AP Biology Cell Communication and Cell Cycle — Worked Answer Explanations

Unit 4 · 12 questions explained

Below is a complete answer key for our AP Biology Cell Communication and Cell Cycle practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Cell Communication and Cell Cycle practice test and come back here to review, or head back to the Cell Communication and Cell Cycle unit overview.

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Question 1 · Easy
Which of the following best describes a ligand in cell signaling?
- A
  A membrane protein that spans the lipid bilayer and detects signals
  Why not A: This describes a receptor, not a ligand.
- B
  A signaling molecule that binds specifically to a receptorCorrect
- C
  An intracellular enzyme that amplifies a signal cascade
  Why not C: Intracellular amplification enzymes are transducers, not ligands.
- D
  A protein released at the synapse to change membrane potential
  Why not D: This describes a neurotransmitter, which is one type of ligand, but does not define ligand broadly.
Explanation
A ligand is any signaling molecule — such as a hormone, growth factor, or neurotransmitter — that binds specifically and non-covalently to a complementary receptor. The ligand-receptor interaction initiates signal transduction. The ligand itself does not need to enter the cell; binding alone triggers a conformational change in the receptor that starts the signaling cascade.

Key takeaway
A ligand is a signaling molecule that binds to a specific receptor to initiate cell communication.
Question 2 · Easy
During which phase of the cell cycle is DNA replicated?
- A
  G₁ phase
  Why not A: G₁ is a gap phase for cell growth and preparation, not DNA replication.
- B
  S phase (synthesis phase)Correct
- C
  G₂ phase
  Why not C: G₂ is the second gap phase where the cell prepares for mitosis; DNA has already been replicated.
- D
  M phase (mitosis)
  Why not D: Mitosis is the process of segregating already-replicated chromosomes into daughter nuclei, not replicating DNA.
Explanation
The cell cycle consists of interphase (G₁, S, G₂) and the M phase (mitosis/cytokinesis). DNA replication occurs specifically during S phase (synthesis phase) of interphase. During S phase, each chromosome is duplicated to produce two identical sister chromatids joined at the centromere, ensuring each daughter cell receives a complete genome after mitosis.

Key takeaway
DNA replication occurs exclusively during S phase of interphase, before the cell enters mitosis.
Question 3 · Easy
Epinephrine binds to a G protein-coupled receptor (GPCR) on a liver cell, leading to glycogen breakdown. Which of the following correctly describes the role of cAMP in this pathway?
- A
  cAMP acts as the first messenger that initially binds the receptor
  Why not A: Epinephrine is the first messenger; cAMP is produced inside the cell after receptor activation.
- B
  cAMP is a second messenger that activates protein kinase A, amplifying the signalCorrect
- C
  cAMP directly breaks down glycogen by acting as a phosphorylase enzyme
  Why not C: cAMP does not directly digest glycogen; it activates a kinase cascade that eventually activates glycogen phosphorylase.
- D
  cAMP inhibits adenylyl cyclase to provide negative feedback immediately
  Why not D: cAMP is produced by adenylyl cyclase; it activates downstream kinases rather than immediately inhibiting its own production.
Explanation
In the epinephrine-GPCR pathway: (1) Epinephrine (first messenger) binds the receptor; (2) the activated G protein stimulates adenylyl cyclase; (3) adenylyl cyclase converts ATP to cyclic AMP (cAMP, the second messenger); (4) cAMP activates protein kinase A (PKA); (5) PKA phosphorylates and activates glycogen phosphorylase, triggering glycogen breakdown. This cascade amplifies the signal because each activated PKA can phosphorylate many targets.

Key takeaway
cAMP is a second messenger that relays and amplifies external signals inside the cell by activating protein kinase A.
Question 4 · Easy
Which of the following is an example of negative feedback in cell signaling?
- A
  Oxytocin release during labor stimulating more uterine contractions, which stimulate more oxytocin release
  Why not A: This is positive feedback, where the response amplifies the original stimulus.
- B
  High blood glucose stimulating insulin release, which lowers blood glucose, reducing insulin secretionCorrect
- C
  A growth factor binding a receptor and triggering DNA replication
  Why not C: This describes signal transduction leading to a cellular response, not a feedback mechanism.
- D
  Calcium ions entering a muscle cell and triggering the release of more calcium
  Why not D: Calcium-induced calcium release is a positive feedback mechanism.
Explanation
Negative feedback occurs when the product or output of a pathway inhibits or reduces the original stimulus, maintaining homeostasis. When blood glucose rises, beta cells in the pancreas release insulin; insulin promotes glucose uptake into cells, lowering blood glucose; the lowered glucose reduces the stimulus for further insulin release. This loop dampens the initial signal and keeps glucose within normal range — the defining feature of negative feedback.

Key takeaway
Negative feedback reduces a stimulus once the desired response is achieved, maintaining homeostasis.
Question 5 · Medium
A researcher treats dividing cells with a drug that prevents the degradation of cyclin B. Based on your knowledge of the cell cycle checkpoint system, which of the following best predicts the result?
- A
  Cells arrest at the G₁/S checkpoint because cyclin B is needed for S phase entry
  Why not A: Cyclin B partners with CDK1 (Cdc2) to form MPF, which drives entry into M phase, not S phase.
- B
  Cells continuously enter and cannot exit mitosis, because MPF activity remains highCorrect
- C
  Cells skip S phase because high MPF signals that DNA is already replicated
  Why not C: Sustained high cyclin B/CDK1 does not skip S phase; it locks cells in mitosis.
- D
  DNA replication occurs multiple times per cell cycle due to continuous cyclin B signaling
  Why not D: Cyclin B/CDK1 drives mitotic entry/exit, not additional rounds of DNA replication.
Explanation
Cyclin B pairs with CDK1 to form MPF (maturation/M-phase promoting factor), which phosphorylates substrates required for mitotic entry (chromosome condensation, spindle formation, nuclear envelope breakdown). Exit from mitosis requires APC/C-mediated degradation of cyclin B, which inactivates CDK1. If cyclin B cannot be degraded, MPF activity remains high, and the cell is trapped in a permanent mitotic state — it can enter mitosis but cannot complete anaphase and cytokinesis, as those require cyclin B destruction.

Key takeaway
Cyclin B degradation is required to inactivate MPF and allow exit from mitosis; its stabilization locks cells in the M phase.
Question 6 · Medium
A lipid-soluble steroid hormone passes through the plasma membrane and binds to an intracellular receptor. How does this differ from signaling through a membrane-bound receptor tyrosine kinase (RTK)?
- A
  Steroid hormones signal faster because they do not require a receptor
  Why not A: Steroid hormones do require a receptor; they simply use an intracellular one, and their effects are typically slower because gene transcription takes time.
- B
  Steroid signaling directly alters gene expression without a second messenger; RTK signaling uses a phosphorylation cascade in the cytoplasmCorrect
- C
  RTK signaling requires the hormone to enter the nucleus, whereas steroid hormones act only at the plasma membrane
  Why not C: It is steroid hormone-receptor complexes that enter the nucleus, not RTK ligands.
- D
  Both pathways use cAMP as a second messenger to transmit their signals
  Why not D: RTKs use phosphorylation cascades (not cAMP) as primary transducers; steroid hormones do not use cAMP at all.
Explanation
Steroid hormones are hydrophobic and diffuse through the lipid bilayer. Their intracellular receptors function as transcription factors: once bound by the hormone, the receptor-hormone complex enters the nucleus and directly regulates gene expression. RTKs, by contrast, are membrane-spanning receptors that, when activated by a ligand, autophosphorylate tyrosine residues and initiate cytoplasmic kinase cascades (e.g., Ras/MAPK pathway). RTK signaling can produce rapid non-transcriptional responses as well as longer-term gene expression changes.

Key takeaway
Steroid hormones act through intracellular receptors that directly regulate transcription; RTKs act through cytoplasmic phosphorylation cascades.
Question 7 · Medium
During the spindle assembly checkpoint (SAC), the cell detects that one kinetochore is not attached to a spindle microtubule. Which of the following correctly describes what happens next?
- A
  The cell immediately undergoes apoptosis because improper attachment signals DNA damage
  Why not A: Unattached kinetochores trigger cell cycle arrest, not immediate apoptosis.
- B
  The cell arrests in metaphase by inhibiting the APC/C until all kinetochores are properly attachedCorrect
- C
  The cell advances to anaphase while attempting to correct the attachment
  Why not C: The SAC prevents anaphase onset until all kinetochores are bi-oriented and under tension.
- D
  The unattached chromosome is degraded by proteosomes before cell division continues
  Why not D: Chromosomes are not degraded during checkpoint arrest; the checkpoint provides time for attachment to be corrected.
Explanation
The spindle assembly checkpoint (SAC) ensures that all sister chromatid pairs are bi-oriented on the metaphase plate before anaphase begins. Unattached kinetochores generate a 'wait' signal by producing the mitotic checkpoint complex (MCC), which inhibits the APC/C (Anaphase-Promoting Complex/Cyclosome). The APC/C normally ubiquitinates securin and cyclin B for degradation, triggering anaphase. As long as even one kinetochore is unattached, MCC inhibits APC/C, arresting the cell in metaphase. Once attachment is established and tension is sensed, MCC production ceases, APC/C is activated, and anaphase proceeds.

Key takeaway
The SAC arrests cells in metaphase by inhibiting APC/C until all kinetochores are properly attached to the spindle.
Question 8 · Medium
A mutation in a proto-oncogene converts it to an oncogene, causing the encoded growth factor receptor to be constitutively active (always 'on') regardless of whether a ligand is bound. Which of the following best explains why this promotes uncontrolled cell division?
- A
  The constitutively active receptor signals the cell to skip checkpoints by degrading p53
  Why not A: A constitutively active receptor mimics growth factor signaling; direct p53 degradation is a separate mechanism used by tumor suppressors.
- B
  The receptor continuously transmits growth and division signals even without a growth factor ligandCorrect
- C
  The receptor migrates into the nucleus and directly activates cell cycle genes
  Why not C: Receptor tyrosine kinases act at the plasma membrane, not by translocating to the nucleus.
- D
  The receptor recruits additional copies of itself, increasing the number of functional receptors
  Why not D: Constitutive activity refers to signaling independent of ligand, not receptor overproduction per se.
Explanation
Proto-oncogenes encode proteins that promote cell growth and division, including growth factors, receptors, and signal transduction proteins. Under normal conditions, receptor tyrosine kinases are activated only when a ligand binds. A gain-of-function mutation converting a proto-oncogene to an oncogene may lock the receptor in an active conformation — it continuously fires downstream growth and division signals (e.g., Ras/MAPK, PI3K/Akt) even without a ligand. This drives the cell to divide relentlessly, a hallmark of cancer.

Key takeaway
Constitutively active receptor oncogenes send continuous cell division signals independent of ligand, bypassing normal growth controls.
Question 9 · Medium
A signaling molecule binds a receptor on Cell A, triggering a phosphorylation cascade that activates a transcription factor. The transcription factor enters the nucleus and upregulates a gene encoding a secreted inhibitory protein. This protein then binds receptors on Cell A itself, reducing receptor activity. What type of signaling and feedback does this describe?
- A
  Paracrine signaling with positive feedback
  Why not A: Paracrine signaling targets neighboring cells; the secreted protein acts back on the original cell (autocrine), and the feedback is negative, not positive.
- B
  Autocrine signaling with negative feedbackCorrect
- C
  Endocrine signaling with negative feedback
  Why not C: Endocrine signaling uses the bloodstream to reach distant cells; this secreted protein acts locally on the cell that produced it.
- D
  Synaptic signaling with positive feedback
  Why not D: Synaptic signaling occurs at synapses between neurons; the feedback described is negative (inhibitory).
Explanation
Autocrine signaling occurs when a cell releases a signal that acts back on itself — through the same or nearby receptors on its own surface. The scenario describes Cell A producing a secreted inhibitory protein that reduces its own receptor activity, which is a classic example of autocrine negative feedback. This type of self-limiting loop is common in growth factor signaling and helps cells modulate their own responses to prevent runaway stimulation.

Key takeaway
Autocrine signaling involves a cell responding to signals it produces itself; negative feedback occurs when those signals reduce the original response.
Question 10 · Hard
Apoptosis (programmed cell death) is triggered in a cell with extensive DNA damage. Which of the following best describes why apoptosis is preferable to continued cell division in this scenario?
- A
  Apoptosis recycles all cellular components into new cells more efficiently than mitosis
  Why not A: While apoptosis does allow phagocytes to recycle materials, efficiency of recycling is not why it is preferred over unchecked division.
- B
  Continued division of damaged cells risks transmitting mutations to daughter cells, potentially creating tumor cellsCorrect
- C
  DNA damage prevents the cell from completing S phase, so apoptosis allows other cells to divide in its place
  Why not C: While the G₁/S checkpoint can halt cells with DNA damage, this does not explain why apoptosis — rather than repair — is chosen for extensive damage.
- D
  Apoptosis sends chemical signals that directly reprogram neighboring cells to replicate the damaged cell's genome
  Why not D: Apoptosis does not involve transfer of genomic information to neighboring cells.
Explanation
When DNA damage is extensive or irreparable, the cell faces a dilemma: attempt continued replication (risking transmitting mutations and chromosomal abnormalities to daughter cells) or activate apoptosis. The tumor suppressor p53 is a key mediator: it detects DNA damage and either upregulates DNA repair genes (if damage is limited) or activates pro-apoptotic genes such as Bax (if damage is irreparable). Apoptosis eliminates the cell in a controlled manner — without inflammation — preventing the propagation of damaged, potentially oncogenic genomes. This is why apoptosis functions as a critical anti-cancer mechanism.

Key takeaway
Apoptosis prevents transmission of irreparable DNA damage to daughter cells, functioning as a critical tumor-suppression mechanism.
Question 11 · Hard
Loss-of-function mutations in both copies of the Rb (retinoblastoma) gene lead to uncontrolled cell division. Based on the role of Rb protein in the cell cycle, explain which checkpoint is affected and how.
- A
  The G₂/M checkpoint is affected; loss of Rb allows cells to enter mitosis with unreplicated DNA
  Why not A: Rb controls the G₁/S checkpoint, not the G₂/M checkpoint; the G₂/M checkpoint monitors DNA damage after replication.
- B
  The G₁/S checkpoint is affected; loss of Rb constitutively releases E2F, driving unscheduled S phase entryCorrect
- C
  The spindle assembly checkpoint is affected; Rb normally prevents kinetochore assembly
  Why not C: Rb is a transcriptional regulator in interphase, not involved in spindle or kinetochore assembly.
- D
  All checkpoints are simultaneously inactivated because Rb coordinates all CDK-cyclin pairs
  Why not D: Rb specifically controls the G₁/S transition; other checkpoints involve distinct regulatory proteins (e.g., Chk1/2, APC/C).
Explanation
Rb (retinoblastoma protein) is a tumor suppressor that regulates the G₁/S checkpoint. In its hypophosphorylated (active) form, Rb binds and sequesters the transcription factor E2F, preventing transcription of genes needed for S phase entry (e.g., cyclin E, DNA polymerase components). In response to appropriate growth signals, cyclin D–CDK4/6 phosphorylates Rb, releasing E2F, which then activates S phase genes. Loss of functional Rb means E2F is constitutively free to drive S phase gene transcription, regardless of growth signals or checkpoint status. This is a gain-of-function for E2F's proliferative activity — the classic two-hit tumor suppressor model in retinoblastoma cancer.

Key takeaway
Rb suppresses cell cycle progression at the G₁/S checkpoint by sequestering E2F; loss of Rb releases E2F constitutively, driving uncontrolled S phase entry.
Question 12 · Hard
A researcher uses FRET (fluorescence resonance energy transfer) to show that a receptor dimerizes upon ligand binding. They then show that a truncated receptor that cannot dimerize fails to activate downstream targets even in the presence of ligand. Which of the following mechanisms is most directly disrupted by preventing dimerization?
- A
  Ligand binding, because dimerization is required for the receptor to adopt a ligand-binding conformation
  Why not A: The truncated receptor still binds ligand (the experiment states ligand is present); dimerization is required for the downstream step, not ligand binding.
- B
  Receptor autophosphorylation and kinase activation, because transphosphorylation between dimerized subunits is required to activate the kinase domainCorrect
- C
  Second messenger release, because dimerization directly synthesizes cAMP
  Why not C: cAMP is synthesized by adenylyl cyclase downstream of G proteins; receptor dimerization does not directly synthesize cAMP.
- D
  Receptor endocytosis, because dimerized receptors cannot be internalized
  Why not D: Receptor internalization (endocytosis) occurs after activation and actually terminates signaling; preventing dimerization inhibits activation, a distinct upstream step.
Explanation
Receptor tyrosine kinases (RTKs) typically function as monomers in the absence of ligand. Ligand binding induces dimerization (or higher-order oligomerization) of the receptor. Once dimerized, the two kinase domains are brought into close proximity and each phosphorylates tyrosine residues on its partner — a process called transphosphorylation or autophosphorylation. These phosphotyrosine residues become docking sites for adaptor proteins (e.g., Grb2) and enzymes that propagate the intracellular signal. Without dimerization, transphosphorylation cannot occur, the kinase domains remain inactive, and no downstream signal is produced despite successful ligand binding — which is precisely what the truncated receptor experiment demonstrates.

Key takeaway
RTK dimerization is required for transphosphorylation of kinase domains; this is the critical step that activates downstream signaling cascades.