#cell cycle arrest

Aptitude and Reasoning - Part 2

Colchicine treatment blocks the polymerization of microtubules whereas Taxol blocks the depolymerization of microtubules

This outcome reflects quantitative reasoning, where ratio, percentage and series logic determine correct calculation. Option B captures this correctly because it meets mathematical and logical criteria, consistent with textbook descriptions and experimental observations in aptitude and reasoning.

Ref: Wren and Martin, High School English Grammar and Composition, Revised Edition, Chapter 12, discusses vocabulary and comprehension providing guidance for verbal aptitude and logical inference assessment.

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Cell Biology and Genetics

Which of the following repair mechanisms is absent in a cell arrested in cell cycle?

Mechanistically, cellular compartmentalization and cell cycle involves SNARE-mediated trafficking, Hayflick limit, P53 and cyclin regulation control proliferation. This validates DNA synthesis annealing repair mechanism because it aligns with membrane trafficking and cell cycle control, a pattern repeatedly demonstrated in cell biology research.

Ref: Alberts et al., Molecular Biology of the Cell, 6th Edition, Chapter 17, Cell Cycle and Regulation, explains Cdc2, cyclins, P53, proteasomes, Hayflick limit, primary culture and immortalization mechanisms governing proliferation.

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Cell Cycle Regulation

The major function of p21 in the cell cycle is to:

p21WAF1/CIP1 encoded by CDKN1A represents broad-spectrum cyclin-dependent kinase inhibitor linking DNA damage response to cell cycle arrest in G1 and G2. Upon double-strand break detection, ATM kinase phosphorylates Chk2 and p53 at serine 15, blocking MDM2-mediated ubiquitination and stabilizing p53. ATR responds to single-stranded DNA coated with RPA. Accumulated p53 binds response elements in CDKN1A promoter, markedly upregulating p21 protein that contains N-terminal CDK inhibitory domain with Cy1 and Cy2 cyclin-binding motifs. p21 inserts into catalytic cleft of cyclin E-CDK2, cyclin D-CDK4

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: p53-p21 DNA Damage Axis.

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Cancer

Active p53 can induce cell cycle arrest by upregulating

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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