Nucleic Acid Structure:- DNA Molecules Have Distinctive Base Compositions
A most important clue to the structure of DNA came from the work of Erwin Chargaff and his colleagues in the late 1940s. They found that the four nucleotide bases of DNA occur in different ratios in the DNAs of different organisms and that the amounts of certain bases are closely related. These data, collected from DNAs of a great many different species, led Chargaff to the following conclusions:
1. The base composition of DNA generally varies from one species to another.
2. DNA specimens isolated from different tissues of the same species have the same base composition.
3. The base composition of DNA in a given species does not change with an organism’s age, nutritional state, or changing environment.
4. In all cellular DNAs, regardless of the species, the number of adenosine residues is equal to the number of thymidine residues (that is, A=T), and the number of guanosine residues is equal to the number of cytidine residues (G=C). From these relationships it follows that the sum of the purine residues equals the sum of the pyrimidine residues; that is, A+G=T+C.
FIGURE 8–13 The Hershey-Chase experiment. Two batches of iso topically labeled bacteriophage T2 particles were prepared. One was labeled with 32P in the phosphate groups of the DNA, the other with 35S in the sulfur-containing amino acids of the protein coats (capsids). (Note that DNA contains no sulfur and viral protein contains no phosphorus.) The two batches of labeled phage were then allowed to infect separate suspensions of unlabeled bacteria. Each suspension of phage-infected cells was agitated in a blender to shear the viral capsids from the bacteria. The bacteria and empty viral coats (called “ghosts”) were then separated by centrifugation. The cells infected with the 32P-labeled phage were found to contain 32P, indicating that the labeled viral DNA had entered the cells; the viral ghosts contained no radioactivity. The cells infected with 35S-labeled phage were found to have no radioactivity after blender treatment, but the viral ghosts contained 35S. Progeny virus particles (not shown) were produced in both batches of bacteria sometime after the viral coats were removed, indicating that the genetic message for their replication had been introduced by viral DNA, not by viral protein.
