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Cytology L3 & L4 - Cell Structures & Cytoskeleton

Exam Summary

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LECTURE 3 - Cell Structures

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1. Eukaryotic Cell Compartments

• Cells isolate reactions in membrane-enclosed organelles to concentrate molecules and optimize biochemical reactions.
• Key strategy: confine reactions into large, multicomponent complexes.

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2. Cytosol

• Cytoplasm = entire cell contents minus the nucleus.
• Cytosol = liquid part of cytoplasm (gel-like); surrounds organelles.
• Contains water, dissolved ions, molecules, and enzymatic complexes.
• Site of protein synthesis and degradation.
• ~10 billion protein molecules in a single animal cell.
• Cytosol is in constant motion.

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3. Nucleus

• Command center for cellular activity; houses genomic DNA.
• Site of gene expression and RNA synthesis.
• Size: 1-10 µm in diameter (~10% of average somatic cell volume).
• Most cells have one nucleus.
  • No nucleus: red blood cells, platelets.
  • Multiple nuclei (syncytia): skeletal muscle cells, osteoclasts.
  • Two nuclei: liver cells (by endomitosis).

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4. Nuclear Components

• DNA stored as chromatin.
• Enclosed by a nuclear envelope (double lipid bilayer).
• Nuclear pores connect nucleus to cytoplasm.
• Nuclear lamina (intermediate filaments) supports the envelope.
• Nuclear envelope likely evolved by invagination of the plasma membrane.

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5. Nuclear Envelope

• Double membrane separated by a 20-40 nm perinuclear space.
• Outer membrane is continuous with rough ER.
• Inner membrane associates with the nuclear lamina (intermediate filaments).
• Nuclear Pore Complexes (NPCs) built from transmembrane proteins called nucleoporins (Nups).
  • At least 456 individual Nup molecules from ~34 distinct Nup proteins.
• Particles < 9 nm pass by passive diffusion.
• Larger molecules undergo active transport powered by GTP/ATP hydrolysis.

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6. Chromatin

• DNA + associated proteins.
• Human DNA: 2 m long, 3.2 billion base pairs.
• Wrapped around 8 histone proteins → nucleosome.
• Nucleosomes linked by linker DNA, stabilized by H1 linker histone.
• Organized into 23 pairs of chromosomes (humans).
• Euchromatin - loosely packed, transcriptionally active.
• Heterochromatin - densely packed, transcriptionally silent.

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7. Karyotype

• Full set of chromosomes visualized during cell division.
• Humans: 22 pairs of autosomes + 1 pair of sex chromosomes (XX female / XY male).
• Diploid cells carry both homologous chromosomes.
• Haploid gametes (sperm/egg) carry half the number.
• Karyotyping detects genetic disorders (indels, aneuploidy).

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8. Chromosomal Territories

• Each chromosome occupies a distinct, non-overlapping territory in the nucleus during interphase.
• Position influenced by interactions with the nuclear lamina and nuclear matrix.
• Affects gene expression, DNA replication, and genomic integrity.

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9. Nucleolus

• Most prominent basophilic structure in the nucleus.
• Site of rRNA transcription, processing, and ribosomal subunit assembly.
• ~50% of all RNAs in a cell are rRNAs.
• Three zones:
  • Fibrillar Centers (FCs) - contain rDNA; initial rRNA transcription.
  • Dense Fibrillar Component (F) - accumulated rRNAs.
  • Granular Component (G) - assembled ribosomal subunits.

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10. Other Nuclear Structures

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11. Mitochondria

• Elongated organelles, 0.5-1 µm in diameter.
• Double membrane: inner membrane folded into cristae (site of oxidative phosphorylation).
• Two compartments: matrix (innermost) and intermembrane space.
• Only organelle outside the nucleus with its own DNA (mtDNA).
• mtDNA: double-stranded, circular; ~5 copies per mitochondrion; 37 genes.
• mtDNA defects accumulate over time (possibly linked to aging).
• All mitochondria inherited from the female parent.
• Cells with high energy demand (e.g., heart muscle) have many mitochondria.

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12. ATP/Energy

• ATP is the main chemical energy source in cells.
• Most ATP produced by oxidative phosphorylation from ADP (in mitochondria).
• Energy used for metabolism, transport, contraction, and cell division.

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13. Endoplasmic Reticulum (ER)

• Continuous membrane system throughout the cytoplasm.
• Connected to Golgi apparatus and nucleus via transport vesicles.

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14. Golgi Apparatus

• Receives proteins from RER via transport vesicles.
• Composed of flattened membrane sacs (cisternae) in a crescent shape near the nucleus.
• Functions:
  • Post-translational modification of proteins and lipids.
  • Sorting and packaging of proteins for secretion (secretory granules) or lysosome formation.

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15. Lysosomes

• Spherical organelles, 0.05-0.5 µm diameter.
• Sites of intracellular digestion and turnover of cellular components.
• Contain ~40 hydrolytic enzymes (proteases, nucleases, phosphatases).
• High acidity due to H+ import.
• Most abundant in phagocytic cells (macrophages, neutrophils).
• Involved in autophagy (fusing with autophagosomes to recycle components).

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16. Peroxisome

• Small, membrane-enclosed vesicles.
• Contain catalases and peroxidases to break down H₂O₂ and ROS.
• Involved in fatty acid breakdown for energy.
• Enzymes enter directly from the cytosol.
• Part of the endomembrane system (with ER, Golgi, endosomes, lysosomes).

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17. Ribosomes

• Two subunits: small (40S) and large (60S).
• Made of 4 rRNAs and ~80 ribosomal proteins.
• Small subunit: decodes mRNA.
• Large subunit: catalyzes peptide bond formation.
• Free ribosomes → proteins used inside the cell (no post-translational modification).
• ER-bound ribosomes → proteins for membranes or secretion (post-translational modification).

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18. Proteasome

• 26S, 33-subunit protein complex.
• Degrades damaged, misfolded, or unwanted proteins.
• Recognizes proteins tagged with ubiquitin (ubiquitination).
• Proteins are unfolded and cleaved into short peptides.
• Dysfunction linked to neurodegeneration, immunopathology, cancer, and aging.

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19. Cytoplasmic Inclusions

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LECTURE 4 - Cytoskeleton

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20. Cytoskeleton Overview

• Network of protein filaments throughout the cytoplasm.
• Provides shape, mechanical interaction, organized movement, and internal transport.
• Highly dynamic - constantly reorganized as the cell changes, divides, or responds to its environment.

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21. Three Components of the Cytoskeleton

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22. Microtubules

• Hollow cylinders made of 13 parallel protofilaments.
• Built from α and β tubulin heterodimers.
• More rigid than other filaments; rupture under stretch.
• Exhibit dynamic instability (constant switching between growth and shrinkage).
• Growth occurs at the (+) end; requires GTP.
• Depend on concentrations of tubulin, Ca²+, Mg²+, and MAPs.
• Found in cilia and flagella as axonemes (rigid structures).

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23. Centrosome (MTOC)

• Microtubule Organizing Center (MTOC) - directs tubulin polymerization.
• Built around two centrioles (9 microtubule triplets each).
• Surrounded by matrix containing γ-tubulin ring (starting point for microtubule growth).
• Duplicates during DNA replication; forms the mitotic spindle during cell division.

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24. Microtubule Motors

• Both are dimers with 2 heads (bind microtubule) and 1 tail (binds cargo).
• ATPase activity in the heads drives movement.

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25. Microtubule Functions

• Maintain cell shape and polarity.
• Provide tracks for cytoplasmic transport.
• Chromosome movement during mitosis.
• Move cilia and flagella.

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26. Actin Filaments (Microfilaments)

• Helical polymers of actin protein, 5-7 nm diameter.
• G-actin (globular monomers) polymerize in the presence of K+ and Mg²+ into F-actin (filamentous double-stranded helix).
• ATP-bound monomers added at (+) end; ADP-bound monomers lost at (−) end → treadmilling.
• Most concentrated just beneath the plasma membrane (cortex).
• Motor proteins: myosin family.

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27. Actin Binding Proteins

• Cross-link actin into bundles or networks.
• Determine physical properties and cellular behaviors of filaments.

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28. Actin-Myosin Contraction

• Myosin: ATPase head domain binds actin; tail attaches to cargo.
• Head rotation pulls cargo along actin track.
• Responsible for muscle contraction and intracellular transport.

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29. Actin Filament Functions

• Mechanical strength of plasma membrane.
• Cytoplasmic streaming (organelle/vesicle transport).
• Endocytosis (cell surface changes).
• Form microvilli (stable structures - A).
• Contractile bundles in most animal cells (B).
• Lamellipodia/filopodia at leading edge of crawling cells (C).
• Contractile rings for cytokinesis during mitosis (D).

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30. Intermediate Filaments

• Rope-like fibers, ~10 nm diameter.
• Very flexible and strong; deform under stress without rupturing.
• Stable (unlike microtubules and actin filaments).
• Form a network surrounding the nucleus and extending to cell periphery.
• Only filament found inside the nucleus of animal cells.

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31. Intermediate Filament Assembly

1. Many different fibrous protein subunits form coiled dimers.
2. Dimers form antiparallel tetramers.
3. Non-polar assembly into 10 nm filaments.

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32. Intermediate Filament Types

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Quick Reference - Key Numbers to Remember

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MCQ Questions - Cytology L3 & L4

Cell Structures & Cytoskeleton

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LECTURE 3 - Cell Structures

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• A) The entire contents of the cell including the nucleus
• B) The liquid part of the cytoplasm surrounding the organelles ✅
• C) The fluid inside the nucleus
• D) The space between the inner and outer nuclear membranes

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• A) 1 million
• B) 1 billion
• C) 10 billion ✅
• D) 100 million

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• A) Liver cells
• B) Osteoclasts
• C) Skeletal muscle cells
• D) Red blood cells ✅

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• A) Liver cells and neurons
• B) Skeletal muscle cells and osteoclasts ✅
• C) Red blood cells and platelets
• D) Epithelial cells and hepatocytes

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• A) 2-5 nm
• B) 10-15 nm
• C) 20-40 nm ✅
• D) 50-100 nm

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• A) Smooth endoplasmic reticulum
• B) Golgi apparatus
• C) Rough endoplasmic reticulum ✅
• D) Lysosome

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• A) Histones
• B) Nucleoporins (Nups) ✅
• C) Lamins
• D) Tubulins

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• A) 2 nm
• B) 5 nm
• C) 9 nm ✅
• D) 15 nm

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• A) 0.2 m
• B) 2 m ✅
• C) 20 m
• D) 200 nm

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• A) 4
• B) 6
• C) 8 ✅
• D) 10

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• A) Heterochromatin
• B) Euchromatin ✅
• C) Barr body
• D) Constitutive chromatin

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• A) 21 pairs
• B) 22 pairs
• C) 23 pairs ✅
• D) 46 pairs

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• A) XY
• B) XX ✅
• C) XO
• D) YY

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• A) A condensed Y chromosome
• B) A condensed inactive X chromosome in female cells ✅
• C) A structure in the nucleolus
• D) A type of heterochromatin found in male cells

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• A) DNA replication
• B) Protein degradation
• C) rRNA transcription and ribosomal subunit assembly ✅
• D) Lipid synthesis

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• A) Granular component (G)
• B) Dense fibrillar component (F)
• C) Fibrillar centers (FCs) ✅
• D) Nuclear lamina

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• A) 10%
• B) 25%
• C) 50% ✅
• D) 75%

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• A) Peroxisome
• B) Lysosome
• C) Mitochondria ✅
• D) Endoplasmic reticulum

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• A) 13 genes
• B) 23 genes
• C) 37 genes ✅
• D) 46 genes

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• A) The male parent only
• B) Both parents equally
• C) The female parent only ✅
• D) Neither parent (they are synthesized de novo)

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• A) The outer membrane
• B) The intermembrane space
• C) The matrix
• D) The inner membrane (cristae) ✅

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• A) Rough ER (RER)
• B) Smooth ER (SER) ✅
• C) Both RER and SER equally
• D) Neither; this occurs in the Golgi

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• A) Cristae
• B) Cisternae ✅
• C) Vesicles
• D) Thylakoids

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• A) Lysosomes
• B) Autophagosomes
• C) Secretory granules ✅
• D) Peroxisomes

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• A) 10
• B) 20
• C) 40 ✅
• D) 80

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• A) Liver cells and neurons
• B) Macrophages and neutrophils ✅
• C) Muscle cells and adipocytes
• D) Epithelial cells and fibroblasts

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• A) Peroxisomes
• B) Mitochondria
• C) Autophagosomes ✅
• D) Secretory granules

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• A) Protein synthesis
• B) Lipid and steroid biosynthesis
• C) Breakdown of H₂O₂ and ROS, and fatty acid oxidation ✅
• D) Packaging proteins for secretion

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• A) ER
• B) Golgi apparatus
• C) Mitochondria ✅
• D) Lysosomes

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• A) 30S and 50S
• B) 40S and 60S ✅
• C) 40S and 80S
• D) 60S and 80S

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• A) Small subunit (40S)
• B) Large subunit (60S) ✅
• C) Both subunits equally
• D) Neither; this is done by the ribosome as a whole

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• A) Destined for secretion
• B) Used inside the cell, without post-translational modification ✅
• C) Incorporated into cell membranes
• D) Sent directly to lysosomes

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• A) Phosphate groups
• B) Methyl groups
• C) Ubiquitin molecules ✅
• D) Acetyl groups

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• A) 20S
• B) 26S ✅
• C) 40S
• D) 80S

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• A) Glycogen granules
• B) Melanin pigment
• C) One large lipid droplet ✅
• D) Multiple small peroxisomes

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LECTURE 4 - Cytoskeleton

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• A) Microtubules
• B) Actin filaments
• C) Intermediate filaments
• D) Ribosomes ✅

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• A) 5-7 nm
• B) 10 nm
• C) 25 nm ✅
• D) 50 nm

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• A) Actin and myosin
• B) α and β tubulin ✅
• C) Keratin and vimentin
• D) Lamin A and lamin B

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• A) The (−) end
• B) The (+) end ✅
• C) Both ends equally
• D) Neither end; it occurs at the middle

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• A) ATP hydrolysis
• B) GTP bound to incoming tubulin subunits ✅
• C) NADH oxidation
• D) Phosphocreatine

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• A) Treadmilling
• B) Polymerization instability
• C) Dynamic instability ✅
• D) Polar assembly

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• A) Nucleolus
• B) Centrosome ✅
• C) Centriole alone
• D) Kinetochore

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• A) 9 microtubule doublets
• B) 9 microtubule triplets ✅
• C) 13 microtubule singlets
• D) 7 microtubule doublets

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• A) Dynein
• B) Myosin
• C) Kinesin ✅
• D) Actin

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• A) Kinesin
• B) Dynein ✅
• C) Myosin II
• D) Spectrin

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• A) 2-3 nm
• B) 5-7 nm ✅
• C) 10 nm
• D) 25 nm

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• A) Na+ and Ca²+
• B) K+ and Mg²+ ✅
• C) Ca²+ and Cl−
• D) Na+ and K+

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• A) (−) end; (+) end
• B) (+) end; (−) end ✅
• C) Both ends; neither end
• D) Neither end; both ends

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• A) Around the nucleus
• B) Inside the nucleus
• C) Just beneath the plasma membrane (cortex) ✅
• D) In the mitochondrial matrix

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• A) Kinesins
• B) Dyneins
• C) Myosins ✅
• D) Lamins

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• A) Lamellipodia
• B) Microvilli
• C) Stress fibers
• D) Contractile ring ✅

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• A) 5-7 nm
• B) ~10 nm ✅
• C) 25 nm
• D) 50 nm

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• A) Vimentin
• B) Neurofilaments
• C) Keratins ✅
• D) Lamins

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• A) Keratins
• B) Vimentins
• C) Neurofilaments
• D) Lamins ✅

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• A) Antiparallel tetramers → coiled dimers → filaments
• B) Coiled dimers → antiparallel tetramers → filaments ✅
• C) G-actin → F-actin → bundles
• D) α/β tubulin → protofilaments → hollow cylinder

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• A) They are dynamic and constantly reorganized
• B) They are stable and do not rapidly depolymerize ✅
• C) They are involved in chromosome movement
• D) They use GTP for assembly

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• A) Epithelial cells
• B) Connective tissue
• C) Nerve cells ✅
• D) Muscle cells

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• A) Keratin
• B) Vimentin family (muscle) ✅
• C) Lamin
• D) Neurofilament

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• A) Microtubules
• B) Actin filaments
• C) Intermediate filaments ✅
• D) All three are found in the nucleus

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• A) rRNA transcription only
• B) DNA repair exclusively
• C) Biogenesis of ribosomes/spliceosomes and telomere maintenance ✅
• D) Packaging proteins for secretion

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Answer Key (Quick Reference)

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