Biology 1958

The Replication of DNA in Escherichia coli

Matthew Meselson & Franklin Stahl

DNA copies itself by keeping one old strand and building one new — proved by weighing it.

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In depth · the introduction

When DNA copies itself, does it keep the old or build entirely new? Two young scientists answered the question by weighing the molecule — and watching where it settled.

The big idea

Watson and Crick had said DNA is two strands twisted together, each a mirror of the other. They guessed that to copy itself, the molecule would unzip and let each old strand act as a stencil for a new partner. If true, every fresh copy would be half old and half new. But that was a guess; nobody had seen it.

Meselson and Stahl found a way to make the answer visible. They grew bacteria so that their DNA was 'heavy', then let them grow in normal 'light' food and watched, generation by generation, exactly how heavy the DNA stayed. The verdict was clean: after one round of copying, every molecule was precisely half-heavy — one old strand kept, one new strand built — exactly as Watson and Crick had predicted.

How it came about

In the mid-1950s, after the double helix was published, biologists faced a sharp open question: how does a molecule like DNA actually duplicate? Three answers seemed possible, and there was no way to choose between them. Matthew Meselson and Franklin Stahl, two young researchers at Caltech, had just invented a new tool: spin DNA in a salt solution so fast that the salt forms a density gradient, and each molecule floats to the layer that matches its own weight.

They realised this could settle the debate. By feeding bacteria a heavy form of nitrogen, then switching to the ordinary light form, they could literally watch the heavy old material get diluted with light new material — and the pattern of bands would reveal which copying scheme nature used. The 1958 result was so clear and elegant that it is often called 'the most beautiful experiment in biology.'

Why it mattered

The double helix was a beautiful structure, but a structure is not a mechanism. This experiment turned a hopeful guess into established fact: DNA really does copy itself by keeping one old strand in each new molecule. That single result anchors how we understand heredity, mutation, DNA repair, and the copying machinery in every living cell — and it stands as a model of how to design an experiment so clean that the answer is almost impossible to argue with.

A way to picture it

Imagine a zip made of two halves that fit only each other. To make two new zips, you don't build both from scratch. Instead you pull the zip apart and give each old half a brand-new matching half. Now you have two complete zips, and each one is half old, half new. Do it again, and the original two old halves are still in there — passed down intact, one to each generation — while more and more brand-new material surrounds them. That faithful keeping of one old half is exactly what Meselson and Stahl caught the molecule doing.

Where it sits

This experiment is the bridge between two milestones in this Library: it confirmed the mechanism implied by Watson and Crick's 1953 double helix, and it set the stage for the molecular machines — polymerases, repair enzymes — and eventually for tools like CRISPR that read and rewrite the very strands shown here to be conserved. Where Watson and Crick gave the structure, Meselson and Stahl gave the proof of how that structure works in life.

The original document

Original source text

The question

Matthew Meselson & Franklin W. Stahl · Proc. Natl. Acad. Sci. USA 44(7) (1958): 671–682 · Communicated by Max Delbrück, May 1958

Watson and Crick had proposed in 1953 that DNA is two complementary strands wound into a double helix, and suggested that each strand could serve as a template for a new partner. That implied a precise prediction about how the molecule should divide when it copies itself — but in 1958 no experiment had yet shown which of the competing schemes nature actually used.

Three schemes were on the table. In semiconservative replication each daughter double helix keeps one whole parental strand and pairs it with a freshly built one. In conservative replication the parent stays wholly intact and an entirely new double helix is made alongside it. In dispersive replication the parental material is broken up and scattered piecemeal through both daughters.

The method — labelling and weighing DNA

Escherichia coli was grown for many generations on a medium whose only nitrogen source carried the heavy isotope N15, so that the bacteria's DNA became uniformly dense. The culture was then abruptly switched to ordinary light N14 medium, and samples were taken after successive generations of growth.

Each sample's DNA was spun for some twenty hours in a concentrated cesium chloride solution. Under the immense centrifugal force the salt forms a smooth density gradient, and every DNA molecule drifts to the level where its own buoyant density matches the surrounding fluid, settling into a sharp band photographed by ultraviolet absorption.

[ … ]

What the bands showed

Fully N15-labelled DNA banded at a higher density than ordinary N14 DNA, the two differing in buoyant density by about 0.014 g/cm³ — a separation wide enough to resolve cleanly. After one generation of growth in N14, all of the DNA banded at a single intermediate position, exactly halfway between heavy and light: every molecule was now half-heavy. After two generations, two bands of equal amount appeared — one half-heavy, one fully light.

A half-heavy molecule, heat-denatured into single strands, separated into one heavy strand and one light strand — direct evidence that the intermediate band was a hybrid of one old and one new strand, not a uniform blend.

The two conclusions

Conclusions (verbatim)

1. The nitrogen of a DNA molecule is divided equally between two subunits which remain intact through many generations.

2. Following replication, each daughter molecule has received one parental subunit.

The result is in exact accord with the expectations of the Watson–Crick model for the duplication of DNA.

Gates and Crellin Laboratories of Chemistry, California Institute of Technology · 1958