MODULE 37
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MODULE 37

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What happens during the G1 phase of the cell cycle?

  • Chromosomes condense.
  • DNA replication occurs.
  • The cell grows and prepares for DNA synthesis. (correct)
  • Sister chromatids separate.

    • During which phase does DNA replication occur?

  • M phase
  • G1 phase
  • G2 phase
  • S phase (correct)

    • What is indicated by a DNA content of 1 during flow cytometry?

  • The cell is in S phase.
  • The cell is undergoing cytokinesis.
  • The cell is in G0 phase.
  • The cell is in G1 phase. (correct)

    • Which of the following events occurs during prophase of mitosis?

    <p>Chromosomes condense.</p> Signup and view all the answers

    What does the Sub-G1 phase indicate?

    <p>The cell is undergoing apoptosis or necrosis.</p> Signup and view all the answers

    Which phase follows the metaphase during mitosis?

    <p>Anaphase</p> Signup and view all the answers

    What is the purpose of radioactive thymidine labeling?

    <p>To track DNA replication during the S phase.</p> Signup and view all the answers

    What is the final phase of mitosis before cytokinesis?

    <p>Telophase</p> Signup and view all the answers

    During which phase of interphase do centrosomes duplicate?

    <p>S phase</p> Signup and view all the answers

    What characterizes the G2 phase of the cell cycle?

    <p>The cell prepares for mitosis with growth and protein synthesis.</p> Signup and view all the answers

    What does the G2 phase represent in the cell cycle?

    <p>The gap of time separating the S phase from the M phase</p> Signup and view all the answers

    How is the length of the S phase determined?

    <p>By measuring the percentage of cells in the S phase</p> Signup and view all the answers

    What happens to a G2-phase cell when fused with an M-phase cell?

    <p>It is pushed into the M phase</p> Signup and view all the answers

    What is the role of Mitosis Promoting Factor (MPF) in cells?

    <p>To drive the cell into the M phase</p> Signup and view all the answers

    At what stage is the level of S phase-promoting factor (SPF) highest?

    <p>In late G1 phase</p> Signup and view all the answers

    What is the consequence of a G2-phase cell undergoing DNA replication?

    <p>It cannot undergo replication again</p> Signup and view all the answers

    Which kinases are involved in the G2 to M transition?

    <p>Wee1 and CAK</p> Signup and view all the answers

    What happens when a cell cycle phase is longer regarding cell detection?

    <p>There is a greater probability of detecting cells in that phase</p> Signup and view all the answers

    How is G1 phase duration determined in relation to total cell cycle time?

    <p>By the difference of the total cycle length minus S, M, and G2 phases</p> Signup and view all the answers

    Which statement is true regarding the effects of cell fusion experiments?

    <p>They prove that M-phase cells drive G2-phase cells into M phase</p> Signup and view all the answers

    What occurs when Wee1 phosphorylates cdc2 kinase at tyrosine 15 (Y15)?

    <p>Inactivation of the cdc2 kinase complex</p> Signup and view all the answers

    Which modification is considered activating for the cdc2 kinase?

    <p>Phosphorylation at T161</p> Signup and view all the answers

    What is the effect of simultaneous phosphorylation of cdc2 at Y15 and T161?

    <p>The Y15 phosphorylation becomes dominant and the complex is inactive</p> Signup and view all the answers

    What happens to a Wee1 mutant in temperature-sensitive yeast?

    <p>The cell grows small and enters the M phase too quickly</p> Signup and view all the answers

    What is the role of Cdc25 in the cell cycle?

    <p>To remove the phosphate group from Y15</p> Signup and view all the answers

    Which of the following represents the G1/S cyclins involved in regulating the transition from G1 to S phase?

    <p>Cyclin D / CDK6</p> Signup and view all the answers

    What characterizes G2/M cyclins in relation to the cell cycle?

    <p>They rapidly decrease in concentration at the end of the M phase</p> Signup and view all the answers

    What is the significance of cyclin-dependent kinases (Cdks) in normal mammalian cells?

    <p>They facilitate movement through the cell cycle phases</p> Signup and view all the answers

    How is flow cytometry used in the context of the cell cycle?

    <p>To determine cell cycle parameters</p> Signup and view all the answers

    What defines the relationship between S-phase and G2-phase cells during cell fusion experiments?

    <p>G2-phase cells prevent S-phase cells from reversing their state</p> Signup and view all the answers

    How can the duration of the G1 phase be calculated?

    <p>Total cell cycle length minus the sum of S, M, and G2 phases</p> Signup and view all the answers

    Which factor is responsible for pushing a cell from G1 phase into S phase?

    <p>S phase Promoting Factor (SPF)</p> Signup and view all the answers

    What does a higher percentage of cells in M phase indicate regarding the cell cycle?

    <p>The transition from G2 to M phase is occurring effectively.</p> Signup and view all the answers

    What is the role of M phase in the context of cell cycle length measurement?

    <p>It marks the completion of one full cell cycle.</p> Signup and view all the answers

    What happens to a G2-phase cell when fused with an S-phase cell?

    <p>The G2-phase cell remains in G2 phase.</p> Signup and view all the answers

    What characterizes the interphase of the cell cycle?

    <p>It is divided into G1, S, and G2 phases.</p> Signup and view all the answers

    Why is the complex of cdc2 kinase and mitotic cyclins important in cell cycle progression?

    <p>It is necessary for the transition from G2 to M phase.</p> Signup and view all the answers

    What effect does cell cycle phase length have on cell detection methodologies?

    <p>Longer phases increase the chance of detecting cells in that phase.</p> Signup and view all the answers

    What happens during the prometaphase of mitosis?

    <p>The mitotic spindle attaches to kinetochores.</p> Signup and view all the answers

    What identification does a DNA content of 2 during flow cytometry indicate?

    <p>The cell is in the G2/M phase.</p> Signup and view all the answers

    In yeast, when are Mitosis Promoting Factor (MPF) levels highest?

    <p>During late G2 phase</p> Signup and view all the answers

    What is a key characteristic of the relationship between cell phases during fusion experiments?

    <p>S-phase cells cannot push G2 cells backward in the cycle.</p> Signup and view all the answers

    Which phase of the cell cycle involves continued growth and final preparations for mitosis?

    <p>G2 Phase</p> Signup and view all the answers

    What does the Sub-G1 phase indicate in the context of flow cytometry?

    <p>Cells are undergoing apoptosis or necrosis.</p> Signup and view all the answers

    How is the percentage of cells in the S phase used to calculate its duration?

    <p>It is expressed as a ratio of total cell cycle time.</p> Signup and view all the answers

    How is the length of the G2 phase determined using radioactive thymidine labeling?

    <p>By detecting when the label appears in all mitotic cells.</p> Signup and view all the answers

    Which event occurs during the anaphase of mitosis?

    <p>Sister chromatids separate and move to opposite poles.</p> Signup and view all the answers

    What occurs during the S phase of the cell cycle?

    <p>DNA replication and centrosome duplication.</p> Signup and view all the answers

    What is true about the percentage of cells in mitosis at any given time?

    <p>A small percentage of cells are normally in mitosis.</p> Signup and view all the answers

    During which mitotic phase does the nuclear envelope re-form?

    <p>Telophase</p> Signup and view all the answers

    What is the effect of a Cdc25 mutant on the cell cycle?

    <p>The cell becomes stuck in G2 phase.</p> Signup and view all the answers

    What role does the phosphatase Cdc25 play in the cell cycle?

    <p>It removes the phosphate group from Y15 on cdc2.</p> Signup and view all the answers

    Which cyclin is associated with regulating the transition from G2 to M phase?

    <p>Cyclin B</p> Signup and view all the answers

    How do temperature-sensitive yeast mutants demonstrate cell cycle regulation?

    <p>They vary in cdc2 activity based on temperature.</p> Signup and view all the answers

    What happens when both Y15 and T161 are phosphorylated simultaneously on cdc2?

    <p>The kinase becomes inactive.</p> Signup and view all the answers

    What distinguishes G1/S cyclins from G2/M cyclins?

    <p>G2/M cyclins are essential for G2/M transition.</p> Signup and view all the answers

    What is the expected phenotype of a Wee1 mutant?

    <p>Cells that are small and enter M phase too quickly.</p> Signup and view all the answers

    What is the primary function of cyclin-dependent kinases (Cdks) in the mammalian cell cycle?

    <p>To facilitate transitions between cell cycle phases.</p> Signup and view all the answers

    What occurs in a CAK mutant compared to a Cdc25 mutant?

    <p>Cells cannot phosphorylate at T161.</p> Signup and view all the answers

    Study Notes

    The Cell Cycle

    • The cell cycle consists of two main phases: interphase and the M phase.
    • Interphase is further subdivided into G1, S, and G2 phases.
    • G1 phase involves cell growth and preparation for DNA synthesis.
    • S phase involves DNA replication and centrosome duplication.
    • G2 phase involves further growth and protein synthesis in preparation for mitosis.
    • The M phase encompasses mitosis and cytokinesis.
    • The G0 phase represents a resting state where cells are not actively proliferating.
    • Mitosis is divided into prophase, prometaphase, metaphase, anaphase, and telophase.
    • Cytokinesis follows telophase, completing cell division.

    Measuring the Cell Cycle

    • Flow cytometry measures DNA content per cell to determine cell cycle phases.
    • DNA content of 1 corresponds to the G1 phase.
    • DNA content increases from 1 to 2 during the S phase.
    • DNA content of 2 corresponds to the G2/M phase.
    • Sub-G1 with DNA content less than 1 indicates apoptosis or necrosis.
    • Radioactive thymidine labeling tracks DNA synthesis during the S phase.
    • The time it takes for the radioactive label to appear in all mitotic cells represents the length of the G2 phase.
    • The total length of the cell cycle is measured from one mitotic peak to the next.
    • The percentage of cells in a particular phase of the cell cycle is equal to the length of that phase.

    Key Regulators of the Cell Cycle

    • Both a mitosis promoting factor (MPF) and an S phase-promoting factor (SPF) exist.
    • Cell fusion experiments demonstrated that fusing an M-phase cell with another cell in a different cycle phase triggers the second cell to enter the M phase.
    • An S-phase cell can induce the S phase in G1-phase cells.
    • The cell cycle progresses forward, not backward.

    Yeast as a Model System

    • In yeast, MPF is a complex of cdc2 kinase and mitotic cyclins.
    • MPF levels peak in late G2 and drive the cell into mitosis.
    • SPF is the same kinase complexed with G1 cyclins.
    • SPF levels peak in late G1 and drive the cell into the S phase.
    • Complex formation is essential for cell cycle progression but not sufficient.
    • Two kinases, Wee1 and CAK, regulate the G2 to M transition in yeast.
    • Wee1 phosphorylates cdc2 at tyrosine 15 (Y15), inactivating the complex.
    • CAK phosphorylates cdc2 at threonine 161 (T161), activating the complex.
    • A phosphatase, Cdc25, removes the phosphorylation at Y15 enabling the active complex to promote mitosis.
    • Temperature-sensitive yeast mutants are used to study key cell cycle regulators.

    Mammalian Cell Cycle Regulation

    • Mammalian cells exhibit a similar cell cycle regulation to yeast but with a greater diversity of cyclin-dependent kinases (Cdks) and cyclins.
    • Cyclin A / CDK2 is active during the S phase.
    • Cyclin D / CDK4, Cyclin D / CDK6, and Cyclin E / CDK2 regulate the transition from G1 to S phase.
    • Cyclin B / CDK1 regulates the progression from G2 to M phase.
    • Cyclins accumulate during specific stages of the cell cycle and are destroyed upon exiting the corresponding phase.

    Fundamental Points

    • The cell cycle is comprised of the interphase and M phase.
    • Each phase is composed of distinct events.
    • Cell cycle parameters are determined through flow cytometry, radioactive labeling, and cell fusion experiments.

    Cell Cycle Overview

    • The cell cycle is comprised of two main phases: interphase and M phase.
    • Interphase is further divided into three stages: G1, S, and G2.
    • G1 phase: The cell grows and prepares for DNA synthesis.
    • S phase: DNA replication and centrosome duplication occurs.
    • G2 phase: Final preparations for mitosis, continued growth, and protein synthesis occur.
    • M phase consists of mitosis (nuclear division) and cytokinesis (cytoplasm division).
    • Mitosis is further divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.
    • Cells in a resting state, not actively proliferating, are considered to be in the G0 phase.

    Cell Cycle Measurement Techniques

    • Flow cytometry is used to measure the cell cycle by staining DNA and tracking the amount of DNA per cell.
    • Cells in G1 phase have a DNA content of 1, while cells in the S phase have DNA content ranging from 1 to 2 due to DNA replication.
    • Cells in the G2/M phase have a DNA content of 2.
    • Sub-G1 cells (with DNA content less than 1) indicate apoptosis or necrosis.
    • Radioactive thymidine labeling is another method to determine cell cycle phases and their durations.
    • Thymidine, a DNA base, is incorporated into DNA during the S phase.
    • The time it takes for the radioactive label to appear in all mitotic cells determines the duration of the G2 phase.
    • The time between two mitotic peaks represents the length of a complete cell cycle.

    Regulation of the Cell Cycle

    • MPF (Mitosis Promoting Factor) and SPF (S-phase promoting factor) play crucial roles in the cell cycle.
    • MPF, a complex of cdc2 kinase and mitotic cyclins, triggers the transition from G2 to M phase.
    • SPF, the same kinase complexed with G1 cyclins, promotes the transition from G1 to S phase.
    • Cell fusion experiments show that an M-phase cell can induce the M phase in cells at other stages but not vice versa.
    • This suggests that the cell cycle progresses in a forward direction, preventing repeated DNA replication.

    Yeast Cell Cycle Regulation

    • In yeast, MPF is essential for the G2 to M transition.
    • Wee1 kinase inactivates cdc2 kinase by phosphorylation at Y15, while CAK kinase activates it by phosphorylation at T161.
    • The dominant phosphorylation at Y15 keeps the MPF inactive, maintaining the cell in G2.
    • Cdc25 phosphatase removes the Y15 phosphorylation, activating MPF and triggering mitosis.
    • Temperature-sensitive yeast mutants have provided insights into these regulatory mechanisms.

    Mammalian Cell Cycle Regulation

    • Mammalian cell cycle is similar to yeast but with more complex regulatory mechanisms.
    • Different sets of cyclin-dependent kinases (Cdks) and cyclins control different cell cycle phases, preventing backward transitions.
    • G1/S cyclins (Cyclin A/CDK2, Cyclin D/CDK4/6, Cyclin E/CDK2) regulate the G1 to S phase transition.
    • G2/M cyclins (Cyclin B/CDK1) control the G2 to M phase transition.
    • G2/M cyclins accumulate during G2 and are degraded at the end of M phase.

    Key Takeaways

    • The cell cycle is regulated through a complex network of events and factors.
    • Different cell types and phases have varying durations.
    • Flow cytometry, radioactive labeling, and cell fusion experiments provide valuable insights into the cell cycle regulation.
    • The accurate understanding of the cell cycle is crucial for understanding normal cell division and identifying cancer cells.

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