IAE Academy

GATE 2026 BT Q.63: solution and answer

GATE 2026 · BTQ.63 · 2 marks · NAT

In the process of generating a clonal mammalian cell line, a single cell was seeded in a well of a cell culture plate. After the first 48 hours, one of the progeny cells underwent apoptosis due to a new mutation. If the doubling time of the cells is 24 hours and no more cell death occurs, the total number of cells after a total of 7 days from seeding will be ________. (answer in integer)

See the question in the paper
GATE 2026 BT question 63Tap to open full size

Answer and solution

ANSWER

The total number of cells after 7 days is 96.

Official answer: 96Source: Official answer key, GATE 2026 BT (IIT Guwahati)

  1. Start: 1 cell seeded at t = 0 h, doubling time = 24 h.

  2. At t = 24 h: 1 × 2 = 2 cells.

  3. At t = 48 h: 2 × 2 = 4 cells (the progeny at this point).

  4. At t = 48 h, one of these 4 cells undergoes apoptosis: 4 − 1 = 3 viable cells remain.

  5. No further cell death occurs, so growth resumes exponentially from 3 cells with the same 24 h doubling time.

  6. Total time = 7 days = 168 h; remaining time after the loss = 168 − 48 = 120 h.

  7. Number of doublings in 120 h = 120 / 24 = 5.

  8. Final cell number = .

KEY IDEA

Track cell number through exponential doubling, applying the one-time apoptotic loss exactly when it occurs (at t = 48 h), then resume doubling from the reduced population for the remaining time.

WATCH OUT FOR

Applying the apoptosis loss at the END of the 7-day period (168 − 1 = ... ) instead of at t = 48 h, or forgetting that "no more cell death occurs" means only ONE cell is ever lost, not a repeated loss at every doubling.

From the GATE 2026 BT paper. Every question of this paper · Download the full paper · All 15 Cell biology questions, with notes

More Cell biology questions