The Double Helix

The Double Helix

by

James D. Watson

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Adenine, Thymine, Guanine, and Cytosine Term Analysis

Adenine, thymine, guanine, and cytosine (generally labeled A, T, G, and C) are the four nitrogenous bases that occur in DNA and carry an organism’s genetic code.

Adenine, Thymine, Guanine, and Cytosine Quotes in The Double Helix

The The Double Helix quotes below are all either spoken by Adenine, Thymine, Guanine, and Cytosine or refer to Adenine, Thymine, Guanine, and Cytosine. For each quote, you can also see the other terms and themes related to it (each theme is indicated by its own dot and icon, like this one:
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).
Chapter 18 Quotes

The moment was thus appropriate to think seriously about some curious regularities in DNA chemistry first observed at Columbia by the Austrian-born biochemist Erwin Chargaff. Since the war, Chargaff and his students had been painstakingly analyzing various DNA samples for the relative proportions of their purine and pyrimidine bases. In all their DNA preparations the number of adenine (A) molecules was very similar to the number of thymine (T) molecules, while the number of guanine (G) molecules was very close to the number of cytosine (C) molecules. Moreover, the proportion of adenine and thymine groups varied with their biological origin. The DNA of some organisms had an excess of A and T, while in other forms of life there was an excess of G and C.

Related Characters: James D. Watson (speaker), Francis Crick , Erwin Chargaff
Page Number: 125-126
Explanation and Analysis:
Chapter 25 Quotes

My aim was somehow to arrange the centrally located bases in such a way that the backbones on the outside were completely regular—that is, giving the sugar-phosphate groups of each nucleotide identical three-dimensional configurations. But each time I tried to come up with a solution I ran into the obstacle that the four bases each had a quite different shape. Moreover, there were many reasons to believe that the sequences of the bases of a given polynucleotide chain were very irregular. Thus, unless some very special trick existed, randomly twisting two polynucleotide chains around one another should result in a mess. In some places the bigger bases must touch each other, while in other regions, where the smaller bases would lie opposite each other, there must exist a gap or else their backbone regions must buckle in.

Related Characters: James D. Watson (speaker), Francis Crick
Page Number: 182-183
Explanation and Analysis:

Despite the messy backbone, my pulse began to race. If this was DNA, I should create a bombshell by announcing its discovery. The existence of two intertwined chains with identical base sequences could not be a chance matter. Instead it would strongly suggest that one chain in each molecule had at some earlier stage served as the template for the synthesis of the other chain. Under this scheme, gene replication starts with the separation of its two identical chains.

Related Characters: James D. Watson (speaker), Francis Crick
Page Number: 184-186
Explanation and Analysis:

As the clock went past midnight I was becoming more and more pleased. There had been far too many days when Francis and I worried that the DNA structure might turn out to be superficially very dull, suggesting nothing about either its replication or its function in controlling cell biochemistry. But now, to my delight and amazement, the answer was turning out to be profoundly interesting. For over two hours I happily lay awake with pairs of adenine residues whirling in front of my closed eyes. Only for brief moments did the fear shoot through me that an idea this good could be wrong.

Related Characters: James D. Watson (speaker), Francis Crick
Page Number: 188
Explanation and Analysis:
Chapter 26 Quotes

Suddenly I became aware that an adenine-thymine pair held together by two hydrogen bonds was identical in shape to a guanine-cytosine pair held together by at least two hydrogen bonds. All the hydrogen bonds seemed to form naturally; no fudging was required to make the two types of base pairs identical in shape.

[…]

The hydrogen-bonding requirement meant that adenine would always pair with thymine, while guanine could pair only with cytosine. Chargaff’s rules then suddenly stood out as a consequence of a double-helical structure for DNA. Even more exciting, this type of double helix suggested a replication scheme much more satisfactory than my briefly considered like-with-like pairing.

Related Characters: James D. Watson (speaker), Francis Crick , Erwin Chargaff
Related Symbols: The Double Helix Structure, Molecular Models
Page Number: 194-196
Explanation and Analysis:
Get the entire The Double Helix LitChart as a printable PDF.
The Double Helix PDF

Adenine, Thymine, Guanine, and Cytosine Term Timeline in The Double Helix

The timeline below shows where the term Adenine, Thymine, Guanine, and Cytosine appears in The Double Helix. The colored dots and icons indicate which themes are associated with that appearance.
Chapter 18
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Scientific Collaboration, Competition, and Community Theme Icon
DNA and the Secret of Life Theme Icon
...biochemist Erwin Chargaff’s discovery that most DNA tended to have around the same amount of adenine (A) as thymine (T), and the same amount of guanine (G) as cytosine (C). At... (full context)
Chapter 20
Research, Adventure, and the Thrill of Discovery Theme Icon
Scientific Collaboration, Competition, and Community Theme Icon
DNA and the Secret of Life Theme Icon
...back to DNA. Between his thesis research, he was trying to test his hypothesis about adenine’s attraction to thymine and guanine’s attraction to cytosine. He went to discuss this with Maurice... (full context)
Chapter 25
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DNA and the Secret of Life Theme Icon
A week later, Watson thought up a possible solution. He remembered that adenine, thymine, guanine, and cytosine can all hydrogen bond with themselves. Therefore, perhaps the two strands... (full context)
Chapter 26
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Scientific Collaboration, Competition, and Community Theme Icon
DNA and the Secret of Life Theme Icon
...Biochemistry of Nucleic Acids, but Donohue strongly believed that the book listed incorrect forms for guanine and thymine . It put a crucial hydrogen atom in the wrong place. Donohue didn’t have definitive... (full context)
Research, Adventure, and the Thrill of Discovery Theme Icon
Scientific Collaboration, Competition, and Community Theme Icon
DNA and the Secret of Life Theme Icon
...that Watson’s model couldn’t explain Erwin Chargaff’s finding that DNA has the same amount of adenine as thymine and guanine as cytosine. After lunch, Watson started avoiding work because he was... (full context)
Research, Adventure, and the Thrill of Discovery Theme Icon
DNA and the Secret of Life Theme Icon
In the morning, Watson started experimenting with his cardboard models. He soon realized that adenine and thymine could be linked by a hydrogen bond, as could guanine and cytosine. Moreover,... (full context)
Chapter 28
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Scientific Collaboration, Competition, and Community Theme Icon
DNA and the Secret of Life Theme Icon
...Watson realized that if it weren’t for Jerry Donohue’s comment about the different forms of guanine and thymine , he never would have discovered the double helix structure. Next, Wilkins returned to London... (full context)