Chapter 12: Mitosis and Regulation of the Eukaryotic Cell Cycle

Chapter 12: Mitosis and Regulation of the Eukaryotic Cell Cycle

Overview of Mitosis and the Cell Cycle

Mitosis is a fundamental process in eukaryotic cells that ensures the accurate distribution of duplicated genetic material into two daughter cells. The cell cycle is a highly regulated sequence of events that includes growth, DNA replication, and cell division. Proper regulation is essential for growth, repair, and maintenance of multicellular organisms.

The Cell Cycle: Phases and Checkpoints

• Interphase: The longest phase, comprising G1 (cell growth), S (DNA synthesis), and G2 (preparation for mitosis).

• Mitotic (M) Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

• Checkpoints: Control points (G1, G2, and M) ensure the cell only proceeds when conditions are optimal.

Key Point: Checkpoints monitor cell size, DNA integrity, and chromosome attachment to spindle fibers, preventing progression if errors are detected.

Structure and Function of Chromosomes

Chromosomes are highly organized structures composed of DNA and proteins. During cell division, chromosomes condense to facilitate accurate segregation.

• Chromatin: DNA-protein complex that forms chromosomes.

• Sister Chromatids: Identical copies of a chromosome, joined at the centromere after DNA replication.

• Cohesins: Protein complexes that hold sister chromatids together.

• Centromere: Region where chromatids are most tightly attached; site of kinetochore formation.

Example: Human somatic cells have 46 chromosomes (23 pairs); gametes have 23 chromosomes.

Phases of Mitosis

Mitosis is divided into five distinct stages, each with specific events that ensure equal distribution of genetic material:

• Prophase: Chromosomes condense, spindle fibers begin to form.

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

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Chromosomes decondense, nuclear envelopes reform.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Key Point: Microtubules (spindle fibers) and motor proteins (e.g., dynein) are essential for chromosome movement and segregation.

Regulation of the Cell Cycle: Cyclins, Cdks, and Checkpoints

Progression through the cell cycle is controlled by cyclins and cyclin-dependent kinases (Cdks). These proteins act as molecular switches, ensuring that each phase is completed before the next begins.

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

• Cyclin-dependent kinases (Cdks): Enzymes that, when bound to cyclins, phosphorylate target proteins to drive cell cycle transitions.

• MPF (Maturation-Promoting Factor): A cyclin-Cdk complex that triggers entry into mitosis.

• CAK (Cdk-activating kinase): Adds an activating phosphate to the cyclin-Cdk complex, making it fully active.

Example: If the CAK gene is non-functional, the cyclin-Cdk complex remains only partially active, preventing the cell from passing the G2 to M checkpoint and leading to cell cycle arrest.

Fluctuations of Cyclin and MPF Activity

The concentrations of cyclins and the activity of MPF rise and fall in a cyclical manner, tightly regulating cell cycle transitions.

• During G2, cyclin accumulates and binds to Cdk, forming MPF.

• MPF activity peaks at the onset of mitosis, promoting chromosome condensation and spindle formation.

• After mitosis, cyclin is degraded, inactivating MPF and allowing the cell to exit mitosis.

Key Point: The rise and fall of cyclin levels ensure that cell cycle events occur in the correct order and only once per cycle.

External and Internal Regulation: Growth Factors and Density-Dependent Inhibition

Cell division is regulated by both internal molecular signals and external environmental cues.

• Growth Factors: Proteins that stimulate cell division by binding to cell-surface receptors and activating signaling pathways.

• Density-Dependent Inhibition: Cells stop dividing when they contact neighboring cells, preventing overcrowding.

• Example: Estrogen acts as a growth factor, promoting cyclin production and cell division in certain tissues.

Key Point: These mechanisms ensure that cell division occurs only when and where it is needed, maintaining tissue organization.

Relationship Between Cell Cycle Regulation and Cancer

Cancer arises from the breakdown of normal cell cycle regulation, leading to uncontrolled cell division and tumor formation.

• Loss of Checkpoint Control: Cancer cells often bypass checkpoints, allowing division despite DNA damage or incomplete replication.

• Defective Regulatory Proteins: Mutations in genes encoding cyclins, Cdks, or checkpoint proteins disrupt normal control.

• Loss of Density-Dependent Inhibition: Cancer cells ignore signals to stop dividing, resulting in disorganized tissue growth.

• Altered Growth Factor Signaling: Cancer cells may produce their own growth factors or overreact to external signals.

Key Point: The accumulation of mutations and loss of regulatory control underlie the development and progression of cancer.

Summary Table: Key Components of Cell Cycle Regulation

Component	Function	Role in Cell Cycle
Cyclin	Regulatory protein	Activates Cdks; levels fluctuate
Cdk	Protein kinase	Phosphorylates target proteins when bound to cyclin
MPF	Cyclin-Cdk complex	Triggers entry into mitosis
CAK	Cdk-activating kinase	Fully activates cyclin-Cdk complex
Checkpoint	Control point	Monitors and regulates cell cycle progression
Growth Factor	External signal	Stimulates cell division
Density-Dependent Inhibition	External signal	Prevents overcrowding by inhibiting division