Lecture Notes: Control of Cell Division

Introduction

• Topic covers higher-level concepts of cell and nuclear division, specifically the control of cell division.
• Focus on examples of cell proliferation and reasons why it occurs.

Reasons for Cell Proliferation

1. Growth

• Multicellular organisms grow by adding more cells.
• Surface area to volume ratio is a key concept.
• In animals, exemplified by growth from a zygote via mitosis.
• In plants, occurs in meristems (especially apical meristems at shoots and roots).
  • High rates of mitosis lead to growth (shoots grow taller, roots extend).

2. Cell Replacement

• Cells have definitive lifespans and need replacement.
• Example: Skin cells are replaced continuously.
  • Outer layer: dead cells.
  • Inner layer (dermis): new cells are produced and push upwards.

3. Tissue Repair

• Occurs following injury or wounds.
• Requires increased mitosis to replace dead cells.
• Varies by tissue type (e.g., skin heals faster than bones).

The Cell Cycle

• Cycle includes interphase and mitosis.
• Interphase: Longest phase, divided into:
  • G1 Phase: Growth, normal cellular functions.
  • S Phase: DNA synthesis and replication.
  • G2 Phase: Preparation for mitosis (enzyme production, organelle duplication).
• Interphase is not a resting phase; high metabolic activity.

Regulation of the Cell Cycle

• Cyclins: Proteins controlling the cell cycle.
  • Different cyclins for each checkpoint (G1 to S, S to G2, etc.).
  • Ensure cell readiness before progressing to the next stage.
• Uncontrolled cell division is prevented by specific genes:
  • Proto-oncogenes: Regularly control the cell cycle.
  • Tumor Suppressor Genes: Prevent unnecessary cell division and repair DNA damage.

Tumors and Mutations

• Tumors: Result from uncontrolled cell division, can be benign or malignant.
• Mutations in division-controlling genes can occur randomly, be heritable, or caused by mutagens (chemicals, radiation).
• Proto-oncogenes may mutate into oncogenes, promoting uncontrolled division.
• Tumor Suppressor Genes mutations are recessive; require two copies for tumor formation.

Types of Tumors

• Primary Tumor: Initial group of tumor cells.
• Secondary Tumor: Tumors spreading to other locations via metastasis.
  • Malignant tumors are capable of metastasizing.

Mitotic Index

• Used to gauge tumor aggression.
• Calculated as the ratio of mitotic cells to total number of cells.
• Higher mitotic index indicates faster cell division, potentially more aggressive tumor.

Conclusion

• Understanding cell division and its control is crucial for continuity and change in biological systems.
• Ensures accurate replication of DNA and proper cell function across generations.

• Theme D: Continuity and Change - Key thematic approach in understanding cell division processes.