Ch. 2 — Introduction • 35 
I'he structure that Cirick and V\'atson uncov- 
ered sohed part of the genetic puzzle. Accord- 
ing to them, the phosphates and sugars formed 
two long chains, or backbones, with one nitrog- 
enous base attached to each sugar. The two 
backbones were held together like the supports 
of a ladder by weak attractions between tbe 
bases protruding from the sugar molecules. Of 
the four different nitrogenous bases— adenine, 
thymine, guanine, and cytosine— attractions e.\- 
isted only between adenine(.A) aiid thymine(T), 
and between guanineKi) and cytosine(C'). (See 
figure 8a) Thus, if a stretch of nucleotides on 
one backbone ran: 
.\-T-(.-c:-T-r-.\ -.\ 
the other backbone had to contain the directly 
opposite complementary setjuence: 
T-.-\-C (;-.\ A- r- r. . 
The complementary pairing between bases run- 
ning down the center of the long molecule was 
responsible for holding together the two other- 
wise independent chains. (See figure 8b.) Thus, 
the Di\A molecule was rather like a zipper, with 
the bases as the teeth and the sugar-phosphate 
chains as the strands of cloth to which each zip- 
per half was sewn. Crick and \\ atson also found 
that in the presence of water, the two poly- 
nucleotide chains did not stretch out to full 
length, but twisted around each other, forming 
what has undoubtedly become the most glori- 
fied structure in the history of biology— the dou- 
ble helix. (See figure 8c.) 
The structure was scientifically elegant. But it 
was received enthusiastically also because it im- 
plied how DNA worked. As Crick and Watson 
themselves noted: 
If the actual order of the bases on one of the 
pair of chains were given, one could write down 
the exact order of the bases on the other one, 
because of the specific pairing. Thus one cliain 
is, as it were, the complement of the other, and 
it is this feature which suggests how the desoxy- 
ribonucleic acid molecule might duplicate 
itself.' 
V\'hen a double-stranded DNA molecule is un- 
zipped, it consists of two separate nucleotide 
chains, each with a long stretch of unpaired 
bases. In the presence of a mixture of nucleo- 
tides, each base attracts its complementary 
match in accordance with the inherent affinities 
of adenine for thymine, thymine for adenine, 
guanine for cytosine, and cytosine for guanine. 
The result of this re[)lication is two DNA mole- 
cules, both precisely identical to each other and 
to the original molecule— which explains the 
faithful duplication of the gene for passage from 
one generation to the next. (See figure 9.) 
Crick and Watson’s work solved a major rid- 
dle in genetic research. Because George Beadle 
and Edward Tatum had recently discovered 
that genes control the appearance of specific 
proteins, and that one gene is responsible for 
producing one specific protein, scientists now 
knew what the genetic material was, how it rep- 
licated, and what it produced. But they had yet 
to determine how genes expressed themselves 
and produced proteins. 
'James D. Watson and Francis Crick, "Genetical Implications of 
the Structures of Deoxyribose Nucleic Acid," Nature 171, 1953. pp. 
737-8. 
