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Question

To demonstrate the semi-conservative mode of replication Meselson and Stahl used

A
Autoradiography
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B
Ultracentrifugation
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C
Both A and B
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D
X-ray crystallography only
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E
Equilibrium density gradient centrifugation
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Solution

The correct option is E Equilibrium density gradient centrifugation

The Meselson-Stahl showed the semiconservative mechanism of DNA replication. They cultured E. coli cells initially in a medium containing ammonium salts prepared with heavy nitrogen N15 to label all cellular DNA. Then cells were transferred to a medium containing the normal light isotope N14.

DNA was analyzed by equilibrium density-gradient centrifugation in periodically collected sample to separate heavy-heavy (H-H), light-light (L-L), and heavy-light (H-L) duplexes into distinct bands.

Equilibrium density gradient centrifugation is used for separating molecules based on their densities. The experimental objects meet the points in the tube at which they are in density equilibrium with the surrounding solvent. The ultracentrifugation separates things by spinning a rotor at very high speeds. Autoradiography uses the pattern of decay emissions by the distribution of a radioactive substance and produces an image on an x-ray film by the pattern. X-ray crystallography identifies the atomic and molecular structure of a crystal by a differential pattern of diffraction of incident X-rays by crystalline atoms cause. Meselson and Stahl used a static system of equilibrium density gradient centrifugation to separate the heavy and light DNA strands. Correct answer is E


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