The Cell Cycle & Mitosis - Study Notes

The Cell Cycle & Mitosis - Study Notes

Chapter 7: The Cell Cycle and Cell Division Key Concepts

  • 7.1 Reproduction May Be Asexual or Sexual
  • 7.2 Asexual Reproduction Results in Genetically Identical Daughter Cells
  • 7.3 Sexual Reproduction by Meiosis Halves the Number of Chromosomes and Generates Genetic Diversity
  • 7.4 Errors during Cell Division Can Result in Changes in Chromosome Number
  • 7.5 The Cell Cycle and Cell Death Are Highly Regulated in Eukaryotes

Concept 7.1a: Reproduction May Be Asexual or Sexual

  • Cell division pertains to the process where a single mother cell divides to yield two daughter cells.
  • Necessity of Cell Division:
    1. Reproduction
    • Essential for passing genetic information to offspring in both prokaryotes and eukaryotes.
    1. Growth
    • Allows for an increase in cell numbers in multicellular organisms (eukaryotes).
    1. Regeneration
    • Involves the regrowth of damaged or lost cells/tissues in eukaryotes.

Concept 7.1b: Asexual vs. Sexual Reproduction

  • Organisms transmit genetic information to offspring using cell division:
    1. Asexual Reproduction
    • Produces offspring that are clones of the parent organism.
    1. Sexual Reproduction
    • Involves the production of gametes, which through fertilization lead to genetically varied offspring.
  • Diagrammatic Representation of Offspring:
    • Asexual reproduction results in genetically identical offspring.
    • Sexual reproduction leads to genetically diverse offspring.

Concept 7.1c: Types of Asexual Reproduction

  1. Binary Fission (in prokaryotes)
    • Seen in unicellular bacteria.
  2. Mitosis (in eukaryotes)
    • Observed in unicellular organisms like yeast and multicellular entities like aspen trees.
  • Genetic variation in asexual reproduction occurs solely due to mutations, which can arise from:
    1. Environmental factors
    2. Errors during DNA replication

Concept 7.1d: Sexual Reproduction

  • Meiosis and Fertilization specifically occur in eukaryotes.
    1. Meiosis:
    • Indicates two rounds of cellular division that produce gametes.
    1. Fertilization:
    • Involves the fusion of two gametes created via meiosis.
  • This process generates genetically unique offspring due to the combination of parent DNA.

Concept 7.2a: Asexual Reproduction Results in Genetically Identical Daughter Cells

  • All cells undertake a common series of events during cell division:
    1. Cell Division Signals: Initiate the process.
    2. DNA Replication: Copies the cell’s genetic information.
    3. DNA Segregation: Distributes copies of DNA into two new cells.
    4. Cytokinesis: Divides the cytoplasm leading to the formation of two cells.
  • Specific differences exist among binary fission, mitosis, and meiosis.

Concept 7.2b: Binary Fission in Prokaryotes

Process:
  1. Cell Division Signals: Prokaryotic cells continuously divide if environmental conditions are suitable.
  2. DNA Replication: Begins at the origin (ori) site on the circular DNA strand, moving towards the terminus (ter) site.
  3. DNA Segregation: As replication occurs, ori regions migrate to opposite ends of the cell following cytoskeletal filaments.
  4. Cytokinesis: Cytoplasm divides, resulting in daughter cells that are genetically identical to one another and the original mother cell.

Concept 7.2c: Mitosis in Eukaryotes

Differences compared to Binary Fission:
  1. Cell Division Signals: May relate to the multicellular organism’s overall function.
  2. DNA Replication Initiation: Occurs at multiple sites on the chromosomes.
  3. Chromosome Count: Eukaryotes possess multiple chromosomes to be segregated.
  4. Cytokinesis Variation: In plant cells, a membrane (cell wall) forms in addition to the cytoplasm division.

Concept 7.2d: Components During the Cell Cycle

  • Consider the fate of the following during the cell cycle:
    1. Cell membrane
    2. Nuclear envelope
    3. Centrosomes
    4. Chromosomes
  • Media Reference: Video on the Phases of Mitosis

Concept 7.2e: Cell Cycle Phases

  • The cell cycle consists of two primary phases:
    1. Interphase:
    • Subdivided into three stages:
      • G1 Phase (Gap 1): Cells execute normal function.
        • G0 Phase: A resting stage outside of the cycle; cells can exit from here based on signals.
      • S Phase (Synthesis): Where DNA replication occurs.
      • G2 Phase (Gap 2): The cell prepares for mitosis and checks replicated DNA for any errors.
    1. M Phase:
    • Includes mitosis and cytokinesis, with the cell spending most of its time in interphase.

Concept 7.2f: Functions during Interphase

  1. G1 (Gap 1): In this phase, cells carry out standard metabolic functions.
    • G0: The resting phase from which cells can re-enter the cycle based on division signals.
  2. S (Synthesis): Contains the critical role of DNA replication.
  3. G2 (Gap 2): The cell's preparatory phase leading to mitosis; replicated DNA is verified for correctness.

Concept 7.2g: Mitosis Overview

  • Mitosis is characterized as a set of processes where chromosomes condense and are distributed into two new nuclei.
Stages of Mitosis:
  1. Prophase
  2. Prometaphase
  3. Metaphase
  4. Anaphase
  5. Telophase
  • Followed by Cytokinesis, which divides the cytoplasm yielding two daughter cells.

Concept 7.2h: Prophase

  • Chromosomes condense, and the spindle apparatus begins to form:
  1. Chromatin coils and supercoils resulting in condensed chromosomes.
  2. Kinetochores form at the centromeres of sister chromatids.
  3. Centrosomes migrate towards the poles and develop spindle fibers (microtubules) necessary for division.

Concept 7.2i: Prometaphase

  • Characterized by the breakdown of the nuclear envelope and attachment of chromosomes to the spindle:
  1. The nuclear envelope that surrounded the chromosomes fully degrades.
  2. Spindle fibers start connecting to chromosomes at the kinetochores.
  3. Sister chromatids attach to opposite poles.

Concept 7.2j: Metaphase

  • Chromosomes align along the equatorial plane of the cell:
  1. All spindle fibers attach to the kinetochores of sister chromatids.
  2. A cellular ‘tug of war’ aligns chromosomes along the cell’s equator.

Concept 7.2k: Anaphase

  • Sister chromatids separate and migrate toward opposing poles:
  1. Sister chromatids are divided and move apart towards opposite poles of the cell.
  2. Non-kinetochore spindle fibers elongate, aiding in cell elongation.

Concept 7.2l: Telophase

  • Chromosomes decondense, and nuclear envelopes begin to reform:
  1. The cell continues to elongate.
  2. Nuclear envelopes start forming around each set of chromosomes.
  3. Chromosomes revert to a less condensed chromatin state.
  4. The cell initiates cytokinesis to separate itself.

Concept 7.2m: Cytokinesis

  • Final separation of the cell's cytoplasm:
  1. In Animal Cells:
    • Uses a contractile ring composed of actin filaments to constrict and separate cells.
  2. In Plant Cells:
    • An additional membrane forms (cell wall); vesicles cooperate to establish a cell plate for dividing the cytoplasm.

iClicker - Questions

  • Question #2: Exploration of Sexual vs. Asexual Reproduction.
  • Question #3: Reviewing the stages of Mitosis identified by the acronym PPMAT (Prophase, Prometaphase, Metaphase, Anaphase, Telophase).