The Watson Crick Model (AQA A Level Biology): Revision Note
Exam code: 7402
Evaluating the Watson-Crick model
In 1953, Watson and Crick confirmed the double-helix structure of DNA
They also proposed a model for DNA replication called semi-conservative replication:
Each new DNA molecule contains one original strand and one newly synthesised strand
This is based on complementary base pairing (A–T and C–G)
The conservative model suggested that the original DNA remains intact, and the new DNA is made entirely of new strands
However, this was just a theory, another theory suggested DNA replicated ‘conservatively’
The theory of conservative DNA replication suggested that the strands of the original DNA molecule would stay together, and the new, replicated DNA molecule would be made out of two brand-new strands
Two other scientists, Meselson and Stahl, tested the theories using bacteria grown in heavy nitrogen (¹⁵N) followed by growth in light nitrogen (¹⁴N)
Their results supported the semi-conservative model, confirming Watson and Crick’s theory
Meselson and Stahl’s experiment
Bacteria are grown in a broth containing the heavy (15N) nitrogen isotope
DNA contains nitrogen in its bases
As the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides
After some time, the culture of bacteria had DNA containing only heavy (15N) nitrogen
A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge
This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
The bacteria containing only 15N DNA were added to a broth containing only the lighter 14N nitrogen
The bacteria were left for one round of DNA replication to occur before their DNA was extracted and spun in a centrifuge
If conservative DNA replication had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
If semi-conservative replication had occurred, all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube
One strand of each DNA molecule would be from the original DNA containing the heavier nitrogen, and the other (new) strand would be made using only the lighter nitrogen
Meselson and Stahl confirmed that the bacterial DNA had undergone semi-conservative replication.
The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes
If more rounds of replication were allowed to take place, the ratio of 15N:14N would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third
This experiment proved Watson and Crick's theory correct


Evaluation of Meselson & Stahl's experiments
The work of Meselson and Stahl was crucial in validating the semi-conservative model
The process highlights the scientific method: hypotheses are proposed based on reasoning, then tested and confirmed through carefully designed experiments
In addition to this,
the method could be replicated using other organisms or molecules
the experiments were well-controlled
the centrifugation method produced observable, measurable bands
the results disproved the conservative and dispersive models.
However,
the experiment only tested one organism (E. coli), while this strongly supported the model, it does generalise to all life, potentially requiring further research
the experiment confirmed the pattern of replication, but didn’t show how enzymes like DNA helicase and DNA polymerase worked
The experiment by Messelson and Stahl is widely regarded as conclusive proof of Watson and Crick’s semi-conservative replication model
Their studies are considered one of the most convincing experiments in molecular biology due to their
Examiner Tips and Tricks
You need to be able to evaluate the work of scientists in validating the Watson–Crick model of DNA replication.
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