AP Biology, AP exams, 10–15%

Unit 4: Cell Communication and Cell Cycle

Unit 4 links signaling to control: how cells send and receive signals, how a signal is transduced and amplified inside the cell, and how feedback loops keep systems in range. It ends with the cell cycle and its checkpoints, including what happens when regulation fails. Many questions here present an unfamiliar pathway and ask you to reason about a disruption.

Position: 4 of 8 units
Published weighting: 10–15% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Cell communication: Direct contact, local regulators, and long-distance chemical signaling, with the receptor types each uses.
Signal transduction: Reception, transduction through second messengers and phosphorylation cascades, amplification, and cellular response.
Changes in signal transduction pathways: How mutations, drugs, or missing components alter a pathway, and how to predict the downstream effect.
Feedback mechanisms: Negative feedback as the maintainer of homeostasis and positive feedback as an amplifier toward an endpoint.
Cell cycle: Interphase and mitotic phases, the events of each stage, and how DNA content changes across the cycle.
Regulation of the cell cycle: Checkpoints, cyclins and cyclin-dependent kinases, and the consequences of losing checkpoint control.

Study tips

How to practice Communication and Cell Cycle.

Section-specific guidance that follows from how this material is usually tested.

Practice the standard prompt pattern: a component is blocked or made constitutively active, and you predict the response.
Keep negative and positive feedback on one card with a physiological example of each; the distinction shows up in Units 4 and 8.
Graph DNA content per cell across the cell cycle once from memory; it clarifies both mitosis and, later, meiosis.

Where this fits

Unit 4: Cell Communication and Cell Cycle is 4 of 8 units in the AP Biology guide, and it carries a published weighting of 10–15% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Biology guide Official exam information

Sample Communication and Cell Cycle questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What type of regulatory loop returns a variable toward its set point after a disturbance, as when insulin release lowers elevated blood glucose?
1. APositive feedback
2. BNegative feedback
3. CSignal amplification
4. DApoptosis
Show answer
Answer: B. Negative feedback
Negative feedback opposes the change that triggered it, which is what keeps homeostatic variables inside a narrow range. Positive feedback is the tempting distractor, but it amplifies the original change and drives a process to completion, as in childbirth or blood clotting. Signal amplification describes how one signal molecule produces many downstream products and says nothing about direction.
Key insight: Negative feedback stabilizes; positive feedback accelerates toward an endpoint.
Question 2Foundational
During which phase of the cell cycle is a cell's DNA replicated?
1. AG1 phase
2. BG2 phase
3. CMitosis
4. DS phase
Show answer
Answer: D. S phase
S phase is the synthesis portion of interphase, during which each chromosome is copied so that it consists of two identical sister chromatids. G1 and G2 are growth and preparation phases with no DNA synthesis. Mitosis is the tempting distractor because chromosomes are most visible then, but by that point replication has already finished.
Key insight: DNA content per cell doubles during S phase and halves again only at the end of mitosis.
Question 3Intermediate
What is the general term for a small intracellular molecule such as cyclic AMP that relays and amplifies a signal after a ligand binds a membrane receptor?
1. AA second messenger
2. BA ligand
3. CA transcription factor
4. DA membrane receptor
Show answer
Answer: A. A second messenger
Second messengers such as cyclic AMP and calcium ions spread rapidly through the cytosol and activate many target proteins, which is how one bound ligand can produce a large response. The ligand itself is the first messenger and usually never enters the cell. Transcription factors act at DNA at the end of many pathways rather than serving as the diffusible relay.
Key insight: Amplification is why a few hormone molecules outside the cell can change millions of molecules inside it.
Question 4Intermediate
Which cell cycle checkpoint verifies that every chromosome is attached to spindle fibers before the cell is allowed to separate sister chromatids?
1. AThe G1 checkpoint
2. BThe M checkpoint
3. CThe G2 checkpoint
4. DThe S checkpoint
Show answer
Answer: B. The M checkpoint
The M checkpoint operates at the metaphase-to-anaphase transition and holds the cell until every kinetochore is correctly attached, which prevents chromosomes from being distributed unevenly. The G2 checkpoint is the tempting distractor, but it confirms that DNA replication finished without damage, before the spindle even forms. The G1 checkpoint assesses cell size, nutrients, and growth signals.
Key insight: G1 asks whether to divide, G2 asks whether the DNA is ready, and M asks whether the spindle is attached.
Question 5Advanced
A mutation locks a G protein in its active form so that it no longer requires a ligand-bound receptor. What is the most likely effect on the pathway's cellular response?
1. AThe response never occurs under any conditions
2. BThe response occurs only at unusually high ligand concentrations
3. CThe response occurs continuously even without a signal
4. DThe receptor is destroyed and the pathway is permanently silenced
Show answer
Answer: C. The response occurs continuously even without a signal
A component locked in the on state passes the signal along regardless of what happens upstream, so the cell behaves as though the ligand were always present. The tempting answer is that the response never occurs, but that is the outcome of a loss-of-function mutation, which is the opposite situation. Constitutive activation of growth pathways is one common route to uncontrolled cell division.
Key insight: A pathway component stuck on mimics a constant signal; a missing component mimics no signal at all.

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Unit 4: Cell Communication and Cell Cycle