Regulation of Cell Cycle
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AP Biology › Regulation of Cell Cycle
In a cell-free extract, cyclin binds CDK to form an active complex that phosphorylates multiple substrates. A phosphatase removes these phosphates, opposing CDK signaling. When cyclin concentration is held constant, adding a phosphatase inhibitor causes substrate phosphorylation to remain high for longer, even though CDK activity is unchanged. In intact cells, sustained phosphorylation of mitotic substrates correlates with delayed exit from mitosis. Which change would most likely shorten mitotic duration in cells treated with the phosphatase inhibitor?
Increase kinase activity that adds inhibitory phosphate onto CDK to block mitotic entry
Increase DNA replication origin firing to ensure rapid completion of S phase
Decrease APC/C activity so cyclin persists and CDK signaling remains elevated
Stabilize cohesin to prevent chromatid separation and accelerate mitotic exit
Reduce cyclin availability so CDK-dependent phosphorylation occurs at a lower rate
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, focusing on CDK-cyclin dynamics and phosphorylation balance during mitosis. The phosphatase inhibitor sustains high phosphorylation of mitotic substrates by preventing dephosphorylation, leading to delayed mitotic exit in intact cells. Reducing cyclin availability decreases CDK-dependent phosphorylation rates, allowing existing phosphatase activity to lower phosphorylation levels more quickly. This shortens mitotic duration by reducing the overall signaling strength opposing exit. A tempting distractor is B, which suggests decreasing APC/C activity to persist cyclin, but this would prolong high CDK activity, stemming from the misconception that inhibiting degradation accelerates exit rather than delays it. To approach similar problems, consider the balance between kinase and phosphatase activities and select changes that tip it toward the desired outcome.
A signaling module at the G2/M transition uses positive feedback: active CDK–cyclin activates a phosphatase that removes the CDK inhibitory phosphate, and simultaneously inhibits the kinase that adds the inhibitory phosphate. This creates a rapid switch from low to high CDK activity. A mutation prevents CDK from activating the phosphatase, but cyclin binding and the inhibitory-kinase inhibition remain normal. Which effect would most likely be observed in the mutant cells?
A slower, less abrupt rise in CDK activity, delaying entry into mitosis
Failure of DNA replication because kinetochore attachment signaling is reduced in G2
Permanent G1 arrest because separase cannot cleave cohesin without phosphatase activation
Faster mitotic entry because inhibitory phosphorylation accumulates more quickly on CDK
Immediate anaphase onset because APC/C is activated earlier by the phosphatase
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, focusing on positive feedback in CDK activation at G2/M. The mutation prevents CDK from activating the phosphatase that removes its inhibitory phosphate, disrupting the feedback loop for rapid CDK activation. This results in a slower, less abrupt rise in CDK activity as inhibitory phosphate removal is impaired, delaying mitotic entry. The remaining inhibition of the kinase provides some activation but lacks the switch-like rapidity. A tempting distractor is D, which suggests faster entry due to phosphorylation accumulation, but accumulation inhibits, stemming from the misconception that losing positive feedback accelerates rather than slows transitions. To approach similar problems, dissect feedback mechanisms and evaluate how partial disruptions affect timing and sharpness of phase changes.
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?
Delayed anaphase onset because APC/C stays inhibited and securin is not degraded.
Immediate cytokinesis because unattached kinetochores activate myosin phosphorylation pathways.
Earlier sister chromatid separation because APC/C remains active longer at metaphase.
Increased DNA replication because cohesin cleavage directly initiates origin firing in S phase.
Normal anaphase timing because checkpoint signaling only monitors DNA damage, not attachment.
Explanation
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.
In an in vitro assay, cyclin A–CDK2 phosphorylates a substrate required for progression through S phase. A second protein binds cyclin A–CDK2 and blocks access to the substrate without removing cyclin A. When this blocker is added to cells, DNA content analysis shows fewer cells with intermediate (between 2N and 4N) DNA. Which change would most likely restore S-phase progression while the blocker remains present?
Add a small molecule that prevents the blocker from binding cyclin A–CDK2
Increase cyclin A synthesis to overwhelm the blocker through gene regulation
Inhibit cyclin B–CDK1 dephosphorylation to extend G2 phase
Prevent kinetochore attachment so cells remain longer in metaphase
Activate APC/C early to degrade securin before chromosomes align
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, specifically S-phase progression requiring cyclin A–CDK2 substrate phosphorylation. The blocker binds cyclin A–CDK2, preventing substrate access and reducing cells with intermediate DNA content. Adding a small molecule that prevents blocker binding, as in choice A, would restore substrate access and S-phase progression. This directly counters the blocker's mechanism in the assay. A tempting distractor is choice D, which proposes increasing cyclin A synthesis, but this misconceives that higher levels overcome binding without displacing the blocker. A transferable strategy is to use inhibitors that target accessory proteins rather than core complexes in signaling disruptions.
In a cultured mammalian cell line, entry into mitosis depends on a checkpoint where cyclin B binds Cdk1 to form active MPF. MPF activity rises sharply only when inhibitory phosphates on Cdk1 are removed by a phosphatase, and MPF activity falls when cyclin B is rapidly degraded by a ubiquitin-ligase complex activated later in mitosis. A researcher adds a small molecule that prevents removal of the inhibitory phosphates on Cdk1 without changing cyclin B abundance.
Which change would most likely occur in cell cycle progression after treatment?
Cells complete mitosis faster because inhibitory phosphorylation stabilizes spindle microtubule attachments.
Cells arrest in G1 because cyclin B degradation prevents activation of G1/S cyclin-Cdk complexes.
Cells bypass the G2 checkpoint and enter mitosis early due to increased MPF activity.
Cells accumulate in G2 because Cdk1 remains inhibited despite cyclin B binding.
Cells accelerate S phase because cyclin B directly activates DNA polymerase at replication forks.
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, emphasizing the activation of mitosis-promoting factor (MPF) for G2/M transition. Normally, MPF forms when cyclin B binds Cdk1, but full activity requires removal of inhibitory phosphates by a phosphatase to trigger mitosis entry. The small molecule prevents this dephosphorylation, keeping Cdk1 inhibited even with cyclin B present, causing cells to accumulate in G2 without progressing to mitosis, as in choice A. This blocks the sharp rise in MPF activity needed for mitotic entry. A tempting distractor is choice B, suggesting early mitosis due to increased MPF, but this ignores the necessity of dephosphorylation for activation, confusing binding with full enzymatic function. A transferable strategy is to identify key activation steps in cyclin-CDK complexes and evaluate how inhibitors affect checkpoint passage.
A lab measures CDK activity using phosphorylation of a peptide substrate. In control cells, cyclin B–CDK1 activity is high in metaphase and then drops after APC/C activation. In a mutant, securin is present but cannot bind separase. APC/C activation and cyclin B degradation still occur normally. Which prediction is most consistent with the mutant’s signaling state during metaphase-to-anaphase transition?
Reduced APC/C activation because securin binding to separase is required for APC/C signaling
Failure to activate cyclin E–CDK2 because securin normally activates CDK2
Earlier cohesin cleavage because separase is not inhibited by securin binding
G1 arrest because securin is required to phosphorylate the G1 checkpoint protein
Delayed cyclin B degradation because separase must cleave cohesin first
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, specifically securin's role in inhibiting separase until APC/C-mediated degradation. In the mutant, securin cannot bind separase, leaving separase uninhibited. This allows earlier cohesin cleavage, as in choice A, even with normal APC/C and cyclin B dynamics. The prediction fits the binding defect during the transition. A tempting distractor is choice B, which links cyclin B degradation to separase, but this misconceives the independent regulation of securin and cyclin B by APC/C. A transferable strategy is to isolate the functional impact of binding mutations on downstream effectors.
Cells exposed to a DNA-damaging agent activate a checkpoint kinase that phosphorylates and inhibits a Cdc25-like phosphatase. When inhibited, the phosphatase cannot remove inhibitory phosphates from Cdk1, keeping M-cyclin–Cdk1 inactive. In a follow-up treatment, a second drug blocks the checkpoint kinase’s catalytic activity without affecting other kinases. The damaged DNA remains unrepaired during the observation window.
Which effect would most likely occur after adding the second drug?
Cells permanently inactivate Cdk1 because M-cyclin cannot bind without phosphorylation
Cells delay cytokinesis because cohesin cleavage requires checkpoint kinase activation
Cells more readily enter mitosis because inhibitory signaling to Cdc25-like phosphatase is reduced
Cells arrest in S phase because APC/C is directly inhibited by the checkpoint kinase
Cells prevent DNA damage by increasing repair enzyme transcription before mitosis
Explanation
Signaling-based regulation of the cell cycle is a key skill in understanding how DNA damage checkpoints prevent mitotic entry with unrepaired DNA. The second drug blocks the checkpoint kinase, preventing inhibition of the Cdc25-like phosphatase, so inhibitory phosphates are removed from Cdk1, activating M-cyclin–Cdk1 and allowing mitotic entry despite damage. This overrides the normal G2 arrest signal. The damaged DNA persists, but the treatment removes the inhibitory block on phosphatase activity. A tempting distractor is choice B, suggesting S-phase arrest because APC/C is inhibited, but this confuses G2 checkpoint mechanisms with mitotic regulators, a misconception about phase-specific controls. To approach similar problems, trace how inhibiting an inhibitor affects the pathway's net output.
During G2, a CDK–cyclin complex is present but kept inactive by an inhibitory phosphate. When DNA damage is detected, signaling increases activity of the kinase that adds the inhibitory phosphate and decreases activity of the phosphatase that removes it, maintaining CDK inactivity. A mutant cell line cannot increase the inhibitory-kinase activity after DNA damage, but the phosphatase regulation is normal. Which effect is most likely in the mutant after DNA damage occurs?
More cells arrest in G1 because securin cannot be degraded by APC/C
More cells enter mitosis because inhibitory phosphorylation is not reinforced despite damage signaling
More cells complete cytokinesis early because kinetochores remain unattached
More cells halt in anaphase because cyclin binding prevents CDK activation
More cells remain in S phase because separase cleaves cohesin before replication
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, focusing on DNA damage response and inhibitory phosphorylation of CDK in G2. The mutant cannot increase inhibitory-kinase activity after DNA damage, failing to reinforce CDK inhibition despite normal phosphatase regulation. This allows more cells to enter mitosis because the inhibitory phosphorylation is not sufficiently maintained, reducing the effectiveness of the G2 arrest signal. Consequently, CDK activation proceeds more readily despite damage. A tempting distractor is B, which suggests G1 arrest due to securin issues, but securin is mitotic, stemming from the misconception that G2 damage signals affect unrelated downstream proteins. To approach similar problems, evaluate how mutations disrupt feedback in damage responses and anticipate effects on cell cycle progression.
In cultured animal cells, entry into mitosis depends on activation of a cyclin-dependent kinase (CDK) by binding to mitotic cyclin and removal of an inhibitory phosphate by a phosphatase. A separate kinase adds the inhibitory phosphate, keeping the CDK inactive when conditions are unfavorable. Researchers add a small molecule that prevents the phosphatase from binding the CDK, while cyclin levels and ATP remain unchanged. Cells accumulate with duplicated DNA and intact nuclear envelopes, indicating arrest before mitosis. Which change would most likely restore progression into mitosis under these conditions?
Add a CDK variant that cannot be inhibited by phosphorylation at the inhibitory site
Increase activity of the kinase that adds the inhibitory phosphate to the CDK
Enhance DNA replication initiation to ensure cells pass the restriction point
Increase cyclin breakdown to promote CDK activation at the G2 checkpoint
Block spindle microtubule attachment to kinetochores to silence the checkpoint
Explanation
This question assesses understanding of signaling-based regulation of the cell cycle, focusing on CDK activation for mitotic entry. The small molecule prevents the phosphatase from removing the inhibitory phosphate on CDK, causing cells to arrest in G2 with duplicated DNA and intact nuclear envelopes despite cyclin presence. Adding a CDK variant that cannot be inhibited by phosphorylation at the inhibitory site bypasses the need for dephosphorylation, allowing cyclin-bound CDK to remain active. This restores progression into mitosis by circumventing the block in the activation pathway. A tempting distractor is A, which suggests increasing the inhibitory kinase activity, but this would further inhibit CDK, stemming from the misconception that enhancing negative regulation could promote activation. To approach similar problems, identify the blocked regulatory step and select interventions that directly bypass it without reinforcing inhibition.
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?
Sustained Cdk1 signaling because M-cyclin is tagged but not degraded, preventing activity decline
Normal exit because nuclear envelope reformation depends only on kinetochore attachment signals
Mitotic exit occurs because APC/C tagging alone is sufficient to inactivate Cdk1
Early G1 arrest because proteasome inhibition activates origin firing kinases at centrosomes
Reduced Cdk1 signaling because proteasome inhibition removes cyclins from cyclin–Cdk complexes
Explanation
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.