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 DNA replication was conservative, after 1 round of replication in 14N, the density of the double stranded DNA would be:
|C||half intermediate, half heavy|
|D||half heavy, half light|
Conservative replication implies that the template strands remain together and the copied strands form the new double stranded DNA molecule. If this were true, then the template strands would contain only 15N, the heavy isotope, and the copy strands would contain only 14N, the light isotope.