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Credit Sprint's Guide to Biology Unit 4: Cell Communication and Cell Cycle

AP® weight 10–15%CLEP® weight 12–16%

Cells talk to each other with chemical signals that are received, passed along inside the cell and turned into a response. Feedback keeps the body stable, and the cell cycle, guarded by checkpoints, controls when cells divide.

The cell cycleG1SG2MInterphaseCheckpoints at G1, G2 and M decide whether the cycle continues

What you need to know

  • Signaling can be direct contact, local (paracrine, synaptic) or long-distance (hormones in the blood).
  • Signal transduction has three stages: reception (ligand binds a receptor), transduction (a relay, often a phosphorylation cascade or second messengers like cAMP) and response.
  • Nonpolar signals (steroid hormones) cross the membrane and bind receptors inside the cell; polar signals bind membrane receptors.
  • A cascade amplifies the signal: one ligand can activate many molecules.
  • Negative feedback brings a variable back toward a set point (blood glucose, body temperature). Positive feedback pushes a change further (labor contractions, blood clotting).
  • The cell cycle: G1 (growth), S (DNA replication), G2 (preparation), then M (mitosis) and cytokinesis. Many cells leave the cycle and rest in G0.
  • Mitosis: prophase, metaphase (chromosomes line up), anaphase (sister chromatids separate), telophase. Result: two genetically identical diploid cells.
  • Checkpoints (G1, G2, M) check DNA damage and spindle attachment. Cyclins and cyclin-dependent kinases (CDKs) drive the cycle forward.
  • Cancer comes from mutations that disable checkpoints: oncogenes push division on; damaged tumor-suppressor genes (such as p53) stop braking it.
  • Apoptosis is programmed cell death, which removes damaged or unneeded cells.

Equations and rules

Mitotic index = cells in mitosis ÷ total cells counted
Chromosome count stays 2n through mitosis; DNA content doubles in S phase

Exam traps

Watch forSister chromatids separate in anaphase of mitosis; homologous chromosomes separate in anaphase I of meiosis.
Watch forA mutation that blocks a receptor stops the response even if plenty of ligand is present.
Watch forPositive feedback doesn't mean 'good'. It means the response amplifies the original change.

Key terms

  • Ligand: A signaling molecule that binds a receptor.
  • Signal transduction: The chain of steps that turns a received signal into a cell response.
  • Second messenger: A small molecule (such as cAMP or Ca²⁺) that spreads a signal inside the cell.
  • Phosphorylation cascade: Kinases add phosphates to the next protein in line, amplifying a signal.
  • Negative feedback: A response that reverses a change, keeping conditions near a set point.
  • Positive feedback: A response that amplifies a change, such as contractions during childbirth.
  • Interphase: G1, S and G2: the cell grows and copies its DNA.
  • Mitosis: Nuclear division that makes two identical nuclei: prophase, metaphase, anaphase, telophase.
  • Cyclin: A protein whose level rises and falls through the cycle, activating CDKs.
  • Checkpoint: A control point (G1, G2, M) that stops the cycle if something is wrong.
  • Apoptosis: Programmed cell death.
  • Homeostasis: Keeping internal conditions stable.
  • Xylem: Plant tissue that carries water and minerals up from the roots.
  • Phloem: Plant tissue that carries sugars from sources (leaves) to sinks (roots, fruits).
  • Stomata: Leaf pores that let in CO₂ and let out water vapor and O₂. Guard cells open and close them.
  • Neuron: A nerve cell that sends signals with action potentials.
  • Action potential: A rapid change in membrane voltage that travels down an axon.
  • Antibody: A protein made by B cells that binds a specific antigen.

Practice questions

Five of the 32 Unit 4 questions in Credit Sprint. Every choice is explained in the app.

1. Which sequence correctly lists the three stages of cell signaling?
  1. Reception, transduction, response
  2. Response, reception, transduction
  3. Transcription, translation, response
  4. Transduction, reception, amplification
Show answer

Reception, transduction, response. The signal is received, relayed and acted on.

2. Which is an example of negative feedback?
  1. Rising body temperature triggers sweating, which lowers temperature
  2. Contractions during childbirth trigger more oxytocin
  3. Platelets at a wound attract more platelets
  4. Ripening fruit releases ethylene that ripens nearby fruit
Show answer

Rising body temperature triggers sweating, which lowers temperature. The response reverses the change.

3. A drug prevents spindle fibers from forming. Cells treated with it will most likely stop in
  1. S phase
  2. cytokinesis
  3. G1
  4. metaphase, because the M checkpoint detects unattached chromosomes
Show answer

metaphase, because the M checkpoint detects unattached chromosomes. No spindle, no anaphase.

4. A neurotransmitter crossing a synapse to a neighboring cell is an example of
  1. direct contact through gap junctions
  2. endocrine signaling through the blood
  3. short-distance (local) signaling
  4. transcription
Show answer

short-distance (local) signaling. Synaptic signaling is local.

5. Antibodies are produced by
  1. helper T cells directly
  2. plasma cells that develop from B cells
  3. red blood cells
  4. platelets
Show answer

plasma cells that develop from B cells. They're part of the adaptive response.