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Questions and Answers
What is the primary purpose of the cell cycle?
What is the primary purpose of the cell cycle?
What characterizes cells in the G1 phase of the cell cycle?
What characterizes cells in the G1 phase of the cell cycle?
Which phase of the cell cycle is known for quality control?
Which phase of the cell cycle is known for quality control?
What is the significance of using bromodeoxyuridine (BrdU) in cell cycle studies?
What is the significance of using bromodeoxyuridine (BrdU) in cell cycle studies?
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What does the MTT assay primarily measure?
What does the MTT assay primarily measure?
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In which phase are round cells typically observed under a microscope?
In which phase are round cells typically observed under a microscope?
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How does the cell cycle contribute to tissue health?
How does the cell cycle contribute to tissue health?
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What is a common feature of model systems like yeast and Xenopus laevis in cell cycle studies?
What is a common feature of model systems like yeast and Xenopus laevis in cell cycle studies?
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What does the MTT assay measure in terms of cell viability?
What does the MTT assay measure in terms of cell viability?
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During which phase does the DNA content double?
During which phase does the DNA content double?
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Which flow cytometry feature is essential for measuring DNA content?
Which flow cytometry feature is essential for measuring DNA content?
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What does a flow cytometer categorize cells into based on DNA content?
What does a flow cytometer categorize cells into based on DNA content?
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What is a characteristic feature of G1 phase cells?
What is a characteristic feature of G1 phase cells?
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How does the cell-cycle control system operate?
How does the cell-cycle control system operate?
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Which phase indicates that G1 is longer than G2 and M phases?
Which phase indicates that G1 is longer than G2 and M phases?
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What is a potential consequence of incomplete chromosome condensation?
What is a potential consequence of incomplete chromosome condensation?
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What is the role of p27kip1 in relation to CDK and cyclin complexes?
What is the role of p27kip1 in relation to CDK and cyclin complexes?
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How does phosphorylation affect the activity of a cyclin-Cdk complex?
How does phosphorylation affect the activity of a cyclin-Cdk complex?
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What mechanism allows cells to pass the restriction point and enter the S phase?
What mechanism allows cells to pass the restriction point and enter the S phase?
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Which of the following describes the role of the E2F family?
Which of the following describes the role of the E2F family?
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Which statement about the Retinoblastoma (Rb) pathway is accurate?
Which statement about the Retinoblastoma (Rb) pathway is accurate?
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What happens to Rb during the cell cycle at the G1/S transition?
What happens to Rb during the cell cycle at the G1/S transition?
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What is a common misconception about phosphorylation in relation to cyclin-CDK complexes?
What is a common misconception about phosphorylation in relation to cyclin-CDK complexes?
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What is the effect of Cdc25 on CDK activity?
What is the effect of Cdc25 on CDK activity?
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What triggers the metaphase-to-anaphase transition during cell division?
What triggers the metaphase-to-anaphase transition during cell division?
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Which complex is primarily responsible for the regulated protein destruction that occurs in mitosis?
Which complex is primarily responsible for the regulated protein destruction that occurs in mitosis?
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What role does Cdc25 play in the regulation of the CyclinB-Cdc2 complex?
What role does Cdc25 play in the regulation of the CyclinB-Cdc2 complex?
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Which of the following proteins does APC/C target for destruction to facilitate the exit from mitosis?
Which of the following proteins does APC/C target for destruction to facilitate the exit from mitosis?
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How does APC/C contribute to the separation of sister chromatids during mitosis?
How does APC/C contribute to the separation of sister chromatids during mitosis?
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What is the result of the destruction of Securin during cell division?
What is the result of the destruction of Securin during cell division?
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What is the role of Cdc20 in the function of the APC/C during mitosis?
What is the role of Cdc20 in the function of the APC/C during mitosis?
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Which of the following is NOT a function of the APC/C during cell division?
Which of the following is NOT a function of the APC/C during cell division?
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What role do F-box proteins play in the activity of the SCF ubiquitin ligase?
What role do F-box proteins play in the activity of the SCF ubiquitin ligase?
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What is the primary function of the SCF ubiquitin ligase during the cell cycle?
What is the primary function of the SCF ubiquitin ligase during the cell cycle?
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Which phase initiates the formation of the pre-replicative complex (preRC)?
Which phase initiates the formation of the pre-replicative complex (preRC)?
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What triggers the activation of DNA helicases during DNA replication initiation?
What triggers the activation of DNA helicases during DNA replication initiation?
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During which phase is the destruction of G1/S-cyclins primarily controlled?
During which phase is the destruction of G1/S-cyclins primarily controlled?
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How does the cell cycle control system respond to various extracellular cues?
How does the cell cycle control system respond to various extracellular cues?
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What is a characteristic of DNA replication in eukaryotic cells?
What is a characteristic of DNA replication in eukaryotic cells?
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In which cell types might inhibitory mechanisms of the cell cycle control system be absent?
In which cell types might inhibitory mechanisms of the cell cycle control system be absent?
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Study Notes
Cell Cycle: Duplication and Division
- The cell cycle is a sequence of events that includes duplication and division of cells
- Cell division is essential for growth and replacement of cells
- Millions of cells are produced every second to compensate for loss
- Certain events in the cell cycle are universal across organisms
- Proteins that control and carry out cell cycle events are highly conserved across evolution and can function in different species
Eukaryotic Cell Cycle Phases
- The eukaryotic cell cycle is divided into four phases: G1, S, G2, and M
- The Gap phases (G1 and G2) are not simple time delays but are essential for quality control and allow for cell growth and preparation for the next phase
- The S phase is dedicated to DNA replication and lasts approximately 10-12 hours
- The M phase (mitosis) is the shortest phase, lasting less than an hour
Studying Cell Cycle Progression
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Microscopical methods:
- Rounded up cells in a culture indicates mitosis
- Cytokinesis can be observed under a microscope
- Budding in fungi can be observed under a microscope, with bud size varying depending on the cell cycle stage
- Unbudded cells are in G1
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BrDU incorporation:
- Supplying cells with BrdU (bromodeoxyuridine) allows for visualization of newly synthesized DNA during the S phase
- BrdU is incorporated into newly synthesized DNA
- Anti-BrdU antibodies can be used to detect and visualize BrdU in cells under a microscope
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MTT Assay:
- Measures cellular metabolic activity as an indicator of cell viability, proliferation, and cytotoxicity
- This colorimetric assay relies on the reduction of a yellow tetrazolium salt (MTT) to purple formazan crystals by metabolically active cells
- Viable cells contain NAD(P)H-dependent oxidoreductase enzymes that reduce MTT to formazan
- The intensity of the purple color is directly proportional to the number of viable cells
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Flow Cytometry:
- Allows measurement of DNA content in cells
- Cells are stained with fluorescent DNA-binding dyes
- A flow cytometer analyzes the fluorescence intensity of individual cells, determining DNA content
- By measuring DNA content in synchronized cell populations, the lengths of G1, S, and G2 + M phases can be determined
- Cells in G1 have unreplicated DNA, cells in S phase have intermediate DNA content, and cells in G2 or M phase have doubled DNA content
Cell Cycle Control System
- The cell cycle control system is a series of biochemical switches that initiate specific cell cycle events
- The switches are typically binary (on/off) and trigger irreversible events
- Proliferative and anti-proliferative signals influence the activity of cyclins
Negative Regulators of the Cell Cycle
- Negative regulators of the cell cycle include CDK inhibitors (CKIs)
- CKIs prevent the activation of CDKs (cyclin-dependent kinases)
Switching between Inhibitor and Substrate Functions
- p27kip1 is a CDK inhibitor that can either inhibit or activate CDKs depending on its phosphorylation state
- When p27kip1 is phosphorylated, it is inactive and the CDK is active
- When p27kip1 is not phosphorylated, it inhibits the CDK
Phosphorylation and Activation
- Phosphorylation of key amino acids within a kinase active site can either activate or inhibit its activity
- Wee1 kinase inhibits CDK activity by phosphorylating specific amino acids
- Cdc25 phosphatase activates CDK activity by dephosphorylating those same amino acids
Retinoblastoma Pathway
- The Retinoblastoma (Rb) protein is a key regulator of the G1/S transition
- When Rb is hypophosphorylated, it binds to E2F transcription factors, inhibiting their activity and preventing S phase entry
- In late G1, phosphorylation of Rb by CDKs releases E2F, allowing for S phase entry
- Rb acts as a tumor suppressor, preventing uncontrolled cell growth
G2/M Checkpoint
- Cyclin B-cdc2 activity is specific to the G2/M checkpoint
- Cdc2 activity is regulated by phosphorylation/dephosphorylation of its activators and inhibitors
- Cdc25 activates Cyclin B-Cdc2 by dephosphorylating inhibitory sites
- Wee1 and Myt1 inhibit Cyclin B-Cdc2 by phosphorylating inhibitory sites
Other Regulatory Mechanisms
- Regulated protein destruction by ubiquitination plays a significant role in cell cycle control
- Ubiquitination targets proteins for degradation by proteasomes
Metaphase-to-Anaphase Transition
- Proteolysis triggers the metaphase-to-anaphase transition
- The Anaphase Promoting Complex/Cyclosome (APC/C) is a ubiquitin ligase that degrades specific proteins during mitosis
- APC/C targets securin and M-cyclins
- Securin degradation activates separase, a protease that separates sister chromatids
- M-cyclin degradation inactivates most CDKs, allowing the cell to exit mitosis
SCF Ubiquitin Ligase
- SCF ubiquitin ligase is another ubiquitin ligase involved in cell cycle control
- SCF targets specific CDK inhibitors in late G1, ensuring proper degradation of G1/S-cyclins in early S phase
Interplay between Phosphorylation and Ubiquitination
- The activity of SCF depends on F-box proteins, which bind to specific phosphorylated targets
- Phosphorylation of a target protein allows recognition by a specific F-box subunit, leading to ubiquitination and degradation
S-Phase: DNA Replication
- DNA replication begins at origins of replication scattered throughout chromosomes
- The initiation of DNA replication requires the activation of DNA helicase, which unwinds the DNA double helix
- DNA replication occurs only once per cell cycle and is tightly regulated
Initiation of DNA Replication
- Licensing: During late mitosis and early G1, inactive DNA helicases are loaded onto replication origins, forming a pre-replicative complex (preRC)
- Activation: In S phase, DDK (Dbf4-dependent kinase) activates the DNA helicases, leading to DNA unwinding and initiation of DNA synthesis
- Once a replication origin has been fired, it cannot be reused until a new preRC is assembled at the end of mitosis.
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Description
Explore the essential stages of the eukaryotic cell cycle, including duplication and division processes. Understand the significance of each phase and how proteins are conserved across species to regulate these events. This quiz will test your knowledge of cell cycle progression and its implications in biology.
