The Watson Crick Model (AQA A Level Biology): Revision Note

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