DNA Replication: Semi-conservative Model — NEET Biology
✅ Asked in NEET 2022Ten E.coli cells with ¹⁵N–DNA are incubated in ¹⁴N medium. After 60 minutes, how many cells will have DNA free from ¹⁵N? (NEET 2022)
Ten E.coli cells with ¹⁵N–DNA are incubated in ¹⁴N medium. After 60 minutes, how many cells will have DNA free from ¹⁵N? (NEET 2022)
Answer & NCERT explanation
Correct answer: C — 60 cells
E.coli generation time is 20 minutes. In 60 minutes = 3 generations. Starting with 10 cells: after 20 min = 20 cells (hybrid DNA), after 40 min = 40 cells (50% hybrid, 50% light), after 60 min = 80 cells (25% hybrid, 75% light). 60 cells will be completely free from ¹⁵N (only ¹⁴N-¹⁴N).
Read more NCERT concept on the PYQ
Thus, the DNA that was extracted from the culture one generation after the transfer from 15N to 14N medium [that is after 20 minutes; E. coli divides in 20 minutes] had a hybrid or intermediate density. DNA extracted from the culture after another generation [that is after 40 minutes, II generation] was
This paragraph tests the Meselson-Stahl experiment, which proves DNA replication is semi-conservative. After one generation in 14N medium, DNA from 15N-labeled bacteria shows hybrid (15N-14N) density, meaning each new DNA molecule contains one old and one new strand. Students often confuse this with conservative or dispersive replication models. The key to remember: E. coli divides every 20 minutes, so after 20 minutes you get hybrid DNA, and after 40 minutes (second generation) you get 50% hybrid and 50% light DNA. This experiment is crucial for understanding DNA structure and replication mechanism—a frequent NEET trap question.
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