Meselson–Stahl experiment facts for kids

The Meselson–Stahl experiment was a famous science test done by Matthew Meselson and Franklin Stahl in 1958. They used DNA from tiny germs called E. coli. Their experiment showed how DNA makes copies of itself, a process called DNA replication. They proved that DNA copies itself in a "semiconservative" way. This means that when DNA makes a copy, each new DNA strand gets one old part and one brand-new part.

Understanding DNA and How it Copies

DNA is like a twisted ladder, known as a double helix. It holds all the instructions for a living thing. When a cell divides, its DNA must make an exact copy. This copying process is called DNA replication. Before Meselson and Stahl, scientists had a few ideas about how this happened.

A summary of the three suggested methods of DNA synthesis

Scientists thought DNA might copy itself in one of three ways:

• Conservative replication: In this idea, the original DNA molecule would stay completely together. A brand-new, exact copy would be made from scratch. So, you'd have one old DNA and one new DNA.
• Semi-conservative replication: This idea suggested that the original DNA ladder would split down the middle. Each half would then get a new partner built next to it. This means each new DNA molecule would be half old and half new.
• Dispersive replication: This was a more mixed-up idea. It suggested that both new DNA molecules would have bits and pieces of old and new DNA all mixed together.

The Clever Experiment

Meselson and Stahl needed a way to tell "old" DNA apart from "new" DNA. They used different types of Nitrogen atoms, called isotopes. Nitrogen is a key part of DNA.

They started with E. coli bacteria grown in a special liquid containing a "heavy" type of nitrogen, called N¹⁵. The bacteria used this heavy nitrogen to build their DNA. So, all their DNA became "heavy."

Next, they moved these "heavy" bacteria into a new liquid that only had "light" nitrogen, called N¹⁴. The bacteria kept growing and copying their DNA in this new liquid.

What They Found

After the bacteria copied their DNA once in the N¹⁴ liquid, Meselson and Stahl checked the DNA. They found that all the new DNA molecules were exactly halfway between "heavy" (N¹⁵) and "light" (N¹⁴). This meant each new DNA molecule had one strand of N¹⁵ (the old part) and one strand of N¹⁴ (the new part).

This result immediately ruled out the "conservative" idea. If it were conservative, they would have seen two separate types of DNA: one fully heavy and one fully light.

They then let the bacteria copy their DNA a second time in the N¹⁴ liquid. This time, they found two types of DNA:

• Some DNA was still halfway between heavy and light (N¹⁵/N¹⁴).
• Some DNA was completely "light" (N¹⁴/N¹⁴).

This second result ruled out the "dispersive" idea. If it were dispersive, all the DNA would still be a mix, but just a bit lighter. The fact that some DNA was completely light proved that the strands stay together.

Why This Discovery Matters

The Meselson–Stahl experiment clearly showed that DNA copies itself in a "semiconservative" way. This was a very important discovery because it helped scientists understand exactly how genetic information is passed on from one generation of cells to the next. It confirmed a key part of how life works!

See also

In Spanish: Experimento de Meselson-Stahl para niños