Cell Cycle
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AP Biology › Cell Cycle
Cells in a tissue are exposed to ionizing radiation that produces double-strand breaks in DNA during G1. A normal G1 checkpoint can delay entry into S phase when DNA damage is detected, preventing replication of damaged templates. In one experimental group, a protein required for the G1 checkpoint is inhibited, so cells enter S phase on schedule even though breaks remain unrepaired. DNA replication proceeds across the genome, and cells later enter G2 with duplicated chromosomes. Which outcome is most likely for these cells as they progress through the cycle?
They enter S phase and replicate DNA that still contains break sites
They skip S phase and enter mitosis with 2C DNA and uncondensed chromatin
They increase spindle attachment, preventing any chromosome condensation in M phase
They delay in G1 until all breaks are repaired before DNA replication begins
They undergo homolog pairing and reduction division to form haploid cells
Explanation
This question assesses the skill of analyzing the cell cycle, focusing on checkpoint roles in response to DNA damage. The stimulus explains that inhibiting the G1 checkpoint allows cells with radiation-induced double-strand breaks to enter S phase without repair. In AP Biology, the G1 checkpoint normally delays progression to prevent replication of damaged DNA, but when inhibited, replication proceeds over break sites, leading to duplicated chromosomes with inherited damage in G2. Consequently, these cells enter S phase and replicate the damaged DNA templates. A tempting distractor is A, suggesting delay in G1 until repairs, but this is incorrect due to a misunderstanding of inhibition effects, specifically a teleological misconception that cells inherently prioritize repair over progression. To approach similar questions, distinguish between normal and disrupted checkpoint functions, then trace the cycle's progression and potential genomic consequences.
In a dividing cell, chromosomes condense and the nuclear envelope breaks down. During metaphase, all chromosomes align at the metaphase plate. Imaging reveals that for several chromosomes, both sister kinetochores are attached to microtubules from the same spindle pole, while other chromosomes show attachments to opposite poles. The cell proceeds into anaphase without a prolonged metaphase delay. After cytokinesis, one daughter cell contains extra copies of some chromosomes while the other is missing those chromosomes. Which checkpoint failure most directly explains the observed daughter-cell chromosome imbalance?
G2 checkpoint failure allowing homologous chromosomes to separate instead of sister chromatids.
G2 checkpoint failure allowing mitosis to start before chromosomes condense.
G1 checkpoint failure allowing cytokinesis before metaphase alignment occurs.
Spindle checkpoint failure allowing anaphase with incorrect kinetochore attachments.
G1 checkpoint failure allowing entry into S phase before DNA replication enzymes assemble.
Explanation
This question assesses the skill of analyzing the cell cycle, focusing on mitotic fidelity and checkpoint failures. The stimulus depicts chromosomes with monopolar kinetochore attachments proceeding to anaphase without delay, leading to aneuploid daughter cells. This indicates a failure of the spindle checkpoint, which normally delays anaphase until all kinetochores are bipollarly attached, as in choice C, aligning with AP Biology's role of the checkpoint in preventing missegregation. The observed imbalance directly results from premature chromatid separation with incorrect attachments. A tempting distractor is choice B, claiming G2 checkpoint failure allows mitosis before condensation, but this involves a structure-function confusion, as G2 checks replication completeness, not attachment during metaphase. To handle similar questions, identify the checkpoint that monitors the specific defect and evaluate effects of its failure on daughter cells.
A researcher tracks a cell that has just completed cytokinesis. The cell enters G1 with unreplicated chromosomes and begins to grow. Later, the cell enters S phase and duplicates its DNA, forming sister chromatids for each chromosome. The cell then enters G2, where DNA replication is complete but chromosomes remain uncondensed. Immediately before mitosis, the researcher detects that one chromosome has a double-strand break and remains unrepaired. The cell does not proceed into mitosis during the observation window. Which checkpoint most likely prevented entry into M phase?
Spindle checkpoint, because microtubules cannot attach until DNA damage is repaired.
G2 checkpoint, because damaged DNA is detected after replication is complete.
G1 checkpoint, because chromosomes are already condensed before DNA replication.
Spindle checkpoint, because homologous chromosomes are not synapsed in prophase.
G1 checkpoint, because sister chromatids must separate before S phase can begin.
Explanation
This question assesses the skill of analyzing the cell cycle, highlighting checkpoint responses to DNA damage. The stimulus describes a cell progressing from cytokinesis through S phase to G2, where an unrepaired double-strand break on one chromosome prevents entry into mitosis. This is because the G2 checkpoint detects DNA damage post-replication and blocks mitotic entry to allow repair, matching choice B and AP Biology's concept of checkpoints preserving genome stability. The uncondensed chromosomes and intact envelope support that the cell remains in G2 without proceeding. A tempting distractor is choice A, claiming the spindle checkpoint blocks due to unrepaired damage affecting attachments, but this reflects a teleology misconception, assuming checkpoints anticipate future issues rather than responding to current conditions like attachments in mitosis. For these questions, link the timing of damage or issues to the checkpoint that monitors that phase's completion.
A cell is observed at the G2 checkpoint with 4C DNA content. DNA replication is complete, but one chromosome is not fully decatenated, and sister chromatids remain physically intertwined along a region. The cell delays entry into mitosis while chromosomes remain uncondensed and the nuclear envelope stays intact. After the issue is resolved, the cell enters mitosis and proceeds normally. Which outcome is most likely if the cell had entered mitosis without this G2 delay?
Homologous chromosomes would pair and recombine, producing genetically identical daughter cells.
The nuclear envelope would remain intact throughout mitosis, preventing chromosome alignment.
Sister chromatids could fail to segregate properly, producing daughter cells with unequal chromosome sets.
Chromosomes would replicate again during prophase, doubling DNA content to 8C.
DNA content would drop to 2C before mitosis begins, preventing spindle formation.
Explanation
This question assesses the skill of analyzing the cell cycle, evaluating G2 checkpoint importance. With intertwined chromatids at G2, delay allows resolution before mitosis; without it, entry would cause improper segregation and unequal chromosome sets in daughters, as in choice A, per AP Biology's checkpoint ensuring decatenation for faithful division. The uncondensed state and intact envelope during delay support G2 arrest. This prevents aneuploidy. A tempting distractor is choice B, suggesting homolog recombination leading to identical daughters, but this arises from a teleology misconception, assuming entanglement promotes meiosis-like events rather than disrupting mitosis. When considering checkpoint bypass, forecast risks to segregation accuracy based on the monitored condition.
In an experiment, cells are treated with a reversible inhibitor that blocks DNA polymerase activity. Cells enter S phase normally, but replication stalls early and most of the genome remains unreplicated. After the inhibitor is removed, some cells quickly complete replication and proceed to G2. Other cells retain long stretches of unreplicated DNA but still attempt to enter mitosis because the G2 checkpoint is experimentally disabled. In those cells, chromosomes begin to condense even though replication is incomplete. Which outcome is most likely after mitosis begins in these checkpoint-disabled cells?
Segregation occurs with some chromosomes lacking fully duplicated sister chromatids
Homologous chromosomes pair and separate, yielding four nuclei after cytokinesis
Cells remain permanently in G1 with 2C DNA and never initiate replication
DNA replication resumes only after anaphase, restoring identical chromatids at poles
Spindle fibers fail to form because DNA replication is required for microtubule polymerization
Explanation
This question assesses the skill of analyzing the cell cycle, exploring effects of incomplete replication in checkpoint-disabled cells. The stimulus describes cells entering mitosis with unreplicated genome sections due to a disabled G2 checkpoint after inhibitor removal. In AP Biology, the G2 checkpoint ensures replication completion before M phase; without it, mitosis proceeds with partially duplicated chromosomes, leading to flawed segregation. Therefore, segregation occurs with some chromosomes lacking fully duplicated chromatids. A tempting distractor is B, claiming permanent G1 arrest, but this is incorrect due to a structure-function confusion, assuming replication blocks prevent all progression rather than allowing faulty mitosis. To approach similar questions, compare normal checkpoint functions to experimental disruptions, forecasting impacts on chromosome integrity during division.
A researcher measures DNA content and observes spindle formation. Cells with 4C DNA begin forming mitotic spindles and condensing chromosomes. In one experimental group, cells with 2C DNA also begin forming mitotic spindles and condensing chromosomes shortly after the treatment. After division, many daughter cells in the experimental group are missing chromosomes. The treatment did not affect microtubule attachment once spindles formed. Which checkpoint was most likely bypassed by the treatment?
G2 checkpoint, allowing entry into mitosis without completed DNA replication.
Spindle checkpoint, allowing cytokinesis to occur before DNA synthesis begins.
G1 checkpoint, allowing entry into S phase without sufficient DNA content.
Spindle checkpoint, allowing DNA replication to begin before chromosome alignment.
G2 checkpoint, allowing homologous chromosomes to pair before metaphase.
Explanation
This question assesses the skill of analyzing the cell cycle, assessing checkpoint bypass effects. The treatment induces cells with 2C DNA to form spindles and condense, leading to mitotic entry without replication and aneuploid daughters, indicating G2 checkpoint bypass allowing mitosis with incomplete DNA synthesis, as in choice B, consistent with AP Biology's G2 role in verifying replication. The unaffected attachments post-treatment pinpoint the entry defect. This results in segregation of unreplicated chromosomes. A tempting distractor is choice A, claiming G1 bypass allows S without sufficient DNA, but this involves a level-of-organization error, as 2C is normal for G1, and the issue is skipping replication entirely. When treatments alter progression, identify the checkpoint overridden and link to observed abnormalities.
A cell completes S phase, producing sister chromatids for each chromosome, and enters G2 with 4C DNA content. Just before mitosis, the centrosomes separate and begin forming a bipolar spindle. A toxin is added that prevents sister chromatids from separating by blocking the proteolysis needed to release cohesion at centromeres. The spindle checkpoint is satisfied because kinetochores are attached and under tension at metaphase. Anaphase is initiated, but cohesion is not removed. Which outcome is most likely observed during anaphase in this cell?
Chromosomes decondense immediately and the nuclear envelope reforms at metaphase
DNA content doubles again as the cell repeats S phase during anaphase
Homologous chromosomes separate, producing two haploid nuclei at telophase
Sister chromatids remain paired and fail to move to opposite poles
Kinetochores detach and the cell returns to G1 with 2C DNA without division
Explanation
This question assesses the skill of analyzing the cell cycle, highlighting mechanisms of chromatid separation in anaphase. The stimulus details a toxin blocking proteolysis of cohesin at centromeres, preventing sister chromatid separation despite a satisfied spindle checkpoint. In AP Biology, anaphase requires cohesin degradation to release chromatids, allowing microtubule pulling to opposite poles; without it, paired chromatids cannot separate. Thus, during anaphase, sister chromatids remain paired and fail to move apart. A tempting distractor is B, proposing immediate decondensation and envelope reformation, but this is incorrect due to a structure-function confusion, mistaking cohesion's role for overall mitotic exit signals. To approach similar questions, identify key molecular events in each mitotic phase and predict effects of targeted disruptions on chromosome behavior.
Cells from a tissue are treated with a drug that prevents microtubule polymerization. After treatment, many cells are found with condensed chromosomes and a fully formed metaphase plate is absent. Each chromosome has two sister chromatids, and kinetochores are visible but not attached to spindle microtubules. DNA content measurements indicate the cells have completed S phase and are not in G1. The cells remain in this state without chromatid separation for an extended period. Which checkpoint is most directly preventing progression to anaphase?
Spindle checkpoint, because homologous chromosomes are not paired at the equator.
G1 checkpoint, because DNA content is too high to begin replication.
Spindle checkpoint, because kinetochores are not properly attached to microtubules.
G1 checkpoint, because cytokinesis cannot occur before chromosome condensation.
G2 checkpoint, because DNA replication has not initiated.
Explanation
This question assesses the skill of analyzing the cell cycle, focusing on mitotic checkpoints and spindle dynamics. The stimulus shows cells treated with a drug preventing microtubule polymerization, resulting in condensed chromosomes, absent metaphase plate, and unattached kinetochores, with no progression to anaphase. This indicates the spindle checkpoint is activated, halting the cell in metaphase until all kinetochores are properly attached to microtubules from opposite poles, as in choice B, consistent with AP Biology's emphasis on the checkpoint ensuring accurate chromosome segregation. The post-S phase DNA content and condensed state confirm the cells are in mitosis, where the spindle checkpoint operates. A tempting distractor is choice C, suggesting the G2 checkpoint due to uninitiated replication, but this involves a phase confusion misconception, as G2 checks occur before mitosis, not after chromosome condensation has begun. To solve similar problems, correlate cellular observations like condensation and attachments to specific mitotic stages and identify the relevant checkpoint.
A cell is engineered so that it cannot pass the G1 checkpoint unless sufficient nutrients are present. In low-nutrient conditions, the cell remains in G1 with unreplicated chromosomes and 2C DNA. When nutrients are added, the cell passes the checkpoint and proceeds through S phase, duplicating its DNA to 4C, then enters G2. No other checkpoints are altered. Which outcome is most likely immediately after the cell passes the G1 checkpoint under nutrient-rich conditions?
Cytokinesis divides the cytoplasm, producing two daughter cells with 2C DNA
The nuclear envelope breaks down and chromosomes condense into visible structures
DNA replication begins, increasing DNA content as sister chromatids are synthesized
Kinetochores attach to spindle fibers and align chromosomes at the metaphase plate
Sister chromatids separate and move to opposite poles of the cell
Explanation
This question assesses the skill of analyzing the cell cycle, examining G1 checkpoint responses to nutrients. The stimulus describes a cell stalled in G1 under low nutrients, then progressing upon nutrient addition, initiating S phase. In AP Biology, passing the G1 checkpoint triggers DNA replication, increasing content from 2C to 4C via sister chromatid synthesis. Thus, DNA replication begins immediately after passing the checkpoint. A tempting distractor is B, proposing chromatid separation, but this is incorrect due to a teleological misconception that checkpoints directly control division rather than replication entry. To approach similar questions, identify environmental cues affecting checkpoints and predict immediate downstream processes like replication.
A student compares two cells. Cell X is in G1 with 2C DNA and one chromatid per chromosome. Cell Y is in G2 with 4C DNA and two sister chromatids per chromosome. Both cells are then induced to enter mitosis at the same time. Cell X enters mitosis without first replicating DNA, while Cell Y enters mitosis normally. After cytokinesis, Cell X produces daughter cells with fewer total DNA molecules than typical, while Cell Y produces typical daughter cells. Which conclusion best explains the difference between Cell X and Cell Y outcomes?
Cell Y underwent meiosis, so homologs separated and reduced DNA content in daughters.
Cell X repeated G2, so DNA content increased beyond 4C before mitosis began.
Cell Y bypassed the spindle checkpoint, so chromatids separated before alignment occurred.
Cell X skipped S phase, so chromosomes lacked sister chromatids for equal segregation.
Cell X remained in telophase, so DNA replication continued during nuclear reformation.
Explanation
This question assesses the skill of analyzing the cell cycle, comparing outcomes of mitosis with and without replication. Cell X, entering from G1 without S phase, lacks sister chromatids, leading to improper segregation and daughters with fewer DNA molecules, while Cell Y proceeds normally post-G2, supporting choice A and AP Biology's requirement for duplication before mitotic division. The stimulus emphasizes Cell X's unreplicated state causing atypical outcomes. This highlights S phase's necessity for equal partitioning. A tempting distractor is choice B, suggesting Cell Y underwent meiosis with homolog separation, but this stems from a structure-function confusion, misapplying meiotic reduction to mitotic contexts. For comparative questions, trace each cell's path and identify deviations from standard cycle that explain differences.