Notes on The Cell Cycle and Cell Division

The Cell Cycle

Why Cells Are Small

  • Cells need to maintain a small size for efficiency.

  • As a cell increases in size, the volume (amount of material inside) increases faster than the cell membrane surface area.

  • If too large, the cell membrane cannot support nutrient intake and waste removal effectively, risking cell death.

  • Diffusion of nutrients and wastes becomes slower with larger cell size.

Why Do Cells Divide?

  • Limits to Growth: Cells must divide instead of just growing larger.

  • Survival Necessity: Maintaining small size ensures adequate supply of:

  • Nutrients

  • Oxygen

  • Water

  • Efficient waste removal requires cells to divide into smaller daughter cells.

Cell Division Process

  • Definition: Cell division is the process by which a parent cell divides to produce two identical daughter cells.

  • Terminology:

  • Parent Cell: The original cell before division.

  • Daughter Cells: The resulting identical cells post-division.

Purpose of Cell Division

  • Functions:

  • Growth: Essential for development from a single fertilized egg into a complex multi-celled organism.

  • Repair: Replaces lost or damaged cells.

    • Example: Daily shedding of skin cells, healing of cuts, broken bones.

  • Reproduction:

    • Asexual Reproduction: One parent; offspring identical to parent.

    • Common in single-celled organisms and most body cells (excluding sex cells).

    • Sexual Reproduction: Involves two parents' sex cells (egg & sperm) resulting in offspring with unique genetic combinations.

Cell Lifespans

  • Different cells have varying lifespans:

  • Red blood cells: 120 days

  • Liver cells: 200 days

  • Intestine lining cells: 3 days

  • Skin cells: 20 days

  • Stomach lining cells: 2 days

  • Brain cells: 30-50 years

The Life Cycle of a Cell

  • Two main stages:

  1. Growth Stage - Interphase:

    • Accounts for approximately 90% of a cell's life.

    • The cell performs regular functions while preparing for division (copying organelles and DNA).

  2. Division Stage - Mitosis:

    • Involves the actual separation of the cell into two identical daughter cells.

Mitosis Stages

  • Mitosis consists of four key stages:

  1. Prophase:

    • Chromatin condenses to form chromosomes.

    • Each chromosome consists of two sister chromatids connected by a centromere.

    • Centrioles move to opposite ends of the cell; spindle fibers begin to form.

  2. Metaphase:

    • Chromosomes align along the cell's midline to ensure proper separation.

    • Spindle fibers attach to the chromosomes.

  3. Anaphase:

    • Sister chromatids are pulled apart toward opposite poles of the cell.

  4. Telophase:

    • Daughter cell nuclei form as chromosomes reach poles.

    • Chromosomes relax back into chromatin.

Cytokinesis

  • Final stage of cell division where the cytoplasm splits.

  • In animal cells, the cleavage furrow forms, pinching off the cell into two distinct daughter cells.

Cell Components Passed to Daughter Cells

  • Each daughter cell receives:

  • An exact copy of the DNA

  • Organelles and cytoplasm

  • Cell membrane

  • Requires: Copying DNA, creating extra organelles, dividing them evenly using the cytoskeleton.