Biology 12

7.2 The Structure of Nucleic Acids

224 MHR • Unit 3 Molecular Genetics

By the late 1940s, it was known that DNA was

made up of a strand of nucleotides, and that each

nucleotide was made up of a sugar, a phosphate

group, and a particular nitrogenous base. Exactly

how the strand was arranged, however, remained

a mystery. The methodical work undertaken by

British scientists Rosalind Franklin, pictured in

Figure 7.9, and Maurice Wilkins to photograph

and analyze X-ray diffraction images of DNA

molecules added a number of new observations

that helped other scientists to finally deduce the

molecule’s structure.

Figure 7.9Rosalind Franklin’s work was a major factor in

the effort to determine the structure of DNA. Her contribution

was not widely recognized at the time, in part because of

prevailing attitudes toward women in science in the 1950s.

Franklin died of cancer at age 38, shortly before the Nobel

prize was awarded to Watson and Crick. Her many years of

work with X rays may have contributed to her illness.

The pattern of shaded areas in the image shown

in Figure 7.10, for example, indicated that DNA

had a helical structure. From the nature of these

X-ray “shadows,” Franklin was able to identify two

distinct but regularly repeating patterns in the

structure — one pattern recurring at intervals of

0.34 nm, and another at intervals of 3.4 nm. As she

prepared her samples for photographing, Franklin

also observed how DNA reacted to water. From

this evidence she deduced that the hydrophobic

nitrogenous bases must be located on the inside

of the helical structure, and that the hydrophilic

sugar-phosphate backbone must be located on the

outside, facing toward the watery nucleus of the

cell. Her observations proved to be important keys

to understanding the structure of DNA.

Figure 7.10The shaded areas in this deceptively simple

image indicate the pattern formed by X rays as they diffract

through crystallized DNA. This photograph was made by

Rosalind Franklin in 1953, and provided a number of

important clues about DNA’s molecular structure.

The partnership between the American geneticist

James Watson and the British physicist Francis

Crick was the first to produce a structural model of

DNA that could account for all the experimental

evidence at hand. Watson and Crick worked with

physical models, as shown in Figure 7.11, trying

different arrangements until they decided on the

double-helix model that soon became established

as the definitive structure for DNA. They published

their results in a two-page paper in Naturemagazine

in 1953.

EXPECTATIONS

Describe and compare the molecular structure and function of DNA and RNA.

Compare and contrast the arrangement of genetic material in prokaryotes

and eukaryotes.

Explain how changes in the molecular arrangement of genetic material are

linked to particular stages in the cell cycle.