MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2c Cancer Cell Cycle Control

What this quiz covers

This quiz focuses on 2c Cancer Cell Cycle Control, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. CDK4/6 inhibition reduces S-phase gene expression in both lines because E2F is a kinase that is directly inhibited by CDK4/6 drugs.
2. CDK4/6 inhibition increases S-phase gene expression in the control cells because Rb becomes more phosphorylated and releases E2F.
3. CDK4/6 inhibition reduces S-phase gene expression in the Rb-truncated cancer cells because cyclin D is a tumor suppressor that blocks E2F.
4. CDK4/6 inhibition fails to reduce S-phase gene expression in the Rb-truncated cancer cells because E2F is constitutively unrestrained. (correct answer)

Explanation: This question assesses knowledge of the Rb-E2F pathway and its deregulation in cancer. Cancer often involves loss of Rb function, which normally represses E2F to control S-phase gene expression, leading to uncontrolled proliferation when mutated. The experiment shows CDK4/6 inhibition reduces S-phase genes in control cells with intact Rb but not in Rb-truncated cancer cells. Choice D is correct as Rb truncation allows constitutive E2F activity, rendering CDK4/6 inhibition ineffective since Rb is needed to bind and repress E2F. Choice B fails by reversing the mechanism; CDK4/6 inhibition decreases Rb phosphorylation, enhancing E2F repression in controls, not increasing it. For similar problems, confirm if the pathway requires the mutated protein for drug efficacy. Also, evaluate whether the outcome matches expected deregulation, like persistent gene expression despite treatment.

Question 2

1. The p53 mutation likely prevents transcriptional activation of genes that enforce the G1/S checkpoint after DNA damage, allowing continued S-phase entry. (correct answer)
2. The p53 mutation likely increases cyclin-dependent kinase (CDK) catalytic activity directly, accelerating progression through mitosis after IR.
3. The p53 mutation likely hyperactivates Rb, strengthening E2F repression and reducing S-phase entry after IR.
4. The p53 mutation likely blocks cyclin synthesis in G1, which would explain why the tumor cells fail to arrest and instead stop dividing.

Explanation: This question tests understanding of cell cycle checkpoints and tumor suppressor genes in cancer. In cancer, mutations in genes like p53 disrupt cell cycle control, allowing cells to proliferate despite DNA damage by failing to activate checkpoints that halt progression. The data show that ionizing radiation induces p53 accumulation and reduces S-phase entry in healthy cells, but not in tumor cells with a p53 DNA-binding domain mutation. Choice A is correct because the mutation impairs p53's ability to transcribe checkpoint genes, such as p21, preventing G1/S arrest and permitting S-phase entry post-damage. Choice B is incorrect as it misrepresents p53's role; p53 does not directly increase CDK activity but rather inhibits it through downstream effectors, and the effect is on G1/S, not mitosis. To verify similar questions, check if the mutation aligns with loss of checkpoint function leading to unchecked progression. Additionally, ensure the explanation matches the specific checkpoint affected, here G1/S rather than others like G2/M.

Question 3

1. Nondegradable cyclin B increases p53 transcriptional activity, which triggers apoptosis and eliminates mitotic cells before chromosome condensation.
2. Persistent cyclin B–CDK1 activity strengthens the G1/S checkpoint, causing cells to arrest in G1 with low DNA content.
3. Nondegradable cyclin B directly inhibits CDK1, preventing chromosome condensation and blocking entry into mitosis.
4. Persistent cyclin B–CDK1 activity prevents mitotic exit, leading to arrest in M phase with failure to complete cytokinesis. (correct answer)

Explanation: 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.

Question 4

1. Premature anaphase onset despite unattached chromosomes, increasing mis-segregation and aneuploidy. (correct answer)
2. Enhanced G1/S arrest after DNA damage due to increased p53 stabilization, decreasing aneuploidy.
3. Reduced cyclin D–CDK4/6 activity, leading to hypophosphorylated Rb and decreased E2F target gene expression.
4. Failure to enter mitosis due to inability of cyclin B to bind CDK1, resulting in accumulation of cells in G1.

Explanation: This question investigates mitotic checkpoints and aneuploidy in cancer. Loss of spindle assembly checkpoint proteins allows premature anaphase, causing chromosome mis-segregation and aneuploidy, a cancer hallmark. The cell line exhibits aneuploidy, mis-segregation, and mutation in a kinetochore checkpoint protein. Choice A is correct because the mutation permits anaphase without full attachment, increasing errors. Choice B is incorrect as it confuses checkpoints; spindle defects do not enhance G1/S arrest or reduce aneuploidy but promote it. To solve similar items, identify if the mutation weakens fidelity at the affected stage, like mitosis. Also, link the phenotype to consequences like genomic instability.

Question 5

1. Unchanged cyclin D indicates that CDK2 cannot be activated, so the tumor cells should arrest in G2 under low growth factor conditions.
2. Elevated cyclin E is consistent with enhanced spindle assembly checkpoint activity, delaying anaphase and reducing proliferation.
3. Elevated cyclin E is consistent with increased G1/S transition by promoting CDK2 activity and S-phase entry despite weak mitogenic signaling. (correct answer)
4. Elevated cyclin E is most consistent with increased degradation of cyclins, which would prevent entry into S phase in low growth factor conditions.

Explanation: This question evaluates comprehension of cyclin roles in cell cycle progression and their dysregulation in tumors. In cancer, elevated cyclins like cyclin E can bypass growth factor requirements, driving inappropriate G1/S transition through CDK activation. The data indicate elevated cyclin E in tumors, with unchanged cyclin D, and S-phase entry under low growth factors that arrest normal cells. Choice C is correct because high cyclin E activates CDK2, promoting Rb phosphorylation and E2F release for S-phase genes, enabling proliferation without mitogens. Choice B is wrong as it confuses cyclin E with mitotic functions; cyclin E acts at G1/S, not the spindle checkpoint, and would increase, not decrease, proliferation. In analogous questions, verify if the cyclin matches the checkpoint deregulated, here G1/S. Moreover, assess if the data support bypass of normal regulatory signals like growth factors.

Question 6

1. Loss-of-function mutation in a p53-regulated CDK inhibitor gene that normally suppresses CDK2 at the G1/S checkpoint. (correct answer)
2. Gain-of-function mutation in Rb that increases binding to E2F, preventing S-phase gene transcription.
3. Loss-of-function mutation in cyclin B that prevents CDK1 activation and blocks mitotic entry.
4. Gain-of-function mutation in a spindle checkpoint protein that delays anaphase until all kinetochores attach.

Explanation: 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.

Question 7

1. Cyclin D amplification increases CDK4/6-mediated phosphorylation of Rb, releasing E2F to drive transcription of genes required for S phase. (correct answer)
2. Cyclin D amplification directly repairs DNA lesions, reducing p53 activation and thereby slowing the cell cycle.
3. Cyclin D amplification inhibits CDK4/6, keeping Rb hypophosphorylated and preventing E2F from activating S-phase genes.
4. Cyclin D amplification activates the spindle assembly checkpoint, preventing chromosome segregation and reducing proliferation.

Explanation: This question assesses how cyclin amplifications drive oncogenic cell cycle progression. Cyclin D overexpression in cancer hyperactivates CDK4/6, phosphorylating Rb to release E2F and promote S-phase genes, fueling growth. The tumor shows cyclin D amplification, high Rb phosphorylation, and proliferation decrease upon cyclin D reduction. Choice A is correct as amplified cyclin D enhances CDK4/6 activity, freeing E2F for S-phase transcription and tumor proliferation. Choice C is incorrect because cyclin D activates, not inhibits, CDK4/6, leading to Rb hyperphosphorylation, not hypophosphorylation. In similar cases, confirm if the alteration accelerates the specific transition, like G1/S here. Additionally, evaluate if reversing the change restores normal control, as with cyclin D reduction.

Question 8

1. Oncogene-driven proliferation in this model depends on cyclin–CDK2 activity for S-phase entry, whereas the tumor-suppressor loss bypasses this control point. (correct answer)
2. Tumor suppressor loss increases reliance on CDK2, so CDK2 inhibition should selectively block the tumor-suppressor–mutant cells.
3. CDK2 inhibitors act by restoring tumor suppressor gene transcription, so they should only affect tumor-suppressor–mutant cells.
4. The oncogene mutation decreases cyclin levels, making CDK2 inhibition unnecessary to reduce S-phase entry.

Explanation: This question tests understanding of how different genetic alterations create distinct dependencies on cell cycle regulators. The oncogene mutation increases cyclin expression, making cells dependent on cyclin-CDK2 activity for S-phase entry, so CDK2 inhibition effectively blocks their proliferation. The tumor suppressor loss likely removes a checkpoint that normally restrains S-phase entry upstream of or parallel to CDK2, allowing cells to bypass the need for CDK2 activity through alternative pathways. The correct answer (A) accurately explains that oncogene-driven proliferation depends on CDK2 for S-phase entry while tumor suppressor loss bypasses this control point. Choice B reverses the logic by suggesting tumor suppressor loss increases CDK2 reliance, choice C incorrectly proposes CDK2 inhibitors restore tumor suppressor transcription, and choice D contradicts the experimental setup. To understand synthetic lethality and targeted therapy, recognize that different mutations create different vulnerabilities - oncogene addiction often creates targetable dependencies while loss of tumor suppressors may eliminate those same dependencies.

Question 9

1. Demethylation likely reactivates p21 transcription, enabling p21-mediated inhibition of cyclin–CDK complexes and promoting G1 arrest. (correct answer)
2. Demethylation likely silences p53, preventing apoptosis and thereby reducing colony formation.
3. Demethylation increases cyclin E stability, which decreases CDK2 activity and causes G1 arrest.
4. Demethylation activates telomerase, which directly inhibits CDK2 and reduces colony formation by shortening S phase.

Explanation: This question tests understanding of epigenetic silencing in cancer and how reversing promoter methylation can restore tumor suppressor function. DNA methylation of CpG islands in gene promoters silences transcription, a common mechanism for inactivating tumor suppressors like p21 without genetic mutation. The demethylating agent removes methyl groups from the p21 promoter, reactivating transcription and restoring p21 protein expression, which then inhibits CDK2 to enforce G1 arrest and reduce proliferation. The correct answer (A) accurately describes demethylation reactivating p21 transcription, enabling CDK inhibition and G1 arrest. Choice B incorrectly suggests demethylation silences p53 and confuses reduced proliferation with reduced apoptosis, choice C proposes an illogical mechanism where cyclin E stability decreases CDK2 activity, and choice D invokes telomerase in an unrelated context. When analyzing epigenetic therapies, remember that demethylating agents can reactivate silenced tumor suppressors, potentially restoring checkpoint control without requiring gene therapy.

Question 10

1. Decreased p53 levels due to enhanced ubiquitination, leading to increased cyclin-CDK activity and faster G1/S progression.
2. Stabilized p53 leading to increased p21, which inhibits cyclin-CDK complexes and slows cell cycle progression. (correct answer)
3. Direct activation of CDK4/6, leading to increased Rb phosphorylation and reduced p21 transcription.
4. Inhibition of E2F DNA binding, which decreases p53 levels because E2F is required for p53 translation.

Explanation: This question assesses MDM2's regulation of p53 in cell cycle arrest and apoptosis in cancer. In cancer, MDM2 degrades p53; inhibitors stabilize p53 to induce p21 for arrest and pro-apoptotic genes. Here, MDM2 inhibition increases p53, p21, apoptotic transcripts, and reduces proliferation. The correct answer (B) follows because stabilized p53 activates p21 to inhibit cyclin-CDKs, slowing progression. A distractor like (A) fails by proposing decreased p53 from enhanced ubiquitination, predicting opposite effects and misconceiving the inhibitor's stabilizing role. For comparable questions, trace from regulator (MDM2) to effector (p53-p21) and verify downstream outcomes like arrest. Also, correlate protein levels with transcriptional changes.

Question 11

1. Wee1 normally activates CDK1 to promote mitosis; inhibiting Wee1 should delay mitosis and reduce DNA damage.
2. Wee1 normally inhibits CDK1 to help enforce the G2/M checkpoint; inhibiting Wee1 can trigger premature mitosis and lethal damage. (correct answer)
3. Wee1 is a cyclin that binds CDK2; inhibiting Wee1 should primarily block G1/S entry.
4. Wee1's main role is to ubiquitinate p53; inhibiting Wee1 should reduce p53 and prevent cell death after replication stress.

Explanation: 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.

Question 12

1. Chk1 loss primarily blocks p53 degradation, increasing p21 and thereby increasing CDK1 activity in mitosis.
2. Chk1 loss likely strengthens the G2/M checkpoint by increasing Wee1 activity, preventing mitosis in damaged cells.
3. Chk1 loss likely decreases E2F activity, causing reduced cyclin B levels and preventing mitotic entry.
4. Chk1 loss likely impairs checkpoint signaling, allowing inappropriate CDK1 activation and mitotic entry despite DNA damage. (correct answer)

Explanation: This question assesses knowledge of cell cycle checkpoints in cancer, focusing on how mutations disrupt DNA damage responses to allow uncontrolled division. In cancer, loss of checkpoint kinases like Chk1 can abolish safeguards that halt the cell cycle at G2/M in response to DNA damage, permitting mitotic entry with unrepaired DNA via unchecked CDK1 activation. The data reveal high cyclin B-CDK1 activity and mitotic progression despite DNA damage in cells with a Chk1 loss-of-function mutation, consistent with impaired checkpoint enforcement. Choice D is correct as it explains that Chk1 loss weakens signaling, enabling inappropriate CDK1 activation and mitosis under damage conditions, matching the core concept. Choice B acts as a distractor by incorrectly stating that Chk1 loss strengthens the checkpoint via Wee1, reversing the actual role of Chk1 in activating inhibitory pathways, which misleads on loss-of-function outcomes. For similar problems, evaluate if the mutation allows cycle progression despite stress signals, indicating checkpoint failure typical in cancer. A transferable check is to confirm the kinase's role in inhibiting mitotic CDKs during damage; loss should promote entry, not prevent it.

Question 13

1. Mutant APC decreases β-catenin signaling, lowering cyclin D1 transcription and slowing G1 progression.
2. Mutant APC increases β-catenin–dependent transcription, which can elevate cyclin D1 and promote G1/S entry. (correct answer)
3. β-catenin directly inhibits CDK4/6, so increased nuclear β-catenin should reduce proliferation.
4. APC mutation primarily blocks the spindle checkpoint, so the key effect should be decreased cyclin B degradation.

Explanation: This question evaluates APC-β-catenin pathway's influence on cyclin D1 and G1 progression in cancer. In cancer, APC loss stabilizes β-catenin, inducing cyclin D1 transcription to promote G1/S via CDK4/6. Here, mutant APC increases nuclear β-catenin, cyclin D1, and proliferation rate. The correct answer (B) follows because APC mutation enhances β-catenin signaling, elevating cyclin D1 for faster G1/S. A distractor like (A) fails by suggesting decreased β-catenin and cyclin D1, predicting slower progression and misconceiving APC as a positive regulator. In comparable questions, map pathway to cyclin targets and verify proliferation effects. Additionally, use localization data like nuclear β-catenin for activation confirmation.

Question 14

1. High cyclin B indicates cells are in G1, since cyclin B activates CDK2 to initiate DNA replication.
2. Dominant-negative CDK1 accelerates mitotic entry, causing loss of cyclin B and accumulation in G1 with 2N DNA content.
3. Dominant-negative CDK1 increases E2F activity, causing premature S-phase entry and decreased cyclin B levels.
4. Dominant-negative CDK1 prevents mitotic entry, causing accumulation in G2 with high cyclin B and 4N DNA content. (correct answer)

Explanation: 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.

Question 15

1. The CDK4 mutation likely decreases Rb phosphorylation, leading to reduced E2F activity and slower proliferation.
2. The CDK4 mutation likely increases resistance to p16 inhibition, increasing Rb phosphorylation and E2F-driven S-phase gene expression. (correct answer)
3. The CDK4 mutation likely prevents cyclin D binding, so Rb remains unphosphorylated and E2F is constitutively repressed.
4. High p-Rb is most consistent with increased p21 activity, since p21 phosphorylates Rb to enforce the G1 checkpoint.

Explanation: This question examines gain-of-function CDK4 mutations in Rb-E2F deregulation in cancer. In cancer, hyperactive CDK4 resists p16, phosphorylating Rb to free E2F for S-phase genes. Here, CDK4 mutation reduces p16 binding, with high p-Rb and E2F expression, no RB1 mutation. The correct answer (B) follows because the mutation enhances CDK4 activity, increasing Rb phosphorylation and E2F despite p16. A distractor like (A) fails by predicting decreased phosphorylation, ignoring gain-of-function and misconceiving it as loss. To address similar queries, evaluate mutation type and impact on inhibitor binding. Additionally, link to downstream markers like p-Rb and gene expression.

Question 16

1. p21 knockdown strengthens the DNA-damage checkpoint because p21 is required to activate cyclin-CDK complexes.
2. p21 knockdown weakens cell cycle arrest downstream of p53, allowing damaged cells to progress despite persistent p53. (correct answer)
3. p21 knockdown prevents DNA damage signaling upstream by blocking ATM activation.
4. High p53 alone is sufficient to block CDKs directly; therefore p21 knockdown should have no effect on re-entry.

Explanation: 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.

Question 17

1. Loss of RB1 function, causing E2F activity to remain high even when cyclin D and Rb phosphorylation decrease. (correct answer)
2. Gain of p16 function, preventing cyclin D decline from affecting Rb phosphorylation.
3. Loss of cyclin E, allowing E2F target genes to remain expressed despite reduced Rb phosphorylation.
4. Wild-type p53, which should maintain E2F target gene expression when cyclin D decreases.

Explanation: 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.

Question 18

1. Tumor cells likely have intact p53 signaling but defective Chk1, explaining the lack of G1 arrest and the transient S-phase slowing.
2. Tumor cells likely have impaired p53→p21 signaling for G1 arrest but retain replication-stress checkpoint signaling through Chk1 in S phase. (correct answer)
3. Normal cells arrest in G1 because Chk1 phosphorylates Rb, while tumor cells cannot because cyclin B is low.
4. The data imply that p21 activates CDK2 to enforce G1 arrest, which is lost in tumor cells.

Explanation: This question tests differential checkpoint integrity in tumor vs. normal cells in cancer. In cancer, p53-p21 defects impair G1 arrest, but ATR-Chk1 can preserve S-phase slowing under stress. Here, tumor cells lack G1 arrest and p21 induction but show S-phase slowing with Chk1 phosphorylation, unlike normal cells' G1 arrest. The correct answer (B) follows because tumors have impaired p53→p21 for G1 but retain Chk1 for S-phase checkpoint. A distractor like (A) fails by proposing intact p53 but defective Chk1, predicting no S-phase response and misconceiving the pathways. For analogous questions, map damage responses to checkpoints and identify retained vs. lost functions. Also, use phosphorylation markers for pathway activity.

Question 19

1. Rb-nonphos increases E2F activity by mimicking hyperphosphorylated Rb, promoting S-phase entry after serum addition.
2. Rb-nonphos prevents growth-factor-induced G1/S progression by keeping E2F repressed despite serum stimulation. (correct answer)
3. Rb-nonphos primarily blocks the G2/M checkpoint by inhibiting cyclin B–CDK1 activation.
4. E2F activity is independent of Rb status; the observed effects must be due to altered actin normalization.

Explanation: This question tests Rb phosphorylation's role in E2F regulation and G1/S progression in cancer. In cancer, nonphosphorylatable Rb constitutively represses E2F, blocking G1/S even with growth signals. Here, Rb-nonphos cells maintain low E2F activity and poor S-phase entry post-serum, unlike controls. The correct answer (B) follows because Rb-nonphos prevents signal-induced E2F release, inhibiting progression. A distractor like (A) fails by claiming Rb-nonphos increases E2F by mimicking phosphorylated Rb, predicting progression and misconceiving nonphos as inactive. In similar problems, assess if Rb variants lock it in repressive or inactive states and link to E2F activity. Additionally, use reporters to confirm transcriptional effects.

Question 20

1. More frequent entry into S phase due to prolonged cyclin D–CDK4/6 activity and sustained inactivation of Rb. (correct answer)
2. Reduced proliferation because cyclin D degradation is required to activate CDK4/6 and phosphorylate Rb.
3. Specific arrest in mitosis because cyclin D directly activates CDK1 and blocks anaphase onset.
4. No change in proliferation because cyclin D functions only in DNA repair, not in cell cycle progression.

Explanation: 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.