DNA Replication and Meselson And Stahl's Experiment

Literally, replication means the process of duplication. In molecular biology, DNA replication is the primary stage of inheritance. Central dogma explains how the DNA makes its own copies through DNA replication which then codes for the RNA in transcription and further, RNA codes for the proteins by translation.

Let’s go through DNA replication and Meselson and Stahl’s Experiment.

DNA Replication

DNA is the genetic material in the majority of the organisms.  Structurally, it is a double-stranded helical structure which can replicate. DNA replication is the process by which the DNA makes multiple copies of itself. It was originally proposed by Watson and Crick. DNA replication proceeds as follows:

  • Primarily during this process, two DNA strands will open and separate.
  • As the strands are separated, the enzymes start synthesizing the complementary sequence in each of the strands. That is, each parental strand will act as a template for the newly synthesized daughter strands.

Since the new DNA strands thus formed have one strand of the parent DNA and the other is newly synthesized, the process is called semiconservative DNA replication.

Semiconservative DNA replication

Meselson and Stahl’s Experiment

Meselson and Stahl’s Experiment was an experimental proof for semiconservative DNA replication. In 1958, Matthew Meselson and Franklin Stahl conducted an experiment on E.coli which divides in 20 minutes, to study the replication of DNA.


15N (heavy) and 14N (normal) are two isotopes of nitrogen which can be distinguished based on their densities by centrifugation in cesium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15NH4Cl over many generations. As a result, 15N was integrated into the bacterial DNA. Later, they revised the 15NH4Cl medium to normal 14NH4Cl. At a regular interval of time, they took the sample and checked for the density of DNA.


Sample no. 1 (after 20 minutes): The sample had bacterial DNA with an intermediate density. Sample no. 2 (after 40 minutes): The sample contained DNA with both intermediate and light densities in the same proportion.

Meselson and Stahl’s Experiment


Based on observations and experimental results, Meselson and Stahl concluded that DNA molecules can replicate semi-conservatively. Investigation of semiconservative nature of replication of DNA or the copying of the cells DNA didn’t end there. Followed by Meselson and Stahl experiment, Taylor and colleagues conducted another experiment on Vicia faba (fava beans) which again proved that replication of DNA is semiconservative.

Stay tuned with Byju’s to learn more about DNA Replication and its experiments.

Practise This Question

Regarding the assertion and reason, select the correct option.

Assertion [A]: DNA replication is semiconservative in nature.

Reason [R]: One of the DNA strands is synthesised continuously, while the other is synthesised discontinuously.