BIOL 1009 - Cell Cycle

BIOL 1009 - Cell Cycle Overview

1. Overview of the Cell Cycle

  • The cell cycle is integral for:
    • Asexual reproduction in unicellular organisms.
    • Development from a fertilized cell in multicellular organisms.
    • Growth and repair of tissues.

2. Importance of Cell Division

  • Vital for organisms that reproduce asexually.
  • Required for:
    • Growth and development from a single fertilized cell.
    • Replacement of cells lost to damage or old age (e.g., 300 million cells are replaced every minute).

3. Requirements for Cell Division

  • Prior to completing division, a cell must:
    1. Reach a minimum size.
    2. Complete DNA replication.
    3. Ensure each daughter cell receives a complete copy of the cell's DNA.

4. Phases of the Cell Cycle

  • The cell cycle consists mainly of two phases:
    1. Interphase
    2. Mitotic Phase

A. Interphase

  • Duration: Approximately 90% of the cell cycle.
  • Sub-phases:
    • G1 Phase (Gap 1)
    • S Phase (Synthesis)
    • G2 Phase (Gap 2)

B. G1 Phase (Gap Phase 1)

  1. Cells undergo growth.
  2. G1 Checkpoint:
    • Checks size, nutrients, and presence of growth factors.
    • Critical in determining continuation of the cell cycle.
    • If conditions are met, the cell proceeds to S phase; otherwise, it exits to G0 phase.
  3. G0 Phase (Non-dividing phase):
    • Most cells spend majority of their time here.
    • Cells can re-enter the cycle upon stimuli like injury.

C. Cyclins and CDKs

  • Passage through checkpoints is regulated by cyclins and cyclin-dependent kinases (Cdks):
    • Cyclin levels fluctuate through the cell cycle.
    • Cdks, present at constant levels, require binding to cyclins for activity.
    • Cyclins promote progression through specific checkpoints by activating Cdks, which phosphorylate target proteins, regulating the cell cycle.

D. S Phase (Synthesis Phase)

  1. DNA replication occurs, leading to the formation of sister chromatids.
  2. Centrosome is duplicated, leading to two centrosomes.
  3. Cells continue to grow during this phase.

E. G2 Phase (Gap Phase 2)

  1. Cells continue to grow and synthesize materials.
  2. G2 Checkpoint:
    • Ensures DNA replication is complete and checks cell size before proceeding to mitosis.

5. Mitotic Phase

  • The mitotic phase consists of multiple overlapping sub-phases divided into:
    1. Mitosis (nucleus division)
    2. Cytokinesis (cytoplasm division)

A. Mitosis Sub-phases

  1. Prophase:

    • Chromosomes condense.
    • Mitotic spindle forms (composed of centrosomes and microtubules).
    • Centrosomes move apart as microtubules grow.

  2. Prometaphase:

    • Nuclear envelope breaks down.
    • Kinetochores form on chromatids.
    • Microtubules attach to kinetochores.

  3. Metaphase:

    • Chromosomes align at the center (metaphase plate).
    • M Checkpoint:
    • Ensures microtubules are attached to kinetochores; vital for DNA distribution to daughter cells.

  4. Anaphase:

    • Cleavage of cohesin proteins; sister chromatids separate.
    • Motor proteins pull chromatids towards centrosomes.
    • Cell elongates due to non-kinetochore microtubules.

  5. Telophase:

    • Nuclear envelopes form around each daughter nucleus.
    • Chromosomes de-condense.

B. Cytokinesis

  1. Division of cytoplasm occurs, completing formation of two daughter cells.
  2. In animal cells, a cleavage furrow forms, pinching the cell in two.

6. Checkpoints in the Cell Cycle

  • Control Points where cells decide whether to proceed with division.
  • Major checkpoints include:
    1. G1 Checkpoint:
    • Ensure sufficient size, nutrients, and growth factors.
    1. G2 Checkpoint:
    • Verifies DNA replication and cell size.
    1. M Checkpoint:
    • Confirms microtubule attachment to kinetochores to secure DNA distribution.

7. Implications of Cell Division Regulation

  • Regulation breakdown can lead to excessive cell division implicated in cancer development.

8. Additional Resources

  • Online games and animations, such as on MasteringBiology, provide interactive learning about the cell cycle.
  • Books like “The Immortal Life of Henrietta Lacks” explore cell biology topics deeply.

9. Conclusion

  • Understanding the cell cycle is crucial for grasping core biological processes, developmental biology, and pathological conditions associated with cell division abnormalities.