10/10/25

Overview of the Cell Division Cycle

  • The cell division cycle can be conceptualized as a circular process marked by distinct phases.

Phases of the Cell Division Cycle

  • Interphase

    • Cell is not actively dividing.
    • Sub-phases include:
    • G1 Phase: First Gap phase where the cell grows.
    • S Phase: Synthesis phase where DNA replication occurs.
    • G2 Phase: Second Gap phase, preparing for mitosis.

  • M Phase (Mitotic Phase)

    • The actual process of cell division.
    • Mitosis divides the genetic material into two daughter cells.
    • Cytokinesis is the physical division of the cytoplasm.

Mitosis and Its Phases

  • Mitosis is also referred to as Karyokinesis, focusing on the separation of chromosomes.
  • Mitosis is divided into several sub-phases:
    • Prophase
    • Chromatin condenses into visible chromosomes.
    • Nucleolus disassembles.
    • Spindle apparatus begins to form.
    • Prometaphase
    • Nuclear envelope breaks down (nuclear envelope disassembly).
    • Microtubules attach to chromosomes at kinetochores.
    • Metaphase
    • Chromosomes align at the metaphase plate due to opposing forces from microtubules.
    • Anaphase
    • Sister chromatids are pulled apart and segregated toward opposite poles of the cell.
    • Telophase
    • Chromosomes de-condense back into chromatin.
    • The nuclear envelope re-forms around each set of chromosomes.

The Spindle Apparatus

  • The spindle apparatus is composed of microtubules responsible for chromosome movement.
    • Types of Microtubules:
    • Polar Microtubules: Extend from one pole of the cell to the other, stabilizing the spindle apparatus.
    • Astral Microtubules: Position the spindle apparatus in relation to the cell membrane.
    • Kinetochore Microtubules: Attach to kinetochores on chromosomes to facilitate their movement.

Cytokinesis

  • The process of cytoplasmic division, differs between animal and plant cells.
    • Animal Cells:
    • Use a contractile ring made of actin and myosin to pinch the cell into two.
    • This remodeling causes the cell to elongate and constrict at the center.
    • Plant Cells:
    • Form a cell plate using vesicles from the Golgi apparatus to create a new cell wall between daughter cells.

The Decision to Divide

  • Cells can decide to continue dividing or enter a quiescent state (G0 phase).
  • Factors influencing the decision include external signals known as Growth Factors.

Cyclin and Cyclin-Dependent Kinases (CDKs)

  • The regulation of the cell cycle is controlled by proteins known as cyclins and cyclin-dependent kinases (CDKs).
    • Cyclin: Levels rise and fall throughout the cell cycle, activating CDKs when present.
    • CDK: Enzymatically adds phosphates to target proteins, driving the cell cycle.

Restriction Point

  • Point in the cell cycle (end of G1) that determines if a cell will proceed to S phase.
  • Activation of the RB protein (retinoblastoma protein) is crucial in controlling this decision.
    • RB Protein: Inhibits progression to S phase until phosphorylated by active CDK-cyclin complexes.

Meiotic Cell Division**

  • Distinct from mitosis, meiosis generates haploid cells (gametes) necessary for sexual reproduction.
    • Involves two rounds of division: Meiosis I and Meiosis II.
    • Results in four non-identical haploid cells from a diploid precursor.

Life Cycles and Sexual Reproduction

  • Life cycles involve a succession of generations through mitosis, meiosis, and fertilization.
    • Involves complex processes that vary between animals, plants, and fungi.
    • The sequence includes meiosis for gamete production and the fertilization process for diploid reconstitution.