Retta - L15

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Questions and Answers

What is the primary role of the contractile ring during cytokinesis?

To synthesize new DNA for daughter cells

To redistribute organelles to daughter cells

To facilitate the movement of chromosomes to opposite poles

To pull the plasma membrane inward, dividing the cell (correct)

Which cellular structure is primarily responsible for determining the plane of cytoplasmic cleavage?

The mitotic spindle (correct)

The nuclear envelope

The centrioles

The Golgi apparatus

What type of filaments make up the contractile ring in animal cells?

Actin and myosin filaments (correct)

Collagen and elastin fibers

Only myosin filaments

Microtubules and intermediate filaments

During which stage of the cell cycle does the cleavage furrow begin to form?

Anaphase Signup and view all the answers

What happens to the nuclei formed during telophase in the context of cytokinesis?

They are distributed to the daughter cells Signup and view all the answers

In which type of cells is the concept of the contractile ring particularly relevant?

Animal cells and many unicellular eukaryotes Signup and view all the answers

What is the significance of the cleavage furrow in the process of cytokinesis?

It is where the contractile ring exerts force to divide the cell Signup and view all the answers

What is the primary function of the Cdc20-APC complex in the cell cycle?

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

During the early stages of Drosophila embryonic development, what is the primary mechanism that drives synchronized nuclear division?

The rhythmic contraction of the entire embryo. Signup and view all the answers

What triggers the transition from mitosis to G1 phase in the cell cycle?

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

Which complex is active during G1 phase and maintains low levels of M-cyclin?

Cdh1-APC Signup and view all the answers

What role do mitogens play in relation to the cell cycle?

They stimulate the synthesis of cycle-regulating proteins. Signup and view all the answers

Why is the G1 phase absent in rapidly dividing embryonic cells during development?

To allow for very rapid succession of cell divisions. Signup and view all the answers

How do the functions of Cdc20-APC and Cdh1-APC differ?

Cdc20-APC responds to M-cyclin Cdk while Cdh1-APC does not. Signup and view all the answers

What triggers the rhythmic division of nuclei in the Drosophila embryo?

The mitotic spindle's pulling of chromosomes. Signup and view all the answers

Why are the levels of M-cyclin low in G1 phase cells?

The Cdh1-APC complex prevents M-cyclin accumulation. Signup and view all the answers

What role do DNA-PK, ATM, and ATR play in the context of DNA damage?

They are involved in the regulation of DNA repair and cell cycle control. Signup and view all the answers

Which statement about p53 is accurate regarding its function in response to DNA damage?

It initiates apoptosis if DNA repair mechanisms are ineffective. Signup and view all the answers

How do CKI p21 and the cyclin CDK complex interact under conditions of DNA damage?

p21 inhibits the cyclin CDK complex, resulting in cell cycle arrest. Signup and view all the answers

What triggers the activation of ATR as a response to DNA damage?

Single strand DNA (ssDNA) alteration at the replication fork. Signup and view all the answers

Which of the following best describes the physiological process linked to Myc in cell proliferation regulation?

Myc is a transcription factor that promotes excessive cell proliferation if unregulated. Signup and view all the answers

What primary function do mitogens serve in the cell cycle?

Stimulating cell division Signup and view all the answers

How do growth factors primarily contribute to cell growth?

By promoting protein synthesis Signup and view all the answers

What is the role of survival factors in cellular processes?

Suppressing apoptosis Signup and view all the answers

What misconception is associated with the term 'growth factor'?

It is often used incorrectly for mitogens Signup and view all the answers

What is the significance of cyclin CDK complexes in the cell cycle?

They promote entry into the cell cycle Signup and view all the answers

Which molecule is an example of a growth factor that also stimulates cell proliferation?

Platelet-derived growth factor (PDGF) Signup and view all the answers

What is the main function of myosin II during cytokinesis?

To assist in the formation of the contractile ring and contribute to cell division Signup and view all the answers

Which molecule is primarily responsible for regulating the activation of myosin II during cytokinesis?

RhoA Signup and view all the answers

What role do sensor molecules play in the context of DNA damage?

They block the cell cycle Signup and view all the answers

In what way can membrane-enclosed organelles be distributed during cytokinesis?

Either symmetrically or asymmetrically Signup and view all the answers

What effect does the degradation of CKI have in the cell cycle?

It allows progression through the cell cycle Signup and view all the answers

Which signaling cascade is activated by mitogens to promote mitosis?

MAP kinase cascade Signup and view all the answers

What roles do the interpolar microtubules play in cell division?

They assist in the positioning of the contractile ring and contribute to chromosome separation Signup and view all the answers

Which statement about cell growth and cell proliferation is accurate?

Cell growth can refer to an increase in cell number. Signup and view all the answers

How does RhoGEF affect RhoA?

It promotes the exchange of GDP for GTP, activating RhoA Signup and view all the answers

What observation can be made regarding the distribution of actin during cytokinesis?

Actin is present both at the cleavage furrow and at the cell cortex Signup and view all the answers

Which of the following best describes the effect of centrosomes on the formation of the contractile ring?

Centrosomes contribute by localizing regulatory proteins necessary for contractile ring formation Signup and view all the answers

What is a significant outcome of mitosis occurring without cytokinesis?

The creation of a syncytium with numerous nuclei Signup and view all the answers

What experimental evidence supports the idea that both astral and interpolar microtubules contribute to the positioning of the cleavage furrow?

Analysis of centrosome positioning and microtubule behavior during cell division Signup and view all the answers

Study Notes

Cytokinesis Overview

Cytokinesis completes the cell cycle and occurs after M phase.
The cleavage plane during cytokinesis is guided by the mitotic spindle.
In animal cells, a contractile ring composed of actin and myosin filaments facilitates cell division.
Essential organelles must be appropriately distributed to daughter cells during division.

Contractile Ring Formation

The contractile ring forms at the cell's equator beneath the plasma membrane and plays a crucial role in cytokinesis.
Actin and myosin, present in the ring, contribute to the ring's contraction and subsequent furrowing of the cell membrane.
Electron microscopy reveals distinct cleavage furrows, exemplified by studies of fertilized frog eggs.

Monitoring Cell Division

The cell cycle of eukaryotic cells, such as mouse fibroblasts, illustrates stages from interphase to cytokinesis.
Mitosis progresses through phases including anaphase and culminates in the visible cleavage furrow formed during cytokinesis.

Actin and Myosin Role

Fluorescent microscopy can label actin with red fluorophores and myosin with green to visualize the contractile ring.
Myosin predominantly associates with the cleavage furrow, while actin is present at both the cleavage furrow and cell cortex.

Cytokinesis Regulation

RhoA, a small GTPase, is crucial for regulating cytokinesis. Activated by RhoGEF, RhoA facilitates actin filament formation and myosin II activation via phosphorylation.
This signaling cascade ensures proper contractile ring function during cytokinesis.

Mitotic Spindle and Division Plane

The plane of animal cell division is influenced by microtubules of the mitotic spindle.
Models of contractile ring formation include contributions from astral microtubules and interpolar microtubules that localize regulatory proteins.

Distribution of Organelles

Membrane-enclosed organelles may be distributed symmetrically or asymmetrically during cytokinesis.
Asymmetric distribution occurs in organisms like C. elegans, guiding differentiation during embryogenesis.

Mitosis Without Cytokinesis

In certain instances, mitosis can happen without cytokinesis, creating multinucleated cells, as observed during Drosophila embryonic development.

Exit from Mitosis

To exit mitosis and enter G1 phase, M-cyclin must be degraded and M-CDK inactivated through the Cdc20-APC complex, enabling proper transition and regulation of the cell cycle.

Role of Mitogens

Mitogens stimulate cell division by relieving negative controls that inhibit cell cycle progression.
Growth factors promote cell size increase, while survival factors prevent apoptosis.

Examples of Mitogens

Platelet-derived growth factor (PDGF) stimulates cell growth and proliferation, indicating the multifaceted roles of certain growth factors.

Cell-cycle Regulation by Mitogens

Mitogen signals activate cyclin/CDK complexes that regulate cell cycle progression, involving transcription factors that promote cyclin synthesis and inhibit CKI proteins.

DNA Damage Response

DNA damage serves as a checkpoint that halts cell division, detected by specific sensor molecules that trigger a response to maintain genomic integrity.### DNA Damage Detection and Response
Sensors identify various types of DNA damage, including replication fork alterations and double-strand breaks.
Detection of DNA damage activates signal transduction pathways, regulating cell cycle blockers until repair is achieved.
Major checkpoints in the cell cycle halt progression if DNA damage is sensed.
Ineffective repair mechanisms lead to apoptosis instead of the proliferation of cells with damaged DNA.

Major Regulators of DNA Damage Response

Key regulators include DNA-PK, ATM, and ATR; all possess kinase domains crucial for phosphorylating target proteins.
These enzymes are central to DNA damage response, regulating repair, cell cycle control, and replication.
ATM was discovered in 1996, highlighting ongoing research in DNA damage mechanisms.

Different pathways are activated for double-strand (DSB) and single-strand (ssDNA) DNA damage.
Specific sensors detect and activate distinct pathways leading to cellular responses.
Effectors like p53 play critical roles in coordinating these cellular responses.

Role of p53 in DNA Damage Response

p53 is a major regulator of DNA damage response, inhibiting cell cycle progression and activating DNA repair mechanisms.
Mutations in p53 can prevent effective DNA damage protection, leading to tumor formation.
p53 activates proteins involved in DNA repair and regulatory checkpoints, blocking cyclin CDK complexes through p21 activation.

Cell Cycle Arrest Mechanisms

DNA damage causes cell cycle arrest in G1 phase through phosphorylation of p53, resulting in CKI p21 synthesis.
CKI p21 inhibits cyclin CDK complexes, preventing their activation and halting the cell cycle.
Various factors, including UV rays and radiation, can induce DNA damage.

Tumor Suppressors and Oncogenes

Myc, a transcription factor, influences cell proliferation and is regulated to prevent excessive growth.
Arf acts as a tumor suppressor, countering excessive Myc levels by activating p53.
Activating mutations in oncogenes like Ras and Myc lead to unregulated proliferation if protective mechanisms fail.

Coordination of Cell Proliferation and Growth

Cell growth accompanies proliferation; without it, daughter cells would decrease in size after each division.
Growth factors stimulate both growth and division by enhancing ribosome and protein production and expanding the cell membrane.
Distinction among factors: some promote growth, some promote division, and others do both.

Conclusion

Understanding the complex pathways involved in DNA damage response and cell cycle control reveals mechanisms critical to preventing abnormal cell proliferation and tumorigenesis.

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Description

Explore the critical phase of cytokinesis in the cell cycle. This lecture outlines the mechanisms involved in the cleavage plane determination and organelle distribution during cell division. Gain insights into the role of the contractile ring made of actin and myosin filaments in animal cells.