5.4.1. The Experimental Proof

It is now proven that DNA replicates semiconservatively. It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells.

Semiconservative replication means that when DNA replicates, the two strands of the original DNA molecule separate and each strand serves as a template for a new strand. This results in two new DNA molecules, each containing one old strand and one newly synthesized strand. This method was first demonstrated in E. coli bacteria and later confirmed in more complex organisms.

Think of it like a zipper on a jacket. When you unzip the jacket, you have two sides, but both sides maintain the original pattern. When you zip it back up, you have one old side and one new side that looks exactly like the old one, symbolizing how DNA keeps its information intact while duplicating itself.

Matthew Meselson and Franklin Stahl performed the following experiment in 1958... (i) They grew E. coli in a medium containing 15NH4Cl (15N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that 15N was incorporated into newly synthesised DNA (as well as other nitrogen containing compounds). This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride (CsCl) density gradient...

In their experiment, Meselson and Stahl used a heavy isotope of nitrogen (15N) to label newly synthesized DNA in E. coli. They grew the bacteria in a medium containing only this heavy nitrogen for several generations. After numerous divisions, the DNA incorporated the heavy nitrogen, which made it denser than normal DNA (which uses the lighter nitrogen isotope, 14N). By employing cesium chloride density gradients, they could separate the heavy DNA from the light DNA. This separation allowed them to trace the densities of the DNA after different generations of replication to support the semiconservative model.

Imagine two types of sand—one fine and light (like DNA in normal conditions) and the other coarse and heavy (like DNA in the experiment). If you layered them in water, the coarse sand would sink to the bottom while the fine sand would float. Similarly, Meselson and Stahl could visually separate the heavy and light DNA after replication using this density difference.

(ii) Then they transferred the cells into a medium with normal 14NH4Cl and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices...

After the E. coli had replicated in the heavy nitrogen medium and incorporated it into their DNA, Meselson and Stahl then moved the bacteria to a new medium containing the normal nitrogen. As the bacteria continued to grow and divide, they sampled the DNA at various intervals. They observed that after one generation, the DNA consisted of one heavy strand and one light strand (hybrid DNA). After two generations, half of the DNA samples were hybrid, and the other half was light DNA. This result validated that DNA replication is semiconservative, meaning each new molecule contains one strand from the original DNA.

Consider mixing a blue dye into a clear liquid. The first time you stir it, you create a shade of purple—this is like the hybrid DNA. The longer you keep mixing (or, in the case of the DNA experiment, how many generations the cells go through), the less blue and more clear the liquid becomes. In this analogy, the stirring and mixing demonstrate how the DNA strands are conserved and modified throughout replication.

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