Question

2-2.  In the 1950’s Matthew Meselson and Franklin Stahl did an experiment in which they took bacteria that had grown for many generations in medium containing the stable nitrogen isotope 15N, and transferred them into medium containing only the 14N isotope of nitrogen.  They purified DNA from these bacteria at time zero (before transfer to 14N) and after 1 and 2 rounds of DNA replication in 14N medium.

1. When they analyzed the density of DNA molecules at time zero (before bacteria had been in the 14N medium), what did they observe?

2. When they analyzed the density of DNA molecules after one round of DNA replication, what did they observe?

3. When they analyzed the density of DNA molecules after two rounds of DNA replication, what did they observe?

4. What was the major conclusion from the Meselson-Stahl experiment?

Answer 1

2-2.  In the 1950’s Matthew Meselson and Franklin Stahl did an experiment in which they took bacteria that had grown for many generations in medium containing the stable nitrogen isotope 15N, and transferred them into medium containing only the 14N isotope of nitrogen.  They purified DNA from these bacteria at time zero (before transfer to 14N) and after 1 and 2 rounds of DNA replication in 14N medium.

1. When they analyzed the density of DNA molecules at time zero (before bacteria had been in the 14N medium), what did they observe?

At time zero, they observed the DNA is replicated and both strands contain 15N isotope of nitrogen (15N15N)

2. When they analyzed the density of DNA molecules after one round of DNA replication, what did they observe?

After one round of replication, half of the DNA strands will be with 15N isotope of nitrogen and other half with 14N isotope of nitrogen, ie DNA duplex with 15N14N will be obtained.

3. When they analyzed the density of DNA molecules after two rounds of DNA replication, what did they observe?

During the second round of replication most of the DNA contain both the strands with 14N isotope (14N14N), while there are few DNA samples in which hybrid DNA with 14N15N composition of isotope of nitrogen

4. What was the major conclusion from the Meselson-Stahl experiment?

There were three models initially, that explained the process of DNA replication; which includes 1. Conservative model of DNA replication, 2. Dispersive model of DNA replication and 3. Semi conservative DNA replication.

In conservative replication, the entire double-stranded DNA act as a template for the synthesis of a whole new molecule of DNA, that means the original DNA molecule is fully conserved during replication.

In dispersive model of replication, both nucleotide strands break down into several fragments, these broken fragments serve as templates for the synthesis of new DNA strands, and the finally reassemble into two complete DNA molecules.

According to semi conservative model of replication, during replication, each strand of DNA can serve as a template for the synthesis of a new strand. Due to the specificity of base pairing (A-T, G-C) the two DNA molecules synthesised on the pair of templates will be identical with the original strand. This process is called semi conservative replication because each of the parental strands remains intact or the original DNA molecule is half (semi) conserved during replication.

From Meselson-Stahl experiment,it is concluded that DNA replication is semi-conservative.