Question

State the aim and describe Messelson and Stahl’s experiment.

Answer 1

Messelson and Stahl in 1958 aimed at proving that the DNA replicates in a semi-conservative fashion. The semi-conservative DNA replication suggests that after the completion of replication, each DNA molecule will have one parental and one newly-synthesised strand.

Experimental proof

(1) E.coli was grown in a medium containing ¹⁵NH₄Cl (¹⁵N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. As a result, ¹⁵N was incorporated into

the newly-synthesised DNA. This heavy DNA could be distinguished by centrifugations in CsCl density gradient.

(2) Then, these E. coli cells were transferred to a medium with normal ¹⁴NH₄ Cl and the DNA was extracted as double-stranded helix. The various samples were separated on CsCl gradients for measuring the density of DNA.

(3) After 40 minutes, the DNA of the second generation was extracted from the

¹⁴NH₄Cl medium and was found to have equal amounts of hybrid and light DNA.