Meselson and Stahl experiment
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of DNA structure and DNA replication. You can test your knowledge on these below.
What is the role of DNA helicase in DNA replication?
DNA helicase breaks the hydrogen bonds between complementary base pairs, separating the two DNA strands.
Describe what is meant by semi-conservative replication.
Semi-conservative replication means that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.
What are the complementary base pairings in DNA?
Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G), held together by hydrogen bonds.
Explainer Video
Background
DNA replication was hypothesised to occur in one of three ways:
-
Conservative replication – One entirely new DNA molecule is made, and the original remains intact.
-
Semi-conservative replication – Each new DNA molecule consists of one old strand and one new strand.
-
Fragmented or Dispersive replication – DNA strands are mixtures of old and new DNA.
The Experiment
Meselson and Stahl (1958) conducted an experiment using the bacterium E. coli and different isotopes of nitrogen to determine the correct model of DNA replication.
They used two isotopes of nitrogen: heavy nitrogen (15N) and light Nitrogen (14N)
Bacteria will incorporate nitrogen into the bases in their DNA
Steps of the Experiment:
-
Growing Bacteria in Heavy Nitrogen (15N):
-
E. coli was first grown in a medium containing only the heavy isotope of nitrogen (15N), which was incorporated into their DNA.
-
This made the DNA denser.
-
Switching to Light Nitrogen (14N):
-
The bacteria were then transferred to a medium containing only the lighter isotope of nitrogen (14N) and allowed to replicate once.
-
Centrifugation to Analyse DNA Density:
-
DNA was extracted and centrifuged to observe its position in the test tube.
-
It does this by spinning the extracted DNA at high speed. Causing the DNA to collect in different areas, depending on the density.
Key Findings & Final Conclusion
Key Findings:
- After one round of replication in 14N:
-
DNA formed a single intermediate band in the centrifuge tube.
-
This disproved the conservative model, as there was no fully light (14N) or fully heavy (15N) DNA.
-
This supported the semi-conservative replication model, as the DNA consisted of one original (15N) strand and one newly synthesised (14N) strand.
-
- After two rounds of replication in 14N:
-
Two distinct bands were observed
-
One intermediate density consisting of one original (15N) strand and one newly synthesised (14N) strand.
-
One less dense band that consisted of two strands containing 14N.
-
Final Conclusion:
-
The experiment provided strong evidence that DNA replication occurs via the semi-conservative model.
Key Terms
-
Semi-conservative replication: Each new DNA molecule contains one original (parental) strand and one newly synthesized strand.
-
Isotope: Different forms of an element with the same number of protons but a different number of neutrons.
-
Nitrogen-15 (15N): A heavy isotope of nitrogen used to label DNA in the experiment.
-
Nitrogen-14 (14N): A lighter isotope of nitrogen used in the second stage of the experiment.
-
Centrifugation: A process that separates DNA by density using a high-speed spinning force.
No answer provided.
Exam Tip
In exam questions, always link Meselson and Stahl’s experiment to semi-conservative replication and give a definition of what semi-conservative replication means.
No answer provided.
There are two isotopes of nitrogen. 15N is a heavier isotope than the normal isotope 14N.
In an investigation, a culture of bacteria was obtained in which all the nitrogen in the DNA was of the 15N form. The bacteria (generation 0) were transferred to a medium containing only the normal isotope, 14N, and allowed to divide once. A sample of these bacteria (generation 1) was then removed. The DNA in the bacteria of generation 1 was extracted and spun in a high-speed centrifuge.
The bacteria in the 14N medium were allowed to divide one more time. The DNA was also extracted from these bacteria (generation 2) and spun in a high-speed centrifuge.
Which part of the DNA molecule contains nitrogen? (1 mark)
Explain why the DNA from generation 1 is found in the position shown. (3 marks)
Draw bands on the final tube to show the position of DNA molecules from bacteria which have divided twice in 14N. (2 marks)
-
The bases in DNA contain nitrogen. (1)
-
DNA with one heavy and one light strand. (1)
-
The newly synthesised strand was made with 14N. (1)
-
DNA replication is semi-conservative. (1)
- One band should be drawn at the same height as generation 1 and another band should be drawn above (higher up the tube).
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!