six regulatory tiers to preserve systemic homeostasis.