Hybrid DNA and Semi-Conservative Replication — NEET Biology
📚 Practice ConceptComposed of equal amounts of this hybrid DNA and of 'light' DNA.
Which of the following statements accurately describes the expected outcomes of different hypothetical DNA replication models in the context of Meselson and Stahl's experiment, or the actual findings, are NOT correct? S1: In the semi-conservative mode of replication, after one generation of E. coli grown in 14N medium following initial growth in 15N, all DNA molecules would be of intermediate density. S2: If DNA replication were conservative, the DNA extracted after two generations in 14N medium would yield three distinct bands: heavy, hybrid, and light density DNA. S3: Had DNA replication been dispersive, then after two generations of E. coli in 14N medium, all DNA molecules would appear as a single band with an intermediate density, progressively lighter than the first generation. S4: The observation of equal amounts of hybrid and light DNA after the second generation in 14N medium provided evidence consistent with a conservative mode of DNA replication. S5: The initial growth of E. coli solely in a 15N medium ensured that all parental DNA had a uniformly heavy density.
View solution & NCERT explanation
Correct answer: D — S2 and S4 only
The core concept examines the predicted results of conservative, semi-conservative, and dispersive DNA replication in Meselson and Stahl's experiment. S1: In semi-conservative replication, after one generation in 14N medium, each new DNA molecule consists of one old (15N) strand and one new (14N) strand, resulting in hybrid (intermediate) density DNA. This statement is correct (NCERT page 89, Figure 5.7). S2: In conservative replication, after two generations in 14N medium, the original heavy DNA (from 15N) would still exist (heavy band), and all newly synthesized DNA would be light (14N/14N, light band). There would be no hybrid DNA, thus only two bands (heavy and light), not three. This statement is NOT correct. S3: In dispersive replication, every DNA molecule is a random patchwork of old and new DNA. After two generations, all DNA molecules would still be a mix, resulting in a single intermediate density band, but the overall density would be lighter than the first generation as more 14N is incorporated. This statement is correct (implied from the nature of dispersive replication). S4: The observation of equal amounts of hybrid and light DNA after the second generation is the key evidence for semi-conservative replication, not conservative replication (NCERT page 89, Figure 5.7). This statement is NOT correct. S5: Growing E. coli in 15N for many generations ensures that both strands of all DNA molecules are composed of 15N, making the DNA uniformly heavy. This statement is correct (NCERT page 89). Therefore, statements S2 and S4 are not correct.
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Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.