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3 Information: genes and genomes

Table 1 Elementary chemical analysis of purified preparations of the transforming substance

Preparation No.

Carbon (%)

Hydrogen (%)

Nitrogen (%)

Phosphorus (%)

N/P ratio

37

34.27

3.89

14.21 15.93 15.36 13.40 15.32

8.57 9.09 9.04 8.45 9.05

1.66 1.75 1.69 1.58 1.69

38B

_

_

42 44

35.50

3.76

_

_

Theory for sodium desoxyribonucleate

34.20

3.21

Table 2 The inactivation of transforming principle by crude enzyme preparations

Crude enzyme preparation

Phosphatase

Tributyrinesterase

Depolymerase for desoxyribo-nucleate

Inactivation of transforming principle

Dog intestinal mucosa Rabbit bone phosphatase Swine kidney phosphatase Pneumococcus autolysates Normal dog and rabbit serum

+ + + _ +

+ + _ + +

+ _ _ + +

+ _ _ + +

smooth bacterial capsule. They also note that production of the capsule must involve the transformed calls carrying out a series of enzyme catalysed reactions. At that stage the structure of DNA was unknown, and the researchers had no idea of the mechanism by which DNA could ena ble this to occur. We now understand how DNA encodes proteins, as described in Chapter 5. The researchers also noted that the transformation, once it had occurred, was ‘stable’: that is, the newly acquired characteristics were passed on as the bacteria divided. They went so far as to state that ‘The inducing sub stance has been likened to a gene, and the capsular antigen which is induced in response to it has been regarded as a gene product’. However, it would take several more years and further research before it was fully established and widely accepted that DNA is the genetic material. 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. The DNA double helix is shown in Figure 3.9b and 3.9c. Its key features are: 1. It consists of two polynucleotide chains or strands, linked together by hydrogen bonds between the bases (complemen tary base pairing). 2. Adenine is linked to thymine by two hydrogen bonds, while guanine is linked to cytosine by three hydrogen bonds. 3. The hydrophilic sugar–phosphate backbones of the two strands are on the outside of the molecule, in contact with its aqueous environment. 4. The two strands are antiparallel: that is, they are arranged in different directions with respect to the 5 ′ to 3 ′ polarity of the sugar–phosphate backbone.

provided evidence that at least the majority of the substance in the puri fied extract was DNA. This was consistent with the results of tests of its physical characteristics, which showed that it was a large molecule and absorbed ultraviolet light of the same wavelength as does DNA. The results in Table 2 show that enzyme extracts are only able to destroy the activity of the transforming substance if they possess DNase activity (described in the table as ‘depolymerase for desoxyribonucleate’). The researchers also showed that treatment with protease and ribonucle ase enzymes did not destroy the activity of the substance. Conclusion Avery and his colleagues point out in the discussion section of their paper that DNA, a nucleic acid, has produced a change in S. pneumoniae bacte rial cells that enables them to produce an entirely different substance, the The DNA double helix The story of the race to discover the structure of DNA has been told many times. Key evidence came from X-ray diffraction images produced by Rosalind Franklin at King’s College London, includ ing the famous ‘Photo 51’ (as shown in Figure 3.9a). The cross shape in the image is characteristic of a helical structure. In 1953, James Watson, who was working on the structure of DNA at the University of Cambridge, was shown the unpublished X-ray pho tograph. Watson and his collaborator Francis Crick were quickly able to create a model of the DNA molecule that incorporated pre viously established data and was compatible with the X-ray image. Their structure for DNA was published in April 1953 and in the decades since it has proved substantially correct.

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