Chapter 18 : Cell-Cycle Control and Cell death

Cell Cycle Dynamics

• Definition: Cells reproduce by duplicating their DNA and dividing in two, a process known as the cell cycle.

• Chromosome structure:

  • 1 chromatid per chromosome initially,

  • 2 chromatids per chromosome (sister chromatids) during duplication.

• Phases of the Eukaryotic Cell Cycle ( 4 phases) :

• Interphase (includes G1, S, and G2 phases)

• M phase (mitosis and cytokinesis)

Cell-Cycle Control

• Internal and external conditions are monitored during G1, G2, and M phases to ensure preparedness for passing through key steps of the cell cycle.

• Checkpoints: Mechanisms that arrest the cycle at critical control points.

• How Regulation was Discovered:

  • Mitosis observable in Xenopus frog eggs.

  • Injection of cytoplasm from M-phase cell induced mitosis.

  • Inject cytoplasm from interphase cell did not induce mitosis.

  • Fractionation of M-phase cytoplasm to identify factors inducing mitosis.

Cyclins and CDKs

Progression through the cell is controlled by cyclically activated protein kinases.

• Role of CDKs:

  • Cyclin-dependent protein kinases (CDKs) regulate key processes (DNA replication, mitosis, cytokinesis) through phosphorylation/dephosphorylation cycles.

  • CDKs are present throughout the cell cycle but are activated only at specific times by binding to cyclins.

  • Timing of CDK activation is determined by cyclin concentration, which varies throughout the cycle.

• Cyclin-CDK complexes: Unique complexes are responsible for progression through each phase of the cell cycle.

• M-Cyclin:

  • Concentration and activity vary.

  • Gradual increase (due to transcription and synthesis) followed by rapid degradation (targeted destruction).

• Activation of M-Cdk: Involves phosphorylation and is characterized by positive feedback amplification.

• Allow massive amplification of the signal to initiate M phase processes.

• Disappearance of M-cyclin : the ubiquitylating protein complex is also activated by cyclin-Cdk, so Cdk contributes to its own delayed inactivation = Negative feedback.

Checkpoint Control in G1 Phase

The cell cycle can be arrested at multiple checkpoints. Some cells can also completely withdraw from the cell cycle, and enter in G0= non-dividing cells ( neurons, skeletal muscle)

• G1 phase activities include:

  • Increased metabolic activity to enlarge the cell and duplicate compartments (e.g., mitochondria).

  • DNA is checked for integrity before duplication; issues in DNA quality will result in G1 arrest.

• Cdk inhibitors:

  • p53: Mutated in 50% of human cancers, plays a significant role in arresting G1 for DNA damage repair.

  • p21 ( cdk inhibitor protein): Works with p53 to inhibit CDKs.

• Mitogens ( induce mitosis): Stimulate production of cyclins that promote cell division.

• Rb = Retinoblastoma protein: A key regulator of the cell cycle that inhibits progression from the G1 to S phase by binding to transcription factors.

DNA Replication in S Phase

• Initiation Control: Replication initiated only once per cell cycle.

  • Key proteins involved: Cdc6, ORC, and DNA helicase phosphorylated by S-cyclin/CDK complex.

  • Phosphorylation of Cdc6 promotes degradation, preventing re-initiation.

M Phase Details ( M-Cdk : positive feedback)

• M Phase Description: Consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis).

• Mitosis Stages:

  • Prophase: Chromosomes condense and spindle apparatus begins forming.

  • Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

  • Metaphase: Chromosomes aligned at the spindle equator.

  • Anaphase: Sister chromatids separate and move to opposite poles.

  • Telophase: Chromosomes arrive at poles, nuclear envelope reassembles, marks end of mitosis.

• Cytokinesis: Cytoplasm divided by a contractile ring of actin and myosin filaments, resulting in two daughter cells.

Apoptosis and Cell Death

• Apoptosis: Process of programmed cell death for:

  • Development

  • Balancing cell number with division

  • Eliminating cells competing for limited resources.

• Morphological changes include: Blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and DNA fragmentation.

• Contrast with Necrosis: Apoptosis is non-traumatic and controlled; necrosis is a form of traumatic cell death that results from acute cellular injury.

Pathways of Apoptosis

• Intrinsic Pathway: Triggered by mitochondrial damage.

• Extrinsic Pathway: Induced by death signals from other cells.

• Both pathways involve cell death, and organization of the dead cell into vesicles that other cells can neatly phagocytose.

• Defects in both pathways are associated with tumors.

Caspase Cascade

• Caspases: Family of cysteine proteases that mediate apoptosis.

  • Initiator caspases activate executioner caspases, amplifying the apoptotic signal.

Role of Bcl-2 in Apoptosis

Suppression of apoptosis occurs via receptor-mediated cell survival signals to block procaspase activitation.

• Bcl-2: Anti-apoptotic protein that prevents apoptosis by blocking procaspase activation.

• Activation of survival signals can enhance Bcl-2 activity, fostering cell survival.

• Cancerous if high amount of Bcl-2

p53 in Cell Cycle and Apoptosis

• p53: Key regulatory protein.

  • Under mild stress: p53 halts the cell cycle.

  • Under severe stress: p53 induces apoptosis.

  • p53 mutations are frequently associated with tumors.

Appendix: Phases of Mitosis and Cytokinesis

• Interphase: Cell increases in size, replicates DNA, and duplicates centrosomes.

• Mitosis:

  • Prophase → Prometaphase → Metaphase → Anaphase → Telophase.

• Cytokinesis: Division of cytoplasm to create daughter cells.

  • Actin and myosin create a contractile ring to separate cells.

Conclusion

• Understanding cell cycle regulation and apoptosis is crucial in biological research, clinical applications, and understanding of diseases such as cancer and developmental disorders.