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GATE 2024 BT Q.48: solution and answer

GATE 2024 · BTQ.48 · 2 marks · MSQ

Under which of the following conditions, a mammalian somatic cell fails to undergo mitosis during cell cycle?

  1. (A)

    Initiation of cell plate formation

  2. (B)

    Incomplete DNA replication

  3. (C)

    Chiasmata formation

  4. (D)

    Irreparable DNA damage

See the question in the paper
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Answer and solution

ANSWER

GATE BT 2024 Q.48 (MSQ, 2 marks): The mammalian somatic cell cycle fails to proceed into mitosis when (B) DNA replication is incomplete or (D) DNA damage is irreparable — both trigger cell-cycle checkpoint arrest before M-phase. Correct answer: B, D.

Official answer: B, DSource: Official answer key, GATE 2024 BT (IISc Bengaluru)

  1. Read the question: identify the condition(s) under which a mammalian SOMATIC cell fails to undergo MITOSIS during the cell cycle. This is an MSQ, so more than one option may be correct.

  2. Option A — 'Initiation of cell plate formation': cell plate formation is the plant-specific cytokinesis mechanism (built via phragmoplast microtubules and Golgi-derived vesicles at the equatorial plane). Mammalian somatic cells instead complete cytokinesis by cleavage-furrow constriction driven by an actomyosin contractile ring. Since cell plate formation is not even part of mammalian mitotic/cytokinetic biology, it cannot be a cause of mitotic failure in such a cell -> A is incorrect.

  3. Option B — 'Incomplete DNA replication': the cell cycle's G2/M checkpoint (with the intra-S checkpoint, mediated by ATR-Chk1 signalling) monitors completion of genome duplication and blocks activation of the CDK1-cyclin B (M-phase-promoting) complex if replication is incomplete, arresting the cell before mitosis -> B is correct.

  4. Option C — 'Chiasmata formation': chiasmata are the physical sites of crossing-over between homologous chromosomes formed during prophase I of MEIOSIS. Somatic mitotic division has no homolog pairing or synapsis, so chiasmata formation never occurs there and is irrelevant to whether a somatic cell can undergo mitosis -> C is incorrect.

  5. Option D — 'Irreparable DNA damage': the DNA-damage checkpoint (ATM/ATR sensing lesions, signalling via Chk1/Chk2 to p53 and effectors such as p21) arrests the cell cycle before mitosis when damage is detected; when the damage is irreparable, the cell is driven toward senescence or apoptosis instead of being allowed to divide -> D is correct.

  6. Combining the verdicts: the conditions that genuinely cause a mammalian somatic cell to fail to undergo mitosis are incomplete DNA replication and irreparable DNA damage.

  7. Final answer: B and D.

KEY IDEA

The eukaryotic cell cycle has surveillance checkpoints (G1/S, intra-S, G2/M, and the spindle-assembly checkpoint) that halt progression into or through mitosis whenever the genome is not yet safely ready to be segregated — most relevantly here, when DNA replication is not finished (G2/M / replication checkpoint, ATR-Chk1 mediated) or when DNA damage cannot be repaired (DNA-damage checkpoint, ATM/ATR-p53 mediated, often followed by apoptosis or senescence). Conditions that are simply not part of mammalian mitotic biology (plant-specific cell-plate cytokinesis; meiosis-specific chiasmata) are not "conditions that cause mitotic failure" — they are non-sequiturs for a somatic mitotic cell.

WATCH OUT FOR

Confusing cell-cycle checkpoint failures (which block mitosis) with processes that are simply absent or irrelevant in mammalian somatic mitosis — picking (A) by forgetting cell plate formation is a plant-cytokinesis mechanism (not part of animal mitosis at all), or picking (C) by forgetting chiasmata form during meiotic prophase I, not mitosis, so it has no bearing on whether a somatic cell enters or completes mitosis.

From the GATE 2024 BT paper. Every question of this paper · Download the full paper · All 15 Cell biology questions, with notes

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