Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transmission
DNA: structure of chromosomes and genes
DNA stores genetic information

The story of DNA begins with Friedrich Miescher, a Swiss biologist who was the first to conduct a systematic Study of the Cell Nucleus. In 1868, Miescher isolated a phosphorus-containing substance from the nuclei of pus Cells obtained from used surgical dressings, which he named "nucleイン" (nuclein). (Pus cells are white Blood Cells, or leukocytes.) Miescher discovered that nuclein contained an acidic component, now known as DNA, and a basic component corresponding to protein. Later, he found a similar substance in the heads of salmon spermatozoa. Although Miescher isolated the nucleic acid fraction and studied its properties, the Covalent Structure of DNA, shown in Fig. 27-5, remained elusive until the late 1940s.

Miescher, and many other researchers following him, suspected that nuclein or nucleic acid was somehow related to cellular heredity; however, the first direct proof that DNA is the carrier of Genetic information was obtained only in 1943 through a discovery made by Oswald T. Avery, Colin MacLeod, and Maclyn McCarty of the Rockefeller Institute. They found that DNA extracted from a virulent (disease-causing) strain of the bacterium Streptomyces pneumoniae, also known as the pneumococcus, invariably converted a non-virulent strain of this bacterium into the virulent form (Fig. 27-6). Avery and his colleagues concluded that the DNA isolated from the virulent strain carries the heritable genetic information responsible for virulence, and that this information is permanently incorporated into the DNA of the recipient non-virulent cells. Initially, not everyone agreed with this assertion. Some critics believed that trace amounts of protein impurities present in the DNA might be the actual carriers of genetic information. This possibility was soon ruled out when it was shown that treating DNA with deoxyribonucleases destroys the transforming activity, whereas Proteolytic Enzymes have no effect on it.

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Fig. 27-6. The experiment of Avery, MacLeod, and McCarty. Infection of mice with an encapsulated strain of pneumococcus (A) causes the death of the animals, whereas the non-encapsulated (B) and heat-killed encapsulated strains (C) are harmless. D. Earlier, the bacteriologist Frederick Griffith showed that adding heat-killed virulent pneumococci (which by themselves are harmless to mice) to live non-virulent cells invariably transformed the latter into virulent encapsulated cells. He concluded that some transforming factor is present in the heat-killed virulent cells, which enters the live non-virulent cells and imparts virulence and The ability to form capsules to them. Avery and his colleagues established that this transforming factor is DNA. E. They isolated DNA from heat-killed virulent pneumococci, purified it from Proteins as thoroughly as possible, and added it to non-virulent cells. As a result, the non-virulent pneumococci were transformed into virulent ones. Apparently, the DNA entered the non-virulent cells, and the genes responsible for virulence and capsule formation became integrated into the Chromosomes of the non-virulent cells.

Later, in another landmark experiment, independent evidence was obtained supporting the concept that DNA carries genetic information. In 1952, Alfred D. Hershey and Martha Chase, using radioactive labeling experiments, demonstrated that upon infection with the T2 bacteriophage, it is specifically the DNA of the T2 viral particle—rather than its protein coat—that enters the E. coli host cell and delivers the genetic information required for the Replication of the virus (Fig. 27-7).

Fig. 27-7. General scheme of the Hershey-Chase experiment. The experiment was performed using two preparations of bacteriophage labeled with radioactive isotopes. In one of them, the ³³P isotope was used to label the phosphate groups of the phage DNA, while in the other, the ³⁵S isotope was incorporated into the Sulfur-Containing Amino Acids of the phage coat protein. Each of the radioactively labeled phages was separately added to a suspension of unlabeled Bacteria. Afterwards, both groups of phage-infected bacterial cells were blended in a mixer. It was found that cells infected with ³²P-labeled Viral Particles contained ³²P, meaning that the labeled viral DNA had entered them. The phage "ghosts" (empty viral coats) separated from the cells contained no radioactivity. In contrast, cells infected with ³³S-labeled viral particles showed no radioactivity; instead, radioactivity was recovered in the phage ghosts after they were separated from the cells using the blender. Since progeny viral particles were produced in both cases, this experiment proved that the genetic information necessary for viral replication is carried by viral DNA rather than viral protein.

As a result of these crucial early experiments and a wealth of subsequent evidence, it is now established beyond doubt that DNA is the specific chromosomal component that carries the genetic information of living cells.