Meselson and Stahl carried out centrifugation in CsCl₂ density gradient to separate :

Correct Answer: The normal DNA from ¹⁵N-DNA

Explanation:
The famous experiment conducted by Matthew Meselson and Franklin Stahl in 1958 provided definitive proof that DNA replication is semi-conservative.

To demonstrate this, they grew the bacterium Escherichia coli (E. coli) in a culture medium containing ammonium chloride (NH₄Cl) labeled with the heavy, non-radioactive isotope of nitrogen, ¹⁵N, as the sole nitrogen source. Over many generations, the bacteria incorporated this heavy nitrogen isotope into their DNA, making the DNA denser than normal DNA which contains the lighter, naturally occurring isotope, ¹⁴N.

To analyze the DNA density at different replication cycles after transferring the bacteria back to a normal ¹⁴N-containing medium, Meselson and Stahl utilized a technique called density gradient centrifugation in a cesium chloride (CsCl) solution. When CsCl is spun at very high speeds in an ultracentrifuge, it establishes a continuous density gradient, where the concentration and density of CsCl increase progressively from the top to the bottom of the centrifuge tube.

During centrifugation, DNA molecules migrate through the gradient until they reach a position where the density of the surrounding CsCl solution equals their own buoyant density (equilibrium):
1. Heavy DNA (containing only ¹⁵N) has a higher density and forms a band closer to the bottom.
2. Normal or light DNA (containing only ¹⁴N) has a lower density and forms a band closer to the top.
3. Hybrid DNA (containing one strand of ¹⁵N and one strand of ¹⁴N) forms a band at an intermediate position in the middle.

Therefore, CsCl density gradient centrifugation was specifically used to separate normal DNA from ¹⁵N-DNA based on their differences in mass and buoyant density, validating the semi-conservative model of DNA replication.