92 Chapter 3. The molecular dance[[Student version, December 8, 2002]]
The idea that genes were big molecules was now on a strong footing. Combining this idea
with the linear arrangement of genes found from Sturtevant’s linkage mapping (Section 3.3.2) led
Delbr ̈uck to his main conclusion:
The physical object carrying genetic factors must be a single long-chain
molecule, orpolymer. The genetic information is carried in the exact iden-
tities (and sequence) of the links in this chain. This information is long-lived
because the chemical bonds holding the molecule together require a large acti-
vation energy to break.(3.30)
Toappreciate the boldness of this proposal, we need to remember that the very idea of a
long-chain molecule was quite young and still controversial at the time. Despite the enormous
development of organic chemistry in the nineteenth century, the idea that long chains of atoms could
retain their structural integrity still seemed like science fiction. Eventually, careful experiments by
H. Staudinger around 1922 showed how to synthesize polymer solutions from well-understood small
precursor molecules by standard chemical techniques. Staudinger coined the wordmacromolecule
to describe the objects he had discovered. These synthesized polymers turned out to mimic their
natural analogs: For example, suspensions of synthetic latex behave much like natural rubber-tree
sap.
In a sense, Delbr ̈uck had again followed the physicist’s strategy of thinking about a simple model
system. A humble sugar molecule stores some energy through its configuration of chemical bonds.
In the language of Section 1.2, this energy is of high quality, or low disorder, and in isolation the
sugar molecule can retain this energy practically forever. The individual units, ormonomers,of
the genetic polymer also store some chemical energy. But far more importantly, they store the
entire software needed to direct the construction of the redwood tree from atmospheric CO 2 ,water
with dissolved nitrates, and a source of high-quality energy. Section 1.2.2 on page 10 argued that
the construction itself is an act of free-energy transduction, as is the duplication of the software.
The idea of an enormous molecule with permanent structural arrangements of its constituent
atoms was certainly not new. A diamond is an example of such a huge molecule. But nobody (yet)
uses diamonds to store and transmit information. That’s because the arrangement of atoms in a
diamond, while permanent, isboring.Wecould summarize it by drawing a handful of atoms, then
adding the words “et cetera.” A diamond is a periodic structure. Schr ̈odinger’s point was that huge
molecules need not be so dull: We can equally well imagine anonperiodicstring of monomers, just
like the words in this book.
Today we know that Nature uses polymers for an enormous variety of tasks. Humans, too,
eventually caught on to the versatility of polymers, which now enter technology everywhere from
hair conditioner to bulletproof vests. Though we will add little to Schr ̈odinger’s remarks on the
information-storage potential of polymers, the following chapters will return to them over and over
as we explore how they carry out the many tasks assigned to them in cells.
Schr ̈odinger’s summary of the state of knowledge focused the world’s attention on the deepest,
most pressing questions: If the gene is a molecule, then which of the many big molecules in the
nucleus is it? If mitosis involves duplication of this molecule, then how does such duplication work?
Many young scientists heard the call of these questions, including the geneticist James Watson. By
this time further advances in biochemistry had pinpointed DNA as the genetic information carrier:
It was the only molecule that, when purified, was capable of permanently transforming cells and their
progeny. But how did it work? Watson joined the physicist Francis Crick to attack this problem,
using the relatively young physical technique of X-ray diffraction. Integrating recent physics results