Meselson and Stahl Experiment Explained
How Meselson and Stahl used nitrogen isotopes and density gradient centrifugation to test the conservative, semi-conservative and dispersive models of replication. Includes the 4-mark exam question, a worked model answer and the examiner trap on predicting generation 2 correctly.
Chapters (9)
Exam question [4 marks]
Bacteria were grown for many generations in a medium containing heavy nitrogen, then transferred to a medium containing light nitrogen. DNA was extracted after one generation and spun in a density gradient. All the DNA formed a single band of intermediate density. Explain what this result shows about the way DNA replicates.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Each DNA molecule contains one heavy strand and one light strand (intermediate density) | "one heavy and one light strand", "half heavy, half light" if it clearly refers to the strands or each molecule, "one original and one new strand", "each molecule contains both heavy and light DNA/nitrogen" | "the DNA is half heavy" or "half heavy, half light" with no reference to strands or molecules |
| 2 | The conservative model predicts one heavy band and one light band (two bands), which was not seen, so it is ruled out | "conservative would give two separate bands", "conservative rejected" | "conservative is wrong" with no reason |
| 3 | The original (parent) strands stay heavy and the new strands are made from light nitrogen | "old strands heavy, new strands light", "new nucleotides use light nitrogen" | "the DNA changes from heavy to light" |
| 4 | This supports semi-conservative replication, where each new molecule keeps one original strand | "semi-conservative", "consistent with / supports semi-conservative", "each daughter has one parent strand"; a candidate who adds that dispersive is not ruled out by this result should not be penalised | "dispersive" supported; "conservative" supported; saying generation one rules out dispersive |
The result at generation one cannot on its own rule out the dispersive model, because that also predicts a single intermediate band; the generation two result is needed for that. Do not credit a generation two prediction showing a heavy band, which is the conservative model.
Model answer (4/4)
After one generation all the DNA has an intermediate density, so each molecule contains one heavy strand and one light strand. The conservative model predicts one fully heavy molecule and one fully light molecule, giving two bands, so it is ruled out. The original strands stay heavy and the new strands are built from light nitrogen. This fits semi-conservative replication, where each new molecule keeps one original strand.
Examiner trap
- Giving a prediction for generation 2 that matches the conservative model instead of the semi-conservative one
Key terms
- semi-conservative
- conservative
- dispersive
- nitrogen isotopes
- density gradient
- centrifugation
- generations
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:31The Watson-Crick model
Watson and Crick suggested that DNA replicates semi-conservatively. Each new molecule keeps one old strand and gains one new strand. But other models were possible. In the conservative model, the original molecule stays whole and a completely new molecule is made.
In the dispersive model, the strands break up, and each new molecule is a patchwork of old and new sections. A good idea needs a test that can tell the models apart.
1:00Heavy and light nitrogen
In nineteen fifty-eight, Meselson and Stahl used isotopes of nitrogen. Ordinary nitrogen is light, nitrogen fourteen. Nitrogen fifteen is heavy. Both behave the same chemically, but DNA made with heavy nitrogen is denser than DNA made with light nitrogen. Nitrogen is in the bases, so any new DNA takes it up from the growth medium.
1:24Growing and centrifuging bacteria
They grew bacteria for many generations in heavy nitrogen, so all their DNA was heavy. Then they moved the bacteria into light nitrogen, and took samples after each generation. The DNA was extracted and spun in a centrifuge, in a tube holding a solution whose density increases from top to bottom. That is a density gradient. Each type of DNA settles at the height where its density matches the solution. Heavy DNA sits low. Light DNA sits high.
1:56The bands they saw
Here is what they saw. Generation zero: one band, low down. All heavy. Generation one: still one band, but halfway up. Intermediate density. Generation two: two bands. One intermediate, and one light.
2:13Testing generation one
Now use the results to test the models. The conservative model predicts, in generation one, one fully heavy molecule and one fully light molecule. That means two bands. They saw one intermediate band. So the conservative model is rejected. But semi-conservative and dispersive both predict a single intermediate band. So generation one alone cannot choose between them.
2:41Generation two settles it
Generation two settles it. The semi-conservative model predicts two kinds of molecule. Half have one original strand and one new strand, which give an intermediate band. Half have entirely new strands, which give a light band. That is exactly what they saw. The dispersive model predicts one band only, lighter than intermediate, because every molecule would keep a quarter of the old material. They did not see that. So dispersive is rejected. Only the semi-conservative model fits both generations. That supports the Watson and Crick model.
3:56Examiner trap and recap
The trap. In generation two, semi-conservative gives one intermediate band and one light band. A heavy band and a light band is the conservative prediction. Recap. Heavy nitrogen labels old strands, a density gradient shows them, and two generations rule out two rival models. Next, we meet the molecule that powers all of this: ATP.
Watch first
This lesson builds on:
- DNA Replication: Helicase and Polymerase · AQA 7402 · 3.1.5.2
Keep going
More from 3.1 Biological molecules
- DNA and RNA Nucleotides: Structure Compared · AQA 7402 · 3.1.5.1
- DNA Double Helix and Phosphodiester Bonds · AQA 7402 · 3.1.5.1
- DNA Replication: Helicase and Polymerase · AQA 7402 · 3.1.5.2
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-018.