Regulation of Cell Cycle - AP Biology

1. A lab uses a fluorescent reporter that increases signal when Cdk1 phosphorylates a specific peptide. In control cells, reporter fluorescence rises sharply at mitotic entry and falls at mitotic exit. In experimental cells, fluorescence rises normally but fails to fall, and cells do not reform nuclear envelopes. APC/C substrate tagging is normal, but proteasome function is impaired. Which molecular signaling state best matches the experimental cells?
   Answer: Sustained Cdk1 signaling because M-cyclin is tagged but not degraded, preventing activity decline Signaling-based regulation of the cell cycle is a key skill in understanding how degradation is essential for activity shifts, like mitotic exit. Impaired proteasome function allows APC/C tagging but prevents M-cyclin degradation, sustaining Cdk1 signaling and high fluorescence, blocking envelope reformation. This matches the persistent phosphorylation observed. Tagging alone isn't sufficient; removal is needed. A tempting distractor is choice B, claiming reduced signaling from cyclin removal, but inhibition stabilizes cyclins, misconstruing blockage as enhancement of degradation. To approach similar problems, differentiate between preparatory tagging and the degradative step in regulation.
2. In a signaling assay, a CDK inhibitor binds and inactivates multiple cyclin–CDK complexes. The inhibitor is normally sequestered by a phosphorylated docking protein during late G1, reducing inhibitor availability and increasing CDK activity. A mutation prevents phosphorylation of the docking protein, so the inhibitor remains free. Which outcome is most likely in mutant cells?
   Answer: Reduced cyclin–CDK activity and delayed progression from G1 into S phase This question assesses understanding of signaling-based regulation of the cell cycle, specifically inhibitor sequestration by phosphorylated docking proteins in G1. The mutation prevents docking protein phosphorylation, keeping the inhibitor free to inactivate CDKs. This reduces cyclin–CDK activity and delays G1 to S progression, as in choice A. The outcome aligns with increased inhibitor availability in the assay. A tempting distractor is choice C, which predicts earlier mitosis, but this misconceives that free inhibitor activates rather than inhibits CDKs. A transferable strategy is to trace mutations affecting sequestration to their impact on inhibitor efficacy.
3. In early mitosis, high Cdk1 activity promotes spindle formation. The spindle checkpoint prevents APC/C activation until all kinetochores are attached. A researcher artificially activates APC/C in prometaphase using a chemical dimerizer, without changing kinetochore attachment status. APC/C then ubiquitinates securin and M-cyclin. Which outcome is most likely immediately after APC/C activation?
   Answer: Chromatid separation begins despite unattached kinetochores because securin is removed Signaling-based regulation of the cell cycle is a key skill in understanding how premature regulator activation disrupts checkpoint timing. Artificial APC/C activation in prometaphase ubiquitinates securin, freeing separase to cleave cohesin and separate chromatids before full attachment. This overrides the spindle checkpoint's delay. M-cyclin is also targeted, but separation happens immediately. A tempting distractor is choice B, suggesting increased spindle assembly from M-cyclin accumulation, but APC/C degrades M-cyclin, misconstruing activation as stabilization. To approach similar problems, predict immediate downstream effects of forcing a regulator active out of sequence.
4. Cyclin levels can change rapidly when cyclins are tagged for degradation by ubiquitin ligases. In a population of dividing cells, cyclin B normally drops sharply at the metaphase-to-anaphase transition due to APC/C-mediated ubiquitination. A mutation reduces APC/C ubiquitin-ligase activity but does not affect microtubule attachment to kinetochores. Which effect is most likely observed in the mutant cells?
   Answer: Prolonged high cyclin B–CDK1 activity and delayed exit from mitosis This question assesses understanding of signaling-based regulation of the cell cycle, specifically how APC/C-mediated ubiquitination controls cyclin levels for mitotic exit. The mutation reduces APC/C activity, preventing sharp cyclin B degradation at the metaphase-to-anaphase transition. This results in prolonged high cyclin B–CDK1 activity and delayed mitotic exit, as in choice A, despite normal kinetochore attachment. The effect stems from impaired ubiquitination in the stimulus. A tempting distractor is choice C, which predicts immediate anaphase, but this misconceives that reduced APC/C accelerates rather than hinders securin degradation. A transferable strategy is to consider how ligase mutations disrupt degradation timing and extend phase-specific activities.
5. APC/C activity is regulated by association with an activator protein present in late mitosis. When the activator binds APC/C, APC/C targets M-cyclin for ubiquitination, reducing Cdk1 activity. A mutation prevents the activator from binding APC/C, but APC/C and substrates are otherwise normal. Which outcome is most likely in mutant cells?
   Answer: Failure to exit mitosis because APC/C cannot efficiently ubiquitinate M-cyclin without its activator Signaling-based regulation of the cell cycle is a key skill in understanding how accessory proteins enable core regulators like APC/C. Preventing activator binding keeps APC/C inactive, blocking M-cyclin ubiquitination and sustaining Cdk1, leading to failure in mitotic exit. Substrates are normal, but activation is lost. This halts progression post-metaphase. A tempting distractor is choice D, claiming normal exit because degradation depends on Cdk1 inhibition, but APC/C is required, misconstruing dependency order. To approach similar problems, identify essential cofactors in multi-component regulation.
6. A researcher tests a Cdk1 variant that cannot be phosphorylated at the inhibitory site but still binds M-cyclin and can be activated at the activating site. In cells with this variant, checkpoint signaling that normally inhibits Cdc25-like phosphatase remains intact. After mild DNA damage, cells with wild-type Cdk1 delay mitotic entry. Which effect is most likely in cells expressing the noninhibitable Cdk1 variant?
   Answer: Reduced G2 delay because inhibitory phosphorylation cannot keep M-cyclin–Cdk1 inactive Signaling-based regulation of the cell cycle is a key skill in understanding how inhibitory modifications enforce checkpoints like G2 DNA damage response. The noninhibitable Cdk1 variant can't be kept inactive by phosphorylation, so even with checkpoint-inhibited Cdc25, Cdk1 activates, reducing G2 delay after damage. Wild-type cells delay via inhibitory phosphate retention. Activation site is intact, allowing function. A tempting distractor is choice B, claiming stronger delay because Cdk1 needs inhibitory phosphorylation, but this misinterprets inhibition as requirement, confusing negative regulation. To approach similar problems, test if removing a brake allows progression despite signals.
7. A signaling module at the G2/M checkpoint integrates two inputs: (1) an activating phosphatase that removes inhibitory phosphate from Cdk1, and (2) an inhibitory kinase that adds the inhibitory phosphate. In an experiment, the inhibitory kinase is hyperactive, but the phosphatase activity is also increased twofold. Active M-cyclin–Cdk1 depends on the balance of these opposing activities. Which change would most likely shift the balance toward mitotic entry?
   Answer: Further increase phosphatase activity so inhibitory phosphate removal outpaces addition on Cdk1 This question assesses understanding of signaling-based regulation of the cell cycle, specifically how opposing kinase and phosphatase activities control the G2/M transition. The hyperactive inhibitory kinase adds more inhibitory phosphates to Cdk1, preventing mitotic entry, but the twofold increase in phosphatase activity partially counters this by removing phosphates. Further increasing phosphatase activity, as in choice A, would enhance phosphate removal, outpacing addition and allowing M-cyclin–Cdk1 activation for mitotic entry. This directly addresses the imbalance in the signaling module described. A tempting distractor is choice E, which suggests increasing cyclin transcription, but this misconceives that higher cyclin levels alone can overcome phosphorylation-based inhibition without altering the kinase-phosphatase balance. A transferable strategy is to identify the core regulatory opposition in checkpoints and prioritize interventions that tip the balance toward progression.
8. A kinase cascade activated by extracellular growth factor signaling increases CDK activity by promoting removal of an inhibitory phosphate from a G1 CDK. In the absence of growth factor, the inhibitory phosphate remains on the CDK, and cells do not pass the G1 checkpoint. Researchers introduce a small molecule that mimics the growth factor signal downstream by directly activating the phosphatase that removes the inhibitory phosphate. Nutrients and ATP are sufficient, and DNA is undamaged. Which outcome is most likely after adding the small molecule?
   Answer: Cells bypass G1 arrest and enter S phase because inhibitory CDK phosphorylation is removed This question assesses understanding of signaling-based regulation of the cell cycle, focusing on growth factor signaling and CDK activation at the G1 checkpoint. The small molecule mimics growth factor signaling by activating the phosphatase that removes the inhibitory phosphate from G1 CDK, even without actual growth factor. This enables CDK activation, allowing cells to pass the G1 checkpoint and enter S phase under sufficient nutrient conditions. Cells bypass G1 arrest because the inhibitory phosphorylation is removed, promoting progression. A tempting distractor is B, which suggests metaphase arrest due to APC/C issues, but APC/C is mitotic, stemming from the misconception that G1 signals directly affect later checkpoints. To approach similar problems, map signaling pathways to specific checkpoints and predict outcomes based on which regulatory step is targeted.
9. A researcher adds a nonhydrolyzable ATP analog that binds a CDK's active site but cannot be used for phosphate transfer. Cyclin binding and CDK phosphorylation states are unchanged, but kinase activity toward substrates is reduced. Cells accumulate before entering S phase. Which change would most likely allow progression into S phase while the ATP analog remains present?
   Answer: Provide an alternative kinase that phosphorylates the same S-phase substrates using normal ATP This question assesses understanding of signaling-based regulation of the cell cycle, specifically ATP-dependent phosphate transfer in CDK catalysis for S-phase entry. The nonhydrolyzable ATP analog binds but blocks transfer, reducing substrate phosphorylation and causing pre-S accumulation. Providing an alternative kinase phosphorylating the same substrates with normal ATP, as in choice A, would restore progression. This bypasses the CDK block in the stimulus. A tempting distractor is choice E, which suggests increasing cyclin levels, but this misconceives that more complex overcomes ATP site occupancy. A transferable strategy is to introduce redundant effectors when core catalytic mechanisms are impaired.
10. Cells treated with a microtubule-depolymerizing drug show many unattached kinetochores. The spindle checkpoint signal inhibits APC/C, stabilizing securin and cyclin B. A second compound is added that forces APC/C to remain active even when checkpoint signaling is present. Which outcome is most likely with both drugs present?
    Answer: Sister chromatids separate despite unattached kinetochores, increasing segregation errors This question assesses understanding of signaling-based regulation of the cell cycle, specifically the spindle checkpoint inhibiting APC/C in response to unattached kinetochores. The microtubule drug creates unattached kinetochores, but the second compound forces APC/C activity, overriding inhibition. This causes chromatid separation with errors, as in choice A, due to missegregation. The outcome stems from checkpoint bypass in the stimulus. A tempting distractor is choice C, which predicts metaphase arrest, but this misconceives that forced APC/C sustains rather than overcomes inhibition. A transferable strategy is to combine perturbations to evaluate checkpoint robustness and failure modes.
11. A cell line is synchronized in G2. Entry into mitosis occurs when cyclin B–CDK1 becomes active after removal of an inhibitory phosphate placed on CDK1. In untreated cells, cyclin B–CDK1 activity rises sharply and cells enter mitosis. A drug is added that keeps the inhibitory phosphate on CDK1 even though cyclin B remains bound. Cells remain in G2 with low CDK1 activity. Which change would most likely allow these cells to enter mitosis despite the drug?
    Answer: Provide a phosphatase activity that removes the inhibitory phosphate from CDK1 This question assesses understanding of signaling-based regulation of the cell cycle, specifically the G2/M transition controlled by inhibitory phosphorylation of CDK1. The drug prevents removal of the inhibitory phosphate from CDK1, keeping cyclin B–CDK1 inactive and cells in G2. Providing a phosphatase activity, as in choice B, would remove the inhibitory phosphate, activating CDK1 and allowing mitotic entry despite the drug. This directly counters the drug's effect on the phosphorylation state detailed in the stimulus. A tempting distractor is choice C, which suggests increasing cyclin B concentration, but this misconceives that higher cyclin levels activate CDK1 without addressing the persistent inhibitory phosphate. A transferable strategy is to focus on the specific molecular modification blocking progression and introduce enzymes that reverse it.
12. A cell-cycle checkpoint at metaphase monitors kinetochore attachment to spindle microtubules. When even one kinetochore is unattached, a signaling complex inhibits the anaphase-promoting complex/cyclosome (APC/C). When all kinetochores are attached, inhibition stops, APC/C becomes active, and APC/C tags securin for degradation; separase then cleaves cohesin, allowing sister chromatids to separate. In an experiment, cells are treated with a drug that keeps the inhibitory kinetochore signal active even after all kinetochores are attached. Which change would most likely affect chromosome segregation in treated cells?
    Answer: Delayed anaphase onset because APC/C stays inhibited and securin is not degraded. This question assesses understanding of signaling-based regulation of the cell cycle, specifically the metaphase checkpoint's control of APC/C activity. Normally, unattached kinetochores signal to inhibit APC/C, preventing securin degradation until all attachments occur, after which APC/C activates to allow chromatid separation. The drug maintains the inhibitory signal post-attachment, keeping APC/C inhibited and securin stable, thus delaying anaphase onset and affecting segregation, as stated in choice B. This mimics a persistent checkpoint activation despite proper attachments. A tempting distractor is choice A, proposing earlier separation from prolonged APC/C activity, but this reverses the drug's effect on inhibition, misconstruing signal persistence as activation. A transferable strategy is to map checkpoint signals to their targets and predict outcomes when signals are artificially sustained or disrupted.
13. A researcher studies the restriction point in late G1, where sustained CDK signaling is required to commit a cell to DNA synthesis. In the experiment, cyclin is present and binds CDK, but a competitive inhibitor occupies the CDK active site, preventing phosphorylation of downstream targets. The inhibitor does not affect cyclin binding or CDK abundance. Cells remain in G1 despite adequate nutrients. Which change would most likely allow the cells to enter S phase while the inhibitor is present?
    Answer: Introduce a CDK allele with reduced inhibitor binding but retained catalytic activity This question assesses understanding of signaling-based regulation of the cell cycle, focusing on CDK regulation at the G1 restriction point. The competitive inhibitor occupies the CDK active site, preventing substrate phosphorylation despite cyclin binding and adequate nutrients, causing G1 arrest. Introducing a CDK allele with reduced inhibitor binding but retained catalytic activity allows the mutant CDK to phosphorylate targets, committing cells to S phase. This bypasses the inhibition without altering cyclin or inhibitor levels. A tempting distractor is A, which suggests adding more cyclin, but excess cyclin would still bind inhibited CDK, stemming from the misconception that increasing ligand overcomes active-site competition. To approach similar problems, distinguish between types of inhibition and select genetic changes that specifically evade the block.
14. A sensor protein at kinetochores recruits checkpoint proteins when microtubules are unattached, generating a diffusible inhibitor of APC/C. In a mutant, the sensor cannot recruit checkpoint proteins even when kinetochores are unattached. During mitosis, many chromosomes remain unattached, but APC/C is not inhibited. Which outcome is most likely in the mutant cells?
    Answer: Anaphase begins with missegregation because securin is degraded despite unattached kinetochores This question assesses understanding of signaling-based regulation of the cell cycle, specifically the spindle checkpoint where unattached kinetochores inhibit APC/C via recruited proteins. The mutant sensor fails to recruit inhibitors, allowing APC/C activity despite unattached kinetochores. This leads to securin degradation and anaphase with missegregation, as in choice A, causing errors. The outcome reflects the loss of checkpoint signaling in the stimulus. A tempting distractor is choice D, which predicts metaphase arrest, but this misconceives that failed recruitment strengthens rather than weakens inhibition. A transferable strategy is to predict checkpoint failures by tracing signal generation to effector inhibition.
15. A kinase that promotes S-phase entry is activated only when bound to its cyclin and when a specific activating phosphate is added. In an experiment, cyclin binding occurs normally, but a drug inhibits the activating kinase that adds the activating phosphate. Cells show low activity of the S-phase CDK complex and accumulate in late G1. Which intervention would most likely restore S-phase CDK activity?
    Answer: Provide a CDK variant that is constitutively active without the activating phosphate This question assesses understanding of signaling-based regulation of the cell cycle, specifically dual requirements for CDK activation: cyclin binding and activating phosphorylation. The drug inhibits the activating kinase, reducing S-phase CDK activity despite cyclin binding. Providing a constitutively active CDK variant, as in choice A, would bypass the need for phosphorylation and restore activity. This addresses the specific block in the stimulus. A tempting distractor is choice E, which suggests increasing cyclin concentration, but this misconceives that more cyclin compensates for missing activation. A transferable strategy is to use modified proteins that eliminate dependency on blocked signaling steps.
16. A cell-cycle inhibitor protein is normally degraded after being ubiquitinated by an E3 ligase, allowing cyclin D–CDK4/6 to become active. A mutation prevents the inhibitor from being ubiquitinated, so it persists and continues binding CDK4/6. Flow cytometry shows many cells remain in G1 with 2N DNA. Which change would most likely increase progression into S phase in the mutant cells?
    Answer: Introduce a CDK4/6 variant that cannot bind the inhibitor protein This question assesses understanding of signaling-based regulation of the cell cycle, specifically ubiquitination controlling inhibitor degradation for cyclin D–CDK4/6 activation. The mutation prevents ubiquitination, allowing the inhibitor to persist and bind CDK4/6, causing G1 arrest. Introducing a CDK4/6 variant that cannot bind the inhibitor, as in choice A, would free CDK4/6 and promote S-phase entry. This evades the persistent inhibitor in the mutants. A tempting distractor is choice E, which suggests increasing E3 ligase transcription, but this misconceives that more ligase overcomes a ubiquitination-defective mutation. A transferable strategy is to alter binding interfaces to prevent inhibitory interactions when degradation fails.
17. A cell population is synchronized in early G1, when Cdk activity is low. A mitogen triggers a signaling cascade that increases the activity of G1 cyclin–Cdk complexes, which phosphorylate a gatekeeper protein that otherwise binds and inhibits S-phase–promoting factors. Phosphorylation reduces the gatekeeper's binding, allowing S-phase signaling to proceed. A drug is added that prevents phosphorylation of the gatekeeper without changing cyclin–Cdk binding. Which outcome would most likely be observed after drug treatment?
    Answer: Cells remain in G1 because S-phase–promoting factors stay bound and inhibited Signaling-based regulation of the cell cycle is a key skill in understanding how mitogens and phosphorylation cascades control transitions like G1 to S phase. The drug prevents phosphorylation of the gatekeeper protein, so it continues binding and inhibiting S-phase–promoting factors, blocking S-phase entry despite normal cyclin–Cdk binding. This results in cells remaining in G1, as the gatekeeper's inhibitory effect persists. The mitogen's signaling increases G1 cyclin–Cdk activity, but without gatekeeper phosphorylation, the downstream release of S-phase factors doesn't occur. A tempting distractor is choice B, which claims cells enter S phase normally because cyclin–Cdk binding is unchanged, but this overlooks the gatekeeper's role in transmitting the signal, a common misconception about binding versus functional activation. To approach similar problems, map out the signaling cascade and identify where the intervention disrupts the flow to the endpoint.
18. A fluorescent reporter indicates cyclin E–CDK2 activity. In control cells, activity rises in late G1 and peaks at the G1/S transition. When cells are treated with a phosphatase inhibitor, CDK2 remains phosphorylated at an inhibitory site, and reporter signal stays low despite normal cyclin E binding. Which change would most likely restore reporter signal in the presence of the phosphatase inhibitor?
    Answer: Add a CDK2 variant that lacks the inhibitory phosphorylation site This question assesses understanding of signaling-based regulation of the cell cycle, specifically inhibitory phosphorylation regulating cyclin E–CDK2 at the G1/S transition. The phosphatase inhibitor keeps CDK2 phosphorylated and inactive, reducing reporter signal despite cyclin E binding. Adding a CDK2 variant lacking the inhibitory site, as in choice A, would prevent phosphorylation and restore activity. This counters the inhibitor's effect on the phosphorylation state. A tempting distractor is choice E, which proposes upregulating cyclin E expression, but this misconceives that higher cyclin levels activate a phosphorylatable CDK2. A transferable strategy is to modify target sites to evade regulatory modifications in signaling pathways.
19. A signaling pathway activates an S-phase cyclin–Cdk complex that phosphorylates proteins at replication origins, enabling origin firing. Later in S phase, the same cyclin–Cdk activity phosphorylates an origin-licensing factor, causing it to dissociate so origins cannot be relicensed. A drug prevents phosphorylation of the licensing factor but does not affect origin firing proteins. Which outcome is most likely in treated cells?
    Answer: Origins may fire more than once because licensing factor remains associated after initiation Signaling-based regulation of the cell cycle is a key skill in understanding how phosphorylation prevents re-replication by regulating origin licensing. The drug prevents phosphorylation of the licensing factor, so it doesn't dissociate after origin firing, allowing origins to be relicensed and potentially fire multiple times in one S phase. This could lead to over-replication and genomic instability. Origin firing proceeds normally since firing proteins are unaffected. A tempting distractor is choice B, claiming failure to enter S phase because firing proteins can't be phosphorylated, but this ignores that the drug only affects licensing, misconstruing the dual roles of cyclin–Cdk in initiation versus prevention. To approach similar problems, separate the timing of phosphorylation effects in early versus late S phase.
20. A lab compares two treatments in synchronized G2 cells. Treatment 1 increases M-cyclin concentration; treatment 2 increases activity of a Cdk1-inhibitory kinase that adds an inhibitory phosphate to Cdk1. In both cases, M-cyclin can bind Cdk1. Nuclear envelope breakdown occurs only when active M-cyclin–Cdk1 exceeds a threshold. Which treatment is more likely to delay nuclear envelope breakdown?
    Answer: Treatment 2, because increased inhibitory phosphorylation reduces active M-cyclin–Cdk1 Signaling-based regulation of the cell cycle is a key skill in understanding how molecular concentrations and modifications set thresholds for events like nuclear envelope breakdown. Treatment 2 increases inhibitory kinase activity, adding more inhibitory phosphates to Cdk1 and reducing active M-cyclin–Cdk1 below the threshold, delaying breakdown. Treatment 1 raises M-cyclin but doesn't address phosphorylation, while treatment 2 directly lowers activity. Both allow binding, but inhibition dominates in treatment 2. A tempting distractor is choice C, claiming treatment 1 delays because M-cyclin prevents phosphorylation, but this confuses cyclin binding with inhibition relief, a misconception about complex dynamics. To approach similar problems, compare how each intervention affects the net activity crossing a threshold.

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