Chapter 1: Stem Cells and Their Differentiation

Stem Cell Fate Decisions

Stem cells are unique cells capable of both self-renewal and differentiation into specialized cell types. The regulation of these two fates underlies development, tissue maintenance, and many disease processes.

• Self-renewal: The process by which a stem cell divides to produce another stem cell, maintaining the stem cell pool.

• Differentiation: The process by which a stem cell gives rise to a more specialized cell type.

Potency Hierarchy

Stem cells are classified by their potency, or the range of cell types they can produce:

• Exam Alert: ESCs are pluripotent (not totipotent); only zygote/early blastomeres are truly totipotent.

Stem Cell Source Comparison

• Exam Alert: iPSCs avoid immune rejection but have higher tumorigenicity risk due to reprogramming factors.

The Niche

• The stem cell niche is an active microenvironment providing signals (e.g., Wnt, Notch, BMP antagonists) that maintain stemness.

• Physical anchoring (e.g., E-cadherin/β-catenin junctions) is essential for stem cell maintenance.

Clinical Correlates

• Metaplasia: Reversible replacement of one differentiated cell type by another, driven by stem cell reprogramming (e.g., Barrett esophagus: squamous → columnar).

• Cancer stem cell hypothesis: Tumor regrowth is driven by a small, self-renewing subpopulation.

• OSKM (Yamanaka) factors: Oct4, Sox2, Klf4, c-Myc reprogram somatic cells to iPSCs; c-Myc is oncogenic.

Additional Regulatory Mechanisms

• MAPK signaling: Promotes differentiation (negatively regulates pluripotency).

• STAT signaling: Supports pluripotency.

• Wnt signaling: Maintains stemness in niche contexts.

• Chromatin state: Undifferentiated cells have open (euchromatin-rich) chromatin.

• microRNAs: Regulate both stemness and differentiation post-transcriptionally.

Chapter 2: Extracellular Matrix and Cell Adhesion

Core ECM Components

The extracellular matrix (ECM) is a dynamic structure providing support, signaling, and tissue organization. Each component has distinct structural and signaling roles.

• Marfan syndrome: Fibrillin-1 defect causes both structural weakness and excess TGF-β signaling.

Genetic ECM Disorders

Adhesion Structures

• Exam Alert: Desmosomes (cell-cell, cadherins) vs. hemidesmosomes (cell-matrix, integrins) both anchor intermediate filaments.

• Tight junctions: Seal paracellular space (barrier function).

• Adherens/desmosomes: Mechanical attachment.

• Gap junctions: Direct cytoplasmic communication.

Collagen Synthesis and Related Deficiencies

• Vitamin C: Cofactor for prolyl/lysyl hydroxylase; deficiency (scurvy) impairs collagen cross-linking.

• Lysyl oxidase: Cross-links collagen/elastin; copper-dependent (defective in Menkes disease).

• GAG sulfation: Hyaluronic acid is not sulfated or core protein-linked (unlike other GAGs).

Adhesion Receptor Specifics

• Integrins: Heterodimers; β2 is leukocyte-specific.

• Selectins: Mediate initial leukocyte rolling on endothelium.

• Basement membrane assembly: Laminin self-assembles first, recruits type IV collagen, bridged by nidogen/perlecan.

Chapter 3: Biological Membranes

Membrane Lipids

• Cholesterol: Only major lipid that flips freely between leaflets.

Membrane Asymmetry and Apoptosis

• Flippases/floppases maintain phospholipid asymmetry.

• During apoptosis, phosphatidylserine (PS) is externalized, signaling phagocytosis.

Membrane Fluidity

• Cholesterol: Buffers fluidity in both directions.

Membrane Transport Types

Lipid Rafts and Caveolae

• Lipid rafts: Cholesterol/sphingolipid-rich microdomains; concentrate signaling proteins.

• Caveolae: Flask-shaped rafts formed by caveolin; involved in endocytosis and signaling.

• Clinical: Pathogens exploit rafts for entry (e.g., SARS-CoV-2 via ACE2 in rafts).

Glycocalyx and Osmotic Balance

• Glycocalyx: Carbohydrate coat for recognition, protection, signaling.

• Osmolality: Governs water distribution; clinical relevance in IV fluids and osmotic shock.

Membrane Protein Mobility

• Proteins anchored to cytoskeleton have restricted lateral movement (measured by FRAP).

• Protein orientation is preserved during vesicle trafficking.

• CFTR (cystic fibrosis): Defect is in protein folding/trafficking, not lipid composition.

Chapter 4: The Cytoskeleton

The Three Filament Systems

• Intermediate filaments: Non-polar, energy-independent assembly.

Actin Dynamics

• Polymerizes by adding ATP-G-actin at the plus end; treadmilling occurs when addition at plus end matches loss at minus end.

• Key actin-binding proteins:

  • Profilin: Promotes polymerization

  • Cofilin: Severs filaments

  • Arp2/3: Nucleates branches

  • Tropomyosin: Stabilizes filaments

  • Phalloidin: Lab tool, stabilizes actin

  • Gelsolin: Severs/caps filaments (Ca2+-regulated)

• Clinical: Listeria uses Arp2/3-mediated actin polymerization for motility.

Motor Proteins

• KIF5A mutations: Impair anterograde transport, causing distal axonopathy.

Intermediate Filament Types

• Nuclear lamins: Organize nuclear envelope and anchor chromatin.

Microtubule Dynamic Instability

• Microtubules grow/shrink via dynamic instability (GTP cap stabilizes; loss = catastrophe).

• Drugs:

  • Colchicine/vinca alkaloids: Block polymerization

  • Taxol: Stabilizes, blocks depolymerization

RBC Membrane Skeleton

• Spectrin network cross-linked to actin, anchored via ankyrin and protein 4.1; defects cause hereditary spherocytosis.

Ciliary Structure

• Motile cilia: 9+2 axoneme, powered by dynein.

• Primary cilia: 9+0, sensory; PKD1/2 mutations cause polycystic kidney disease.

• Primary ciliary dyskinesia: Absent dynein arms, leads to respiratory infections, situs inversus, infertility.

Cytoskeletal Disease Beyond Filament Structure

• Duchenne muscular dystrophy: Dystrophin links actin to ECM; loss destabilizes muscle membrane.

• All cytoskeletal systems are integrated, not independent.

Chapter 5: The Organelles

The Nucleolus

• Site of rRNA synthesis and ribosome subunit assembly.

• Organized around nucleolar organizing regions (NORs) on acrocentric chromosomes (13, 14, 15, 21, 22).

Protein Targeting to the ER

• Proteins destined for secretion/membranes/lysosomes are targeted co-translationally by the signal recognition particle (SRP).

• Defects in SRP impair ER targeting, distinct from glycosylation/folding defects.

The Protein-Processing Chain

• I-cell disease: Mannose-6-phosphate tagging enzyme is absent; lysosomal enzymes are secreted, not delivered to lysosome.

Lysosomal Storage Diseases

• Most are autosomal recessive; substrate accumulation drives disease, especially in neurons.

Mitochondrial Genetics

• Maternal inheritance; heteroplasmy explains variable expressivity.

• High-energy tissues most affected (e.g., ragged red fibers in muscle).

• Examples: Leber hereditary optic neuropathy, Pearson syndrome.

Peroxisomal Disease

• X-linked adrenoleukodystrophy: Impaired very-long-chain fatty acid breakdown → myelin destruction.

• Zellweger syndrome: Defective peroxisomal protein import; multi-organ failure, early death.

Lysosomal Acidification and Pharmacology

• Lysosomal pH (~5) maintained by v-ATPases (active H+ pumping).

• Chloroquine raises lysosomal pH, disrupting parasite digestion (antimalarial mechanism).

ER Stress and the Unfolded Protein Response (UPR)

• UPR reduces global protein synthesis to allow chaperones to catch up with misfolded proteins.

Autophagy and Apoptosis Signals

• Autophagosomes: Deliver damaged organelles/proteins to lysosomes for degradation.

• Cytochrome c release: Triggers intrinsic apoptosis pathway.

Chapters 6 & 7: Eukaryotic Genomes and DNA Replication

Chromatin Packaging Hierarchy

• DNA (2 nm) → nucleosome (~147 bp around histone octamer, 10 nm) → 30 nm fiber → loop domains → chromosome.

• Constitutive heterochromatin: Always silent (centromeres, telomeres).

• Facultative heterochromatin: Reversibly silent (e.g., Barr body).

Epigenetic Regulation

• Acetylation opens, methylation closes chromatin.

DNA Replication: Enzymes and Steps

• Both Pol δ and Pol ε have 3′→5′ exonuclease proofreading.

• PCNA is the sliding clamp for processivity.

• Topoisomerase relieves supercoiling; SSBs prevent reannealing.

Genome Composition and Repetitive DNA

• ~1.5% of genome is protein-coding; rest is regulatory/repetitive DNA.

• LINEs/SINEs: Largest fraction of interspersed repeats.

• Satellite DNA: Highly repetitive, at centromeres/telomeres.

• Trinucleotide repeat expansions: Cause anticipation (e.g., Huntington disease).

Telomere Problem and Solution

• Lagging strand cannot fully replicate 3′ end; telomerase extends telomeres using its own RNA template.

• Active in stem cells, germ cells, most cancers.

DNA Repair Pathways

• MRN complex: Initial sensor for double-strand breaks.

• HR: High-fidelity, uses sister chromatid; NHEJ: Error-prone, no template.

• Synthetic lethality: PARP inhibitors kill BRCA-deficient cells by blocking backup repair.

Chapter 8: Transcription

The Three RNA Polymerases

• Pol II CTD: Organizes capping, splicing, polyadenylation.

Prokaryotic vs. Eukaryotic Transcription

Promoter Recognition and Initiation

• TATA box: TATAAAA, ~25–30 bp upstream of start site.

• TFIID (TBP): Binds TATA box; TFIIH unwinds DNA and phosphorylates Pol II CTD to initiate elongation.

• CTD phosphorylation status regulates transition from initiation to elongation and recruits processing factors.

mRNA Processing Steps

1. 5′ capping: 7-methylguanosine cap added co-transcriptionally; required for translation initiation and stability.

2. Splicing: Spliceosome removes introns (5′ GU...AG 3′ boundaries).

3. Cleavage and polyadenylation: ~200 adenine poly(A) tail added at 3′ end; protects from degradation, regulates translation.

• PRPF8 mutations: Disrupt spliceosome, causing exon skipping or intron retention.

• Alternative splicing: Allows one gene to produce multiple protein isoforms.

Post-Translational Modification Location Map

Transcriptional Regulation

• Transcription factors have DNA-binding and transactivation domains.

• Chromatin accessibility (nucleosome positioning, histone marks, DNA methylation) regulates transcription factor access to promoters.