Cell Cycle and Apoptosis: Chapter 24 Study Notes

Cell Cycle and Apoptosis

Definition of Cell Division Cycle

The cell division cycle (or cell cycle) is the ordered sequence of events that a cell undergoes to duplicate its contents and divide into two daughter cells. This process is fundamental for growth, development, and tissue repair in multicellular organisms.

Phases of the Cell Cycle

The cell cycle is divided into distinct phases, each characterized by specific events:

• G1 Phase (Gap 1): Cell grows and prepares for DNA replication.

• S Phase (Synthesis): DNA replication occurs, resulting in the duplication of chromosomes.

• G2 Phase (Gap 2): Cell continues to grow and prepares for mitosis; checks for DNA damage.

• M Phase (Mitosis): Chromosomes are separated and the cell divides (cytokinesis).

Major events: DNA synthesis in S phase, chromosome segregation in M phase, and cell growth in G1 and G2.

Regulation of the Cell Cycle

Cyclin-dependent kinases (Cdks) and cyclins are highly conserved regulators of cell cycle progression. Cdks are enzymes that, when bound to cyclins, phosphorylate target proteins to drive the cell through different phases.

• Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.

• Cdks: Kinases that require cyclins for activation; control transitions between cell cycle phases.

Cell Population Dynamics

The number of cells in a population is determined by the balance between cell division and cell death. Cell cycle regulation ensures proper cell proliferation.

Purpose of Cyclin Degradation

Cyclin degradation is essential for cell cycle progression. It ensures that cell cycle transitions are irreversible and prevents re-initiation of previous phases.

• Example: Degradation of mitotic cyclins triggers exit from mitosis.

Cell Cycle Checkpoints

Checkpoints are surveillance mechanisms that monitor and regulate the progression of the cell cycle. They ensure that critical events are completed before the cell proceeds to the next phase.

• Major checkpoints in eukaryotic cells:

  • G1/S Checkpoint: Checks for DNA damage and cell size before DNA replication.

  • G2/M Checkpoint: Ensures DNA replication is complete and checks for DNA damage before mitosis.

  • Spindle Assembly Checkpoint (M Checkpoint): Ensures all chromosomes are properly attached to the spindle before anaphase.

Apoptosis and Caspases

Apoptosis is a form of programmed cell death characterized by specific morphological and biochemical features. It is essential for development, tissue homeostasis, and removal of damaged cells.

• Caspases: A family of protease enzymes that play a central role in the execution of apoptosis by cleaving specific cellular substrates.

• Role of caspases: Initiate and carry out the cell death program by dismantling cellular components.

Apoptosis vs. Necrosis

Apoptosis and necrosis are two distinct forms of cell death:

• Apoptosis: Programmed, controlled, non-inflammatory cell death; cell shrinks, chromatin condenses, and membrane blebs.

• Necrosis: Uncontrolled, often pathological cell death; cell swells, membrane ruptures, and inflammation occurs.

Commonality: Both result in cell death, but apoptosis is regulated and beneficial, while necrosis is often harmful.

Function of Apoptosis

Apoptosis removes unnecessary, damaged, or potentially dangerous cells without causing inflammation. It is crucial for development (e.g., digit formation), immune system function, and cancer prevention.

Summary Table: Cell Cycle Checkpoints

Checkpoint	Main Function	Key Regulators
G1/S	Checks for DNA damage, cell size, nutrients	p53, Cyclin D/Cdk4/6
G2/M	Ensures DNA replication is complete, checks for DNA damage	Cyclin B/Cdk1
Spindle Assembly (M)	Ensures chromosomes are attached to spindle	Mad2, Bub1

Key Equations

• Cell cycle progression rate (simplified): $\text{Rate} = k[\text{Cyclin}][\text{Cdk}]$

Additional info: Apoptosis is tightly regulated by intrinsic (mitochondrial) and extrinsic (death receptor) pathways, both converging on caspase activation.