AP Biology Unit 4 review: Cell Communication and Cell Cycle
Unit 4 covers how cells receive signals, change their activity and control division. It carries 10–15% of multiple choice. Expect to predict the effect of blocking a receptor or relay protein, explain feedback and connect checkpoint failure to uncontrolled cell division.
Unit 4 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 10–15% of the multiple-choice section |
| Exam date | Monday, May 3, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 6 questions in 90 minutes, worth 50%. Two long questions worth 9 points each and four short questions worth 4 points each. |
| Calculator | Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027. |
| Class time | About 12–14 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 71% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 4 covers
College Board's course description splits the unit into 6 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 4.1 Cell Communication | Compare direct contact, local signals and long-distance signaling. Explain why only particular target cells respond. |
| 4.2 Introduction to Signal Transduction | Identify reception, transduction and response in a signaling model. |
| 4.3 Signal Transduction Pathways | Predict how a changed receptor, relay protein or second messenger alters signal transmission and amplification. |
| 4.4 Feedback | Use a feedback diagram to predict whether a response counters or reinforces the initiating change. |
| 4.5 Cell Cycle | Track DNA replication, chromosome movement and division through interphase, mitosis and cytokinesis. |
| 4.6 Regulation of Cell Cycle | Explain checkpoints, cyclin-dependent regulation and how altered growth control can cause cancer. |
Trace a disruption through a pathway
Follow the order of events and state which downstream response depends on the disrupted step.
| Step or control | Normal role | Prediction after disruption |
|---|---|---|
| Receptor | Binds a compatible signaling molecule | Loss of binding can prevent a response even when ligand concentration rises. |
| Relay protein | Passes a signal to the next pathway component | A permanently active relay may trigger a response without continued ligand binding. |
| Amplification | One activated molecule activates many downstream molecules | Blocking an early step can reduce a much larger downstream response. |
| Negative feedback | The response reduces the initial disturbance | Failure can prolong a deviation from the regulated range. |
| Positive feedback | The response reinforces the initial disturbance | The response continues until a limiting event or external stop occurs. |
| G1/S checkpoint | Checks conditions before DNA replication proceeds | Failure can allow damaged DNA to enter replication. |
| Spindle checkpoint | Delays chromosome separation until attachment is adequate | Failure can produce unequal chromosome distribution. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Listing checkpoint names without their timing or purpose. State what is checked and which next step is delayed. The CED identifies this error on printed p. 81.
- Assuming ligand binding is the entire response. Explain the relay steps and the final change in enzyme activity or gene expression. See CED p. 81 and topics 4.2–4.3.
- Treating all feedback as stabilizing. A reinforcing response is positive feedback and requires a stopping condition. See CED topic 4.4.
- Doubling chromosome number merely because DNA replicates. Each replicated chromosome has two sister chromatids until their separation. See CED topic 4.5.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 4.1
A hormone reaches liver and skin cells, but only the liver cells respond. Which observation best explains the difference?
- AOnly liver cells contain the hormone in their DNA
- BOnly liver cells translate using the genetic code
- COnly liver cells change the hormone into a receptor
- DOnly liver cells express the required signaling proteins
Show the answer
D. A signal produces a response only in a cell with the required receptor and downstream components. Exposure alone is insufficient. Cells can share a genome while expressing different signaling proteins.
Question 2 · 4.3
In a pathway, receptor R activates kinase A, which activates kinase B. A mutation keeps B active at all times. A drug blocks R. What is the most likely outcome?
- AB becomes inactive immediately
- BB can continue activating its downstream target
- CThe drug changes B’s DNA sequence
- DAll cellular signaling stops
Show the answer
B. The mutation places B in an active state independently of upstream activation. Blocking R therefore may leave the downstream response on. This is why the position of a mutation matters when predicting a drug’s effect.
Question 3 · 4.3
One activated receptor activates 8 molecules of A. Each A activates 15 molecules of B. Under this model, how many B molecules can be activated per receptor?
- A23
- B8
- C120
- D225
Show the answer
C. Multiply the amplification factors, 8 × 15 = 120. Addition counts molecules at separate stages without accounting for each A acting on multiple B molecules. The number assumes every activation in the stated model occurs.
Question 4 · 4.4
A rise in a hormone causes a gland to reduce secretion of that same hormone. Which description fits?
- ANegative feedback
- BPositive feedback
- CIndependent assortment
- DAn irreversible mutation
Show the answer
A. The response reduces the disturbance that initiated it. This is negative feedback. The sign describes the direction of the feedback effect, rather than whether the hormone is beneficial or harmful.
Question 5 · 4.5
A diploid cell has 10 chromosomes in G1. Immediately after S phase and before mitosis, how many chromosomes and chromatids does it contain?
- A20 chromosomes and 20 chromatids
- B10 chromosomes and 10 chromatids
- C20 chromosomes and 40 chromatids
- D10 chromosomes and 20 chromatids
Show the answer
D. DNA replication produces two sister chromatids per chromosome. Before those sisters separate, there are still 10 chromosomes, now consisting of 20 chromatids. Counting centromeres helps prevent doubling too early.
Question 6 · 4.6
A damaged cell enters S phase despite extensive DNA damage. Which control most directly failed?
- AGolgi sorting of newly synthesized membrane proteins
- BG1/S checkpoint control before DNA replication
- CSpindle attachment checking before chromatid separation
- DCytokinesis after the completion of nuclear division
Show the answer
B. The failure occurs before DNA replication. A checkpoint at the G1-to-S transition normally helps prevent damaged DNA from being copied. The timing identifies the relevant control more precisely than saying only that mitosis failed.
How to study Unit 4
Redraw a signaling diagram without its labels. Add the ligand, receptor, relay molecules and response. Then block one component at a time and explain the effect downstream. Include a permanently active protein so that you practice more than loss-of-function cases.
For the cell cycle, make a table of DNA amount, chromosome count and checkpoint function at each stage. Use counts from a small diploid cell to keep homologous chromosomes and sister chromatids distinct.
Ashore's Biology course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
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