Cell Cycle and Mitosis Notes

Overview of the Cell Cycle

  • The cell cycle comprises a series of stages that cells go through to divide and replicate.
Mitotic Cell Cycle
  • Nuclear and Cytoplasmic Events:
    • Normal gene expression decreases in the nucleus.
    • DNA condenses into chromosomes.
    • Kinetochores are formed in the cytoplasm.
    • Cytoskeletal elements are prepared for chromosome movement.

Stages of Mitosis

Interphase
  • Gap 1 (G₁): Cell grows and carries out normal functions.
  • S Phase: DNA synthesis occurs, and centrosomes duplicate.
  • Gap 2 (G₂): Further growth and preparation for mitosis.
Mitosis Phases

  1. Prophase:

    • Chromatin condenses into distinct chromosomes.
    • The nucleolus shrinks, and centrosomes migrate to opposite poles, forming the mitotic spindle.

  2. Prometaphase:

    • Nuclear envelope breaks down.
    • Spindle microtubules attach to chromosome kinetochores.
    • Chromosomes are moved as the spindle microtubules attach.

  3. Metaphase:

    • Chromosomes align at the metaphase plate.
    • Cohesins bind sister chromatids, keeping them together while opposing forces act on them.

  4. Anaphase:

    • Sister chromatids are pulled apart towards opposite poles (now called daughter chromosomes).
    • Chromosome segregation is completed.

  5. Telophase:

    • Spindle apparatus disassembles.
    • Chromosomes decondense back into chromatin.
    • Nuclear envelopes re-form around each set of chromosomes.

Cytokinesis

  • Division of the cytoplasm occurs differently in plant and animal cells:
    • Animal Cells: Form a cleavage furrow through contraction of a contractile ring of microfilaments.
    • Plant Cells: Produce a cell plate that becomes the new cell wall, separating daughter cells.

Spindle Formation

  • Animal Cells:

    • Centrosomes divide and microtubules radiate to form the spindle.

  • Plant Cells:

    • Form spindles from microtubules without centrosomes and include multiple microtubule organizing centers (MTOCs).

Microtubule Dynamics During Mitosis

  • Kinetochore Microtubules:

    • Connect chromosomes to spindle poles, becoming shorter as chromosomes move.

  • Non-Kinetochore Microtubules:

    • Extend between spindle poles, pushing them apart during separation.

Key Terminology

  • Chromosome: Structure made of DNA and protein that carries genetic information.
  • Sister Chromatid: Identical halves of a duplicated chromosome.
  • Centromere: Region where sister chromatids are joined.
  • Kinetochore: Protein structure where microtubules attach to chromosomes during mitosis.

Important Concepts

  • Each stage of mitosis is crucial for ensuring accurate division and distribution of genetic material.
  • Animal and plant cells have different mechanisms for cytokinesis and spindle formation, highlighting the adaptability of cellular processes in different organisms.
  • The dynamics of microtubules play a vital role in the movement of chromosomes and the physical separation of daughter cells.

Outcomes of Mitosis

  • Results in two identical daughter cells, each maintaining the original chromosome number (2n) and genetic information of the parent cell, ensuring consistency in somatic tissues.