Microtubule Structure and Function Quiz

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40 Questions

What are the major structural elements of the cytoskeleton?

Microfilaments, microtubules, intermediate filaments

What are the components of microtubules and microfilaments?

Tubulin subunits, actin subunits

What are the bacterial cytoskeletal elements involved in DNA segregation and cell shape?

Actin-like MreB and tubulin-like FtsZ

Which proteins use ATP to drive vesicle or organelle transport or to generate sliding forces between microtubules?

MAPs

Which proteins promote depolymerization of microtubules by binding to tubulin heterodimers and preventing their polymerization?

Stathmin/Op18

Which proteins act at the ends of microtubules and promote the peeling of subunits from the ends?

Catastrophins

Which proteins sever microtubules?

Katanin

What contributes to the dynamic instability of microtubules?

GTP hydrolysis

What gives microtubules inherent polarity with plus and minus ends?

Protofilaments made of α and β tubulin heterodimers

What nucleates the assembly of new microtubules in microtubule-organizing centers (MTOCs)?

γ-tubulin

What is the primary function of microfilaments?

Muscle contraction

What does actin polymerize to form?

Microfilaments

How are F-actin filaments structured?

Two linear strands of polymerized G-actin wound into a helix

Which protein is found at sites where intermediate filaments connect to microfilaments and microtubules?

Plectin

What is the fundamental subunit of intermediate filament proteins?

Dimers

What role do intermediate filaments play in neurons?

Tension-bearing role

How do two IF polypeptides align in the basic structural unit of intermediate filaments?

Parallel

Which protein binds ADP-G-actin and F-actin, increasing turnover at the minus end of microfilaments and severing filaments to create new plus ends?

ADF/cofilin

What protein is crucial for the formation of loose networks of crosslinked actin filaments, splicing and joining intersecting microfilaments?

Filamin

Which protein facilitates indirect force exertion during cell movement by linking microfilaments to the plasma membrane?

Ezrin

What is the primary function of microfilaments?

Generating movement in muscle cells

What gives microtubules inherent polarity with plus and minus ends?

GTP-bound tubulin at the plus end

What are the roads and cars in the context of cellular movement?

Microtubules and motor proteins

What gives myofibrils a pattern of alternating dark and light bands in skeletal muscle?

A bands and I bands

What constitutes a calcium-sensitive switch that activates contraction in striated muscle?

Tropomyosin, troponin, and F-actin

What model explains muscle contraction as due to thin filaments sliding past thick filaments?

The sliding filament model

What constitutes the thick filaments in skeletal muscle?

Molecules of myosin

Which motor protein is involved in ATP-dependent transport towards the plus ends of microtubules?

Kinesin

What is the structure that consists of an axoneme connected to a basal body and aids in movement through fluid environments?

Cilia

What is the rate of transporting proteins in fast axonal transport?

About 2 μm/sec

What did Chargaff observe about the amount of nucleotides in DNA?

The amount of A = T, and the amount of G = C

What did Watson and Crick propose about the structure of DNA?

DNA is a double helix with a sugar phosphate backbone and nitrogenous bases attached

What was Erwin Chargaff's significant observation regarding DNA composition?

The DNA from different cells of a given species has the same percentage of each of the four bases

What did Chargaff's rules reveal about the base composition of DNA?

A = T and G = C

What was known about the bases in DNA that allowed Watson and Crick to propose the double-helix model?

The bases would be able to form hydrogen bonds with each other

What did X-ray diffraction data from Rosalind Franklin reveal about DNA?

The long thin helical structure

What is the diameter of the double helix model of DNA?

2 nm

What is the primary mechanism suggested for DNA replication by the double helix model?

Purine-pyrimidine pairing consistent with Chargaff’s rules

What is the unit used to measure DNA length?

Base pairs (bp)

What is the role of topoisomerases in DNA?

Inducing and relaxing supercoils

Study Notes

Microtubules: Structure, Assembly, and Function

  • Microtubules are the largest structural elements of the cytoskeleton, involved in various cellular functions.
  • There are two types of microtubules: cytoplasmic and nuclear, responsible for different functions such as maintaining axons and cell shape, and forming spindles.
  • Microtubules are composed of tubulin heterodimers, forming straight, hollow cylinders of varied lengths.
  • Protofilaments, made of α and β tubulin heterodimers, give microtubules inherent polarity with plus and minus ends.
  • Microtubules form through the reversible polymerization of tubulin dimers in the presence of GTP and Mg2+, with distinct lag and elongation phases.
  • Microtubule assembly depends on the concentration of tubulin dimers and the critical concentration, leading to growth or disassembly.
  • The plus end of microtubules grows faster due to the addition of tubulin dimers, while treadmilling occurs when free tubulin concentration varies at the plus and minus ends.
  • GTP hydrolysis contributes to the dynamic instability of microtubules, leading to periods of growth, shrinkage, catastrophe, and rescue.
  • Microtubules originate from microtubule-organizing centers (MTOCs) containing γ-tubulin, which nucleates the assembly of new microtubules.
  • MTOCs organize and polarize microtubules within cells, with the minus ends anchored in the MTOC, leading to dynamic growth and shrinkage at the plus ends.
  • Centrosomes, containing γ-tubulin ring complexes, are MTOCs that nucleate the assembly of new microtubules away from the centrosome.
  • Loss of γ-TuRCs prevents a cell from nucleating microtubules, impacting cellular functions.

Actin and Intermediate Filaments: Structure and Function

  • Myosin subfragment 1 (S1) binds and decorates actin microfilaments in a distinctive arrowhead pattern, defining the plus and minus ends.
  • Actin monomers in the cytosol bind ATP, which is converted to ADP upon complex formation, reflecting the polarity of microfilaments.
  • Actin-binding proteins regulate the polymerization, length, and organization of actin, controlling nucleation, elongation, severing, and network association.
  • Thymosin β4 and profilin compete for G-actin binding, regulating the availability of ATP-bound G-actin for microfilament assembly.
  • ADF/cofilin binds ADP-G-actin and F-actin, increasing turnover at the minus end of microfilaments and severing filaments to create new plus ends.
  • Capping proteins like CapZ and tropomodulins bind to the plus and minus ends of microfilaments, respectively, regulating filament growth.
  • Filamin, a protein, is crucial for the formation of loose networks of crosslinked actin filaments, splicing and joining intersecting microfilaments.
  • Actin can be bundled into highly ordered arrays like focal contacts or focal adhesions, with proteins like α-actinin and fascin playing prominent roles.
  • Microfilaments are connected to the plasma membrane by crosslinks made of myosin I, calmodulin, fimbrin, and villin, exerting force during cell movement.
  • Proteins like band 4.1, ezrin, radixin, moesin, spectrin, and ankyrin link microfilaments to the plasma membrane, facilitating indirect force exertion during cell movement.
  • Actin forms dendritic networks with the help of the Arp2/3 complex, nucleated by proteins like WASP and WAVE/Scar.
  • Intermediate filaments (IFs) are abundant in many animal cells, with keratin being an important IF in structures growing from animal skin, supporting the entire cytoskeleton.

Microtubule-Based and Microfilament-Based Movement Inside Cells

  • Microtubules act as tracks for organelle and vesicle transport, with inbound and outbound traffic
  • Kinesins and dyneins are microtubule-associated motor proteins that enable movement
  • Fast axonal transport involves transporting proteins at a rate of about 2 μm/sec
  • Kinesin is involved in ATP-dependent transport towards the plus ends of microtubules
  • Kinesins are a large family of proteins classified based on amino acid sequence
  • Cytoplasmic dynein moves cargo towards the minus ends, aided by protein complexes called dynactin
  • Microtubule motors shape the endomembrane system and facilitate vesicle transport
  • Cilia and flagella are structures that aid in movement through fluid environments
  • Cilia and flagella consist of an axoneme connected to a basal body
  • Doublet sliding within the axoneme causes cilia and flagella to bend
  • Myosins, a large superfamily, interact with actin microfilaments and are involved in various cellular events
  • Muscle contraction is mediated by intracellular filaments, such as thin and thick filaments in skeletal muscle cells

DNA Structure and Packaging

  • X-ray diffraction data from Rosalind Franklin revealed DNA's long thin helical structure
  • Watson and Crick produced the double helix model based on Franklin's data
  • The double helix model has 10 base pairs per turn and a diameter of 2 nm
  • Purine-pyrimidine pairing in the double helix is consistent with Chargaff’s rules
  • The double helix model suggested a mechanism for DNA replication
  • DNA length is measured in base pairs (bp) with larger stretches in kilobases (kb)
  • DNA can exist in right-handed (BDNA) and left-handed (Z-DNA) helical forms
  • DNA can be interconverted between relaxed and supercoiled forms
  • Supercoiling helps make chromosomal DNA more compact
  • Topoisomerases induce and relax supercoils in DNA
  • Strand separation (DNA denaturation) can be induced by raising temperature or pH
  • Very long DNA molecules must be packaged into the cell's nucleus in eukaryotes

Test your knowledge of microtubules with this quiz on their structure, assembly, and function. Explore the composition of microtubules, their polymerization process, and the impact of GTP hydrolysis. Dive into the role of microtubule-organizing centers (MTOCs) and centrosomes in nucleating and organizing microtubules within cells.

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