Answer the following questions based on Meselson and Stahl’s experiment:
(a) Why did the scientists use 15NH4Cl and 14NH4Cl as sources of nitrogen in the
culture medium for growing E.coli?
(b) Name the molecule(s) that 15N got incorporated into.
(c) How did they distinguish between 15N labelled molecules from 14N ones?
(d) Mention the significance of taking the E.coli samples at definite time intervals for
observations.
(e) Write the observations made by them from the samples taken at the end of 20
minutes and 40 minutes respectively.
(f) Write the conclusion drawn by them at the end of their experiment.
Answers
The solution to the following questions are:
a) Nitrogen(¹⁴N) is a major constituent of the DNA molecule.¹⁵N is the most abundant isotope of Nitrogen. Therefore ¹⁵NH₄Cl and ¹⁴NH₄Cl are used as a source of nitrogen in the culture medium for growing E.coli, for Meselson and Stahl's experiment.
(b) ¹⁵N got incorporated into the newly synthesized DNA molecule as nitrogen is the major constituent of DNA. DNA with the heavier isotope of nitrogen (N¹⁵) is also stable.
(c) ¹⁵N is the heavier isotope of Nitrogen, forming heavier bands after CsCl density gradient centrifugation. While ¹⁴N forms lighter band after density gradient centrifugation.
When these bacteria with incorporated ¹⁵N were placed in medium containing ¹⁴N (¹⁴NH₄Cl), it was noticed newly formed DNA molecules contain one strand heavier than the other.
(d) E. coli culture is taken at equal intervals to know the progress of the experiment as generation time of E. coli is 20 minutes.
(e) At the end of 20 minutes that is one generation time:
¹⁵N in ¹⁴N medium forms a hybrid of N¹⁵N¹⁴, therefore intermediate density band is obtained.
After 40 minutes that is two generation time:
The culture had equal amounts of hybrid DNA( N¹⁵N¹⁴) and the light DNA (N¹⁴N¹⁴) showing two bands, one in the center and one at the bottom.
(f) This experiment was done to proof that DNA replicates semi -conservatively.
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