Exhaustive Study Notes on Mitosis and the Molecular Control of the Cell Cycle

Fundamental Concepts and Definitions of Cell Division

• Mitosis (GCSE Level): The process in which a eukaryotic cell divides its nucleus to produce two genetically identical nuclei.
• Cell Division (GCSE Level): The overall process where one cell splits into two; this includes both mitosis (nuclear division) and cytokinesis (cytoplasm division).
• The Continuity of Life: Based on the reproduction of cells. As German physician Rudolf Virchow stated in 1855, "Every cell from a cell."
• Daughter Cells/Offspring (GCSE Level): The two new cells produced after division. In mitosis, they are genetically identical to each other and the parent cell.
• Genome (A-Level): The complete set of DNA, including all genes and non-coding sequences, in an organism or cell. In a typical human cell, the length of DNA is approximately $2\,m$.
• Chromosomes (GCSE Level): Long, threadlike DNA molecules wrapped around proteins called histones, carrying genetic information. The name comes from the Greek chromo (color) and soma (body).
• Chromatin (A-Level): The complex of DNA and protein that makes up chromosomes. It appears loose and uncoiled during interphase.
• Somatic Cells (GCSE Level): Any body cells except reproductive cells (gametes) that divide by mitosis. Humans have $46$ chromosomes (or $23$ pairs) in somatic cells.
• Gametes (GCSE Level): Sex cells (sperm and egg) that contain half the usual number of chromosomes (haploid) and are produced by meiosis rather than mitosis.
• Chromatids (GCSE Level): Each of the two identical DNA copies formed during DNA replication.
• Sister Chromatids (GCSE Level): A pair of identical chromatids joined at a centromere. They are attached by proteins called cohesins.
• Centromere: The central region where two sister chromatids are most closely attached. The sections on either side of the centromere are referred to as "arms."

The Eukaryotic Cell Cycle

• Cell Cycle Definition (GCSE Level): The ordered sequence of events in a cell's life, divided into interphase and the mitotic (M) phase.
• Interphase Prep Stages (90% of the Cycle):
  • G1 Phase (First Gap): The cell grows in size and synthesizes mRNA and proteins. This phase is the most variable in length among different cell types. If conditions are poor, cells may exit to the G0 phase.
  • S Phase (DNA Synthesis): The cell replicates its genetic material. Chromosome duplication occurs here. This is typically the longest stage of interphase.
  • G2 Phase (Second Gap): The cell continues to grow, produces remaining organelles (like mitochondria and ER), and performs final checks on replicated DNA. Individual chromosomes are not yet visible under a light microscope because they are not yet condensed.
• Mitotic (M) Phase (10% of the Cycle): Includes both mitosis and cytokinesis. It is the shortest part of the cell cycle.

The Five Stages of Mitosis

• Prophase: Chromatin fibers become tightly coiled and condense into visible chromosomes. The nucleolus disappears, and the mitotic spindle begins to form.
• Prometaphase: The nuclear envelope completely breaks down. Spindle microtubules attach to the kinetochores of the chromosomes and begin moving them toward the cell center.
• Metaphase: Chromosomes arrive and align at the metaphase plate, an imaginary line at the cell's equator. This ensures equal distribution.
• Anaphase: The shortest stage of mitosis, lasting only a few minutes. The enzyme separase cleaves the cohesins holding sister chromatids together. The chromatids are pulled toward opposite poles of the cell.
• Telophase: Two new nuclear membranes form around the two sets of separated chromosomes. The chromosomes become less condensed, and the division of the nucleus into two identical nuclei is complete.

The Mitotic Spindle and Cytokinesis

• Mitotic Spindle: A dynamic machine made of microtubules (hollow tubes of tubulin) and proteins that moves chromosomes. It includes:
  • Centrosome: The microtubule-organizing center. It duplicates before mitosis. In animal cells, each centrosome contains two centrioles.
  • Aster: Radial arrays of short microtubules extending from centrosomes.
  • Kinetochore Microtubules: Spindle fibers that attach to the kinetochore (a protein structure on the centromere) to pull chromatids apart.
• Cytokinesis (Animal Cells): Occurs by a process called cleavage. A cleavage furrow (a shallow groove) forms near the old metaphase plate. On the cytoplasmic side, a contractile ring of actin microfilaments and myosin proteins contracts, pinching the cell into two.
• Cytokinesis (Plant Cells): Because plants have rigid cell walls, no cleavage furrow forms. Instead, vesicles from the Golgi apparatus move to the center and fuse to form a cell plate, which eventually becomes the new cell wall.

Prokaryotic Division: Binary Fission

• Binary Fission: The method of asexual reproduction for prokaryotes (bacteria and archaea). It is simpler and faster than mitosis because they lack a nucleus.
• Process Steps:
  1. DNA Replication: Begins at the Origin of Replication. The single circular chromosome duplicates.
  2. Cell Elongation: The cell grows, and the two DNA copies move toward opposite poles.
  3. Septum Formation: The membrane and wall pinch inward, aided by a tubulin-like protein, forming a septum.
  4. Separation: The cell splits into two identical daughter cells.
• Evolutionary Context: Prokaryotes preceded eukaryotes by more than a billion years. It is hypothesized that mitosis evolved from simpler prokaryotic binary fission mechanisms.

Molecular Control System of the Cell Cycle

• Cell Cycle Control System: A molecular regulatory network that governs progression through growth and division using "stop" and "proceed" signals.
• Key Regulatory Molecules:
  • Protein Kinases (Cdks): Cyclin-dependent kinases. They are only active when attached to a cyclin. They work by phosphorylating other proteins.
  • Cyclins: Proteins whose concentrations oscillate rhythmically throughout the cycle.
  • MPF (Maturation-promoting factor): A complex of Cyclin B and CDK1 (or Cdc2). It triggers the passage from G2 into the M phase by phosphorylating proteins involved in nuclear envelope breakdown and chromosome condensation.
• Checkpoints:
  1. G1 Checkpoint (Restriction Point): Known as the "point of no return." Once passed, the cell is committed to division. Regulated by the Rb protein.
  2. G2 Checkpoint: Ensures DNA is fully replicated and undamaged. Controlled by ATM/ATR kinases and Chk1/Chk2, which inhibit Cdc25 to block M-CDK activation.
  3. M Checkpoint (Spindle Assembly Checkpoint/SAC): Ensures all kinetochores are attached to spindle microtubules. Unattached kinetochores activate Mad2, which inhibits the APC/C complex.
• APC/C (Anaphase-Promoting Complex/Cyclosome): A ubiquitin ligase that degrades securin (to activate separase) and destroys cyclins to allow mitotic exit.
• Inhibitors (CKIs): Proteins like p21 (induced by p53 in response to DNA damage) that halt the cycle to allow for repair or apoptosis.

Specialized Cells and Division Frequency

• Variable Division Rates: Human bodies contain approximately $200 \text{ trillion}$ somatic cells, but not all divide.
• Non-dividing Cells:
  • Neurons (Nerve Cells): Mature neurons lack centrioles and cannot divide or regenerate once they exit to G0.
  • Cardiac Muscle (Myocytes): Lose division ability shortly after birth; heart damage results in scar tissue.
  • Red Blood Cells (Erythrocytes): Expel their nucleus to maximize oxygen space; they cannot undergo mitosis.
• Why Frequency Varies:
  • Wear and Tear: Skin and intestinal cells divide rapidly to replace damage.
  • Specialized Function: Complex structures like neural networks would be disrupted by division.

Real-Life Consequences and Applications

• Cancer: Results from the loss of cell cycle control, leading to uncontrolled mitotic division. Chemotherapy and radiation target the cell cycle.
• Medicine: Stem cell therapies utilize cell cycle regulation to treat degenerative diseases.
• Agriculture: Tissue culture use mitosis to clone high-quality plants.
• Forensics: PCR (Polymerase Chain Reaction) mimics DNA replication to amplify samples for profiling.
• Diagnostics: Genetic karyotyping involves halting cells in metaphase to check for abnormalities like Trisomy $21$.

Questions & Discussion

• Question: What part of the cell cycle is shortest?
• Answer: The mitotic (M) phase, accounting for approximately $10\%$ of the cycle.
• Question: Why were the G phases misnamed as "gaps"?
• Answer: Early observers believed the cells were inactive during those times, though they are actually periods of intense metabolic activity.
• Question: What is the point of mitosis if the parent cell doesn't die?
• Answer: It maintains homeostasis. While parent cells become daughter cells, many other cells undergo apoptosis (programmed cell death) to balance the population.
• Question: Do all prokaryotes reproduce by binary fission?
• Answer: No; while it is the primary method, some use alternative strategies like multiple fission, budding, or baeocyte formation.
• Question: Can a cell go backward in the cell cycle?
• Answer: No. The cell cycle is a linear, one-way process driven by checkpoints. Once the G1/S "point of no return" is crossed, it cannot be reversed.