DNA Replication via CsCl Density Separation — NEET Biology
📚 Practice ConceptThen they transferred the cells into a medium with normal 14NH4Cl and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices. The various samples were separated independently on CsCl gradients to measure the densities of DNA.
Arrange the following steps of Meselson and Stahl’s experiment in the correct chronological order: 1. DNA extracted after two generations (40 minutes) showed both hybrid and light DNA. 2. E. coli was grown for many generations in ¹⁵N medium → DNA became heavy. 3. DNA was analysed for densities by CsCl density gradient centrifugation. 4. After transfer to ¹⁴N medium, DNA extracted after one generation (20 minutes) showed hybrid density. 5. Cells were shifted from ¹⁵N medium to ¹⁴N medium and allowed to divide.
View solution & NCERT explanation
Correct answer: D — 2 → 5 → 4 → 1 → 3
First, E. coli was grown in ¹⁵N medium → heavy DNA. Then shifted to ¹⁴N medium. After one generation, DNA showed hybrid density. After two generations, DNA showed hybrid + light DNA. All DNA densities were confirmed by CsCl centrifugation.
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