- Cells exposed to a replication stressor activate the ATR/Chk1 pathway, which reduces origin firing and slows S-phase progression. In a tumor cell line, a researcher knocks down Chk1 and then repeats the replication stress. The Chk1-knockdown cells show increased DNA strand breaks and continue incorporating nucleotides at a high rate despite the stressor.
Which conclusion is most consistent with these results regarding checkpoint control in cancer?
Answer: Chk1 normally enforces an S-phase checkpoint; its loss allows continued DNA synthesis under stress, increasing genomic damage
This question tests understanding of how checkpoint kinases like Chk1 protect genome integrity during replication stress. The ATR/Chk1 pathway normally responds to replication problems by slowing origin firing and S-phase progression, allowing time for repair and preventing catastrophic DNA damage. The data show that Chk1 knockdown cells continue DNA synthesis despite stress and accumulate DNA breaks, demonstrating checkpoint failure. The correct answer (A) accurately describes this: Chk1 enforces an S-phase checkpoint, and its loss allows continued synthesis under stress, increasing damage. Choice B incorrectly suggests Chk1 promotes rather than prevents damage, choice C confuses Chk1 (a kinase) with cyclins and DNA polymerase, and choice D wrongly limits Chk1 to the spindle checkpoint. To solve similar problems, remember that checkpoint kinases like Chk1 act as quality control mechanisms - their loss allows cells to proceed through the cell cycle despite problems, accumulating errors.
- A tumor sample is found to overexpress cyclin E relative to adjacent normal tissue. In cell culture, forced cyclin E overexpression increases CDK2 activity and shortens the time cells spend in G1. When the same experiment is performed in a line engineered to express high levels of p21, the cyclin E–overexpressing cells show much less increase in S-phase entry.
Which interpretation is most consistent with these observations about cell-cycle control?
Answer: Cyclin E promotes G1-to-S progression by activating CDK2, and p21 counteracts this by inhibiting cyclin–CDK activity.
This question tests understanding of cyclin E-CDK2 function in G1/S progression and how CDK inhibitors like p21 regulate this transition. Cyclin E partners with CDK2 to drive G1-to-S progression by phosphorylating substrates that promote S-phase entry, while p21 acts as a CDK inhibitor that binds cyclin-CDK complexes to block their kinase activity. The experiment shows cyclin E overexpression increases CDK2 activity and S-phase entry, but this effect is blunted when p21 is also overexpressed, demonstrating p21's inhibitory role. The correct answer (A) accurately describes cyclin E promoting G1-to-S via CDK2 activation and p21 counteracting this through CDK inhibition. Choice B incorrectly assigns DNA repair function to cyclin E, choice C reverses the relationship by suggesting p21 activates CDK2, and choice D confuses cell cycle progression with apoptosis. To analyze cyclin-CDK regulation, remember that cyclins activate CDKs to drive cell cycle progression, while CDK inhibitors like p21 oppose this activity to enforce checkpoints.
- A tumor line is treated with a drug that stabilizes p53 by blocking its interaction with MDM2 (an E3 ubiquitin ligase that targets p53 for degradation). After treatment, p53 protein levels rise, p21 mRNA increases, and the fraction of cells entering S phase decreases.
Which outcome would be expected if the same drug were applied to an otherwise similar tumor line harboring a p53 mutation that prevents p21 transcription (but still allows p53 protein accumulation)?
Answer: p53 protein would accumulate, but S-phase entry would be less reduced because p21 induction is impaired
This question tests understanding of the p53-p21 axis in cell cycle checkpoint control and how specific mutations affect this pathway. MDM2 inhibition stabilizes p53 protein regardless of its transcriptional activity, but p53's ability to induce cell cycle arrest depends on its capacity to activate p21 transcription. The data from the first cell line show the complete pathway: p53 stabilization leads to p21 induction and reduced S-phase entry. The correct answer (A) accurately predicts that with a p21-transcription deficient p53 mutant, p53 would still accumulate but S-phase reduction would be impaired due to lack of p21. Choice B incorrectly suggests p53 accumulation requires p21, choice C reverses the effect of p21 loss, and choice D introduces unrelated mitotic mechanisms. A key principle is that protein accumulation and functional activity are separate - mutant proteins can accumulate but fail to execute their normal functions.
- In a set of tumor-derived cells, sequencing shows a gain-of-function mutation in cyclin D that prevents its ubiquitination and degradation. Compared with cells expressing degradable cyclin D, mutant cells show persistently high cyclin D levels and increased Rb phosphorylation throughout the cell cycle. Which outcome is most consistent with this mutation?
Core concept: sustained cyclin D can keep CDK4/6 active, maintaining Rb phosphorylation and promoting continued G1/S progression.
Answer: More frequent entry into S phase due to prolonged cyclin D–CDK4/6 activity and sustained inactivation of Rb.
This question examines effects of stabilized cyclins on cell cycle timing in cancer. In cancer, non-degradable cyclin D sustains CDK4/6 activity, maintaining Rb inactivation for frequent G1/S entry. Here, mutation prevents cyclin D ubiquitination, causing persistent high levels and Rb phosphorylation. The correct answer (A) follows because sustained cyclin D promotes ongoing G1/S progression. A distractor like (B) fails by claiming degradation is needed for CDK4/6 activation, predicting reduced proliferation and misconceiving stabilization's effect. In related scenarios, assess if stabilization disrupts oscillations and leads to persistent phase driving. Additionally, link to downstream markers like p-Rb.
- Two lung cancer cell lines are treated with a DNA-damaging chemotherapeutic that causes replication stress. In line 1, treatment leads to strong S-phase slowing and reduced origin firing. In line 2, DNA synthesis continues at near-baseline levels, but cells accumulate DNA breaks and die during the next mitosis.
Line 2 is found to have a loss-of-function mutation in a kinase that normally phosphorylates downstream targets to stabilize stalled replication forks and inhibit late origin firing (a key S-phase checkpoint regulator). Which mutation best explains the phenotype of line 2?
Answer: Loss of an S-phase checkpoint kinase prevents appropriate slowing of DNA replication under stress, increasing damage carried into mitosis.
This question tests understanding of S-phase checkpoint mechanisms that protect genome integrity during DNA replication stress. S-phase checkpoint kinases normally respond to replication stress by stabilizing stalled forks, inhibiting late origin firing, and slowing overall DNA synthesis to prevent replication catastrophe. Line 1 shows normal checkpoint function with S-phase slowing and reduced origin firing, while line 2 lacks this protective response due to loss of the checkpoint kinase, allowing continued DNA synthesis that generates breaks carried into mitosis. The correct answer (A) accurately describes loss of an S-phase checkpoint kinase preventing appropriate replication slowing, increasing damage in mitosis. Choice B incorrectly suggests Rb gain-of-function and misunderstands fork stabilization mechanisms, choice C wrongly involves cyclin D in S-phase regulation, and choice D reverses p21's inhibitory role. When analyzing replication stress responses, remember that checkpoint kinases protect genome stability by slowing replication when problems arise, and their loss allows damage accumulation even if replication appears to continue normally.
- A research group engineers human epithelial cells to express either wild-type p53 or a mutant p53 that cannot bind DNA (p53-DBD−). Cells are exposed to ionizing radiation, which induces double-strand DNA breaks. Four hours later, researchers measure mRNA for p21 (a CDK inhibitor transcriptionally activated by p53) and quantify the fraction of cells in S phase. Results: wild-type p53 cells show a large increase in p21 mRNA and a marked decrease in S-phase fraction after radiation, whereas p53-DBD− cells show little change in p21 mRNA and maintain a high S-phase fraction.
Which conclusion is most consistent with these data regarding cell cycle regulation in cancer?
Answer: Loss of p53 DNA-binding prevents transcriptional induction of p21, weakening the G1/S checkpoint and allowing continued entry into S phase after DNA damage
This question tests understanding of p53's role as a tumor suppressor that enforces cell cycle checkpoints in response to DNA damage. In normal cells, p53 acts as a transcription factor that induces p21 expression following DNA damage, and p21 inhibits cyclin-CDK complexes to prevent G1/S transition until damage is repaired. The data show that cells with DNA-binding deficient p53 cannot induce p21 and continue entering S phase despite radiation damage, demonstrating loss of checkpoint control. The correct answer (A) accurately describes this mechanism: without p53's ability to bind DNA and activate p21 transcription, the G1/S checkpoint is weakened. Choice B incorrectly states that p53 loss strengthens checkpoints and that it directly activates cyclin D kinase, while choices C and D contain fundamental errors about the molecular mechanisms involved. To approach similar questions, identify the normal function of the protein (p53 induces p21 to inhibit CDKs), then predict the consequence of its loss (checkpoint failure).
- A cancer cell line was engineered to overexpress a nondegradable cyclin B variant (cyclin B cannot be ubiquitinated). Normally, cyclin B–CDK1 activity rises to drive entry into mitosis and then falls when cyclin B is degraded, allowing mitotic exit. After induction of nondegradable cyclin B, cells accumulate with condensed chromosomes and fail to complete cytokinesis. Which outcome is most consistent with this manipulation?
Answer: Persistent cyclin B–CDK1 activity prevents mitotic exit, leading to arrest in M phase with failure to complete cytokinesis.
This question probes understanding of mitotic regulation and cyclin degradation in cell cycle control. Cancer can arise from failures in cyclin B degradation, which normally allows mitotic exit after CDK1 activation drives mitosis entry. The manipulation with nondegradable cyclin B leads to cells stuck with condensed chromosomes and incomplete cytokinesis. Choice D is correct as persistent cyclin B-CDK1 activity blocks mitotic exit, causing M-phase arrest without cytokinesis completion. Choice C is incorrect because nondegradable cyclin B sustains CDK1 activity, promoting chromosome condensation, not inhibiting it. For related scenarios, check if the defect prevents progression beyond the affected phase, like mitosis here. Additionally, confirm the role of ubiquitin-mediated degradation in cycle advancement.
- A targeted therapy inhibits a kinase upstream of cyclin D transcription in a subset of tumors. After treatment, responders show decreased cyclin D levels, decreased Rb phosphorylation, and reduced E2F target gene expression. Non-responders show unchanged E2F target gene expression despite decreased cyclin D. Which tumor alteration best explains non-response?
Core concept: if E2F is activated independently of Rb control (e.g., RB1 loss), lowering cyclin D may not reduce E2F target expression.
Answer: Loss of RB1 function, causing E2F activity to remain high even when cyclin D and Rb phosphorylation decrease.
This question probes mechanisms of resistance to therapies targeting cyclin D in Rb pathway cancers. In cancer, RB1 loss allows E2F activity independent of cyclin D and Rb phosphorylation, conferring resistance to upstream inhibition. Here, responders decrease cyclin D, p-Rb, and E2F targets post-therapy, while non-responders maintain E2F despite cyclin D drop. The correct answer (A) follows because RB1 loss bypasses need for cyclin D-mediated Rb inactivation. A distractor like (B) fails by suggesting p16 gain prevents effects, but p16 inhibits CDK4/6, predicting sensitivity, misconceiving resistance. For similar resistance questions, identify if alterations downstream bypass the target. Also, compare responder vs. non-responder molecular changes.
- A study tested whether a tumor suppressor pathway is intact by treating cells with a DNA replication inhibitor that causes replication stress. In normal cells, replication stress activates a kinase cascade that stabilizes p53 and increases transcription of a CDK inhibitor, resulting in reduced CDK2 activity and G1/S arrest. In a cancer cell line, replication stress does not reduce CDK2 activity, and cells continue DNA synthesis with elevated DNA damage markers. Which mutation is most likely to produce this phenotype?
Answer: Loss-of-function mutation in a p53-regulated CDK inhibitor gene that normally suppresses CDK2 at the G1/S checkpoint.
This question tests insight into DNA damage responses and tumor suppressor pathways in cancer. Mutations in p53 or its effectors allow cancer cells to ignore replication stress, evading G1/S arrest and accumulating damage. The cancer cells fail to reduce CDK2 activity or arrest at G1/S despite replication stress, continuing DNA synthesis with damage. Choice A is correct as a loss-of-function in a p53-induced CDK inhibitor like p21 would prevent CDK2 suppression, bypassing the checkpoint. Choice B is flawed by inverting Rb's role; gain-of-function Rb would enhance E2F repression, promoting arrest, not the observed progression. To approach similar questions, identify if the mutation disables a checkpoint response to stress. Furthermore, ensure the phenotype matches unchecked activity at the specific CDK and phase involved.
- A research vignette examined a kinase (Wee1) that phosphorylates and inhibits CDK1. Cancer cells treated with a Wee1 inhibitor showed premature entry into mitosis despite incomplete DNA replication, followed by increased DNA damage markers and cell death. Which conclusion is most consistent with the role of Wee1 in cell cycle control?
Core concept: Wee1 restrains CDK1 to prevent premature G2/M transition; inhibiting Wee1 can force mitotic entry and cause mitotic catastrophe, especially in genomically unstable cells.
Answer: Wee1 normally inhibits CDK1 to help enforce the G2/M checkpoint; inhibiting Wee1 can trigger premature mitosis and lethal damage.
This question probes Wee1's regulation of CDK1 in G2/M checkpoint control in cancer. In cancer, Wee1 inhibits CDK1 to enforce G2 arrest under stress; inhibition forces premature mitosis, causing damage. Here, Wee1 inhibitor causes early mitotic entry with incomplete replication, leading to DNA damage and death. The correct answer (B) follows because Wee1 restraint prevents premature G2/M, and loss triggers catastrophe. A distractor like (A) fails by stating Wee1 activates CDK1, predicting delayed mitosis and misconceiving its inhibitory phosphorylation. To solve analogous issues, identify kinase's effect on CDK activity and link to phase timing. Additionally, check damage markers for checkpoint bypass consequences.
- A research group profiled a tumor and found a frameshift mutation in a gene encoding a CDK inhibitor that normally binds cyclin E–CDK2 complexes. In tumor cells, CDK2 activity was elevated and cells entered S phase rapidly. Which mutation is most likely described, based on the cell cycle effect?
Core concept: p21 (CDKN1A) and related inhibitors can restrain cyclin E–CDK2; loss increases G1/S progression.
Answer: Loss-of-function in CDKN1A (p21), reducing inhibition of cyclin E–CDK2 and promoting S-phase entry.
This question assesses mutations in CDK inhibitors affecting G1/S progression in cancer. In cancer, p21 (CDKN1A) loss reduces cyclin E–CDK2 inhibition, accelerating S-phase entry. Here, a frameshift in a CDK inhibitor gene elevates CDK2 activity and rapid S-phase in tumor cells. The correct answer (D) follows because CDKN1A loss frees cyclin E–CDK2 for G1/S promotion. A distractor like (B) fails by suggesting cyclin B loss increases CDK2, but cyclin B affects G2/M, misconceiving phase specificity. For such questions, match inhibitor to CDK target and predict progression effects from loss. Also, use activity levels to confirm deregulation.
- A melanoma cell line carries a loss-of-function mutation in CDKN2A (encoding p16). Compared with a matched p16-restored line, the p16-mutant cells show higher CDK4/6 activity and increased Rb phosphorylation, with more cells in S phase. Which outcome is most consistent with restoring p16 expression?
Core concept: p16 inhibits CDK4/6; restoring p16 should reduce Rb phosphorylation and slow G1/S progression.
Answer: Decreased CDK4/6 activity and decreased Rb phosphorylation, leading to reduced S-phase entry.
This question examines p16's inhibitory role in CDK4/6 and Rb regulation in cancer. In cancer, p16 loss allows unchecked CDK4/6 to phosphorylate Rb, freeing E2F for G1/S. Here, p16-mutant cells show high CDK4/6 activity, Rb phosphorylation, and S-phase fraction compared to p16-restored. The correct answer (B) follows because restoring p16 decreases CDK4/6, Rb phosphorylation, and S-phase entry. A distractor like (A) fails by predicting increased phosphorylation and S-phase, ignoring p16's inhibition and misconceiving its effect. For related questions, map inhibitors to CDK targets and predict downstream phosphorylation changes. Also, compare mutant vs. restored lines for functional validation.
- A tumor sample contains an RB1 truncation that prevents Rb from binding E2F. In primary culture, these cells show high expression of E2F target genes involved in DNA synthesis even under low growth factor conditions. p53 and p21 are intact and inducible after DNA damage. Which outcome is most consistent with the RB1 mutation's effect on cell cycle checkpoints?
Core concept: Rb restrains E2F to enforce the G1 restriction point; loss leads to inappropriate S-phase gene expression.
Answer: Cells will show diminished dependence on growth factors for G1/S progression because E2F is constitutively active.
This question tests the impact of Rb mutations on G1 restriction point and checkpoint independence in cancer. In cancer, Rb loss frees E2F to drive S-phase genes constitutively, bypassing growth factor requirements while preserving other checkpoints like p53-p21. Here, RB1 truncation prevents E2F binding, leading to high E2F targets even in low growth factors, with intact p53 and p21 inducibility. The correct answer (B) follows because Rb loss causes growth factor-independent G1/S due to active E2F. A distractor like (A) fails by suggesting reduced S-phase from free E2F repressing genes, contradicting E2F's activating role and misconceiving it as a repressor. For related questions, evaluate if mutation bypasses specific signals like growth factors and confirm other checkpoints remain functional. Also, link gene expression data to checkpoint outcomes.
- A small-molecule drug is designed to inhibit CDK4/6 activity in a breast cancer cell line that retains functional Rb. After treatment, researchers observe decreased phosphorylation of Rb and reduced expression of E2F target genes. Cell counts over several days show slowed proliferation without immediate cell death.
Which outcome is most consistent with the mechanism of CDK4/6 inhibition in these cells?
Answer: Reduced Rb phosphorylation keeps E2F inhibited, limiting G1/S transition and slowing proliferation
This question tests understanding of how CDK4/6 inhibitors restore cell cycle control in cancer cells with intact Rb. CDK4/6 complexes with cyclin D normally phosphorylate Rb in early G1, initiating the process that eventually releases E2F and allows S-phase entry. The data show that CDK4/6 inhibition reduces Rb phosphorylation and E2F target gene expression, leading to slowed proliferation without immediate death. The correct answer (A) accurately describes this mechanism: reduced Rb phosphorylation keeps E2F inhibited, limiting G1/S transition. Choice B reverses the relationship between Rb phosphorylation and E2F activity, choice C contains nonsensical molecular mechanisms, and choice D incorrectly limits Rb function to G2/M. A key principle is that CDK4/6 inhibitors work by maintaining Rb in its hypophosphorylated, E2F-inhibitory state - this requires functional Rb to be effective.
- Two tumor cell lines are treated with the same CDK4/6 inhibitor. Line 1 expresses functional Rb; Line 2 has an RB1 loss-of-function mutation. After treatment, Line 1 shows decreased S-phase fraction and slower growth, while Line 2 shows little change in S-phase fraction or growth rate.
Which conclusion is most consistent with these results?
Answer: CDK4/6 inhibition depends on functional Rb to restrain E2F and block G1/S progression, so RB1 loss confers resistance
This question tests understanding of why certain targeted therapies require specific molecular contexts to be effective. CDK4/6 inhibitors work by preventing Rb phosphorylation, thereby keeping Rb bound to E2F and blocking S-phase gene expression - this mechanism absolutely requires functional Rb protein. The data show that Line 1 (Rb+) responds to CDK4/6 inhibition while Line 2 (Rb-) does not, demonstrating that Rb is necessary for drug efficacy. The correct answer (A) accurately explains this: CDK4/6 inhibition requires functional Rb to restrain E2F, so RB1 loss confers resistance. Choice B incorrectly suggests Rb loss increases CDK4/6 dependence, choice C wrongly attributes the mechanism to cyclin E synthesis, and choice D invents an unrelated DNA repair mechanism. A critical principle for precision medicine is that targeted therapies often require intact downstream pathways - if a key downstream effector (like Rb) is lost, the drug cannot work regardless of target inhibition.
- A glioblastoma cell line is exposed to a microtubule-stabilizing drug that prevents proper spindle dynamics. In cells with an intact spindle assembly checkpoint, this drug causes prolonged mitotic arrest and eventual cell death. A resistant subclone continues to divide despite drug exposure and shows frequent chromosome mis-segregation.
Sequencing identifies a loss-of-function mutation in a gene required for spindle checkpoint signaling (assume it normally inhibits anaphase onset until all kinetochores are attached). Which outcome is most consistent with the resistant phenotype?
Answer: Premature anaphase onset occurs despite unattached kinetochores, allowing division under drug treatment but increasing aneuploidy.
This question tests understanding of the spindle assembly checkpoint (SAC) and how its loss enables resistance to antimitotic drugs while causing chromosomal instability. The SAC normally prevents anaphase onset until all kinetochores are properly attached to spindle microtubules, ensuring accurate chromosome segregation. The microtubule-stabilizing drug disrupts proper spindle dynamics, activating the SAC and causing prolonged mitotic arrest in normal cells. The resistant subclone has a loss-of-function mutation in SAC signaling, allowing premature anaphase onset despite unattached kinetochores, which explains both drug resistance and frequent chromosome mis-segregation. The correct answer (A) accurately describes this premature anaphase despite unattached kinetochores, allowing division but increasing aneuploidy. Choice B incorrectly places SAC action in G1, choice C wrongly connects SAC to DNA replication, and choice D contradicts the resistance phenotype. When analyzing mitotic checkpoints, remember that SAC loss allows cells to bypass mitotic arrest but at the cost of chromosomal instability, a common mechanism of drug resistance in cancer.
- In a study of checkpoint adaptation, yeast-inspired terminology was applied to human cancer cells: after prolonged DNA damage, some cells resumed division without fully repairing DNA. In a human cell model, knockdown of p21 increased the fraction of damaged cells that re-entered the cell cycle, while p53 levels remained high. Which conclusion is most consistent with these results?
Core concept: p21 is an effector of p53-mediated cell cycle arrest; reducing p21 can weaken arrest even if p53 is present.
Answer: p21 knockdown weakens cell cycle arrest downstream of p53, allowing damaged cells to progress despite persistent p53.
This question tests p21's effector role in p53-mediated arrest and checkpoint adaptation in cancer. In cancer, p21 loss weakens arrest downstream of p53, allowing damaged cells to resume cycling. Here, p21 knockdown increases cycle re-entry post-damage despite high p53. The correct answer (B) follows because p21 is key for p53-enforced arrest, and reduction permits progression. A distractor like (A) fails by claiming p21 knockdown strengthens checkpoint via CDK activation, contradicting data and misconceiving p21 as activator. For such scenarios, distinguish upstream signaling from effectors and assess if knockdown bypasses arrest. Also, consider adaptation in prolonged damage contexts.
- A lab examines a tumor suppressor that promotes apoptosis or cell-cycle arrest after severe DNA damage. In a sarcoma line, this protein is present but predominantly cytosolic and fails to accumulate in the nucleus after irradiation. A sequencing analysis identifies a mutation that disrupts the protein's nuclear localization signal (NLS). After irradiation, these mutant cells show continued proliferation and reduced expression of a CDK inhibitor normally induced by this tumor suppressor.
Which conclusion is most consistent with the data?
Answer: The NLS mutation prevents nuclear entry needed for transcriptional activation of genes that enforce arrest, impairing the DNA damage checkpoint.
This question tests understanding of how nuclear localization is essential for transcription factor function in DNA damage responses. Many tumor suppressors must enter the nucleus to activate transcription of genes encoding cell cycle inhibitors or pro-apoptotic factors. The NLS mutation traps this tumor suppressor in the cytoplasm, preventing nuclear entry after irradiation and thus blocking transcriptional activation of its target CDK inhibitor, allowing continued proliferation despite DNA damage. The correct answer (A) accurately explains that the NLS mutation prevents nuclear entry needed for transcriptional activation of arrest genes, impairing the checkpoint. Choice B incorrectly suggests increased nuclear entry would cause excessive arrest, choice C proposes an implausible cytoplasmic CDK activation mechanism, and choice D irrelevantly invokes microtubule effects. To analyze tumor suppressor mutations, consider not just whether the protein is present but whether it can reach its site of action - nuclear localization defects can be as detrimental as complete loss for transcription factors.
- Researchers introduce an activating mutation in RAS (a proto-oncogene) into cultured cells. They observe increased expression of cyclin D and increased phosphorylation of Rb. However, when the same experiment is performed in cells with intact p53, many cells undergo growth arrest after several divisions. The arrest is accompanied by increased p21 expression.
Which interpretation is most consistent with these observations about oncogenes and tumor suppressor genes?
Answer: Oncogenic RAS can drive G1 progression, but intact p53 can counterbalance excessive proliferative signaling by inducing p21-mediated CDK inhibition
This question tests understanding of the balance between oncogenic signals and tumor suppressor responses in cell cycle control. Oncogenic RAS promotes proliferation by increasing cyclin D expression and Rb phosphorylation, but intact p53 can sense excessive proliferative stress and induce p21 to inhibit CDKs and cause growth arrest. The data show that RAS drives initial proliferation but p53-competent cells eventually arrest with high p21 expression, demonstrating tumor suppressor-mediated checkpoint activation. The correct answer (A) accurately describes this balance: oncogenic RAS drives G1 progression, but p53 can counteract this through p21-mediated CDK inhibition. Choices B, C, and D all contain fundamental errors about the molecular mechanisms - p53 doesn't increase cyclin D, p21 inhibits rather than increases CDK activity, and p21 doesn't phosphorylate Rb. To approach similar questions, recognize that oncogenes push proliferation while tumor suppressors apply brakes - cancer often requires loss of both systems.
- A researcher introduced a dominant-negative mutant of CDK1 into rapidly dividing cancer cells. After expression, cells accumulated with 4N DNA content and showed reduced mitotic markers. Cyclin B levels were high. Which interpretation is most consistent with these results?
Core concept: cyclin B–CDK1 drives G2/M transition; inhibiting CDK1 prevents entry into mitosis, leading to G2 arrest with 4N DNA.
Answer: Dominant-negative CDK1 prevents mitotic entry, causing accumulation in G2 with high cyclin B and 4N DNA content.
This question assesses CDK1's role in mitotic entry and effects of inhibition in cancer. In cancer, dominant-negative CDK1 blocks cyclin B–CDK1, preventing G2/M and causing G2 arrest with high cyclin B and 4N DNA. Here, expression leads to 4N accumulation, reduced mitotic markers, and high cyclin B. The correct answer (D) follows because inhibiting CDK1 halts mitotic entry, trapping cells in G2. A distractor like (B) fails by claiming acceleration of entry, predicting G1 accumulation and misconceiving dominant-negative as activator. For related problems, identify CDK's phase and predict arrest from inhibition. Also, use DNA content and cyclin levels for phase confirmation.
