BIOLOGY Volume 3 - A Guide to General Biology - 2004

23. THE CONTINUITY OF LIFE

23.6. DNA

23.6.1. Evidence for the Role of DNA in Heredity

In the early 20th century, Sutton and Boveri suggested that Chromosomes are responsible for transmitting Genetic information from one generation to the next. However, many years passed before it was finally established whether the genetic material is DNA or chromosomal protein. Scientists leaned toward the view that protein was the only substance whose molecules possessed sufficient structural diversity to serve as genetic material.

Evidence from Bacteria

In 1928, British microbiologist Frederick Griffith made an observation that would later play a crucial role in solving this problem. In an era before Antibiotics, Pneumonia was frequently fatal. Griffith was attempting to develop a vaccine against pneumococcus—the bacterium responsible for one form of pneumonia. Two forms of this bacterium were known: one enclosed in a mucous capsule and virulent (disease-causing), and the other lacking a capsule and non-virulent. Evidently, the capsule protected the bacterium in some way from the human immune system.

Griffith hoped that if a patient were injected with the non-encapsulated form or the heat-killed encapsulated form, their body would begin producing Antibodies capable of preventing pneumonia. In a series of experiments, Griffith injected mice with both forms of the bacteria, obtaining the results shown in Table 23.3. Upon dissecting the deceased mice, he discovered living encapsulated forms inside them. Based on these findings, Griffith concluded that some factor from the heat-killed encapsulated bacteria was somehow transferred to the living non-encapsulated bacteria, inducing them to form capsules and become virulent. However, The Nature of this transforming factor remained unknown until 1944, when it was successfully isolated and identified.

Class="center">Table 23.3. Results of Griffith's experiments

Over a span of 10 years, Avery, MacLeod, and McCarty worked on isolating and purifying the molecules comprising heat-killed encapsulated pneumococcal Cells and studied their ability to transform non-encapsulated cells. Removal of the polysaccharide capsule and protein fraction from The Cell extracts had no effect on transformation, whereas The addition of deoxyribonuclease (DNase), an enzyme that hydrolyzes DNA, prevented it. The ability of highly purified DNA extracts from encapsulated cells to induce transformation demonstrated that Griffith's transforming factor was DNA. Despite these findings, many scientists still refused to accept that DNA, rather than protein, was the genetic material. In the early 1950s, a wealth of additional data obtained from The Study of Viruses finally demonstrated that DNA is indeed the carrier of genetic information.

Evidence from Viruses

In the 1940s, viruses became one of the primary subjects of experimental genetic research. Viral Particles have a very simple Structure; they consist of a protein coat enclosing a nucleic acid molecule—either DNA or RNA (Section 2.4.2). This makes them ideal model systems for investigating whether the genetic material is protein or nucleic acid. In 1952, Hershey and Chase embarked on a series of experiments using a special type of virus that infects bacterial cells, known as a bacteriophage. Bacteriophage T₂ penetrates Cells of the bacterium Escherichia coli (which inhabits the human gut) and forces them to produce a multitude of T₂ phage particles in a very short time. Hershey and Chase cultured T₂ phage particles in E. coli cells grown in a medium containing radioactive isotopes of either sulfur (³⁵S) or phosphorus (³²P).

Phage Proteins contain sulfur but no phosphorus, whereas DNA contains phosphorus but no sulfur. Consequently, phage particles produced in E. coli labeled with radioactive sulfur incorporated the isotope into their protein coats, whereas particles produced in E. coli labeled with radioactive phosphorus contained DNA labeled with ³²P.

These labeled T₂ phage particles were used to infect unlabeled E. coli cells. After a few minutes, the cells were blended in a mixer to detach the phage particles from the bacterial cell walls. The bacteria were then incubated and tested for radioactivity. The results are shown in Fig. 23.19.

Fig. 23.19. Diagram of the experiments conducted by Alfred Hershey and Martha Chase using phage T₂ and E. coli.

Based on their findings, Hershey and Chase concluded that it is the phage DNA, rather than the protein, that enters the bacterial cell and initiates The production of numerous phage progeny. These experiments provided conclusive evidence that DNA is the hereditary material. Results from Electron Microscopy and more comprehensive data on the viral life cycle further confirm that only the phage DNA enters the bacterial cell. The life cycles of viruses and Bacteriophages are described in Sections 2.4.3 and 2.4.5.