Question

Give an account of Meselson and Stahl’s experiment

Answer 1

Mathew Meselson and Franklin Stahl demonstrated semiconservative replication of DNA in 1958. They grew Escherichia coli for several generations on a medium with a high nitrogen isotope (¹⁵N). Bacterial DNA became fully tagged with a heavy isotope. The labelled bacteria were transferred to a new medium with normal or ¹⁴N nitrogen. DNA samples were collected for each generation and analysed for heavy nitrogen isotopes using density gradient centrifugation with caesium chloride. The first generation’s DNA was a mix of ¹⁵N and ¹⁴N.

The second generation of bacteria has two forms of DNA. 50% light, 50% moderate. This observation is only conceivable if the two strands of a DNA duplex split during replication and serve as a template for the synthesis of new complementary strands with normal or ¹⁴N. This will result in two DNA duplexes: one old strand (¹⁵N) and one new strand (¹⁴N). During second-generation DNA duplex production, the lSN and ¹⁴N strands separate to serve as templates. Approximately 50% of new DNA duplexes contain only normal or ¹⁴N strands, whereas the other 50% have both ¹⁵N and ¹⁴N. During replication, one strand of the parent DNA is preserved in the daughter, while the second strand is synthesised from scratch.