Meselson and Stalh experiment

Meselson and Stalh used two different isotopes of nitrogen in order to give two different densities to cell growth. . The DNA of the cells grown in ¹⁵N medium had a higher density than cells grown in normal ¹⁴N medium.  After that, E. coli cells with only ¹⁵N in their DNA were transferred to a ¹⁴N medium and were allowed to divide; the progress of cell division was monitored by microscopic cell counts and by colony assay.

DNA was extracted periodically and was compared to pure ¹⁴N DNA and¹⁵N DNA. After one replication, the DNA was found to have intermediate density. Since conservative replication would result in equal amounts of DNA of the higher and lower densities (but no DNA of an intermediate density), conservative replication was excluded. 

Conclusion of experiment

• Results show that after one generation, the double stranded DNA is 1/2 heavy (from the parent) and 1/2 light (newly synthesized). This means that 100% of the strands are of intermediate density.
• After a second generation, one half of the new daughter strands are light (using ¹⁴N DNA as template and synthesizing ¹⁴N NA) and one half are intermediate density (using ¹⁵N DNA as a template and ¹⁴N DNA for synthesis). This result is predicted by semiconservative replication.
• Conclusion- as predicted by Watson and Crick, DNA strands serve as templates for their own replication.

Result of experiment

 After one replication, the DNA was found to have intermediate density. Since conservative replication would result in equal amounts of DNA of the higher and lower densities (but no DNA of an intermediate density), conservative replication was excluded. However, this result was consistent with both semiconservative and dispersive replication. Semiconservative replication would result in double-stranded DNA with one strand of ¹⁵N DNA, and one of ¹⁴N DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of ¹⁵N and ¹⁴N DNA, either of which would have appeared as DNA of an intermediate density.