648 Chapter 19 NEL
The Race to Reveal the Structure
When scientists confirmed that DNA was the material of heredity, their focus shifted
to understanding how it works. Part of that understanding would come from knowing
its structure since, as in other subjects, structure in biology provides many clues about
function. In the race to be the first to discover the structure of DNA, scientists around
the world employed emerging technologies to help them gain new insights into this
mysterious “molecule of life.” In the end, the honour would go James Watson and Francis
Crick (Figure 9).
James Watson was considered a child prodigy when he entered the University of
Chicago at the age of 15.He began studying ornithology, but eventually turned his
attention to genetics and molecular biology. In 1951,he began studies at England’s
Cambridge University, where he met Francis Crick, a physicist who had served with the
British army during World War II. Each would bring to bear his experience from a dif-
ferent area of science to interpret and synthesize the experimental data that were rapidly
mounting.
One source of important data came from the Cambridge laboratory of Maurice
Wilkins, where researcher Rosalind Franklin used a technique called X-ray diffraction to
help determine the structure of the DNA molecule. Another source of data involved the
comparison of the chemical structure of DNA molecules in different organisms. By this
time it had long been known that DNA is comprised of chains of molecules called
nucleotides. The nucleotides consist of a 5-carbon cyclic ring structure called a
deoxyribose sugar(Figure 10) having one of four nitrogenous basesattached to its
1 carbon and a phosphate groupattached to it 5carbon (Figure 11). The carbons in
the sugar are identified by the numbers one to five and a prime () symbol to distin-
guish them from the carbons in the nitrogenous base. The four nitrogenous bases are ade-
nine (A), guanine (G), thymine (T), and cytosine (C). Adenine and guanine are
double-ringed structures classed as purines, while thymine and cytosine are single-
ringed structures classed as pyrimidines. The only difference in the nucleotides is in
their bases.Figure 10
A deoxyribose sugar with numbered
carbonsFigure 11
A DNA nucleotide is comprised of a
deoxyribose sugar, a nitrogenous
base, which in this case is adenine,
and a phosphate group.CH
H
CH
CH 2 OH
HC CH
O
OH
OH
4 1 5 3 2 P O CH 2
O
H
H
OH
N
NH 2
N
N
N
O
H
O
O
nitrogen
basedeoxyribose
sugarphosphate
groupHH
nucleotidea molecule having a
five-carbon sugar with a
nitrogenous base attached to its 1
carbon and a phosphate group
attached to its 5carbon
deoxyribose sugara sugar
molecule containing five carbons
that has lost the –OH (hydroxyl
group) on its 2position
nitrogenous basean alkaline,
cyclic molecule containing nitrogen
phosphate groupa group of four
oxygen atoms surrounding a central
phosphorus atom found in the
backbone of DNA
Figure 9
Francis Crick and James Watson
were awarded the Nobel Prize for
Physiology or Medicine in 1962 for
deducing the structure of DNA.