Meselson-Stahl Experiment: Unraveling DNA Replication

Meselson-Stahl Experiment and DNA Replication

The Meselson-Stahl experiment tested the hypotheses of DNA replication. They cultured bacteria in a ¹⁵N medium. ¹⁵N is a heavy isotope of nitrogen, so the synthesized DNA is of heavy density. They then shifted the bacteria to a ¹⁴N medium, and DNA was isolated at different times corresponding to replication cycles 0, 1, and 2.

Results of the Experiment

After one replication cycle, the DNA was all of intermediate density. This rules out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present, but none of intermediate density will be present. This result is consistent with the semi-conservative replication model, which predicts that all DNA molecules will consist of one ¹⁵N-labeled DNA strand and one ¹⁴N-labeled DNA strand. The result does not rule out the dispersive replication model, which also predicts that all DNA will be of intermediate density, consisting of interspersed double-stranded ¹⁵N-labeled and ¹⁴N-labeled segments.

After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density. This result is exactly what the semi-conservative model predicts: half should be ¹⁵N-¹⁴N intermediate density DNA and half should be ¹⁴N-¹⁴N light density DNA. This result rules out the dispersive replication model, which predicts that after replication cycle 1, the DNA density of all DNA molecules will gradually become lower, so no intermediate density DNA should remain at replication cycle 2. The semi-conservative model is correct.

The Process of DNA Replication

Here's an explanation of the process of DNA replication:

1. Helicase binds to the DNA at the origin of replication. Helicase enzymes then travel in both directions away from the origin of replication and break hydrogen bonds between complementary bases.
2. SSBPs (single-stranded binding proteins) bond to the unwound DNA to stabilize it.
3. RNA Primase adds complementary RNA nucleotides in a 5' to 3' direction. These serve as a primer to which DNA can attach. On the leading strand, only one primer is needed, while on the lagging strand, multiple primers are needed.
4. DNA Polymerase III then adds DNA nucleotides to the 3' end of the RNA primers as synthesis occurs in the 5' to 3' direction. On the lagging strand, synthesis occurs discontinuously, forming Okazaki Fragments.
5. DNA polymerase I replaces RNA primers with DNA nucleotides.
6. Along the lagging strand, the Okazaki fragments are joined by DNA Ligase to form a single DNA strand.
7. DNA polymerase I proofreads DNA for errors.