Meselson & Stahl grew E. coli cells in a dense N15 nitrogen medium, then transferred to a lighter N14 nitrogen medium. Describe the observations of their experiment (until the third generation) and use it to explain the mechanism of DNA replication.
Observations:- After each generation, the amount of DNA will be doubled during the S phase of interphase- At the start, all DNA will be labelled N15- In the generation 1, all DNA will be labelled N14N15 since each DNA molecule will consist of one original parental strand (N15) and one newly synthesised strand (N14)- In the generation 2, half of the DNA will be labelled N14N15, while the other half will be labelled N14 since each DNA molecule (from generation 1) will have one template strand labelled N15 and one labelled N14, while the newly synthesised strands will all be labelled N14- In the third generation, 25% of the DNA will be labelled N14N15, while the remaining 75% will be labelled N14 as only 25% of the newly synthesised DNA molecules will contain one DNA strand labelled N15, while the rest will have two DNA strands labelled N14
Explanation: - During DNA replication, the two strands of the DNA double helix separate- Each strand acts as a template for the synthesis of a new strand, which is made by adding nucleotides using complementary base pairing - Each of the replicated DNA molecules are composed of one strand from the original parent DNA molecule and one strand that is newly-synthesised- This mechanism of DNA replication is called semi-conservative replication
