The Quantum-Mechanical Evidence

THE GENERAL PICTURE OF THE HEREDITARY SUBSTANCE

Can these structures made of few atoms withstand for long periods the disturbing heat motion to which the hereditary substance is continually exposed?

The gene is a huge molecule, capable only of discontinuous change, which consists in a rearrangement of the atoms and leads to an isomeric2 molecule.

The rearrangement may affect only a small region of the gene.

Many different rearrangements may be possible.

The energy thresholds, separating the actual configuration from any possible isomeric ones, have to be high enough (compared with the average heat energy of an atom) to make the change-over a rare event. These rare events we shall identify with spontaneous mutations.

The later parts of this chapter will be devoted to putting this general picture ofa gene and of mutation (due mainly to the German physicist M. Delbriick) to the test, by comparing it in detail with genetical facts.

Before doing so, we may fittingly make some comment on the foundation and general nature of the theory.

THE UNIQUENESS OF THE PICTURE

Was it absolutely essential for the biological question to dig up the deepest roots and found the picture on quantum mechanics?

The conjecture that a gene is a molecule is today, I dare say, a commonplace. Few biologists, whether familiar with quantum theory or not, would disagree with it.

On p. 47 we ventured to put it into the mouth of a pre-quantum physicist, as the only reason- able explanation of the observed permanence.

The subsequent considerations about isomerism, threshold energy, the paramount role of the ratio W:kTin determining the probability of an isomeric transition - all that could very well be introduced on a purely empirical basis, at any rate without drawing on quantum· theory.

Why did I so strongly insist on the quantum-mechanical point of view, though I could not really make it clear in this little book and may well have bored many a reader?

Quantum mechanics is the first theoretical aspect which accounts from first principles for all kinds of aggregates of atoms actually encountered in Nature.

The Heitler-London bondage is a unique, singular feature of the theory, not invented for the purpose of explaining the chemical bond.

It comes in quite by itself, in a highly interesting and puzzling manner, being forced upon us by entirely different considera- tions.

It proves to correspond exactly with the observed chemical facts, and, as I said, it is a unique feature, well enough understood to tell with reasonable certainty that ‘such a thing could not happen again’ in the further development of quantum theory.

Consequently, we may safely assert that there is no alternative to the molecular explanation of the hereditary substance. The physical aspect leaves no other possibility to account for its permanence. If the Delbriick picture should fail, we would have to give up further attempts. That is the first point I wish to make.

SOME TRADITIONAL MISCONCEPTIONS

Are there really no other endurable structures composed of atoms except molecules? Does not a gold coin, for example, buried in a tomb for a couple of thousand years, preserve the traits of the portrait stamped on it?

It is true that the coin consists of an enormous number of atoms, but surely we are in this case not inclined to attribute the mere preservation of shape to the statistics of large numbers. The same remark applies to a neatly developed batch of crystals we find embedded in a rock, where it must have been for geological periods without changing.

That leads us to the second point I want to elucidate. The cases of a molecule, a solid, a crystal are not really different. In the light of present knowledge they are virtually the same.

Unfortunately, school teaching keeps up certain traditional views, which have been out of date for many years and which obscure the understanding of the actual state of affairs.

What we have learnt at school about molecules does not give the idea that they are more closely akin to the solid state than to the liquid or gaseous state.

On the contrary, we have been taught to distinguish carefully between a physical change, such as melting or evaporation in which the molecules are preserved (so that, for example, alcohol, whether solid, liquid or a gas, always consists of the same molecules,

C 2 H 6 0), and a chemical change, as, for example, the burning of alcohol, C 2 H 6 0 + 30 2 = 2C0 2 + 3H 2 0 ,

where an alcohol molecule and three oxygen molecules undergo a rearrangement to form two molecules of carbon dioxide and three molecules of water.

About crystals, we have been taught that they form three- fold periodic lattices, in which the structure of the single molecule is sometimes recognizable, as in the case of alcohol and most organic compounds, while in other crystals, e.g. rock-salt (NaCI), NaCI molecules cannot be unequivocally delimited, because every Na atom is symmetrically sur- rounded by six CI atoms, and vice versa, so that it is largely arbitrary what pairs, if any, are regarded as molecular partners.

Finally, we have been told that a solid can be crystalline or not, and in the latter case we call it amorphous.