TLDR

• Cells communicate through signaling pathways: Receive → Relay → Respond.

• The cell cycle is tightly regulated with checkpoints at G₁, G₂, and M.

• Mitosis (PMAT) creates 2 identical diploid cells for growth and repair.

• Meiosis creates 4 unique haploid gametes through crossing over and chromosome halving.

• ✅ Download your free Unit 4 Cheat Sheet PDF above.

Why Unit 4 Matters

Unit 4 bridges the gap between cellular processes and genetics. It explains how cells talk to each other, how they regulate when to divide, and how mistakes can lead to disease. On the exam, you’ll often see questions about signal transduction cascades, or diagrams comparing mitosis and meiosis. Mastering this unit will set you up for success in both MCQs and FRQs.

Cell Communication

Cells use signals to coordinate behavior and respond to their environment.

• Autocrine: cell signals itself.

• Paracrine: local signaling between nearby cells.

• Endocrine: long-distance via hormones.

• Direct contact: gap junctions (animals), plasmodesmata (plants).

Signal Transduction Pathway

1. Reception: ligand binds to receptor.

2. Transduction: signal is amplified via cascades or second messengers (like cAMP).

3. Response: gene activation, protein synthesis, or a change in cell behavior.

👉 Mnemonic: Receive → Relay → Respond.

Cell Cycle Basics

• Interphase (90%):

  • G₁ = growth.

  • S = DNA replication.

  • G₂ = prep for mitosis.

• M phase: mitosis + cytokinesis.

• G₀: resting state (non-dividing).

Mitosis (PMAT)

• Purpose: growth, repair, asexual reproduction.

• Phases: Prophase, Metaphase, Anaphase, Telophase.

• Output: 2 identical diploid cells.

👉 Mnemonic: Please Make A Taco.

Meiosis

• Purpose: gamete production, genetic diversity.

• Two rounds of division:

  • Meiosis I = homologs separate.

  • Meiosis II = sister chromatids separate.

• Key events:

  • Crossing over (Prophase I).

  • Independent assortment (Metaphase I).

• Output: 4 unique haploid cells.

👉 Mnemonic: Mixing & Reducing.

Checkpoints & Regulation

• G₁ checkpoint: checks DNA damage, cell size, nutrients.

• G₂ checkpoint: checks DNA replication errors.

• M checkpoint: ensures spindle fibers are attached.

• Cyclins & CDKs: regulate cycle progression.

• Cancer: caused by failure of checkpoint control → uncontrolled division.

Mini Concept Box

• Interphase ≈ 90% of the cycle.

• Independent assortment = 2ⁿ possible gamete combinations (n = haploid number).

Visual Mnemonics

• Signal Transduction: arrows for Receive → Relay → Respond.

• Mitosis: PMAT lined up in easy sketch.

• Meiosis: Mixing & Reducing → crossing over + chromosome halving.

Common Exam Pitfalls

• Mixing up mitosis vs meiosis products.

• Forgetting meiosis = genetic diversity via crossing over & assortment.

• Assuming all cells divide continuously (many enter G₀).

• Confusing signal reception with response — they’re distinct steps.

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