Retta - L14

Questions and Answers

What is the primary role of kinetochores during mitosis?

Attach chromosomes to the mitotic spindle (correct)

Regulate the cell cycle phases

Synthesize ATP for energy

Facilitate transport of nutrients

How many microtubules can attach to the kinetochore of each sister chromatid in humans?

70-80

30-40 (correct)

50-60

1-2

What role do the proteins at the kinetochores serve during mitosis?

Facilitating DNA replication

Increasing the oxygen levels in cells

Promoting apoptosis in damaged cells

Supporting the assembly of the mitotic spindle (correct)

Which statement accurately describes the interaction between microtubules and kinetochores?

The interaction is mediated by a protein complex that acts as a bridge Signup and view all the answers

What feature of kinetochores differs among various species?

Number of microtubules attaching to the kinetochores Signup and view all the answers

In the context of kinetochores, what does the term 'bridging role' refer to?

Acting as a mediator between the kinetochore and microtubules Signup and view all the answers

What variety of attachment can be observed in C. elegans' kinetochores?

Multiple kinetochores are found on a single centromere Signup and view all the answers

What role does the Ndc80 complex play in microtubule dynamics?

It stabilizes the attachment of microtubules to the kinetochores. Signup and view all the answers

How does the tension generated during chromosome alignment affect microtubule attachments?

It promotes the dephosphorylation of the Ndc80 complex. Signup and view all the answers

What mechanism do microtubules use to attach to chromosomes during mitosis?

A search and capture mechanism. Signup and view all the answers

What is the consequence of low tension on the kinetochores during chromosome segregation?

Phosphorylation of the Ndc80 complex by Aurora-B kinase. Signup and view all the answers

Which protein complex is essential for promoting microtubule polymerization at the plus end?

Ndc80 complex. Signup and view all the answers

What is a key feature of the kinetochore concerning its physical structure?

It is comprised of multiple proteins with regulatory roles. Signup and view all the answers

What is the primary role of Mad2 in the cell cycle?

Inhibits the APC complex from functioning Signup and view all the answers

What issue can arise from incorrect microtubule attachment to kinetochores?

Inability of sister chromatids to orient correctly. Signup and view all the answers

Which mechanism explains the gradual stabilization of microtubule attachment during chromosome alignment?

Stable bi-orientation achieved through trial and error. Signup and view all the answers

What happens to the Cdc20-APC complex when kinetochores are unattached?

It is inhibited by Mad2 and cannot function Signup and view all the answers

What is the role of interpolar microtubules in chromosome movement during mitosis?

They guide the movement of sister chromatids to the poles. Signup and view all the answers

How does Mad2 interact with Cdc20?

By directly binding and inhibiting its function Signup and view all the answers

What effect does a single unattached kinetochore have on the cell cycle?

It sends a negative signal that arrests the cell cycle Signup and view all the answers

What is the consequence if Mad2 is altered or dysfunctional?

Cells will continue through the cell cycle without checks Signup and view all the answers

What role does phosphatase play when kinase is not active?

It dephosphorylates target proteins. Signup and view all the answers

How do motor proteins contribute to chromosome movement during mitosis?

They generate opposing forces on chromosomes. Signup and view all the answers

What is the function of the APC complex in cell division?

To trigger sister chromatid separation. Signup and view all the answers

What specifically activates the APC complex?

The interaction with Cdc20. Signup and view all the answers

What is the primary role of separase during anaphase?

To degrade cohesin proteins. Signup and view all the answers

Which of the following describes the relationship between securin and separase?

Securin inhibits separase. Signup and view all the answers

During which phase do the opposing forces on chromosomes balance to position them correctly?

Metaphase Signup and view all the answers

What happens to the APC complex when it interacts with the M-Cdk complex?

It is phosphorylated and activated. Signup and view all the answers

Which of the following best describes a proteolytic enzyme?

An enzyme that degrades proteins into smaller fragments. Signup and view all the answers

What is the role of cohesin during the M phase of the cell cycle?

To hold sister chromatids together. Signup and view all the answers

What is the role of separase in sister chromatid separation?

It cleaves the cohesin complex allowing separation. Signup and view all the answers

How does APC/C contribute to the separation of sister chromatids?

By ubiquitinating and degrading specific proteins. Signup and view all the answers

What happens if APC is inhibited during mitosis?

Mitosis will be arrested at metaphase. Signup and view all the answers

What is the function of securin in the context of sister chromatid separation?

It inhibits separase to maintain chromatid cohesion. Signup and view all the answers

What must occur for a cell to exit mitosis after anaphase?

The degradation of M-cyclin. Signup and view all the answers

How does the spindle assembly checkpoint (SAC) affect sister chromatid separation?

It ensures all chromosomes are properly attached to the spindle. Signup and view all the answers

What is a consequence of a nondegradable mutant form of M-cyclin added to mitotic extracts?

Sister chromatids will not separate. Signup and view all the answers

What triggers the action of APC/C in promoting sister chromatid separation?

The increase of transcription of Cdc-20. Signup and view all the answers

Which statement best describes the relationship between biochemical and mechanical events during chromatid separation?

Both events are necessary for proper separation. Signup and view all the answers

What is the main function of the M-Cdk complex before anaphase?

To ensure progression and entry into mitosis. Signup and view all the answers

Study Notes

Role of Kinetochores in Mitosis

Kinetochores are protein complexes essential for chromosome segregation during mitosis.
Each kinetochore can attach numerous microtubules; in humans, 30-40 microtubules connect to each sister chromatid's kinetochore.
Different species exhibit variations in kinetochore structure; for example, C. elegans has multiple kinetochores for each centromere.

Mechanism of Microtubule-Kinetochore Interaction

Microtubules bind to kinetochores through a protein complex known as the Ndc80 complex, which acts as a bridge.
The Ndc80 complex facilitates microtubule polymerization by allowing tubulin subunits to be added at the microtubule's plus end.
GTPase Ran promotes microtubule nucleation near chromosomes, enhancing the attachment process.

Chromosome Attachment and Spindle Formation

Microtubule attachment follows a "search and capture" mechanism where growing microtubules interact with chromosomes.
Proper chromosome orientation in the mitotic spindle involves trial and error, establishing bi-orientation for stable attachment.

Tension and Chromosome Bi-Orientation

Tension generated by microtubule attachment is crucial for stable interactions with kinetochores, improving attachment stability.
Low tension indicates unstable connections, while high tension leads to Ndc80 complex dephosphorylation, enhancing microtubule stability.

Forces Acting on Chromosomes

Motor proteins generate opposing forces: some pull chromosomes toward spindle poles while others push them toward the spindle center.
The balance of these forces is critical for ensuring correct chromosome positioning at the spindle equator prior to anaphase.

APC/C and Sister Chromatid Separation

The anaphase-promoting complex (APC/C) is vital for sister chromatid separation and mitosis completion.
APC/C activates during the metaphase-anaphase transition, leading to cohesin degradation, which holds sister chromatids together.

Activation Mechanism of APC/C

APC/C is activated by the co-factor Cdc20; this interaction is promoted by M-Cdk activity.
Target proteins of APC/C include securin, which inhibits separase; upon securin degradation, separase becomes active and cleaves cohesin.

Combined Mechanical and Biochemical Processes

The separation of sister chromatids requires both tension (mechanical) and biochemical degradation of cohesin mediated by separase.
Proper biochemical signaling ensures the mechanical processes lead to effective chromosome separation.

Importance of Proteolytic Events

M-cyclin degradation is necessary to exit mitosis, following the anaphase, demonstrating the role of APC/C in cell cycle regulation.
Studies have shown that disrupting APC function arrests cells in metaphase, indicating its essential role in the metaphase-anaphase transition.

Spindle Assembly Checkpoint (SAC)

Unattached chromosomes activate the spindle assembly checkpoint, preventing sister chromatid separation until all kinetochores are correctly attached.
This checkpoint ensures proper chromosomal alignment and reduces errors during cell division.### Spindle Assembly Checkpoint
Major checkpoint in cell cycle where cells verify all chromosomes are correctly attached to the spindle.
If any chromosome is unattached, an inhibitory signal halts progression in the cell cycle, preventing further transitions.

Role of Mad2 Protein

Mad2 is essential for the spindle attachment checkpoint, acting as a molecular brake for mitosis.
It recruits to unattached kinetochores and inhibits the Cdc20-APC complex, blocking anaphase initiation.
Presence of Mad2 at unattached kinetochores sends a signal that prevents Cdc20-APC activation, essential for chromatid separation.

Mechanism of Action

Unattached kinetochores lack microtubule attachment and recruit proteins like Mad1, which then interacts with Mad2.
This interaction leads to a conformational change in Mad2, promoting its closed form which can sequester Cdc20.
The recruitment of Mad2 induces further conformational alterations in soluble Mad2, propagating the inhibitory signal.

Mitotic Checkpoint Complex (MCC)

Composed of Mad2, Cdc20, and other proteins, the MCC prevents the activation of the APC complex.
By sequestering Cdc20, the MCC blocks the pathway to anaphase, maintaining the cell at metaphase until all kinetochores are attached properly.

Consequences of Inhibition

Inhibition of APC by the MCC leads to the retention of cyclin B and securin, preventing activation of separase.
Since separase is inhibited by securin, anaphase cannot proceed, ensuring chromosome integrity before cell division is completed.

Visualization

Fluorescent microscopy can detect Mad2 presence on unattached kinetochores, confirming its role in the checkpoint process.
The absence of Mad2 when microtubules are attached illustrates its function specifically in monitoring kinetochores.

Summary of Cycle Control

The interaction between Mad2 and Cdc20 acts like a safety belt in a car, restricting Cdc20's activation and thus delaying anaphase.
This regulatory mechanism ensures the cell does not progress until all chromosomes are securely attached to the spindle framework.

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Description

Test your knowledge on the role of kinetochores in mitosis and how they interact with microtubules. This quiz covers key mechanisms involved in chromosome attachment and spindle formation, highlighting important complexes such as Ndc80. Understand the variations in kinetochore structures across different species.