Evidence for DNA replication

What is the semi-conservative hypothesis?

The accepted model for the synthesis of a polypeptide; DNA helicase unwinds the DNA molecule into its two strands by breaking the hydrogen bonds which hold the base pairs together. DNA polymerase catalyses the addition of free nucleotides to the exposed bases, and each chain acts as a template so that the nucleotides can be joined to their complimentary bases by the enzyme DNA polymerase. The result is two DNA molecules (after the rest of the process), each made up of one newly synthesised strand and one old strand (which had been conserved from the original molecule).

Who proposed the theory of how DNA replicates?

Watson and Crick

How was the theory of how DNA replicates confirmed?

The Meselson and Stahl experiment: E-coli is grown in a medium containing the isotope 15N (while the typical isotope for nitrogen is 14N). DNA is extracted from the E-coli, and both strands of DNA contain 15N. The DNA is put in a centrifuge to separate the cellular material based on size and density. This forms a band low in the centrifuge tube (as the 15N is heavier than the 14N). Then the 15N E-coli is grown in medium containing the normal isotope of nitrogen, 14N for one generation, and the DNA is extracted. Now it has one strand of 15N and one of 14N, shown by the centrifuged DNA as it forms a higher, intermediate band as it contains a lighter (less heavy) isotope. Some of this 14N 15N E-coli is grown for another generation in the medium containing the normal 14N isotope, and the DNA is extracted. Half of this now contains only 14N and half contains 14N 15N. To prove this, the DNA is centrifuged; it forms two bands, one of the hybrid middle and one a lighter band at the top. This provides conclusive evidence

What do thicker bands in centrifuged tubes show?

That a higher proportion of DNA is of that weight

What is a fully conservative model?

One double helix is fully conserved and the other double helix is completely new

What would the centrifuged DNA look like for a fully conservative model?

Beginning with only 15N heavy DNA, after one generation, the centrifuged tube would have one light band and one heavy band (since one strand of DNA was synthesised completely from the new 14N isotope while the other helix remained the same with the 15N isotope). After two generations, there would be a thicker band of light DNA and a thin band of heavy DNA

What is the dispersive theory?

That the DNA gets mixed between the strands so there could be any combination of DNA with different isotopes

What would the centrifuged DNA look like for a fully dispersive model?

Beginning with only 15N heavy DNA, after one generation, the centrifuged tube would have a hybrid of 14N and 15N isotopes (as each double helix has mixed its genes randomly so that both have some of each hybrid). After two generations, a new hybrid would be formed with 14N and 15N in different proportions to prove that this is not the correct theory