Q.No. 48 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 _____________.

Human fibroblasts are diploid cells with 46 chromosomes, and each chromosome has one centromere where a kinetochore assembles during the cell cycle, primarily visible and functional from prophase through anaphase. The query describes a culture starting with five cells: two remain undivided (G0/G1 phase, each with 46 kinetochores), while three undergo three full division rounds, yielding 3 × 8 = 24 cells, all post-cytokinesis in G1 with 46 kinetochores each. Thus, total cells = 2 + 24 = 26, and total kinetochores = 26 × 46 = 1196.​

Step-by-Step Solution

Start with five G1 cells, each having 46 chromosomes and thus 46 kinetochores.

• Two cells do not divide: Contribute 2 × 46 = 92 kinetochores.

• Each of the three dividing cells undergoes three rounds: 1 → 2 → 4 → 8 cells per lineage, totaling 24 cells.

• All 24 post-division cells are in G1 (single chromosomes post-S phase replication and mitosis), each with 46 kinetochores: 24 × 46 = 1104.

• Grand total: 92 + 1104 = 1196 kinetochores.​

Kinetochores duplicate during S phase (one per sister chromatid), but after mitosis and cytokinesis, each daughter inherits one per chromosome pair, resetting to 46 per G1 cell.​

Common Misconceptions

No options are provided, but typical errors include:

• Counting only original cells: 5 × 46 = 230 (ignores progeny).

• Assuming post-mitosis cells retain duplicated kinetochores (92 per cell): Wrong, as G1 has unduplicated chromosomes.​

• Miscalculating divisions: Two rounds yield 4 cells/lineage (12 total dividing cells), but query specifies three rounds.​

• Including S-phase cells: Query implies completion (“completed three rounds”), so all in G1.​

Human fibroblast cells, key in cell culture studies and exams like GATE Biotechnology, start diploid with 46 chromosomes—each hosting one kinetochore at the centromere for microtubule attachment during mitosis. In this scenario, five cells begin: two stay undivided, while three complete three rounds of division, raising questions on total kinetochores across all cells.​

Cell Division Breakdown

Three division rounds per active cell follow binary fission: initial cell divides to 2 (round 1), then 4 (round 2), then 8 (round 3), totaling 24 cells from three lineages. Undivided cells remain at 2, for 26 total cells in G1 phase post-division.​

Kinetochore Count per Cell

Each G1 human fibroblast has 46 kinetochores (one per chromosome). Kinetochores assemble anew per mitosis but reset to 46 after cytokinesis—no persistent doubling.​

Final Calculation

• Undivided: 2 cells × 46 = 92 kinetochores.

• Divided progeny: 24 cells × 46 = 1104 kinetochores.

• Total: 1196 kinetochores.​

This problem tests mitosis mechanics, ideal for IIT JAM or GATE prep on cell biology and kinetochores in human fibroblast cell culture.​