84 pages • 2 hours read
James D. WatsonThe Double Helix
Nonfiction | Book | Adult | Published in 1968A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Chapters 25-28Chapter Summaries & Analyses
Chapter 25 Summary
Crick is frustrated by Watson’s balancing of work with tennis, girls, and cinema in the evening. Watson knows the real issue to be solved is the bases. They already have a functioning model for the backbone compatible with the B-structure data from Kings College.
It’s neither Wilkins or Franklin who provide this data, instead, itcomes from Perutz,who is on a committee overseeing the outputs of the Kings College lab. As it wasn’t confidential, he felt able to pass the data on.
In the evenings, Watson tries to puzzle out the bases, the issue being, how do irregular, complex bases fit, in neat three-dimensional arrangement, with what must be a regular, repeatable sugar-phosphate backbone structure. Equally problematic is the question of what binds the bases together.
Increasingly, Watson thinks hydrogen bonds are key, not just between molecules, but occurring between all bases in a DNA molecule.
The next week an idea hits him: the “potentially profound implications” (134) of like-with-like base bonding. This would mean each base is bound to its like-base by two hydrogen bonds at a 180-degree rotation. A with A, T with T, C with C, G with G, as shown in the schematic in this chapter. DNA would then consist of “two [helical] chains with identical base sequences” (134).
This created something of a “messy backbone” (134), but the implications for gene reproduction would be huge.
The two intertwined chains with identical base sequences would strongly suggest that one chain in each molecule had, at an earlier stage, provided the template for the other. Thus, gene replication would start with the separation of the two identical chains of a DNA molecule, and two new daughter strands made of the parent template would complete two new identical molecules, bound like-to-like by hydrogen bonds.
To Watson’s delight, the secret of DNA’s structure and gene replication appear closely linked.
Chapter 26 Summary
By lunch time, Watson’s idea is proven wrong. Before this, he foolishly replies to a letter from Delbruck in the States, and drops a hint he’s onto a new DNA structure.
In the lab, he explains his idea to expert crystallographer Jerry Donohue, who observes that Watson has selected the wrong forms for the hydrogen-bonded base pairs, opting for enol configurations instead of keto. The solution won’t work with keto configurations. When Crick arrives he deals another blow, noting problems to do with angles of rotation, and the fact that it gives no explanation for Chargaff’s rule on the equal proportions of A/T and G/C.
Reluctantly, Watson accepts his like-for-like hypothesis won’t work, and playing with the model confirms this. But the same day, while modelling possible arrangements, another inspiration strikes. An A & T pair held together by two hydrogen bonds is identical in shape to a C & G pair held together by two hydrogen bonds. Thus, “two irregular sequences of bases could be regularly packed into the centre of a helix if a purine always hydrogen bonded to a pyrimidine” (141).
Watson runs the idea past Donohue, who sees no problems this time. Watson also sees that this formation accounts for Chargaff’s rule on equal proportions of purine and pyrimidine bases (A-T and C-G). It also suggests an even more elegant model of gene reproduction. Because A always bonds with T and C with G, one separated chain would determine the shape of its partner, acting as a perfect template for reproduction.
When Crick comes in, Watson excitedly explains, and Crick’s analysis helps to fine-tune the arrangement. He spots that both bonded pairs could be flipped and still fit, chemically speaking, into the backbone (as A-T or T-A and C-G or G-C). This suggests that a given chain can contain both pyrimidine and purine pairs and that the backbones of the two pairs must run in opposite directions. Many details still need working out, but this doesn’t stop Crick’s announcing that they’ve “discovered the secret of life” (144).
Chapter 27 Summary
Watson and Crick waste no time in planning next steps: a full model must be built and all the atomic co-ordinates verified. They’re still waiting for pieces to arrive before the final model can be built; until now, they’ve been improvising with some cardboard atoms.
Watson tells Peter, Elisabeth and Bertrand about their success, a solution that “would revolutionise biology” (146). They’re all pleased to hear it. He wakes up feeling “marvellously alive” (146), takes in the beauty of Cambridge, and reflects that “much of [their] success was due to the long uneventful periods […]” (146) when they walked, or read, and casually passed the time.
Crick is in the lab early, double-checking that the flipped base pairs fit into the backbone configuration. News of their immanent success spreads in the Cavendish, with Crick explaining the solution to Perutz and Kendrew.
Watson fetches the final model atoms and they start constructing the finished article. Watson constructs and Crick checks over all the inter-atomic contacts.
Over dinner, they decide they want to be absolutely sure everything is right before contacting Wilkins. This will involve a day’s work pinpointing atomic coordinates.
Crick gets into the lab early again to work on the model, while Watson daydreams about the letters he’ll write. Bragg comes to have a look, immediately grasps the salient features (how the equivalence of A&T and C&G was a consequence of the regular repeating shape of the backbone) and gets excited by its implications.
The final refinements of the coordinates are done the following evening. It’s decided that Kendrew will call Wilkins. It’s a task neither Cricks nor Watson relishes, as the day before a letter had come from him saying he was ready to go “full steam ahead” with model building (151).
Chapter 28 Summary
Wilkins sees the model, studies it silently, and is convinced. Watson notes the value of Donohue’s contribution, as without his insights on using the keto configuration for the hydrogen bonds, they may still have been way off track.
The next step is to compare in detail the x-ray data with the diffraction pattern predicted by their model. Wilkins returns to London to do this, “without a hint of bitterness…[i]n his subdued way [Wilkins] was thoroughly excited that the structure would prove of great benefit to biology” (153).
Two days later, he telephones to say he and Franklin have found the x-ray data strongly supported the double helix, and they intend to write up their results to publish simultaneously with Crick’s and Watson. In the scientific journal Nature, papers of significance can be published quickly.
Franklin, to Watson’s surprise, also eagerly accepts the solution: “[l]ike almost everyone else she saw the appeal of the base pairs […] and that the structure was too pretty not to be true” (153). Her hostility towards Crick and Watson vanishes. On a later trip to London, Crick notes that she takes pleasure in sharing her data. He saw then how “fool-proof” (155) her assertion was that the backbone must be on the outside of the molecule.
Watson reflects positively about Franklin, acknowledging her talents. Her difficulties with Wilkins and the scientific establishment grew, he says, from “her understandable need for being equal with the people she worked with” (155), something not freely given.
Letters reveal that Pauling is still well off-base. He writes to Watson (having heard the hint from Delbruck in Watson’s last letter) to ask about his new theory. The hint was the one relating to Watson’s doomed like-for-like theory, so it’s a very lucky escape that they now have the actual answer.
Victory now assured, Watson takes a trip to Paris for a week. A letter from Chargaff to Kendrew asks what his two “scientific clowns” (157) are up to.
Chapters 25-28 Analysis
The narrative stays pacey, as we move towards Watson and Crick’s final success. We have continuing energy and drama, first in the false dawn of Watson’s like-with-like theory, and then in the inspiration of the final answer, which follows quickly on that failure. The drama peaks in Chapter 26, with Chapters 27 and 28 tying things up both scientifically and personally. Indeed, there is almost the feel of a mounting happy ending, with personal and scientific resolutions.
While it no sooner arises than it is dismissed, Watson’s brief-lived, like-with-like idea is important for sowing the seeds of the final success. It is a failure, but one that sees Watson closing in on the right answer. It leads to a crucial realisation about the keto formation in hydrogen-bonded base pairs, and it also suggests the wider implications the structure of DNA could have for genetic replication.
We also have personal drama here: the sense of elated discovery, the fear that “an idea this good could be wrong” (136), and the human hope and desire (quite apart from the scientific belief) that the solution could be correct. Watson shows his streak of impetuousness by writing to Delbruck of the new theory before it’s been properly assessed. But as we’ve seen elsewhere, while this impetuousness can land him in trouble, it is also crucial to his talent and achievements.
It is Watson who makes the crucial breakthrough, realising that A-T and C-G bonds are identical in shape, and so could fit into the regular backbone structure of complimentary helix chains. From this “eureka” moment, things start to fall into place.
As Watson notes, the “answer was turning out to be profoundly interesting” (136). DNA, as they had hoped, was not just another molecule to be mapped; their revelation about complimentary chains shows clearly how all genetic material (and from that amino acids, cells, and life forms) can be replicated. As Crick puts it in bombastic fashion, they had “discovered the secret of life” (144).
Another element of the appeal and convincing quality of their final structure is its elegance: it’s prettiness (to use Watson’s favoured term) and simplicity. There is something compelling in the idea that at the heart of nature lies this simple structure capable of containing the seed of so much complexity and variety. From the regular helical molecule, an endless stream of variations becomes possible in the sequencing of the bases.
It is telling that just as the double helix solves scientific problems, it seems to solve something of the personal and professional disputes narrated here as well. Professionally, all participants are convinced by it, full of admiration, and excited by its implications for science. And personally, Wilkins shows no sign of bitterness, quite the opposite. Perhaps he was never as driven by the sense of the race as Watson and Crick in the first place.
A spirit of genuine and open collaboration overtakes the tension, subterfuge and competitiveness of earlier chapters. It’s clear that Wilkins’ and Franklin’s work has a huge hand in the final success, and the decision to publish simultaneously implies a nascent team approach. Perhaps there is something of an idealised, rose tinted view of things creeping in here; this is, after all, the narrative of one of the “winners” in the DNA story.
The reconciliation with Franklin (so long an adversary) is particularly noteworthy. Her hostility to Crick and Watson melts away in Chapter 27. A new spirit of equality informs their interactions. Her opinion of them has doubtlessly risen, but so, too, has Watson’s sympathy for the difficulty of her situation. It was a desire for equal treatment in a professional world so slow to give that to women which motivated her combative stance. And her “uncompromising statements […] represented first-rate science, not the outpourings of a misguided feminist”(155).(This is a problematic statement, and returned to in more detail in the Important Quotes section of this guide.)
How far should we trust this neat and harmonious culmination to the DNA story? Something of the pleasure and good grace of the race winner definitely informs its tone. Watson is gracious in acknowledging the role of the people without whom the solutions wouldn’t have been reached, at least in the way it was. We can see in this the truth that no scientific discovery or breakthrough belongs solely to one person. It is always a combination of influences, perspectives and incremental gains. That said, for reaching the double helix, Watson and Crick are the undisputed race winners, and thoroughly earn their scientific plaudits.
On the evidence of Watson’s narrative, ego and ambition play a large role in his and Crick’s ultimate success. And yet, in these final chapters, the note of something bigger and more important than any one ego can be felt, something perhaps akin to the mutual admiration of nature itself and the shared enterprise of science, which has laid open to human understanding the elegance and creative force of the double helix.
