Cytoskeleton Components and Functions
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Questions and Answers

What role do organelles play in cellular function?

Organelles compartmentalize incompatible functions, increase lipid surface area for membrane-related activities, and concentrate molecules in a confined space.

Define the endomembrane system and its main functions.

The endomembrane system is a group of organelles involved in protein production, quality control, and secretion; it also internalizes substances from outside the cell.

What is the process called that brings molecules into a cell?

Endocytosis.

What mechanism is used to move molecules out of a cell?

<p>Exocytosis.</p> Signup and view all the answers

Identify the three mechanisms by which proteins are sorted within a cell.

<p>Proteins are sorted by transport through nuclear pores, across membranes, and by vesicular transport.</p> Signup and view all the answers

Which protein sorting method requires that proteins remain unfolded?

<p>Transportation across membranes.</p> Signup and view all the answers

What determines a protein's destination within a cell, and how was this discovered?

<p>Signal sequences direct protein localization; experiments showed that lacking the signal sequence prevented proper sorting.</p> Signup and view all the answers

Describe the nuclear pore, its structure, and its function.

<p>The nuclear pore is a double membrane gate with a ring-like protein structure that controls the movement of molecules in and out of the nucleus.</p> Signup and view all the answers

What are the three types of cytoskeleton components, and what is their main function?

<p>The three types are intermediate filaments (withstands mechanical stress), microtubules (intracellular transport), and actin filaments (cell movement and muscle contraction).</p> Signup and view all the answers

Describe the structural assembly of intermediate filaments.

<p>Intermediate filaments are formed by dimers of single polypeptides that wrap around each other, creating a tetramer which binds to form a filament.</p> Signup and view all the answers

How do intermediate filaments structurally differ from actin filaments and microtubules?

<p>Intermediate filaments are made of single proteins in a filament form, while actin filaments are composed of twisted monomers and microtubules are dynamic hollow tubes.</p> Signup and view all the answers

What is the primary role of nuclear lamins?

<p>Nuclear lamins play a role in DNA replication, RNA transcription, and maintaining nuclear and chromatin organization.</p> Signup and view all the answers

Describe the structure and function of microtubules.

<p>Microtubules consist of alpha and beta tubulin subunits that stack and form hollow tubes, functioning in intracellular transport and chromosome segregation.</p> Signup and view all the answers

What defines the plus end of the microtubule?

<p>The plus end of the microtubule is the end that incorporates new dimers at the fastest rate.</p> Signup and view all the answers

What is an MTOC and its function?

<p>An MTOC, or Microtubule Organizing Center, organizes the location, number, and orientation of microtubules in the cell.</p> Signup and view all the answers

Name one protein found in intermediate filaments.

<p>Keratin is one example of a protein found in intermediate filaments.</p> Signup and view all the answers

What is the role of specialized proteins in nuclear import?

<p>They identify addresses and small peptides, allowing proteins to navigate through the nuclear pore.</p> Signup and view all the answers

How do import receptors facilitate the nuclear import process?

<p>Import receptors and their cargo bind and hop between regions of the proteins in the nuclear pore.</p> Signup and view all the answers

What triggers the directionality of nuclear import/export?

<p>A cytosolic GAP initiates GTP hydrolysis by RAN, indicating whether a molecule is in the nucleus or cytoplasm.</p> Signup and view all the answers

Which form of RAN is found in high concentration in the nucleus, and why?

<p>Ran-GTP is high in the nucleus due to the presence of Ran-GEF, which keeps RAN in its GTP-bound form.</p> Signup and view all the answers

Why is Ran-GDP predominant in the cytoplasm?

<p>Ran-GDP is high in the cytoplasm because Ran-GAP converts Ran-GTP to its inactive form upon exiting the nucleus.</p> Signup and view all the answers

What is the purpose of immunoprecipitation in protein studies?

<p>Immunoprecipitation is used to purify specific proteins by utilizing antibody binding to detect protein-protein interactions.</p> Signup and view all the answers

What does it indicate when two proteins immunoprecipitate together?

<p>It indicates that the two proteins are physically associated with one another under the experimental conditions used.</p> Signup and view all the answers

What steps does a protein take to be secreted from the cell?

<p>The protein starts in the nucleus, is synthesized in the rough endoplasmic reticulum, and is modified in the Golgi apparatus before secretion.</p> Signup and view all the answers

What is the process by which a cell engulfs something to become part of itself?

<p>Phagocytosis.</p> Signup and view all the answers

How do early endosomes differ from late endosomes in function?

<p>Early endosomes are involved in sorting and recycling materials, whereas late endosomes specialize in delivering materials for degradation in lysosomes.</p> Signup and view all the answers

What role do lysosomes play in cellular digestion?

<p>Lysosomes contain hydrolytic enzymes that degrade materials taken up by endocytosis.</p> Signup and view all the answers

Describe the pathway of an internalized molecule after it enters the cell.

<p>The molecule enters via endocytosis or phagocytosis, forms an endosome or phagosome, matures into a late endosome, and either gets degraded or recycled.</p> Signup and view all the answers

What maintains the low pH of lysosomes?

<p>Proton pumps maintain the low pH of lysosomes.</p> Signup and view all the answers

What is receptor recycling in the context of cell membranes?

<p>Receptor recycling is the process where surface receptors internalize after binding to their ligands and are returned to the membrane for reuse.</p> Signup and view all the answers

How do lysosomal proteins, such as degradative enzymes, reach lysosomes without degrading there?

<p>They are synthesized in the rough ER, modified in the Golgi, tagged with the M6P marker, and directed to early endosomes, maturing into late endosomes before reaching lysosomes.</p> Signup and view all the answers

What distinguishes pinocytosis from phagocytosis?

<p>Pinocytosis is the random ingestion of fluids and small molecules, while phagocytosis involves the engulfment of larger particles.</p> Signup and view all the answers

Why do enzymes not degrade in the lysosome?

<p>Enzymes are activated only in the acidic environment of the lysosome, and the lysosome membrane protects the cell from their destructive activity.</p> Signup and view all the answers

What is the primary symptom of Familial Hypercholesterolemia (FH) patients?

<p>The primary symptom of FH patients is high blood cholesterol levels.</p> Signup and view all the answers

What happens to cholesterol biosynthesis in normal cells when LDL is absent?

<p>Cholesterol biosynthesis increases in normal cells when there is no LDL.</p> Signup and view all the answers

What occurs to cholesterol biosynthesis in normal cells when LDL is present?

<p>Cholesterol biosynthesis decreases in normal cells when LDL is present.</p> Signup and view all the answers

In FH cells, how does the absence of LDL affect cholesterol biosynthesis?

<p>In FH cells, cholesterol biosynthesis remains high when LDL is absent.</p> Signup and view all the answers

What is the effect of LDL on cholesterol biosynthesis in FH cells?

<p>In FH cells, the presence of LDL leads to unregulated cholesterol biosynthesis.</p> Signup and view all the answers

What did Brown and Goldstein discover regarding cholesterol metabolism in normal individuals?

<p>Brown and Goldstein found that LDL receptors on cells efficiently mediate the uptake of LDL cholesterol.</p> Signup and view all the answers

What did Brown and Goldstein find about LDL receptors in FH patients?

<p>They found that LDL receptors in FH patients are either defective or absent, inhibiting efficient LDL uptake.</p> Signup and view all the answers

Why is p53 referred to as a tumor suppressor?

<p>Mutations in p53 dramatically increase the risk of cancer.</p> Signup and view all the answers

What role does active S-CDK play during the cell cycle?

<p>It initiates and regulates DNA replication during the S phase.</p> Signup and view all the answers

What critical decisions occur during the G2/M checkpoint?

<p>The cell checks if replication is complete and assesses for any DNA damage.</p> Signup and view all the answers

Describe the primary functions of active M-CDK.

<p>Active M-CDK drives the cell into mitosis.</p> Signup and view all the answers

What happens to chromosomes during metaphase and anaphase?

<p>Chromosomes line up in metaphase and are separated during anaphase.</p> Signup and view all the answers

What is the function of kinetochores in the cell cycle?

<p>Kinetochores are where chromosomes attach to spindle microtubules.</p> Signup and view all the answers

When is cohesin loaded onto chromosomes, and when is it removed?

<p>Cohesin is loaded onto sister chromatids during the S phase and is removed in anaphase.</p> Signup and view all the answers

How does cytokinesis differ between animal and plant cells?

<p>In animal cells, a contractile ring aids in the split, while plant cells form a cell plate.</p> Signup and view all the answers

Study Notes

Cytoskeleton Components

  • Three types: intermediate filaments, microtubules, and actin filaments
  • Intermediate filaments: withstand stress, found in muscle and skin cells
  • Microtubules: intracellular transport highways, chromosome segregation, located in centrosomes
  • Actin filaments: cortex stabilization, cell movement, muscle contraction; located beneath the plasma membrane

Intermediate Filaments Structure

  • Single polypeptide wraps around another to form a dimer
  • Dimers attach anti-parallel
  • No polarity
  • Eight dimers form a tetramer
  • Tetramers bind to form a filament
  • Network in cytoplasm, surround nucleus, extend to cell junctions (desmosomes)
  • Keratin and lamin are examples

Intermediate Filaments vs. Actin and Microtubules

  • Intermediate filaments are protein polymers
  • Microtubules are dynamic, hollow tubes made of monomers (flexible or stiff)
  • Actin filaments are flexible chains of monomers

Nuclear Lamins Function

  • DNA replication, RNA transcription
  • Nuclear and chromatin organization
  • Cell cycle regulation
  • Cell development and differentiation
  • Nuclear migration
  • Apoptosis

Microtubules Structure

  • Subunits stack to form a filament
  • Attach to other filaments to form a hollow tube
  • Plus and minus ends
  • Grows at both ends
  • Anchored to the nucleus
  • Alpha-tubulin and beta-tubulin are protein subunits

Microtubule Organizing Center (MTOC)

  • Organizes location, number, and orientation of microtubules
  • Near the nucleus
  • Orient the same way

Microtubules Dynamic Instability

  • Growing and shrinking of microtubules
  • Catch molecules

GTP Hydrolysis in Microtubules

  • Controls dynamic stability

Actin Treadmilling

  • New actin monomers added to plus end faster than minus end
  • Monomers come off of minus end
  • Treadmilling effect

Actin Modification

  • Severing proteins cut filament in half
  • Bundling proteins group filaments together
  • Cross-linking proteins set filaments across each other

Myosin's Role

  • Generates force and movement toward plus end of actin

Cytoskeleton Proteins Comparison

  • Intermediate filaments are symmetric backbones connecting cells and tissues
  • Microtubules are structural backbones with plus and minus ends used in transport activity
  • Actin is a protein polymer used in movement

Motor Proteins (Kinesin and Dynein)

  • Kinesin: plus end-directed; functions in synapse
  • Dynein: minus end-directed; moves towards the cell body
  • ATP hydrolysis powers motor movements
  • Cargo binding domain and cytoskeletal binding domain
  • Kinesin and dynein position organelles (e.g., in neurons)

Myosin Function

  • Binds, releases, rebinds to move along actin filaments
  • Moves cargo

Action Potential Muscle Contraction

  • Electrical signal travels from neuron to muscle
  • Neurotransmitters are released
  • Action potential in the muscle cell's plasma membrane
  • Calcium released into cytoplasm
  • Calcium binding proteins expose myosin binding sites on actin

Calcium's Role in Muscle Contraction

  • Exposes binding sites on actin by binding to troponin, removing tropomyosin
  • Other muscle contractions requirements are Magnesium, potassium, ATP, acetylcholine, and vitamin D

Endomembrane System

  • Organelles compartmentalize incompatible functions, providing more surface area
  • High concentration of molecules in a small space
  • Includes endoplasmic reticulum (ER), Golgi, and lysosomes, responsible for production, modification, sorting and trafficking of proteins, lipids and other materials.

Endocytosis

  • Cellular uptake of molecules from external environment, including phagocytosis, pinocytosis, and receptor-mediated endocytosis processes

Lysosomes

  • Spherical structures with hydrolytic enzymes
  • Degradation of material taken by endocytosis
  • Low pH maintained by proton pumps

Receptor Recycling

  • After binding to ligand, receptor is internalized, then returned to membrane

Protein Degradation in Lysosomes

  • Enzymes are synthesized in the rough ER and tagged with M6P marker
  • Directed to the Golgi and then early endosome becoming a late endosome and finally fusing with lysosome

Receptor-Mediated Endocytosis

  • High blood cholesterol (FH). LDL receptor deficiency causing the cell to not be able to suppress cholesterol production, due to lack of LDL cholesterol
  • Uptake of LDL cholesterol by LDL receptors
  • Cholesterol metabolism regulation by LDL receptors

Cell Cycle

  • The series of events a cell goes through as it grows and divides.
  • Includes G1, S, G2, and M phases.
  • Rate-limiting step is how quickly the cell duplicates the genome.

Mitochondria Replication

  • Fission (similar to bacteria)

Centrosome Duplication

  • S phase

Centrosome Function

  • Growth of microtubules; chromosome segregation during mitosis

Flow Cytometry

  • Application for flow of cells
  • Data possible, to draw logical conclusions

Cyclin-CDK Complexes

  • Cyclins and Cyclin-dependent kinases (CDKs) control the cell cycle.
  • Activation by phosphorylation,
  • Regulated by inhibitors, cell cycle checkpoints and damage
  • Cell cycle arrest (temporary pause) for repair
  • Cell cycle checkpoints regulate transitions between phases

Mitosis and Meiosis

  • Mitosis: growth, repair, asexual reproduction
  • Meiosis: gamete production for sexual reproduction

Mitosis Stages

  • Metaphase: chromosomes line up, separated in anaphase

Kinetochores Function

  • Chromosome attachment to spindle microtubules
  • Generate tension between sister kinetochores

Cohesin Function

  • Loaded onto sister chromatids during the S phase, and removed in anaphase

Spindle Assembly Checkpoints

  • Cell stops in metaphase until all kinetochores are under tension.
  • Anaphase then happens

Cytokinesis

  • Cytoplasm split
  • Animals: contractile ring made of actin filaments, myosin motors
  • Plants: fusion of vesicles with polysaccharides and glycoproteins

Apoptosis

  • Programmed cell death, pathway response to damage or injury, different from necrosis
  • Healthy cell death to maintain tissue homeostasis
  • Cell shrinkage, condensation, apoptosis bodies

Apoptosis Proteins

  • Caspases initiate apoptosis.
  • Initiator caspases activated by apoptosis signals & activating other executioner caspases
  • Active executioner caspases initiate cell death

Apoptosis Signal Pathways

  • Intrinsic: stress/damage, DNA damage, oxidative stress
  • Extrinsic: developmental signals
  • Mitochondrial membrane permeabilization and cytochrome C release are part of the intrinsic pathway

Apoptosis Markers

  • Phosphatidylserine flipped to the outside of the cell: Annexin V dye for detection

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Description

This quiz explores the three main types of cytoskeletal components: intermediate filaments, microtubules, and actin filaments. It will cover their structural characteristics, functions, and differences. A focus is placed on intermediate filaments and their roles in cell integrity and movement.

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