Cell Division and Cell Cycle Regulation Notes

Cell Division and Cell Cycle Regulation

Learning Goals
  • Explain why cells reproduce and the major steps required for one cell to become two daughter cells.
  • Differentiate between prokaryotic and eukaryotic cells, focusing on binary fission vs. mitotic vs. meiotic division.
  • Discuss the role of growth factors and survival factors in cell reproduction.
  • Outline the phases of the mitotic cell cycle and describe events in each phase.
  • Detail the stages of mitosis, drawing and explaining the events and the function of spindle microtubules.
  • Compare cytokinesis in animal and plant cells.
Reasons for Cell Reproduction
  • Growth & Development: Necessary for increasing organism size.
  • Replacement of Lost or Damaged Cells: Essential for tissue maintenance and repair.
  • Organism Reproduction: For species propagation.
Signals for Cell Reproduction
  • Mitogens: Extracellular molecules (growth factors) that bind to receptors, initiating signal transduction, resulting in changes that promote or inhibit cell reproduction.
When do Cells Reproduce?
  • Cells reproduce:
    1. Constantly (e.g. skin cells).
    2. Only once (e.g. neurons).
    3. Upon receiving a growth signal from the environment.
    4. Never (e.g. when differentiation prevents further division).
Mechanisms of Cell Reproduction

  • Asexual Reproduction:

    • Identical offspring (clones) via binary fission in prokaryotes or mitotic division in eukaryotes.
    • Genetic variation occurs mainly through mutations.
    • Advantages include speed, energy conservation, and lack of necessity for courtship.

  • Sexual Reproduction:

    • Formation of genetically unique offspring through meiotic division and fertilization.
    • Advantages include increased genetic variability, adaptation facilitation, and accelerated evolution.
Types of Cellular Reproduction

  • Asexual:

    • Binary Fission:
    • Prokaryotes divide by this method, copying a single circular chromosome.
    • Mitotic Division:
    • Used by eukaryotes for growth and repair; produces genetically identical daughter cells.

  • Sexual:

    • Meiotic Division:
    • Results in daughter cells with half the DNA content, leading to genetic diversity post-fertilization.
Eukaryotic Cell Cycle Overview

  • Interphase (90% of cell cycle):

    • G1 Phase: Cell growth and preparation for DNA replication.
    • S Phase: Duplication of chromosomes.
    • G2 Phase: Continued growth and preparation for mitosis.

  • M Phase (10% of cell cycle):

    • Mitosis occurs in stages: Prophase, Prometaphase, Metaphase, Anaphase, Telophase with Cytokinesis usually overlapping with Telophase.
Phases of Mitosis
  1. Prophase: Chromosomes condense, spindle forms, centrosomes move to opposite poles.
  2. Prometaphase: Nuclear envelope breaks down, spindle microtubules attach to kinetochores.
  3. Metaphase: Chromosomes align at the metaphase plate.
  4. Anaphase: Sister chromatids are pulled apart to opposite poles.
  5. Telophase: Nuclei reform, chromosomes decondense.
Microtubules' Role in Separating Chromosomes
  • Microtubules and associated motor proteins facilitate movement:
    • Kinetochore microtubules shorten, pulling apart sister chromatids.
    • Interpolar microtubules push spindles apart.
    • Astral microtubules help position spindle in the cell.
Cytokinesis

  • In Animals: A contractile ring forms, constricting the cell membrane and creating a cleavage furrow.

    • Triggered by RhoA, leading to contractile ring formation.

  • In Plants: A phragmoplast forms, and vesicles carrying cell wall materials converge at the center to form a cell plate, initiating the new cell wall.

Cell Cycle Regulation: Checkpoints

  • **Main Checkpoints:

    1. G1/S Checkpoint:** Ensures DNA stability and adequate resources for DNA synthesis.

    2. G2/M Checkpoint:** Confirms readiness for mitosis, including DNA damage repair.

    3. M Checkpoint:** Verifies all chromosomes are attached correctly before proceeding to anaphase.

  • Consequences of Errors:

    • Errors detected can lead to pause and repair or apoptosis (programmed cell death).
Role of Cyclins and CDKs in Cell Cycle Progression

  • Cyclins: Proteins whose levels fluctuate to activate CDKs (cyclin-dependent kinases) at specific phases, promoting progression through the cell cycle.

  • pRb (Retinoblastoma Protein): Inhibits cell cycle until phosphorylated by cyclin-CDK complex to allow progression.

    • Acts as a checkpoint regulator; abnormal regulation leads to unregulated cell proliferation.

  • p53: The master regulator for cell repair, halting cell cycle in response to DNA damage and promoting repair mechanisms or apoptosis if damage is severe.

Cancer and Unregulated Cell Reproduction

  • Cancer Characteristics: Result from uncontrolled cell growth due to mutations in proto-oncogenes or tumor suppressor genes, leading to increased cell division and potential metastasis.

  • HPV and Cancer: Directly causes increased proliferation of infected cells or indirectly alters the cellular environment to favor loss of control.

Key Takeaways:
  • Importance of understanding cell cycle regulation in relation to cancer and its implications for medical research and treatment.