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GATE 2020 BT Q.48: solution and answer

GATE 2020 · BTQ.48 · 2 marks · NAT

Assume that a cell culture was started with five human fibroblast cells. Two cells did not divide even once whereas the other three cells completed three rounds of cell division. At this stage, the total number of kinetochores in all the cells put together is ______.

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Answer and solution

ANSWER

The total number of kinetochores in the culture at this stage is 1196.

Official answer: 1196Source: Official answer key, GATE 2020 BT (IIT Delhi)

  1. Human fibroblast cells are diploid: 2n = 46 chromosomes per cell.

  2. A cell sitting in the G1 state — whether it has never divided or has just completed a round of division — has unreplicated chromosomes (one chromatid each), so each such chromosome carries exactly one kinetochore. Hence every cell being counted at this stage has 46 kinetochores, regardless of its division history.

  3. The 2 cells that never divided remain as 2 cells: kinetochores contributed = .

  4. The other 3 cells each undergo 3 complete rounds of division. After rounds a starting cell's lineage yields final cells, so each gives cells.

  5. Total cells produced from these 3 lineages = cells, each carrying 46 kinetochores: .

  6. Total kinetochores in the whole culture = .

  7. Cross-check: total cells now present , and — consistent.

KEY IDEA

A kinetochore forms at the centromere of every chromosome present. A resting (G1-state) cell contains 46 unreplicated chromosomes (single chromatid each), so it has exactly 46 kinetochores — the same number whether the cell has just been born, is about to divide, or never divides at all. So the answer reduces to (total number of cells now present) × 46.

WATCH OUT FOR

Confusing a chromosome's kinetochore count with its DNA/chromatid content: assuming a cell mid-S-phase or in G2 (duplicated chromosomes, two sister chromatids) has 92 kinetochores per cell instead of 46. In this problem all cells being counted are freshly-divided or never-divided cells sitting in G1, so each one — dividing lineage or not — carries only 46 kinetochores; forgetting this and, e.g., doubling the daughter-cell count for "two chromatids" gives 2392 instead of 1196.

From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 15 Cell biology questions, with notes

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