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Volume I · MMXXVI AP Biology
Library› AP Biology› Unit 4: Cell Communication and Cell Cycle
⁂   AP Biology · Unit 4

4. Cell Communication and Cell Cycle

10–15% of the AP exam. 8 key topics. 4 common errors analyzed.

10–15% exam weight 10 instructional periods

Unit 4: Cell Communication and Cell Cycle

Unit 4 covers cell communication and cell cycle — a core component of the AP Biology curriculum. Master these concepts to build the mechanistic vocabulary the FRQ rubrics demand.

Key topics in this unit

  1. Signal transduction pathway stages: reception, transduction, response
  2. Ligand-receptor specificity and signal amplification via second messengers
  3. Types of cell signaling: endocrine (long-distance), paracrine, autocrine, direct contact (juxtacrine)
  4. Cell cycle stages: interphase (G1, S, G2) and mitotic phase (mitosis + cytokinesis)
  5. Mitosis stages: prophase, metaphase, anaphase, telophase (PMAT)
  6. Cell cycle checkpoints: G1, G2, and spindle assembly checkpoints
  7. Proto-oncogenes, tumor suppressor genes, and cancer as uncontrolled cell division
  8. Apoptosis and its role in development and disease prevention

Common errors — what the rubric penalizes

These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.

Common error

Confusing the reception stage (ligand binds receptor) with the transduction stage (signal relay inside the cell) — students often collapse both into a vague 'binding causes a response' description

Common error

Stating that DNA replication occurs during mitosis rather than during the S phase of interphase

Common error

Mixing up sister chromatids (identical, joined at centromere) with homologous chromosomes (matched pairs, one maternal and one paternal) when describing mitosis vs. meiosis

Common error

Incorrectly claiming that loss of a tumor suppressor gene alone is sufficient to cause cancer, without noting that multiple mutations across proto-oncogenes and tumor suppressors are typically required

Study checklist

  • Can you list all key topics from memory without this page?
  • Have you worked through at least one FRQ-style question per topic?
  • Can you explain each common error above — and why the correct answer differs?
  • Have you reviewed the relevant College Board CED section and formula sheet entries?
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