Question

The method developed by Matthew Meselson and Franklin Stahl to separate heavy DNA with N15 from DNA, for providing evidence for semi-conservative replication of DNA is
(a) Ion exchange chromatography
(b) Density gradient centrifugation
(c) Buoyant density centrifugation
(d) Gel filtration
(e) Isopycnic centrifugation

Answer 1

This is spatial variation in density over an area. in the natural sciences, the term density gradient is used to describe the varying density of matter but can apply to any quantity whose density can be measured.

Complete answer:
The density gradient centrifugation is the process developed by Matthew Meselson and Franklin Stahl to separate heavy DNA with N15 from DNA, for providing evidence for semi-conservative replication of DNA. In their experiment, they used the bacteria E.coli as a model system.
-They grow E.coli in a medium that contains a "heavy" isotope of nitrogen (N15).
-The bacteria absorb the nitrogen and use it to synthesize new biological molecules, including DNA.
-After growing many generations in the N15 medium the bacteria is then switched to a medium containing a “light” isotope of nitrogen (N14) and allowed to grow for several generations.
-After the switch, the DNA would have to be made up of N14 because this was the only nitrogen available for DNA synthesis.
-They collected the small samples in every generation and extracted and purified the DNA. After that, they measured the density of the DNA using density gradient centrifugation.
-The process of density gradient centrifugation separates the DNA bands by spinning them at high speeds in the presence of cesium chloride.
-After the analysis of the first four generations of E.coli , the following results were found:
-When centrifuged a single band of DNA is produced after one generation. It contains intermediate density in between the heavy N15 and the light N14 DNA. This generation possesses a hybrid of light and heavy DNA.
-When centrifuged two bands of DNA are produced after the second generation. One band is at the same position as the intermediate band of first-generation and the second band is positioned a little higher at the position of the light molecule. This shows the semi-conservative replication of DNA.
-After centrifugation in the third and fourth generations it has been observed that hybrid DNA molecules will give rise to hybrid one and the light molecules will create lighter molecules.
-Meselson and Stahl after their experiment concluded that each strand in a DNA molecule serves as a template for the synthesis of a new, complementary strand.

So, the correct answer is,’ Density gradient centrifugation.

Note:
-An isotope is that form of an element that differs from other forms of the same element by the number of neutrons in its nucleus.
-Cesium chloride is responsible for the formation of a density gradient from the top to the bottom of the spinning tube.