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3.1 The chemical structures of DNA and RNA

would be unable to distinguish between normal uracil bases and those that arise through cytosine deamination. The occurrence of thymine in DNA allows recognition and replacement of uracil but not thymine, so that repair can take place without accidental alteration of the normal base sequence. Before moving on to discuss the structure of DNA in more detail, in Scientific Process 3.1 we explore the crucial experiments that provided evidence of DNA’s role as the genetic material of the cell. PAUSE AND THINK Looking at Figure 3.6, what is the difference between the structures of thy mine and uracil that allows the DNA repair machinery to distinguish between them?

is chemically a much more stable molecule than RNA. This is because the 2 ′ hydroxyl group on the ribose sugar of RNA is capa ble of interacting with the adjacent phosphorus attached to the 3 ′ carbon, and in doing so it breaks the link between the phosphate group and the 5 ′ carbon of the next sugar. This is probably the rea son why through evolution DNA has replaced RNA as the material of the genome, while, as explained in Chapter 5, RNA retains a role as a more transient ‘messenger’ molecule in protein synthesis. Second, the presence of thymine rather than uracil in DNA is explained by the need to repair damaged DNA to maintain the integrity of the genome as discussed later in this chapter. A care ful look at the base structures in Figure 3.6 will show you that loss of the amino group from cytosine would convert it into uracil. Spontaneous deamination of cytosine occurs in cellular DNA, and this would cause a mutation (a change in the genome sequence) if it were not repaired by removing and replacing the incorrect base. If uracil were present naturally in DNA the repair mechanism Research question In the early part of the 20th century scientists debated the nature of genetic material. Many argued that nucleic acids, with their relatively sim ple chemical composition, could not be the repository of all the informa tion required to direct the development and functions of a complex organism. Instead, they proposed that protein was the substance of genes. A number of experiments, however, led to the conclusion that the genome is indeed composed of DNA. Important among them was the work of Oswald Avery and co-workers, published in 1944. Prediction Avery and his team built upon the work of Frederick Griffiths, who had shown that a virulent strain of the bacterium Streptococcus pneumoniae , termed the S strain due to its smooth outer capsule, could ‘transform’ bac teria of a non-virulent R or rough strain so that they gained its virulent characteristics. Mice injected with either live R strain bacteria, or heat killed bacteria from the S strain, did not develop disease. However, co injection of the live R strain and heat-killed S strain caused the mice to die. The experiments designed by Avery and his co-workers predicted that they could isolate the ‘transforming substance’ from bacteria and analyse its chemical composition. They also decided to treat the purified sub stance with enzymes. If it was DNA, then an enzyme that could break down DNA should destroy its transforming activity. Methods Rather than injecting bacteria into mice, the team used an assay that allowed them to detect whether transformation of the R strain occurred by observing the bacteria in culture (Figure 1). They experimented with a vari ety of methods for purifying crude extracts of heat-killed S strain bacteria SCIENTIFIC PROCESS 3.1

Answer: Thymine has a methyl group attached to the ring at position 5 while uracil does not.

Evidence that DNA is the genetic material

and established a protocol that gave them a relatively pure extract that was still able to transform the R strain when mixed with it. They subjected the purified material to chemical analysis and they also treated it with a panel of enzymes. At that time, they were unable to buy pure DNase enzyme ‘off the shelf’ so they used extracts of enzymes prepared from a number of dif ferent animal tissues. They tested the ability of these extracts to break down DNA by using them on pure DNA samples from other sources. Results Table 1 shows the results of chemical analysis of four preparations of the transforming substance, and compares them to the known composition of the sodium salt of DNA. The similarity in chemical composition to DNA Figure 1 Streptococcus pneumoniae experiment. Colonies of R strain S. pneumoniae grown on a culture plate are shown on the left, while colonies of S strain bacteria that have arisen from transformation of the R strain are shown on the right. Magnification 3.5× Source : Avery, O. T. et al. (1944) Studies on the chemical nature of the substance inducing transformation of pneumococcal types. J. Exp. Med. , 79, 137–157.

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