Week 12 MTC Notes
The Cell Cycle & Its Regulation
Ie. Cell Division/Reproduction
The Cell Cycle:
- The cell doctrine: “where a cell arises there must be a previous cell.”
- For a cell to reproduce, it must duplicate its contents & divide cyclically. (the cell cycle)
Fundamental tasks of the cell cycle:
• Copy and pass on genetic info to next generation.
• Produce 2 genetically identical daughter cells
• DNA in each chromosome must be accurately replicated
• Replicated chromosomes must be equally distributed between daughter cells.
• Coordinate growth with division to maintain size & contents.
4 Phases:
Ø Interphase
- Cell continues to transcribe genes, synthesize proteins, & grow.
1. G1 Phase (gap)
§ Provide additional time for cell to grow & duplicate
Cytoplasmic organelles
2. S Phase (synthesis)
§ DNA Replication
§ Restriction/Commitment Point – no turning back.
3. G2 Phase (gap)
§ Provide additional time for cell to grow & duplicate
Cytoplasmic organelles
§ Replicated chromosomes condense
Ø Mitosis & Cytokinesis
4. M Phase (mitosis & cytokinesis)
§ Prophase – nuclear membrane breaks down
§ Prometaphase – replicated chromosomes are lined up
§ Metaphase – chromosomes separated into 2 sister chromatids
§ Anaphase – chromatids arrive at ends of cell, decondense & form separate nuclear
membranes
§ Telophase – Plasma membrane pinches cell into 2.
5. G0 = Some cells (neurons) that don’t divide, just stop at G0.
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, The Cell-Cycle Control System:
- To ensure these tasks are completed, there exists a complex network of regulatory proteins, known
as the cell-cycle control system.
o Ordered series of biochemical switches that control the main events of the cycle:
§ DNA replication
§ Segregation of duplicated chromosomes
o Responds to various signals from outside & inside the cell
o Critical to regulation of cell numbers
o Malfunction can lead to cancer. (Abnormal number of chromosomes/Mutated DNA)
- Can stop the cycle using molecular ‘brakes’ at 2 important checkpoints.
Molecular Brakes of the Cell-Cycle Control System:
• Govern cell-cycle by activating & inactivating proteins that regulate DNA replication, mitosis &
cytokinesis.
• Activation via phosphorylation à by protein kinases (consume ATP + phosphorylate substrate)
• Deactivation via dephosphorylation à by protein phosphatases
• To become fully active, protein kinases must first become partially active by binding to a cyclin
• Cyclins – have no enzymatic activity.
• Cyclin Dependant Kinases (Cdk’s) – partly activated by cyclin.
o Cyclin + Cyclin Dependant Kinase = Cyclin-Cdk
• Cdk-Activating kinase (CAK) – phosphorylates the partly activated cyclin-Cdk, fully activating it.
o Activation of Cyclin-Cdk complexes can trigger cell-cycle events. (Entry into S-phase or M-
phase.)
§ ‘M’-Cyclin + Cdk = M-Cdk à drives cells into M-phase
§ ‘S’-Cyclin + Cdk = S-Cdk à drives cells into S-phase
Cdk Inactivation: Once ‘?’-Phase is complete, ubiquitin-dependant proteolysis destroys cyclins.
www.MedStudentNotes.com
The Cell Cycle & Its Regulation
Ie. Cell Division/Reproduction
The Cell Cycle:
- The cell doctrine: “where a cell arises there must be a previous cell.”
- For a cell to reproduce, it must duplicate its contents & divide cyclically. (the cell cycle)
Fundamental tasks of the cell cycle:
• Copy and pass on genetic info to next generation.
• Produce 2 genetically identical daughter cells
• DNA in each chromosome must be accurately replicated
• Replicated chromosomes must be equally distributed between daughter cells.
• Coordinate growth with division to maintain size & contents.
4 Phases:
Ø Interphase
- Cell continues to transcribe genes, synthesize proteins, & grow.
1. G1 Phase (gap)
§ Provide additional time for cell to grow & duplicate
Cytoplasmic organelles
2. S Phase (synthesis)
§ DNA Replication
§ Restriction/Commitment Point – no turning back.
3. G2 Phase (gap)
§ Provide additional time for cell to grow & duplicate
Cytoplasmic organelles
§ Replicated chromosomes condense
Ø Mitosis & Cytokinesis
4. M Phase (mitosis & cytokinesis)
§ Prophase – nuclear membrane breaks down
§ Prometaphase – replicated chromosomes are lined up
§ Metaphase – chromosomes separated into 2 sister chromatids
§ Anaphase – chromatids arrive at ends of cell, decondense & form separate nuclear
membranes
§ Telophase – Plasma membrane pinches cell into 2.
5. G0 = Some cells (neurons) that don’t divide, just stop at G0.
www.MedStudentNotes.com
, The Cell-Cycle Control System:
- To ensure these tasks are completed, there exists a complex network of regulatory proteins, known
as the cell-cycle control system.
o Ordered series of biochemical switches that control the main events of the cycle:
§ DNA replication
§ Segregation of duplicated chromosomes
o Responds to various signals from outside & inside the cell
o Critical to regulation of cell numbers
o Malfunction can lead to cancer. (Abnormal number of chromosomes/Mutated DNA)
- Can stop the cycle using molecular ‘brakes’ at 2 important checkpoints.
Molecular Brakes of the Cell-Cycle Control System:
• Govern cell-cycle by activating & inactivating proteins that regulate DNA replication, mitosis &
cytokinesis.
• Activation via phosphorylation à by protein kinases (consume ATP + phosphorylate substrate)
• Deactivation via dephosphorylation à by protein phosphatases
• To become fully active, protein kinases must first become partially active by binding to a cyclin
• Cyclins – have no enzymatic activity.
• Cyclin Dependant Kinases (Cdk’s) – partly activated by cyclin.
o Cyclin + Cyclin Dependant Kinase = Cyclin-Cdk
• Cdk-Activating kinase (CAK) – phosphorylates the partly activated cyclin-Cdk, fully activating it.
o Activation of Cyclin-Cdk complexes can trigger cell-cycle events. (Entry into S-phase or M-
phase.)
§ ‘M’-Cyclin + Cdk = M-Cdk à drives cells into M-phase
§ ‘S’-Cyclin + Cdk = S-Cdk à drives cells into S-phase
Cdk Inactivation: Once ‘?’-Phase is complete, ubiquitin-dependant proteolysis destroys cyclins.
www.MedStudentNotes.com
