5.1 | Cell Cycle & Regulation
🔬 Overview:
Cells do not divide randomly—they follow a highly regulated cycle to ensure growth, repair, and reproduction. This cycle has checkpoints that prevent uncontrolled division (which can lead to cancer).
🔹 Part 1: What is the Cell Cycle?
The cell cycle is the series of stages a cell goes through to grow and divide. It consists of two main phases:
1⃣ Interphase – Growth & preparation (90% of the cycle)
2⃣ Mitotic Phase (M phase) – Division occurs
💡 Example: When your skin is cut, new skin cells go through the cell cycle to replace damaged ones.
🔹 Part 2: Stages of the Cell Cycle
Stage | Description | Key Events |
|---|---|---|
G₁ (Gap 1) | Cell growth & normal functions | Organelle production, protein synthesis |
S (Synthesis) | DNA replication | Each chromosome is duplicated |
G₂ (Gap 2) | Preparation for division | More growth, final error checks |
M (Mitosis & Cytokinesis) | Cell divides | Produces two identical daughter cells |
🔍 G₀ Phase: Some cells, like neurons, exit the cycle permanently and don’t divide.
🔹 Part 3: DNA Replication in the S Phase
During S phase, the cell copies its DNA to prepare for division.
✅ Process:
The enzyme helicase unwinds DNA.
DNA polymerase builds a new complementary strand.
Semi-conservative replication occurs—each new DNA molecule has one old and one new strand.
⚠ If errors occur in replication, mutations can form.
🔹 Part 4: M Phase – Mitosis & Cytokinesis
Mitosis = Division of the nucleus
Cytokinesis = Division of the cytoplasm
📌 Phases of Mitosis (PMAT):
1⃣ Prophase – Chromosomes condense, nuclear envelope breaks down.
2⃣ Metaphase – Chromosomes align in the middle.
3⃣ Anaphase – Sister chromatids pull apart.
4⃣ Telophase – Two new nuclei form.
📌 Cytokinesis: The cytoplasm splits, forming two identical daughter cells.
💡 Example: Skin cells undergo mitosis constantly for renewal.
🔹 Part 5: Cell Cycle Checkpoints & Regulation
Cells must pass checkpoints before dividing to prevent errors.
Checkpoint | Purpose | Controlled By |
|---|---|---|
G₁ Checkpoint | Is the cell big enough? Is DNA undamaged? | Cyclins & CDKs |
G₂ Checkpoint | Is DNA replicated correctly? | p53 tumor suppressor gene |
M Checkpoint | Are chromosomes aligned properly? | Spindle fibers |
💡 If a problem is detected, the cell either fixes it or undergoes apoptosis (programmed cell death).
🔹 Part 6: What Happens When Cell Cycle Regulation Fails?
⚠ Cancer = Uncontrolled Cell Division
Mutations in proto-oncogenes turn them into oncogenes, causing uncontrolled division.
Tumor suppressor genes (like p53) fail, allowing mutations to accumulate.
Cells ignore checkpoints and divide uncontrollably, forming tumors.
💡 Example: p53 mutations are found in over 50% of cancers!
🔹 Part 7: Key Differences – Mitosis vs. Meiosis
Feature | Mitosis | Meiosis |
|---|---|---|
Purpose | Growth & repair | Sexual reproduction |
Number of Divisions | 1 | 2 |
Daughter Cells | 2 identical | 4 genetically unique |
Chromosome Number | Diploid (2n) | Haploid (n) |
📌 Meiosis will be covered in 2.11!
📌 Summary:
🔹 The cell cycle ensures controlled cell growth and division.
🔹 Interphase (G₁, S, G₂) is for growth and preparation.
🔹 M phase (Mitosis & Cytokinesis) produces two identical cells.
🔹 Checkpoints regulate the cycle, preventing errors.
🔹 Uncontrolled cell division leads to cancer.
That’s 2.10 – Cell Cycle & Regulation! 🔄🧬
Next up: 2.11 – Mitosis & Meiosis. Ready to continue? 🚀