Regulating Cell Division and Apoptosis

The cell cycle is heavily regulated at three main checkpoints to ensure abnormal divisions do not occur.
Gap 1 ( $G_1$ ) Checkpoint: Occurs toward the end of interphase; checks for adequate resources ( $O_2$ and proteins for DNA synthesis), cell size, and DNA damage.
Gap 2 ( $G_2$ ) Checkpoint: Verifies resource availability and ensures DNA replication has occurred correctly.
Metaphase ( $M$ ) Checkpoint: Confirms spindle fibers are correctly attached to sister chromatids and chromosomes are aligned at the equator.

Purpose: Eliminates cells with irreparable DNA damage or cells that have divided too many times.
Stage 1: Activation of caspases; mitochondria detect internal damage and release Cytochrome C.
Stage 2: Digestion of cell contents; caspases cleave intracellular proteins, breaking down organelles.
Stage 3: Cell shrinkage; the cell and nucleus decrease in size as material is broken down.
Stage 4: Membrane blebbing and breakage; the cytoskeleton is digested, causing the membrane to bubble and detach into apoptotic bodies.
Final step: Phagocytic cells engulf and destroy the apoptotic bodies.

Necrosis is accidental cell death caused by physical injury or lack of oxygen.
Contrast in Death Mechanism:
- Apoptosis: Regulated, affects single cells, cell fragments are engulfed by phagocytes.
- Necrosis: Accidental, affects clusters of cells, cells "explode"/burst, spilling contents into extracellular fluid (often causing Hematomas).

A Neoplasm is an abnormal growth of tissue forming a mass (tumor) due to uncontrolled cell division.
Benign Tumors: Slow-growing masses enclosed in a capsule that prevents them from invading other body parts.
Malignant Tumors: Harmful masses that can invade nearby tissues or enter the bloodstream/lymphatic system to grow elsewhere.

Inability to form spindle fibers: This would have the greatest effect during anaphase and results in programmed death of the cell.
Blebbing: Characterized by the bulging of the plasma membrane after the cytoskeleton is digested.
Alzheimer’s disease context: Excessive death of neurons via apoptosis would lead to high concentrations of phagocytes in the brain to clear the cell debris.