Cell Biology Quiz on Growth Factors and Division
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Questions and Answers

What initiates the cellular response to platelet-derived growth factor (PDGF) in fibroblasts?

  • The release of nutrients from surrounding cells
  • The deformation of the cell membrane
  • The binding of PDGF to plasma membrane receptors (correct)
  • The binding of PDGF to intracellular proteins

Which of the following statements is true regarding normal mammalian cells?

  • They stop dividing when they come into contact with neighboring cells. (correct)
  • They rely solely on internal factors for division control.
  • They divide constantly, independent of anchorage.
  • They continue to divide indefinitely regardless of density.

How do cancer cells differ from normal cells in terms of division?

  • Cancer cells exhibit density-dependent inhibition.
  • Cancer cells require a substratum for attachment.
  • Cancer cells undergo apoptosis to control growth.
  • Cancer cells do not require external growth factors for division. (correct)

What role do kinases play in the cell cycle?

<p>They transfer phosphate groups to activate target molecules. (B)</p> Signup and view all the answers

What phenomenon related to cell division is still considered poorly understood?

<p>The mechanism by which contact with neighboring cells inhibits growth (D)</p> Signup and view all the answers

Which process is associated with the cyclical production and degradation of proteins that regulate the cell cycle?

<p>Cyclin synthesis (D)</p> Signup and view all the answers

Which mechanism prevents cells from growing too close together?

<p>Density-dependent inhibition (B)</p> Signup and view all the answers

What is a distinct characteristic of cancer cells in relation to the extracellular matrix?

<p>They do not require attachment to the extracellular matrix for proliferation. (B)</p> Signup and view all the answers

What happens to a cell if it does not receive a go-ahead signal at the G1 checkpoint?

<p>The cell enters a nondividing state called G0. (B)</p> Signup and view all the answers

Which of the following is an internal factor that can influence cell division?

<p>Cell size. (A)</p> Signup and view all the answers

What role do growth factors play in cell division?

<p>They serve as chemical signals to promote cell growth. (B)</p> Signup and view all the answers

At which checkpoint is the decision for a cell to continue in the cell cycle most critical?

<p>G1 checkpoint. (B)</p> Signup and view all the answers

What typically happens if a cell experiences DNA damage before the G1 checkpoint?

<p>The cell receives a signal to halt the cell cycle. (A)</p> Signup and view all the answers

Which of the following best describes external factors influencing cell division?

<p>Signaling molecules from other cells that influence growth. (D)</p> Signup and view all the answers

How do cells generally respond to growth factors?

<p>They respond to a combination of several growth factors. (D)</p> Signup and view all the answers

What is the significance of the G1 checkpoint in the cell cycle?

<p>It serves as a point to assess overall cell health and readiness to divide. (C)</p> Signup and view all the answers

What is the relationship between cyclin levels and the cell cycle phases?

<p>Cyclin levels are low during interphase and peak during mitosis. (B)</p> Signup and view all the answers

What role does Cdk-activating kinase (CAK) play in the activation of the cyclin-Cdk complex?

<p>It partially activates the complex by phosphorylation. (D)</p> Signup and view all the answers

What is the mechanism of cyclical proteolysis in the regulation of cyclins?

<p>Cyclins are tagged for destruction through ubiquitylation. (D)</p> Signup and view all the answers

What is the initial state of a cyclin-Cdk complex upon formation?

<p>It is inactive and needs further modification. (D)</p> Signup and view all the answers

What is the role of protein phosphatase in the activation of Cdk?

<p>It removes inhibitory phosphate groups from Cdk. (A)</p> Signup and view all the answers

What happens to cyclins from the previous phase before moving to the next phase of the cell cycle?

<p>They need to be removed from the system. (D)</p> Signup and view all the answers

What triggers the start of M phase in the cell cycle?

<p>The activation of the cyclin-Cdk complex. (A)</p> Signup and view all the answers

Which statement accurately describes the activity levels of Cdks during the cell cycle?

<p>Cdk activity is low during interphase and increases during mitosis. (B)</p> Signup and view all the answers

What role do Cdk inhibitor proteins play in the cell cycle?

<p>They inhibit the cell cycle progression of cyclin/Cdk complexes. (D)</p> Signup and view all the answers

What happens to the p53 protein in response to DNA damage?

<p>It is activated and accumulates to halt cell cycle progression. (D)</p> Signup and view all the answers

What is the purpose of the G1 checkpoint in the cell cycle?

<p>To allow the cell to correct DNA errors or enter a quiescent state. (D)</p> Signup and view all the answers

In the context of the cell cycle, what is the significance of the p21 protein?

<p>It binds to and inactivates G1/S-Cdk and S-Cdk. (A)</p> Signup and view all the answers

What does the cell cycle control system allow cells to do in response to unfavorable conditions?

<p>Pause cell cycle progression until conditions improve. (B)</p> Signup and view all the answers

How do cells in the G0 phase behave in terms of re-entering the cell cycle?

<p>They are capable of re-entering the cycle only when conditions improve. (B), They remain permanently differentiated and exit the cycle. (D)</p> Signup and view all the answers

What triggers the activation of specific protein kinases during the cell cycle?

<p>The presence of DNA damage. (C)</p> Signup and view all the answers

Which of the following statements accurately describes the transition between different phases of the cell cycle?

<p>The transition from G1 to S requires completion of DNA repair. (A)</p> Signup and view all the answers

What role does S-Cdk play in the initiation of DNA replication?

<p>It phosphorylates Mcm proteins to activate them. (B)</p> Signup and view all the answers

What is the function of cohesins during DNA replication?

<p>They hold sister chromatids together. (A)</p> Signup and view all the answers

How does M-Cdk contribute to mitosis?

<p>It assembles the mitotic spindle for chromosome segregation. (C)</p> Signup and view all the answers

What is the consequence of Cdc6 degradation during DNA replication?

<p>It restricts replication to one cycle. (B)</p> Signup and view all the answers

What triggers the condensation of replicated chromosomes in mitosis?

<p>Activation of M-Cdk. (B)</p> Signup and view all the answers

Which of the following is true regarding origin recognition complex (ORC)?

<p>It binds to the replication origin throughout the cell cycle. (B)</p> Signup and view all the answers

What happens to the cohesion between sister chromatids during late mitosis?

<p>It is completely broken to allow separation. (D)</p> Signup and view all the answers

When does the synthesis of M cyclin begin in the cell cycle?

<p>Immediately after S phase. (A)</p> Signup and view all the answers

What triggers the sudden activation of M-Cdk at the end of G2 phase?

<p>Dephosphorylation by Cdc25 (A)</p> Signup and view all the answers

What role does the Cdk-activating kinase (Cak) play in the activation of M-Cdk?

<p>Phosphorylates Cdk at an activatory site (C)</p> Signup and view all the answers

How does activated M-Cdk contribute to its own further activation?

<p>By activating Cdc25, a phosphatase (B)</p> Signup and view all the answers

What is the function of the spindle-attachment checkpoint in cell division?

<p>To monitor kinetochore attachment to spindle (D)</p> Signup and view all the answers

Which protein is recruited to unattached kinetochores as part of the spindle-attachment checkpoint?

<p>Mad2 (C)</p> Signup and view all the answers

What regulatory role does the SCF complex play in the cell cycle?

<p>Targets proteins for degradation (C)</p> Signup and view all the answers

What is the primary consequence of the positive feedback loop created by activated M-Cdk?

<p>Explosive increase in M-Cdk activity (D)</p> Signup and view all the answers

What happens to the cell-cycle control system if kinetochores are improperly attached to the spindle?

<p>It blocks Cdc20-APC activation (D)</p> Signup and view all the answers

Flashcards

G1 Checkpoint

A critical point in the cell cycle where a cell checks for conditions needed to proceed with division. It's like a checkpoint before starting a long journey.

Restriction Point

The point in the G1 checkpoint where a cell commits to the complete cell cycle. This is like a one-way street, once the cell passes this point, it will divide.

Go-Ahead Signal

A signal received by a cell at the G1 checkpoint that tells it to proceed with the cell cycle. It's like a green light for division.

G0 Phase

A non-dividing state that cells enter if they don't receive the go-ahead signal at the G1 checkpoint. It's like a 'pause' button for the cell cycle.

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Internal Signals

Factors from inside the cell that influence its decision to divide. They're like messages within a cell.

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External Signals

Factors outside the cell that tell it to divide, like chemical signals from other cells. Think of them as communication from other cells.

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Growth Factor

A protein signal released by other cells that encourages cell division. It's like a chemical message saying 'divide!'.

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Cell Size

A crucial factor in the G1 checkpoint. Cells must reach a certain size before they can divide. It's like having enough ingredients to make a recipe!

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Density-Dependent Inhibition

Cells stop dividing when they reach a certain density, due to contact with their neighbors.

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Anchorage-Dependent Inhibition

Cells need to be attached to a surface (like a substratum) to divide.

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Cancer Cells

Cells that ignore density-dependent inhibition and anchorage dependence. They divide uncontrollably.

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Kinase

An enzyme that adds a phosphate group to a target molecule. Kinases can activate or deactivate other molecules.

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Cyclin

A protein that regulates the activity of kinases. Cyclins are rapidly produced and degraded.

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Phosphorylation

The process of adding a phosphate group to a molecule, often changing its activity.

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What happens to a cell's growth when it is in contact with other cells?

When a cell comes into contact with other cells, it typically stops dividing due to density-dependent inhibition. This prevents uncontrolled growth and helps maintain tissue structure.

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Pre-RC

A complex of proteins that assembles at the origin of replication, initiating DNA replication. It acts as a starting point for DNA copying.

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ORC

A protein complex that binds to the origin of replication throughout the cell cycle, marking the spot where DNA duplication begins.

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Mcm Proteins

A type of protein recruited by ORC in early G1, forming a DNA helicase that unwinds DNA during replication.

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S-CDK

A kinase that activates DNA replication by phosphorylating ORC and MCM, triggering the start of the S phase.

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Cohesin

A protein complex that holds sister chromatids together after replication, ensuring proper chromosome segregation.

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M-CDK

A kinase that triggers chromosome condensation, mitotic spindle assembly, and entry into mitosis.

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Cyclin Levels in Cell Cycle

Cyclin levels rise and fall cyclically throughout the cell cycle. They are low during interphase but gradually increase to a peak level during mitosis. This increase in cyclin concentration drives the cell into mitosis.

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Cdk Activity in Cell Cycle

Cdk activity mirrors cyclin levels, being low in interphase and increasing in mitosis. The activation of Cdk is essential for the progression of the cell cycle stages.

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Cyclin-Cdk Complex

The cyclin-Cdk complex formation is crucial for cell cycle progression. Cyclin binds to Cdk, partially activating it. Full activation requires further phosphorylation by CAK.

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Post-translational Modification of Cyclin-Cdk Complex

The cyclin-Cdk complex undergoes phosphorylation for its full activation. Phosphorylation at specific sites promotes or inhibits its activity.

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Regulation of Cyclin Abundance

Cyclin levels are regulated by protein degradation. They are ubiquitinated and targeted for destruction through the proteasome pathway.

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Inactivation of Cyclin-Cdk Complex

Cyclin degradation ultimately inactivates the cyclin-Cdk complex. This allows the cell to transition to the next phase of the cell cycle.

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G2 Checkpoint Control

The G2 checkpoint ensures that the cell is ready for mitosis. It checks for DNA replication completion and damage. If the cell is healthy, MPF (Maturation Promoting Factor) triggers the transition into mitosis.

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Maturation Promoting Factor (MPF)

MPF is a cyclin-Cdk complex active in G2/M transition. It phosphorylates a variety of proteins, initiating mitosis and the cell cycle progression.

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Cell Cycle Control System

A complex network of proteins that regulates the timing and order of events in the cell cycle. It ensures a smooth progression of the stages, like a conductor leading an orchestra.

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Cdk Inhibitors

Proteins that halt the cell cycle by binding to Cyclin-dependent kinases (Cdks), preventing them from activating cell division. It's like pressing the pause button on a cell's division.

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What happens when DNA damage occurs at G1?

The cell cycle arrests at G1 when DNA is damaged. The p53 protein activates, halting its normal degradation and leading to the activation of p21, which inhibits the G1/S-Cdk and S-Cdk, thus stopping the cell cycle.

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Why does a cell pause in G1?

The G1 checkpoint allows the cell to pause and assess conditions before committing to another cell cycle. This pause ensures the cell only divides when it has enough resources and the environment is favorable.

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What are the three options a cell has at the G1 checkpoint?

At G1, a cell can decide to proceed to the S phase and replicate its DNA, pause temporarily until conditions are favorable, or withdraw from the cell cycle and enter G0.

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Can cells in G0 re-enter the cell cycle?

Yes, some cells in G0 can re-enter the cell cycle when conditions improve. However, many cell types permanently withdraw from the cell cycle when they differentiate and remain in G0 for their lifespan.

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M-Cdk Activation

The process of activating M-Cdk, a key protein kinase that drives the cell into mitosis, by removing inhibitory phosphates and adding activating phosphates.

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Cdc25

A protein phosphatase that activates M-Cdk by removing inhibitory phosphate groups from its active site.

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Wee1

A protein kinase that inhibits M-Cdk activity by adding phosphate groups at inhibitory sites.

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Positive Feedback Loop

A cycle where activated M-Cdk activates more Cdc25, which then activates more M-Cdk, creating a rapid increase in M-Cdk activity.

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SCF Complex

A protein complex that targets G1/S cyclins and some CKIs for degradation, regulating the cell cycle.

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Spindle Attachment Checkpoint

A checkpoint that ensures all chromosomes are properly attached to the mitotic spindle before sister chromatids are separated.

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Kinetochore

A specialized region on a chromosome where microtubules attach during cell division.

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Study Notes

Cell Cycle Overview

  • The cell cycle is a series of events that results in cell division, essential for life's continuity.

  • It encompasses cell division and cell cycle.

  • Key functions include reproduction, growth, and tissue renewal.

  • A cell precisely duplicates its contents (DNA) and divides into two identical daughter cells.

  • The cell cycle is a cyclic, orderly process, crucial for regulated cell growth.

Cell Cycle Learning Outcomes

  • Understand the purposes of cell division and the cell cycle.
  • Describe the organization of a eukaryotic genome.
  • Explain cytokinesis and its significance.
  • List and describe phases within the cell cycle.

Phases of the Cell Cycle

  • The cell cycle consists of two major phases:
    • Interphase: A preparatory phase. This includes the G1, S, and G2 phases
    • Mitotic (M) phase: The active division phase. This includes prophase, prometaphase, metaphase, anaphase, and telophase.

Interphase

  • A growth period between mitotic divisions
  • The cell increases in size, DNA replicates, and the centrosome duplicates
  • Consists of three phases:
    • G1 (Gap 1) phase: Cell grows and synthesizes proteins.
    • S (Synthesis) phase: DNA replication occurs.
    • G2 (Gap 2) phase: Cell continues to grow and prepares for mitosis.

Mitotic Phase (M Phase)

  • The actual cell division process
  • Divided into five distinct stages:
    • Prophase: Chromatin condenses into visible chromosomes, nuclear envelope breaks down, mitotic spindle formation begins.
    • Prometaphase: Nuclear envelope completely fragments, microtubules attach to kinetochores.
    • Metaphase: Chromosomes align at the metaphase plate, centrosomes are at opposite poles, kinetochores attach to microtubules.
    • Anaphase: Sister chromatids separate and move towards opposite poles, spindle fibers shorten.
    • Telophase: Chromosomes arrive at opposite poles, nuclear envelopes reform, chromosomes decondense, and the cytoplasm divides (cytokinesis).

Cytokinesis

  • The cytoplasm divides to form two separate daughter cells.

  • In animal cells, a cleavage furrow forms and pinches the cell in two.

  • In plant cells, a cell plate forms and divides the cell.

Cell Cycle Control System

  • A control system regulates the cell cycle, ensuring proper order.

  • Chemical signals regulate the progress of the cycle

    • checkpoints regulate progress through the cycle, ensuring proper DNA replication, ensuring each chromosome is correctly attached to the mitotic spindle before moving on to anaphase.

  • Internal and External Factors control the cycle.

    • External signals: growth factors, hormones, nutrients.
    • Internal signals: cell size, nutrients, DNA damage.

G1, G2 Checkpoints

  • Checkpoints occur at three critical points in the cell cycle:
    • G1 checkpoint: primary decision point; it assesses if the cell size and environment are favorable for division.
    • G2 checkpoint: checks for DNA damage and ensures that DNA replication is complete.
    • Spindle checkpoint: ensures that all chromosomes are attached to spindle fibers. This ensures correct chromosome segregation.

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

  • Cyclins and cyclin-dependent kinases (Cdks) are key to regulating the cell cycle.

  • Cdks are present in constant concentrations. They require cyclins to become active.

  • The concentration of cyclins changes throughout the cell cycle.

  • Cyclin concentration impacts Cdk activity, driving the cell through different phases.

  • Different cyclins activate Cdks to control distinct stages, driving the cell cycle forward.

Regulation of the Cyclin-Cdk Complex

  • Post-translational modifications, including phosphorylation and dephosphorylation, regulate cyclin-Cdk complexes directly.

  • Transcription regulation influences gene expression to control cyclin-Cdk activity.

  • Ubiquitin-mediated cyclical proteolysis regulates cyclins.

  • This method of regulating cyclin concentration helps regulate Cdk activity, timing the transition between cell cycle phases.

DNA Damage Arresting the Cell Cycle

  • DNA damage activates damage-specific proteins, p53 being an important one.

  • p53 halts cycle progression, initiating DNA repair mechanisms.

  • p53 can also trigger apoptosis if damage is beyond repair.

External Signals

  • External influences can either stop or promote cell division.

  • Growth factors are proteins that stimulate cell division.

  • Nutrients, hormones, and extracellular signals regulate the cell cycle.

  • Physical factors include density-dependent inhibition, halting cell division as density increases, and anchorage dependence, requiring cells to attach to a surface before dividing.

Importance of the Cell Cycle

  • The cell cycle is essential for growth, development, and maintenance of multicellular organisms.

  • Cellular division relies on the accuracy of DNA replication and segregation.

  • Uncontrolled cell division contributes to diseases.

Cell Cycle Checkpoints

  • Specific proteins and enzymes precisely regulate cell cycle processes.
  • Mechanisms are in place to prevent errors in DNA duplication, enabling correct inheritance of genetic material.
  • They prevent the cycle from continuing when conditions are not optimal.

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The Cell Cycle PDF

Description

Test your knowledge on the cellular response to growth factors like PDGF, and understand the differences between normal and cancer cell behaviors regarding division. Explore the critical checkpoints in the cell cycle and the role of external and internal factors in cell proliferation.

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