Mitosis and Chromosome Dynamics (2C)
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MCAT Biological and Biochemical Foundations of Living Systems › Mitosis and Chromosome Dynamics (2C)
Human epithelial cells were engineered to express a fluorescent kinetochore marker and imaged during mitosis. Investigators added a small molecule that reduces microtubule dynamics without fully depolymerizing the spindle. Compared with vehicle-treated cells, treated cells spent longer between nuclear envelope breakdown and anaphase onset, and many cells displayed chromosomes remaining near one spindle pole rather than forming a tight metaphase plate. Immunostaining showed that these mispositioned chromosomes frequently retained spindle-assembly checkpoint (SAC) proteins at their kinetochores.
Based on the study, which conclusion about chromosome alignment is most consistent with the data?
Stabilizing kinetochore microtubules can delay satisfaction of the SAC by reducing error-correction and the acquisition of stable bi-orientation needed for metaphase alignment.
The treatment primarily blocks sister chromatid separation by preventing cohesin cleavage, so chromosomes remain aligned but cannot enter anaphase.
The treatment causes homologous chromosomes to fail to synapse, leading to persistent SAC signaling and misalignment.
The treatment accelerates anaphase by increasing poleward microtubule flux, pulling chromosomes away from the metaphase plate earlier than normal.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on how reduced microtubule dynamics affects chromosome alignment and spindle assembly checkpoint (SAC) satisfaction. Choice A is correct because it accurately describes how stabilizing kinetochore microtubules prevents the error-correction mechanisms needed for proper bi-orientation, as supported by the data showing mispositioned chromosomes with persistent SAC proteins. Choice B is incorrect because it describes a separase/cohesin issue, but the data shows alignment problems before anaphase, not a block after metaphase alignment. When analyzing mitosis, ensure that treatments affecting microtubule dynamics are understood to impact attachment correction and bi-orientation establishment, which are prerequisites for SAC satisfaction.
In a study of chromosome alignment, researchers cultured cells in low-dose nocodazole that partially suppresses microtubule polymerization but does not eliminate spindles. Many cells formed bipolar spindles with several chromosomes failing to align at the metaphase plate. These cells showed persistent SAC signaling and delayed anaphase onset. When nocodazole was washed out, chromosomes rapidly aligned and anaphase proceeded.
Which event would be expected during metaphase in the control (untreated) cells but is least likely in the nocodazole-treated cells?
Reformation of the nuclear envelope around decondensing chromosomes at each pole.
Pairing of homologous chromosomes into bivalents to promote reductional division.
Stable bi-oriented attachment of sister kinetochores to microtubules from opposite poles with sustained tension across the centromere.
Separation of sister chromatids due to cohesin cleavage and movement of chromatids toward the spindle poles.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on how partial microtubule suppression affects metaphase chromosome alignment. Choice C is correct because it describes stable bi-oriented attachment with tension, which requires robust microtubule dynamics and would be disrupted by nocodazole treatment, as supported by the data showing alignment failure and SAC persistence. Choice B is incorrect because it describes anaphase events, while the question asks about metaphase, and the cells are arrested before anaphase. When analyzing mitosis, understand that proper metaphase requires stable bi-oriented attachments generating tension, which depends on dynamic microtubules for error correction and attachment maturation.
To probe regulation of mitotic progression, investigators expressed a separase variant that cannot be activated by its normal mitotic signals. Cells entered mitosis, formed a metaphase plate, and maintained high tension at kinetochores. Despite apparent SAC satisfaction, sister chromatids did not separate, and cells eventually exited mitosis with a single enlarged nucleus.
Based on the study, which conclusion is most consistent with the data?
Separase activity is required for centrosome separation in prophase; without it, cells form monopolar spindles and never reach metaphase.
Separase activity is required for cohesin cleavage at anaphase onset; without separase activation, sister chromatids remain paired despite normal metaphase alignment.
Separase activity is required for DNA replication origin firing; without it, cells enter mitosis with unreplicated chromosomes that cannot align.
Separase activity is required for kinetochore-microtubule attachment; without it, chromosomes cannot congress and remain near spindle poles.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on separase function in triggering sister chromatid separation at anaphase onset. Choice D is correct because it accurately describes how separase cleaves cohesin to allow sister chromatid separation, as supported by the data showing normal metaphase but failure to separate chromatids despite SAC satisfaction. Choice C is incorrect because it describes a prophase centrosome function, while the data shows normal spindle formation and metaphase plate assembly. When analyzing mitosis, remember that separase activation is the final trigger for anaphase, cleaving cohesin rings that hold sister chromatids together.
Cells were treated with a reversible inhibitor of topoisomerase II during late G2 and early mitosis. Microscopy showed that chromosomes condensed and aligned at the metaphase plate, and the SAC was satisfied. However, at anaphase onset, sister chromatids frequently began to separate but remained connected by thin DNA bridges, leading to chromosome breakage and micronuclei formation.
Which conclusion about the affected mitotic process is most consistent with these observations?
Topoisomerase II is required to resolve residual catenation between sister chromatids; inhibition can cause anaphase bridges despite normal alignment.
Topoisomerase II is required for homolog recombination; inhibition prevents crossing over and causes anaphase bridges in meiosis II.
Topoisomerase II is required to attach microtubules to kinetochores; inhibition prevents metaphase alignment and activates the SAC.
Topoisomerase II is required to degrade cyclin B; inhibition prevents mitotic entry and keeps cells in G1.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on topoisomerase II function during sister chromatid separation. Choice D is correct because it accurately describes how topoisomerase II resolves DNA catenations between sister chromatids that persist after replication, as supported by the data showing normal alignment but DNA bridges during separation. Choice B is incorrect because it describes a kinetochore attachment function, while the data shows normal metaphase alignment indicating proper attachments. When analyzing mitosis, recognize that topoisomerase II is essential for complete chromatid disjunction by resolving topological links between sister DNAs.
A lab used high-speed imaging to compare chromosome movement in early vs late anaphase. They observed an initial rapid poleward movement of chromatids followed by a slower phase, while spindle poles continued to separate throughout. When a drug that selectively disrupts cortical pulling forces on astral microtubules was added, spindle pole separation decreased but initial chromatid poleward movement was less affected. Based on the study, which conclusion is most consistent with the data?
Reduced spindle pole separation indicates failure of homologous chromosomes to disjoin during meiosis I.
Cortical pulling on astral microtubules contributes mainly to anaphase B spindle pole separation, whereas anaphase A can proceed via kinetochore microtubule shortening.
Astral microtubules primarily attach to kinetochores to pull chromatids toward the metaphase plate during anaphase.
Cortical pulling forces are required for metaphase chromosome condensation, explaining the two-phase anaphase movement.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on spindle pole separation during anaphase B, influenced by disruption of cortical pulling on astral microtubules. Choice A is correct because it accurately describes cortical pulling's role in anaphase B while anaphase A proceeds via shortening, as supported by the data showing reduced separation but maintained initial movement. Choice C is incorrect because it describes astral attachment to kinetochores, a common error if astral functions are misplaced. When analyzing mitosis, ensure the phase-specific activities are matched with correct events and components; consider regulatory influences at each stage.
In an investigation of aberrant mitosis, a tumor-derived cell line was treated with a compound that weakens the spindle assembly checkpoint (SAC) without directly affecting microtubule polymerization. Compared with control, treated cells entered anaphase sooner and showed a higher frequency of lagging chromosomes and micronuclei in daughter cells. Metaphase spreads revealed that some chromosomes remained near a spindle pole at anaphase onset. Based on the study, which conclusion is most consistent with the data?
Weakening the SAC prevents cytokinesis by blocking actin ring formation, causing multinucleated cells.
Weakening the SAC causes sister chromatids to rejoin during anaphase, producing lagging chromosomes.
Weakening the SAC allows anaphase onset before all kinetochores achieve proper bipolar attachment, increasing missegregation and micronuclei formation.
Weakening the SAC increases crossing over between homologs, generating micronuclei during meiosis I.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on spindle assembly checkpoint function during metaphase, influenced by SAC weakening. Choice A is correct because it accurately describes how weakened SAC allows premature anaphase with missegregation, as supported by the data showing lagging chromosomes and micronuclei. Choice B is incorrect because it describes SAC's role in cytokinesis via actin, a common error if checkpoint functions are confused with contractile ring assembly. When analyzing mitosis, ensure the phase-specific activities are matched with correct events and components; consider regulatory influences at each stage.
A lab tracked kinetochore-microtubule attachment maturation using a fluorescent marker that accumulates on stable end-on attachments. In control cells, the marker increased as chromosomes congressed and peaked at metaphase. In cells treated with a mild Aurora B kinase activator, the marker remained low and chromosomes frequently failed to align, despite abundant spindle microtubules. Based on the study, which conclusion is most consistent with the data?
Increased Aurora B activity can destabilize kinetochore-microtubule attachments, impairing the transition to stable end-on binding needed for alignment.
Aurora B activation primarily blocks centrosome duplication, so fewer microtubules form and the marker cannot accumulate.
Stable end-on attachments occur only after cytokinesis, so low marker levels indicate failed abscission.
Aurora B activation promotes cohesin cleavage, causing premature anaphase and reduced marker accumulation.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on attachment stability during prometaphase to metaphase, influenced by Aurora B activation. Choice D is correct because it accurately describes how increased activity destabilizes attachments, as supported by the data showing low marker and failed alignment. Choice B is incorrect because it describes promotion of cohesin cleavage, a common error if Aurora B's corrective role is confused with anaphase initiation. When analyzing mitosis, ensure the phase-specific activities are matched with correct events and components; consider regulatory influences at each stage.
In a mitosis regulation study, cells were engineered to express a CDK1 variant with reduced kinase activity. These cells showed delayed entry into mitosis, incomplete chromosome condensation, and frequent spindle defects; however, once a bipolar spindle formed, many cells could align chromosomes. Based on the study, which statement best describes the role of CDK1 activity during mitosis?
CDK1 activity directly forms kinetochore microtubules by acting as a structural component of tubulin.
CDK1 activity triggers homologous chromosome pairing at the metaphase plate, explaining spindle defects in somatic cells.
CDK1 activity is required only after cytokinesis to initiate DNA replication, so reduced activity should not affect mitosis.
CDK1 activity promotes coordinated mitotic entry events such as chromosome condensation and spindle assembly; reduced activity delays and destabilizes early mitosis.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on the role of CDK1 in coordinating early mitotic events like chromosome condensation and spindle assembly during prophase and prometaphase, influenced by its kinase activity. Choice D is correct because it accurately describes the role of CDK1 in promoting these coordinated events, as supported by the observed delays and defects with reduced activity, while later alignment occurs once the spindle forms. Choice B is incorrect because it misattributes CDK1's role to post-mitotic DNA replication, a common error if confusing CDK1 with other cyclins involved in S phase. When analyzing mitosis, ensure the phase-specific activities are matched with correct events and components; consider regulatory influences at each stage.
A team analyzed cells exposed to a mild microtubule-stabilizing drug that reduces microtubule dynamics without fully preventing spindle formation. Treated cells frequently displayed chromosomes that oscillated near the metaphase plate but failed to achieve stable biorientation; anaphase onset was delayed. When the SAC was experimentally bypassed, treated cells entered anaphase with increased chromosome missegregation. Based on the study, which conclusion about metaphase alignment is most consistent with the data?
Reducing microtubule dynamics should accelerate anaphase by increasing separase activity at kinetochores.
Stable biorientation requires homologous chromosomes to pair at the metaphase plate, so stabilization prevents synapsis.
Dynamic microtubule turnover is important for error correction and stable biorientation; reducing dynamics promotes persistent incorrect attachments and SAC delay.
Microtubule stabilization primarily blocks DNA synthesis, indirectly delaying anaphase onset.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on biorientation during metaphase, influenced by reduced microtubule dynamics from stabilization. Choice A is correct because it accurately describes the importance of dynamics for error correction and stable attachments, as supported by the data showing delayed anaphase and missegregation. Choice D is incorrect because it suggests acceleration of anaphase, a common error if stabilization's inhibitory effects are misunderstood. When analyzing mitosis, ensure the phase-specific activities are matched with correct events and components; consider regulatory influences at each stage.
In a mitotic timing experiment, cells were treated with a reversible inhibitor of the APC/C (anaphase-promoting complex/cyclosome). Treated cells accumulated with condensed chromosomes aligned at the metaphase plate, high cyclin B levels, and intact sister chromatid cohesion. Upon washout, cells rapidly initiated chromatid separation and exited mitosis. Which statement best describes the role of APC/C activity during mitosis?
APC/C activity initiates synapsis and crossing over between homologous chromosomes at metaphase.
APC/C activity promotes the metaphase-to-anaphase transition by targeting key proteins for degradation, enabling chromatid separation and mitotic exit.
APC/C activity directly polymerizes actin to form the contractile ring during cytokinesis.
APC/C activity is required for spindle microtubule nucleation at centrosomes during prophase.
Explanation
This question assesses understanding of mitosis and chromosome dynamics within the cell cycle. Mitosis involves precise chromosome alignment and segregation, regulated by specific proteins and checkpoints. In this scenario, the focus is on the metaphase-to-anaphase transition, influenced by APC/C inhibition maintaining cyclin B and cohesion. Choice D is correct because it accurately describes APC/C's role in promoting the transition via degradation, as supported by the data showing metaphase accumulation and rapid progression upon washout. Choice B is incorrect because it describes prophase nucleation, a common error if APC/C timing is misplaced. When analyzing mitosis, ensure the phase-specific activities are matched with correct events and components; consider regulatory influences at each stage.