Multiple choice

Meselson and Stahl proved that DNA replication is semiconservative. E.coli bacteria was grown in N15-NH4Cl medium, then washed and incubated with N14 compounds for 3 generations. Then CsCl density gradient centrifugation of isolated DNA showed semiconservative DNA replication. What is the pattern of band formation, if formed in the following way: N14, N14/15, N15?

  1. Thin band at N14, thick band at N15

  2. Thin bands at N15 and N14/15

  3. Thick band at N14 and thin band at N15

  4. Thick band at N14/15 and thin band at N15

  5. Thick band at N14
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C Correct answer
Explanation

Since, the Bacteria has been transferred from N15 to N14 medium, the source of nitrogen for synthesis of DNA will be non radioactive. Now, the bacteria was grown in N15 medium after which it was grown for several generations in N14 medium. So, after centrifugation thick band will be formed in N14 zone and a thin band will be formed in the N15 zone as initially the DNA contained N15 nitrogen.

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