The Watson Crick Model (AQA A Level Biology)

• Watson and Crick were two scientists who worked together to confirm the double-helix structure of DNA in 1953
• Watson and Crick also came up with a model by which DNA might be replicated:
  • This theory was called semi-conservative DNA replication
  • The theory is based upon the specific hydrogen bonding between pairs of nitrogenous bases (A+T and C+G) being used during replication to conserve the genetic sequence

• 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

• Watson and Crick’s theory of semi-conservative DNA replication was later proved to be correct by the work of two other scientists, Meselson and Stahl
  • They used bacteria and two nitrogen isotopes, a heavy form (¹⁵N) and the normal, lighter form (¹⁴N), to prove this

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 was then taken out of the ¹⁵N broth and added to a broth containing only the lighter ¹⁴N nitrogen. The bacteria were left for enough time 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

Meselson and Stahl’s experiment that showed bacterial DNA replicated via semi-conservative DNA replication