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6.2 The Watson and Crick Double Helix Model of DNA 187

as in a normal phage infection, producing and disgorging Figure 6.8 X-ray diffraction patterns reflect the helical
hundreds of progeny particles. From these observations, structure of DNA. Photograph of an X-ray diffraction pattern
Hershey and Chase concluded that phage genes are made produced by oriented DNA fibers, taken by Rosalind Franklin and
of DNA. Maurice Wilkins in late 1952. The crosswise pattern of X-ray
The Hershey-Chase experiment, although less rigor- reflections indicates that DNA is helical.
ous than the Avery project, had an enormous impact. In the © Science Source
minds of many investigators, it confirmed Avery’s results
and extended them to viral particles. The spotlight was now
on DNA.

essential concepts

• DNA is a polymer of nucleotides joined by phosphodiester
bonds. Nucleotides are made of deoxyribose, phosphate,
and one of four nitrogenous bases.
• DNA is localized almost exclusively in the chromosomes pattern, he realized that it “must have a regular structure
within the nucleus of a cell. that could be solved in a straightforward fashion.”
• Avery and his colleagues showed that a purified DNA In this section, we analyze DNA’s three-dimensional
preparation from S (virulent) bacteria could transform R structure, looking first at significant details of the nucleo-
(nonvirulent) bacteria into the S form; this result was tide building blocks, then at how those subunits are linked
strong evidence for DNA as the genetic material. together in a polynucleotide chain, and finally, at how two
• Hershey and Chase grew T2 bacteriophages in the chains associate to form a double helix.
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presence of either S (labels proteins) or P (labels
32
DNA). They showed that it is the P-tagged viral DNA
that contains the genetic instructions to produce more Nucleotides Are the Building
virus particles.
Blocks of DNA
DNA is a long polymer composed of subunits known as
nucleotides. Each nucleotide consists of a deoxyribose
6.2 The Watson and Crick Double sugar, a phosphate, and one of four nitrogenous bases. De-
tailed knowledge of these chemical constituents and the
Helix Model of DNA way they combine played an important role in Watson and
Crick’s model building.
learning objectives
The components of a nucleotide
1. Describe the key features of the Watson-Crick model Figure 6.9 depicts the chemical composition and structure
for DNA structure. of deoxyribose, phosphate, and the four nitrogenous bases;
2. Explain what is meant by the antiparallel polarity of the how these components come together to form a nucleotide;
two strands of DNA within the double helix. and how phosphodiester bonds link the nucleotides in a
3. Distinguish the different structural forms of DNA from DNA chain. Each individual carbon or nitrogen atom in the
one another. central ring structure of a nitrogenous base is assigned a
number: 1–9 for purines, and 1–6 for pyrimidines. The
carbon atoms of the deoxyribose sugar are distinguished
Under appropriate experimental conditions, purified mol- from atoms within the nucleotide base by the use of primed
ecules of DNA can align alongside each other in fibers to numbers from 1′ to 5′. Covalent attachment of a base to the
produce an ordered structure. And just as a crystal chande- 1′ carbon of deoxyribose forms a nucleoside. The addition
lier scatters light to produce a distinctive pattern on the of a phosphate group to the 5′ carbon forms a complete
wall, DNA fibers scatter X-rays to produce a characteristic nucleotide.
diffraction pattern (Fig. 6.8). A knowledgeable X-ray crys-
tallographer can interpret DNA’s diffraction pattern to de-
duce certain aspects of the molecule’s three-dimensional Connecting nucleotides to form a DNA chain
structure. When in the spring of 1951 the 23-year-old As Fig. 6.9 shows, a DNA chain composed of many nucle-
James Watson learned that DNA could project a diffraction otides has polarity: an overall direction. Phosphodiester