CELL CYCLE & CHECKPOINTS – Comprehensive Bullet-Point Notes

Prior Knowledge & Context

  • Review of Cell Theory
    • All living things made of cells; cells are basic structural & functional units.
    • Cells arise only from pre-existing cells (supports hierarchy \text{cells} \to \text{tissues} \to \text{organs} \to \text{systems}).
    • Key contributors: Robert Hooke (coined “cell”), Anton van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, Rudolf Virchow.
  • Organelle functions to recall for cell-cycle discussions
    • Centrosome ➜ organizes spindle during mitosis & duplicates in S-phase.
    • Centrioles (animal cells) ➜ aid chromosome movement.
    • Nucleolus ➜ ribosome synthesis.
    • Nuclear envelope ➜ regulates traffic in/out of nucleus.
    • DNA/Chromatin ➜ hereditary blueprint controlling all cell activities.

Key Vocabulary (from word-puzzle warm-up)

  • Cell cycle, Interphase, Mitosis, Gap 1 (G1), Synthesis (S), Gap 2 (G2), Gap 0 (G0), Cytokinesis, Control point/Checkpoint, Cyclin, Cyclin-dependent kinase (Cdk), Retinoblastoma protein (Rb), Spindle fibers, Centromere, Mutation, Cancer, Binary fission.

Lesson 1 – Cell Cycle and Its Phases

  • General definition
    • Ordered series of events in eukaryotic cells where the cell grows, duplicates contents, and divides ➜ two genetically identical daughter cells.
  • Two broad stages
    • Interphase (≈ 90\% of cycle) – preparation & growth.
    • Mitotic Phase – actual division (mitosis + cytokinesis).
  • Interphase sub-phases & key events
    • G1 (First Gap)
    • Cell grows; normal metabolism; protein/RNA synthesis; organelles duplicate.
    • Preparation for DNA replication.
    • S (Synthesis)
    • Entire genome DNA replicated; centrosome duplicates.
    • Chromosome still uncondensed (chromatin).
    • G2 (Second Gap)
    • Continued growth; synthesis of proteins required for mitosis.
    • Final error-checking of replicated DNA.
  • Mitotic Phase detail
    • Mitosis (Prophase, Metaphase, Anaphase, Telophase)
    • Chromosomes condense, align, separate equally.
    • Cytokinesis
    • Cytoplasm divides ➜ two daughter cells each with same chromosome number as parent (2n \to 2n in somatic cells).
  • G0 (Rest Phase)
    • Exited cycle; non-dividing state.
    • Temporary (e.g., liver cells) or permanent (e.g., neurons, heart muscle).
  • Time allocation example
    • Typical mammalian cell: 24\text{ h} total; \approx 11\text{ h} G1, 8\text{ h} S, 4\text{ h} G2, 1\text{ h} M.
  • Biological significance
    • Growth (boy ➜ man), tissue repair (wound healing like Andrea’s abrasion), replacement of dead cells (skin, intestine), asexual reproduction (binary fission in bacteria such as Leptospira interrogans), development of embryos.

Lesson 2 – Cell Cycle Control Points (Checkpoints)

  • Purpose: Ensure accuracy of DNA replication & chromosome segregation; prevent propagation of mutations.
  • Locations & primary questions answered
    • G1 Checkpoint (Restriction point)
    • “Is environment favorable? Is DNA undamaged? Do we have enough nutrients & correct size?”
    • Damaged DNA ➜ cycle arrest; repair or apoptosis.
    • G2/M Checkpoint
    • “Is DNA completely & accurately replicated? Cell large enough? Organelles ready?”
    • Ensures readiness for mitosis.
    • Metaphase (Spindle-Assembly) Checkpoint
    • “Are all chromosomes properly attached to spindle at centromeres? Spindles correctly formed?”
    • Prevents unequal chromosome segregation (aneuploidy).
  • Result of failure ➜ uncontrolled division, mutations, cancer.

Molecular Regulators

  • Positive regulators
    • Cyclins
    • Concentrations oscillate; bind Cdks ➜ activate them; give “go-ahead” at G1 & G2.
    • Cyclin-dependent kinases (Cdks)
    • Kinase enzymes; phosphorylate target proteins to advance cycle.
  • Negative regulators
    • Rb (Retinoblastoma protein)
    • Binds transcription factors blocking S-phase genes; released when phosphorylated by Cyclin-Cdk.
    • p53, p21 (mentioned implicitly) – halt cycle or trigger apoptosis on DNA damage.

Cell Cycle & Cancer

  • Cancer described as uncontrolled cell division caused by malfunctioning checkpoints and regulator proteins.
  • Statistics
    • WHO (2018): 9.6\text{ million} cancer deaths worldwide; 1.7\text{ million} from lung cancer.
    • Philippines (2018): 86\,337 cancer deaths (GCO 2019).
  • Tumor formation ➜ mutated cells ignore contact inhibition, continue cycling, form masses that disrupt tissues.
  • External cancer triggers (student brainstorm)
    • Carcinogens (tobacco smoke, UV, radiation, chemicals), viruses (HPV), genetic predisposition, lifestyle (high-fat diet, inactivity).
  • Government actions (example)
    • Graphic health warnings & tax on cigarettes aim to cut lung-cancer incidence.

Assessment-Type Sample Q&A (mirrors module quizzes)

  • True/False & MC items include:
    • DNA copying occurs in S-phase.
    • G2 checkpoint precedes mitosis.
    • Cyclin provides go-ahead at checkpoints.
    • Bacteria do not undergo the eukaryotic cell cycle.
    • Nerve cells exit to G0 permanently.

Suggested Learning Aids

  • Cell-Cycle Wheel Project
    • Use sturdy card, color code (e.g., red = G1, blue = S, brown = G2, green = M, yellow = cytokinesis, violet = G0).
    • Rotate wheel to reveal events & checkpoints per segment.
  • Color-coding mnemonic
    • “Red Starts Big, Green Makes You Violet” ➜ R (G1) S (S) B (G2) G (M) Y (Cytokinesis) V (G0).
  • Concept map outline
    • Central node: Cell Cycle
    • Branch 1: Interphase (G1, S, G2)
    • Branch 2: Mitotic Phase (Mitosis + Cytokinesis)
    • Branch 3: Checkpoints (G1, G2/M, Metaphase)
    • Branch 4: Regulators (Cyclin/Cdk positive; Rb negative)
    • Branch 5: Outcomes (Growth, Repair, Asexual Reproduction, Cancer if faulty).

Ethical, Practical & Real-World Connections

  • Medical
    • Anti-cancer drugs often target rapidly dividing cells by disrupting spindle formation (Taxol) or DNA replication (5-FU).
  • Biotechnology
    • Controlled induction of G0 useful in tissue engineering to maintain cell lines.
  • Public Health
    • Hygiene prevents bacterial binary fission (pimples), illustrating difference between prokaryotic division and eukaryotic cell cycle.

Quick Reference Numbers & Equations

  • Interphase duration ≈ 90\% of cycle.
  • Typical human somatic mitosis maintains 2n chromosome number.
  • Cancer deaths 2018: 9.6 \times 10^{6} globally; Philippines 8.63 \times 10^{4}.

Study Tips

  • Focus on “WHEN & WHAT” questions:
    • WHEN: G1, S, G2, M, G0.
    • WHAT: growth? DNA copy? check DNA? spindle attach?
  • Link checkpoint question keywords:
    • G1 = “Size & Damage?”, G2 = “DNA copied?”, Metaphase = “Spindles attached?”.
  • Practice drawing complete cycle with arrows to cement order & control points.