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Cell Communication and Cell Cycle

Unit 4 of AP Biology, worth 10–15% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.

Cell signalling pathways, signal transduction, feedback, the cell cycle and its regulation, mitosis and checkpoints.

How this unit is tested

Start by mapping every signaling question onto the same skeleton: a ligand binds a specific receptor, the receptor changes shape, and that shape change is relayed inside the cell (often through phosphorylation or second messengers) until it produces a measurable cellular response such as a gene turning on or an enzyme activating. Most exam questions ask you to identify where in that chain something is blocked or overactive and predict the downstream consequence, so practice tracing pathways forward and backward. For the cell cycle, memorize the order of phases (G1, S, G2, M, cytokinesis) and pair each checkpoint with the specific question the cell is 'asking' there: G1 checkpoint asks about cell size and growth signals, G2/M checkpoint asks whether DNA replicated correctly, and the M (spindle) checkpoint asks whether every chromosome is properly attached to the spindle. Cyclin-CDK complexes are the answer to almost any 'what controls this transition' question, so know that cyclin levels rise and fall while CDK levels stay constant, and only the bound complex is active. When a question mentions feedback, first decide whether the response opposes the stimulus (negative feedback, restores a set point) or reinforces it (positive feedback, drives the process further in the same direction). Cell cycle and cancer questions frequently combine both signal transduction and cell cycle regulation, so be ready to explain how a mutation in a signaling protein (like Ras) or a checkpoint protein (like p53) removes normal control and leads to unchecked division.

What you have to know

Signal transduction pathway
A signal transduction pathway is the sequence of molecular events, beginning with ligand-receptor binding and often including relay proteins, second messengers, or phosphorylation cascades, that converts an extracellular signal into a specific intracellular response such as altered gene expression or enzyme activity.
Cyclin-CDK control
Cyclin-dependent kinases (CDKs) are present at constant levels throughout the cell cycle but are only active when bound to cyclins, whose concentrations rise and fall at specific points; the resulting cyclin-CDK complexes phosphorylate target proteins that trigger transitions between cell cycle phases.
Cell cycle checkpoints
The G1 checkpoint (restriction point) assesses cell size, nutrients, and growth factor signals before committing to division; the G2/M checkpoint verifies that DNA has been completely and correctly replicated; the M (spindle) checkpoint ensures every chromosome is properly attached to spindle fibers before anaphase proceeds.
Negative vs positive feedback
Negative feedback occurs when the output of a pathway inhibits an earlier step, returning the system toward a set point (e.g., p53 halting the cell cycle); positive feedback occurs when the output of a pathway stimulates an earlier step, driving the process further in the same direction (e.g., MPF activation accelerating its own production).
Loss-of-function vs gain-of-function mutations in cancer
Proto-oncogenes normally promote cell division; a gain-of-function mutation converts one to an oncogene that is overactive with only one mutated allele needed. Tumor suppressor genes normally restrain division; typically both alleles must be inactivated (loss of function) before cell cycle control is lost.

14 practice questions

  1. A signaling cell secretes a local chemical that diffuses through extracellular fluid and affects only nearby cells. What type of signaling is this?
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    Answer. Paracrine signaling

    Paracrine signaling involves local regulators that act on nearby target cells, unlike endocrine signaling which uses hormones traveling through the bloodstream to distant cells. Synaptic signaling requires a synapse and neurotransmitter release, and autocrine signaling means the cell signals itself.
  2. Describe the sequence of events after a ligand binds a G protein-coupled receptor (GPCR), up to activation of an intracellular enzyme.
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    Answer. Ligand binding changes the GPCR's shape, allowing it to bind and activate a G protein inside the membrane; the activated G protein exchanges GDP for GTP, dissociates, and binds to and activates (or inhibits) a target enzyme, such as adenylyl cyclase.

    GPCRs work through an intermediary G protein rather than acting as enzymes themselves. The conformational change caused by ligand binding is transmitted to the G protein, which then diffuses along the membrane to reach and regulate its target enzyme.
  3. Adenylyl cyclase, activated by a G protein, converts ATP into a second messenger. Name this second messenger and the kinase it typically activates.
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    Answer. Cyclic AMP (cAMP), which activates protein kinase A (PKA)

    cAMP is a common second messenger produced from ATP by adenylyl cyclase. Once formed, cAMP binds to and activates PKA, which then phosphorylates various target proteins to carry out the cellular response.
  4. Why does a phosphorylation cascade amplify a signal rather than simply pass it along unchanged?
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    Answer. Because each activated kinase in the cascade can phosphorylate and activate many copies of the next kinase, so the number of activated molecules multiplies at each step.

    A single activated receptor might activate one relay protein, but that protein can then activate several kinase molecules, each of which activates several more downstream, producing an exponential increase in activated molecules and a large cellular response from a small initial signal.
  5. Put the following cell cycle phases in correct order: mitosis, G2, S, G1.
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    Answer. G1, S, G2, mitosis

    The cell cycle proceeds through interphase first — G1 (growth), S (DNA synthesis/replication), and G2 (growth and preparation for division) — before entering the M phase (mitosis) where the nucleus and cell divide.
  6. What condition must be met for a cell to pass the G1 checkpoint (restriction point)?
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    Answer. The cell must be an adequate size, have sufficient nutrients, and receive appropriate growth factor signals indicating favorable conditions for division.

    The G1 checkpoint is the primary decision point for whether a cell will divide at all; if conditions are unfavorable, the cell can exit the cycle into a non-dividing G0 state instead of proceeding to S phase.
  7. What is MPF, and what checkpoint does it control?
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    Answer. MPF (maturation-promoting factor) is a cyclin-CDK complex that triggers the cell's passage through the G2/M checkpoint into mitosis.

    MPF activity rises as cyclin accumulates during G2 and binds to CDK; once MPF concentration is high enough, it phosphorylates target proteins that initiate chromosome condensation and other mitotic events.
  8. At the M (spindle) checkpoint, what specific condition is verified before the cell is allowed to proceed to anaphase?
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    Answer. That every chromosome's kinetochore is properly attached to spindle microtubules from opposite poles.

    If even one chromosome is unattached or improperly attached, the spindle checkpoint delays anaphase, preventing unequal distribution of chromosomes to daughter cells, which would cause aneuploidy.
  9. The protein p53 detects DNA damage and halts the cell cycle at the G1 checkpoint until repairs are made, or triggers apoptosis if damage is irreparable. Is this an example of negative or positive feedback, and why?
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    Answer. Negative feedback, because the detection of a problem (DNA damage) produces a response (cell cycle arrest) that opposes further progression until the problem is corrected, restoring normal conditions.

    Negative feedback loops counteract a deviation to return the system toward a stable state. Here, the abnormal signal (damaged DNA) is met with a response that stops the process causing the deviation, rather than reinforcing it.
  10. During G2, rising MPF activity phosphorylates and activates more cyclin-CDK complexes, causing MPF concentration to rapidly spike right before mitosis begins. What type of feedback loop does this describe?
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    Answer. Positive feedback

    MPF's own activity increases the production and activation of more MPF, reinforcing the same direction of change rather than opposing it. This rapid, self-amplifying rise is characteristic of positive feedback, which drives a fast, decisive transition into mitosis.
  11. A cell's DNA sustains damage so severe it cannot be repaired. What cellular process is normally triggered as a result, and what molecule commonly initiates it?
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    Answer. Apoptosis (programmed cell death), typically triggered by p53 detecting the irreparable damage.

    Rather than allowing a cell with damaged DNA to continue dividing and potentially pass on mutations, the cell activates a controlled self-destruction pathway. p53 plays a central role in sensing damage and, if repair fails, activating genes that lead to apoptosis.
  12. A tumor cell is found to have both copies of the Rb gene inactivated by mutation. Predict the effect on cell cycle regulation and explain why both copies typically need to be affected.
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    Answer. With both Rb alleles inactivated, the cell loses the brake normally placed on the G1 checkpoint, allowing uncontrolled progression into S phase and division; because Rb is a tumor suppressor, a single functional copy is usually sufficient to maintain control, so both alleles must be lost (two-hit hypothesis) for regulation to fail.

    Tumor suppressor genes work as loss-of-function mutations, and one normal copy is generally enough to perform the braking function. Only when both alleles are disabled does the cell lose this checkpoint control entirely, contributing to unregulated growth characteristic of cancer.
  13. A single point mutation in the Ras gene locks the Ras protein in its active, GTP-bound state. Explain the likely downstream effect on cell division, and classify this as a gain- or loss-of-function mutation.
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    Answer. This is a gain-of-function mutation; because active Ras continuously stimulates the downstream MAP kinase cascade regardless of growth factor presence, genes promoting cell division are expressed constitutively, leading to uncontrolled cell proliferation.

    Ras is a proto-oncogene product that normally cycles between active (GTP-bound) and inactive (GDP-bound) states. A mutation preventing GTP hydrolysis keeps it permanently 'on,' so only one mutated allele is needed to drive continuous signaling, consistent with Ras acting as an oncogene.
  14. How do proto-oncogenes and tumor suppressor genes differ in the type of mutation that leads to cancer?
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    Answer. Proto-oncogenes become cancer-causing through gain-of-function mutations that increase or make constitutive their normal growth-promoting activity, often requiring only one mutated allele; tumor suppressor genes become cancer-causing through loss-of-function mutations that eliminate their normal growth-inhibiting activity, typically requiring both alleles to be inactivated.

    This distinction explains why oncogenes behave dominantly (one bad copy is enough) while tumor suppressor loss usually behaves recessively at the cellular level (both copies must fail). Recognizing which category a mutated gene falls into lets you predict whether the mutation increases or decreases a specific cellular activity.

What people get wrong

  1. Confusing the G1 checkpoint with the G2/M checkpoint on exam questions. Fix: attach a specific question to each checkpoint (G1 asks 'is there enough growth signal/nutrients?', G2/M asks 'is DNA replication complete and undamaged?', M asks 'are all chromosomes attached to the spindle?') rather than memorizing them as an unordered list.
  2. Treating mitosis and the full cell cycle as the same thing, and including S phase (DNA replication) inside 'mitosis.' Fix: remember mitosis (M phase) is only the division of the nucleus and cell, occurring after interphase (G1, S, G2) is already complete.
  3. Assuming a phosphorylation cascade only transmits the signal rather than amplifying it. Fix: state explicitly that one active kinase phosphorylates many copies of the next kinase, so the signal grows in magnitude at each step, not just gets passed along unchanged.
  4. Mixing up proto-oncogenes and tumor suppressor genes when explaining a cancer scenario. Fix: ask whether the mutation makes a gene more active (proto-oncogene to oncogene, gain of function, one allele suffices) or less active (tumor suppressor, loss of function, usually needs both alleles hit).
  5. Labeling every self-reinforcing biological process as 'bad' or every corrective process as 'positive.' Fix: define feedback strictly by direction relative to the original stimulus — if the response counteracts the stimulus it's negative feedback, if it amplifies the stimulus it's positive feedback, regardless of whether the outcome seems beneficial or harmful.

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