DNA Structure and Function
The Watson and Crick model of the structure of DNA eloquently suggested the method of DNA replication. The double helical nature, and the complementary base pairing, implied that one strand of DNA provided the information for making the other strand. Once the strands of DNA separated, each would be used as a template to make a new, complementary strand. Therefore, the two resulting strands of DNA would contain both an old strand and a new strand. This is known as the semiconservative theory of replication. The conservative theory stated that one double helix would contain only the newly synthesized DNA while the other would contain only the original strands.
In 1958, Matthew Meselson and Franklin Stahl confirmed the semiconcervative model. They grew the bacterium E. coli for many generations in a heavy isotope of nitrogen, 15N. The isotope was incorporated into the DNA.
The bacteria were then placed in medium that contained only the light isotope of nitrogen, 14N. The bacteria were sampled over a period of time. Their DNA was extracted and subjected to centrifugation techniques, which would separate the DNA based on density.
DNA containing solely 15N was “heavy” whereas DNA containing solely 14N was “light”. After one round of division in the medium containing 14N, all the DNA was found to be intermediate in size, between the light and heavy types.
This “intermediate” DNA was further analyzed. The hydrogen bonds between the base pairs were broken so the DNA was single stranded. These strands were then centrifuged. Half of the single stranded DNA was in the heavy form, and half was in the light form.
If the replication theory suggested by Watson and Crick’s model was correct, then the density of the double stranded DNA after 2 rounds of replication in 14N medium would be:
|A||half heavy, half light|
|B||half heavy, half intermediate|
|C||half intermediate, half light|
After one round of replication, all the DNA was found to be intermediate in density, with one strand containing the light isotope and one containing the heavy isotope. A second round of replication would mean that these intermediate DNA double strands would now serve as the template strands. The resulting copies would contain the light isotope. Therefore, from the template strand containing the heavy isotope, the resulting double strand would be intermediate, and from the template containing the light isotope, the double strand would contain only the light isotope. This would result in half the DNA being intermediate and half being light.