DNA Replication: Light and Hybrid — NEET Biology
📚 Practice ConceptIf E. coli was allowed to grow for 80 minutes then what would be the proportions of light and hybrid densities DNA molecule?
Which of the following statements regarding the modes of DNA replication and Meselson and Stahl's experiment are NOT correct? S1: If DNA replication were conservative, the first generation of E. coli grown in 14N medium after transfer from 15N would show two distinct bands (heavy and light) in a CsCl density gradient. S2: In a dispersive mode of DNA replication, after two generations in a 14N medium, all DNA molecules would still exhibit a single, albeit lighter, intermediate density band. S3: Meselson and Stahl's experiment established that 15N, a radioactive isotope of nitrogen, allowed for the differentiation of DNA molecules based on their density. S4: The observation of DNA with intermediate density after one generation in the 14N medium supported the hypothesis of conservative DNA replication. S5: The final results of Meselson and Stahl's experiment with E. coli conclusively demonstrated that DNA replication is semi-conservative, with each new molecule containing one parental and one newly synthesized strand.
View solution & NCERT explanation
Correct answer: B — S3 and S4 only
The core concept involves understanding the different hypothetical modes of DNA replication (conservative, semi-conservative, dispersive) and their expected outcomes in Meselson and Stahl's experiment. S1: In conservative replication, the parental DNA remains intact (heavy band), and a completely new DNA molecule is synthesized (light band). Thus, two distinct bands (heavy and light) would appear after one generation in 14N medium. This statement is correct. S2: In dispersive replication, each DNA molecule would be a patchwork of old and new DNA. Even after two generations, all molecules would contain a mix, resulting in a single intermediate density band that would gradually become lighter with more generations in 14N. This statement is correct. S3: Meselson and Stahl used 15N, which is a heavy isotope of nitrogen, not a radioactive isotope. They separated DNA based on density, not radioactivity. This statement is NOT correct (NCERT page 89, '15N is not a radioactive isotope'). S4: The observation of DNA with intermediate density after one generation (hybrid DNA) directly supported the semi-conservative mode of replication, not the conservative mode. This statement is NOT correct (NCERT page 89). S5: Meselson and Stahl's experiment, by showing hybrid DNA after one generation and equal amounts of hybrid and light DNA after two generations, conclusively proved that DNA replication is semi-conservative. This statement is correct (NCERT page 88-89). Therefore, statements S3 and S4 are not correct.
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