Ten E.coli cells with ^15N - dsDNA are incubated in medium containing ^14N nucleotide. After 60 minutes, how many E.coli cells will have DNA totally free from ^15N ?

1. 80 cells

2. 20 cells

3. 40 cells

4. 60 cells

Doubtly Asked 30 Oct 2026 NEET UG 2022 Practice test neet Save WhatsApp Report

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Answer: 4. 60 cells

Key idea: DNA replication is semi-conservative, and the question assumes the usual E. coli doubling time of 20 minutes.

In 60 min a cell divides 3 times, giving 8 cells from each original cell.
Each original DNA has two \(^{15}N\) strands. These end up in only 2 of the 8 daughter DNAs, each a hybrid (\(^{15}N/^{14}N\)).
So 6 of 8 cells have DNA totally free of \(^{15}N\).
From 10 cells: 80 cells in total, of which \(10\times6 = 60\) are free of \(^{15}N\).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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