Lec-5-Cytoskeleton. HARD.OSR

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Questions and Answers

What structural component forms the wall of a microtubule?

Centriole proteins

Tubulin dimers stacked together (correct)

Monomeric tubulin

Microfilaments made from actin

Which part of the microtubule is known as the plus end?

The midpoint of the microtubule

The end where the α-tubulin is located

The end embedded in the centrosome

The end where the β-tubulin is located (correct)

What is the primary function of the γ-tubulin ring complex in the centrosome?

To enable depolymerization of microtubules

To generate energy for microtubule polymerization

To stabilize microtubules once formed

To serve as a nucleation site for microtubule growth (correct)

Which process describes the behavior of microtubules as they grow and shrink?

Dynamic instability Signup and view all the answers

In which cellular location do microtubules primarily grow from the centrosome?

At the plus end extending into the cytoplasm Signup and view all the answers

How many protofilaments are typically found in a microtubule structure?

13 Signup and view all the answers

What type of bonding holds the tubulin dimers together in microtubules?

Noncovalent bonding Signup and view all the answers

What is the role of centrioles in the centrosome?

They organize and anchor microtubules Signup and view all the answers

What is the phenomenon of switching between growth and shrinkage of microtubules called?

Dynamic Instability Signup and view all the answers

What effect does GTP hydrolysis have on microtubule stability?

It weakens the bond between tubulin dimers. Signup and view all the answers

What can prevent a microtubule from disassembling?

Attachment to another molecule or cell structure Signup and view all the answers

Why do rapidly growing microtubules tend to keep growing?

They contain a GTP cap. Signup and view all the answers

What happens to a microtubule when the GTP cap is lost?

It experiences depolymerization and shrinks. Signup and view all the answers

What drives the dynamic instability of microtubules?

The intrinsic capacity of tubulin dimers to hydrolyze GTP Signup and view all the answers

What occurs during the depolymerization phase of microtubules?

Protofilaments peel away from the plus end. Signup and view all the answers

What is a characteristic result of slow microtubule growth?

Early hydrolysis of GTP to GDP Signup and view all the answers

What is the relationship between GTP-bound tubulin dimers and GDP-bound tubulin dimers?

GTP-bound dimers have a stronger bond. Signup and view all the answers

What is the role of the γ-tubulin ring complex in microtubule dynamics?

It initiates the growth of new microtubules. Signup and view all the answers

What characteristic arrangement do eukaryotic cilia and flagella typically exhibit?

A 9 + 2 array of microtubules Signup and view all the answers

What role do dynein molecules play in the movement of cilia and flagella?

They create a sliding force between adjacent microtubules Signup and view all the answers

During the bending movement of cilia, what is the primary effect of the dynein activity?

Relative sliding between microtubule doublets occurs Signup and view all the answers

What occurs when the dynein molecules in a sperm flagellum are freed from their other components and exposed to ATP?

The doublets slide against each other Signup and view all the answers

What happens to the doublet microtubules within an intact flagellum?

They are tied to each other by flexible protein links Signup and view all the answers

What are the accessory proteins associated with microtubules responsible for?

Cross-linking the microtubule bundle and generating force Signup and view all the answers

What is the result of the flexible protein links that tie adjacent doublet microtubules in cilia and flagella?

They convert sliding forces into bending motions Signup and view all the answers

How do dynein heads interact with adjacent microtubules in the ciliary structure?

They reach out and slide over adjacent microtubules Signup and view all the answers

Which of the following statements is true regarding ciliary dynein?

It interacts with adjacent microtubules to enable sliding Signup and view all the answers

What is the consequence of the multiple links holding the microtubule doublets together in cilia and flagella?

It facilitates bending motion rather than sliding Signup and view all the answers

What initiates the contraction of muscle cells?

The shift in tropomyosin position allowing myosin binding to actin Signup and view all the answers

What happens to the cytosolic Ca2+ concentration when the nerve signal terminates?

It is rapidly pumped back into the sarcoplasmic reticulum Signup and view all the answers

Which statement correctly describes smooth muscle activation?

It requires various extracellular signals for contraction Signup and view all the answers

Why is the activation mechanism of smooth muscle slower than that of skeletal muscle?

It requires time for enzyme molecules to diffuse and act on myosin Signup and view all the answers

What role do the troponin and tropomyosin molecules play in muscle contraction?

They block myosin binding to actin when Ca2+ is absent Signup and view all the answers

What is the primary role of the network of fibrous proteins in human red blood cells?

Providing structural support and maintaining cell shape Signup and view all the answers

Which of the following statements correctly describes a process in cell crawling?

The cell's rear end contracts while the front pushes outward. Signup and view all the answers

Which molecule is primarily responsible for facilitating the contraction at the rear of a crawling cell?

Myosin Signup and view all the answers

What occurs at the leading edge of a cell during the process of cell crawling?

Protrusions are formed through actin polymerization Signup and view all the answers

What is the significance of establishing new anchorage points at the front during cell crawling?

It provides stability for the cell to move forward. Signup and view all the answers

What happens to old anchorage points during cell crawling?

They are released as the cell advances. Signup and view all the answers

In addition to providing structural support, what is another likely benefit of the actin cortex in eukaryotic cells?

Enabling rapid responses to environmental changes Signup and view all the answers

What is the result of coordinated changes of molecules in different regions of the cell during crawling?

Efficient forward movement of the cell Signup and view all the answers

How does actin polymerization contribute to the movement of a crawling cell?

It leads to the formation of protrusions that advance the cell. Signup and view all the answers

Which characteristic of human red blood cells is directly supported by the underlying network of actin and spectrin filaments?

The maintenance of a discoid shape Signup and view all the answers

Study Notes

Microtubule Structure and Function

Microtubules are hollow tubes formed by tubulin dimers arranged into 13 parallel protofilaments.
Each protofilament consists of alternating α-tubulin and β-tubulin, exhibiting structural polarity: the β-tubulin end is the plus end, while the α-tubulin end is the minus end.

Centrosome as Microtubule-organizing Center

The centrosome organizes microtubules in animal cells and contains centrioles surrounded by a protein matrix.
γ-tubulin rings in the centrosome serve as initiation sites for microtubule growth, embedding the minus end in the centrosome while allowing growth at the plus end.

Dynamic Instability of Microtubules

Microtubules exhibit dynamic instability, characterized by alternating periods of rapid growth and shrinkage.
Growth is initiated from nucleated microtubules by the addition of αβ-tubulin dimers at the plus end.
Stabilization of the plus end through attachment can prevent disassembly, creating stable links with distant structures.

GTP Hydrolysis and Microtubule Dynamics

GTP hydrolysis is crucial for maintaining microtubule dynamics; tubulin dimers with GTP bind more tightly than those with GDP.
Loss of the GTP cap, due to slower growth rates, leads to depolymerization as GDP-bound dimers cause instability.

Cilia and Flagella Structure

Cilia and flagella contain stable microtubules organized in a "9 + 2" arrangement, producing motion through bending.
Dynein proteins between microtubule doublets generate sliding forces, converting this sliding into bending due to flexible cross-links.

Cell Crawling Mechanism

Cell crawling relies on coordinated changes involving protrusions at the cell's leading edge, adhesion to surfaces, and traction provided by the cell's body.
Actin polymerization pushes the plasma membrane forward and creates new anchorage points, while myosin-mediated contraction at the rear facilitates movement.

Muscle Contraction and Calcium Regulation

Muscle contraction is triggered by a rise in cytosolic Ca2+, spreading quickly through the cell to synchronize contraction across all myofibrils.
Once the nerve signal stops, Ca2+ is pumped back into the sarcoplasmic reticulum, causing tropomyosin to block myosin-binding sites, ending contraction.

Function of Different Muscle Cells

Smooth muscle cells contract slowly and can be activated by various extracellular signals such as adrenaline and serotonin.
This activation mechanism relies on the diffusion of enzyme molecules for phosphorylation and dephosphorylation of myosin heads, differentiating it from the rapid activation found in skeletal muscle cells.

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Description

Test your understanding of microtubule structure and dynamics. This quiz covers the formation of microtubules from tubulin dimers, the role of the centrosome in organization, and the concept of dynamic instability. Dive deep into these essential cellular components and their functions.