F

— protein "ghost" labeled with 3aS

32P tabeted DNA

multiplication of viral chrcmosomt and production of new phage release of new progeny particles i

F IC U K E 2-3 Demonstration that only the DNA component of T2 carries the genetic information and that the protein coal serves only as a protective shell.

X-ray diffraction photographs were taken by Maurice Wilkins and Rosalind Franklin (Figure 2-4). These photographs suggested not only that the underlying DNA structure was helical but that it was composed of more than one polynucleotide chain — cither two or three. At the same time, the covajent bonds of DNA were being unambiguously established. In 1952 a group of organic chemists working in the laboratory of Alexander Todd showed that 3' —*S' phosphodiester bonds regularly link together the nucleotides of DNA (Figure 2-5).

In 1951, because of interest in Linus Pauling's a helix protein motif (which we shall consider in Chapter 5), an elegant theory of diffraction of helical molecules was developed by William Cochran, Francis H. Crick, and Vladimir Vand. This theoiy made it easy to test possible DNA structures on a trial-and-error basis. The correct solution, a complementary double helix (see Chapter b), was found in 1953 by Crick and (ames D. Watson, then working in the laboratory of Max Perutz and john Knndrew. Their arrival at the correct answer depended largely on finding the stereochemical^ most favorable configuration compatible with Ute X-ray diffraction data of Wilkins and Franklin.

In the double helix, the two DNA chains are held together by hydrogen bonds (a weak moneo val ent chemical bond; see Chapter a) between pairs of bases on the opposing strands (Figure 2-6). This base pairing is very specific: The purine adenine only base-pairs to the pyrimidine thymine, while the purine guanine only base-pairs to the pyrimidine cytosme. In double-helical DNA, the number of A residues must be equal to the number of T residues, while the number of C and C residues must likewise he equal (see Box 2-1, Chargaff's Rules). As a result* the sequence of the bases of the Iwu chains oí a given double helix have a complementary relationship and the sequence of any DNA strand exactly defines that of its partner strand.

The discovery of the double helix initiated a profound revolution in the way many geneticists analyzed their data. The gene was no longer a mysterious entity, the behavior of which could be investigated only by genetic experiments. Instead, it quickly became a real molecular object about which chemists could think objectively, as they did about smaller molecules such as pyruvate and ATP. Most of the excitement, however, came not merely from the fact that the structure was solved, but also from the nature of the structure. Before the answer was known, there had always been the worry that if would turn out to be dull, revealing nothing about how genes replicate and function. Fortunately, however, the answer was immensely exciting. The two intertwined strands of

FIC U ft E 2-4 The key x-ray photograph involved in the elucidation of the DNA structure. This photograph taken by Rosalind Franklin at King's College, London, in the winter of 1952 1953, confirmed the guess that DNA was helical. The holies! form is indicated by the crossways pattern of X-ray reflections (photographically measured by darkening of the X-ray fiirn) in the center of the photograph D>e very heavy black regions at the top and bottom tell that the 3.4 A thick purine and pynm>dinc bases are regularly stacked next to each other, perpendicular to the helical axjs. (Source: Reproduced from Franklin R£. and Costing R 1953 Nature 171: 740, with permission.)

3'end

figure 2-5 A portion of a PNA polynucleotide chain, showing the 3 S' phosphodiester linkages that connect the nucleotides. Phosphate groups connect ihe 5' carbon of one nucleotide with the 5' carbon of the next complementary structures suggested that one strand serves as the specific surface (template) upon which the other strand is made (Figure 2-6). If this hypothesis were true, then the fundamental problem of gene replication, about which geneticists had puzzled for so many years, was, in fact, conceptually solved.

Box 2-1 Chargaff's Rules

Biochemist Erwin Chargaff used a technique called "paper chromatography" to analyze the nucleotide composition of DNA. By 1949 his data showed not only that the four different nucleotides are not present in equal amounts, but also that the exact ratios of the four nudeotides vary from one species to another {Box 2 1 Tabic 1). These findings opened up the possibility that it is the precise arrangement of nudeotides within a DNA molecule that confers its genetic specificity.

Chargaff's experiments also shewed that tlie relative ratios of the four bases were not random The number of adenine (A) residues in all DNA samples was equal to the number of thymine (T) residues, while the number of guanine (G) residues equaled the number of cytosine (C) residues. In addition, regardless of the DNA source, the ratio of purines to pynmidines was always approximately one (purines = pyrimidtnes). The fundamental significance of the A - T and G = C relationships (Chargaff s rules) could not emerge, however, until serious attention was given to the three-dimensional structure of DNA

old otd old otd

old new new old

FIGURE 2-6 The replication of DNA.

The newty synthesized strands are shown in orange.

old new new old

FIGURE 2-6 The replication of DNA.

The newty synthesized strands are shown in orange.

Box 2-1 (Continued)

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