Biology 101: Mitosis and The Continuity of Life

Bio 101 Unit III: The Continuity of Life

Cell Theory and the Genome

• Genome Defined: A cell’s genome is its complete collection of DNA.

• The Cell Theory Review: This theory dictates that all cells arise from pre-existing cells. This principle applies to both prokaryotic and eukaryotic organisms.

• Cell Division: This is the process (Mitosis) by which the continuity of life is maintained.

The Eukaryotic Cell Cycle

• Overview: All cells progress through a predictable cycle consisting of two main stages: Interphase and Cell Division.

• Interphase:

  • Represents approximately $90\%$ of a cell’s lifespan.

  • During this phase, the cell performs its normal physiological functions.

  • It is further subdivided into three sub-phases:

    • G1 Phase: The first growth phase, lasting approximately $9$ hours.

    • S Phase (Synthesis): The DNA is duplicated/replicated. This stage lasts approximately $10$ hours. During this phase, DNA is in a "loose" or "unspooled" state.

    • G2 Phase: The second growth phase, lasting approximately $4$ to $5$ hours. This phase prepares the cell for division.

• Cell Division (M Phase & C Phase):

  • The final stage of the cell cycle, typically lasting $0.5$ to $1$ hour.

  • Mitosis (M): The division of the genetic material.

  • Cytokinesis (C): The division of the rest of the cellular material (cytoplasm).

  • Result: The production of daughter cells which are exact copies (clones) of the parent cell.

Chromosome Structure and Morphology

• Condensation: DNA typically exists as a double helix, but during division, it condenses into the chromosomal form.

• Replicated Chromosome Structure:

  • Sister Chromatids: Identical copies of a chromosome produced at the end of interphase via replication. They are joined together lengthwise.

  • Centromere: The region where sister chromatids are most closely attached.

  • Kinetochore: Protein structures located at the centromere where spindle fibers attach during division.

  • Arms:

    • p Arm: The short arm of the chromosome.

    • q Arm: The long arm of the chromosome.

  • Constrictions: Chromosomes feature primary constrictions (at the centromere) and secondary constrictions.

• Ploidy:

  • Diploid ($2n$): Cells containing two copies of each chromosome. Mitosis occurs in these somatic cells.

  • Haploid ($n$): Cells containing a single set of chromosomes (reproductive germ-line cells/gametes).

The Phases of Mitosis (IPMAT)

Many sources use a 4-phase or 5-phase model. The 5-phase model includes Prometaphase.

1. Prophase:

   • Chromosomes condense and become visible under a microscope.

   • Spindle fibers emerge from the centrosomes.

   • The nuclear envelope begins to break down.

   • The nucleolus disappears.

2. Prometaphase:

   • Chromosomes continue to condense.

   • Kinetochores appear at the centromeres.

   • Mitotic spindle microtubules attach to the kinetochores.

   • Centrosomes move toward opposite poles of the cell.

3. Metaphase:

   • The mitotic spindle is fully developed.

   • Centrosomes are established at opposite poles.

   • Chromosomes line up along the metaphase plate (the central plane of the cell).

   • Each sister chromatid is attached to a spindle fiber originating from opposite poles.

4. Anaphase:

   • Cohesin proteins that bind sister chromatids together break down.

   • Sister chromatids (now considered individual chromosomes) are pulled toward opposite poles.

   • Non-kinetochore spindle fibers lengthen, causing the cell to elongate.

5. Telophase:

   • Chromosomes arrive at opposite poles and begin to decondense (uncoil).

   • Nuclear envelope material surrounds each set of chromosomes to form new nuclei.

   • The mitotic spindle breaks down.

• Cytokinesis:

  • Following telophase, the cytoplasm is cleaved in half.

  • Animal Cells: A cleavage furrow separates the daughter cells.

  • Plant Cells: A cell plate forms to separate the daughter cells.

Cell Mortality and Development

• Mortality: Cell division is not indefinite. Most cells only last for a few dozen cycles.

• Programmed Cell Death (Apoptosis): This process keeps multicellular existence "fresh and vibrant."

• Developmental Example: Human fingers begin as webbed "paddles" (a relic of amphibian evolutionary history) at $1.5$ months of development. Later, specific genes trigger the removal of the webbed skin between digits through programmed cell death to form distinct fingers and toes.

Prokaryotic Cell Division

• Binary Fission: This is the process of cell division in prokaryotes (bacteria) and some single-cell protist eukaryotes.

• Prokaryotic DNA: Unlike eukaryotes, prokaryotic DNA is typically a single, circular double-stranded molecule.

• Process Highpoints:

  1. Replication of DNA starts at the Origin of Replication.

  2. The cell elongates.

  3. The cell pinches in two at the center.

  4. Two identical daughter cells are produced.

Asexual Reproduction (Cloning)

• Methods:

  • Binary Fission: Used by bacteria.

  • Budding: An individual grows a whole new individual out of itself (e.g., Hydra).

  • Rhizomes: New shoots emerge from modified underground stems (e.g., grass/vegetative reproduction).

  • Fragmentation: A piece or fragment of an organism can repopulate a new colony (e.g., coral).

  • Runners: Stems that grow along the ground to produce clones (e.g., strawberries).

  • Cuttings: A common horticultural technique where a branch is pushed into soil; the stem cells differentiate into root tissue to grow a clone.

  • Parthenogenesis: Translated from Greek as "of virgin origin." This occurs when females (such as some reptiles)