Tuesday, November 24, 2009

DNA: Not the Master Molecule

Before I begin, let me wish everyone a safe, happy Thanksgiving celebration.

In researching a piece I'm working on, I read Richard Lewontin's "The Dream of the Human Genome." Like all of us, Lewontin has his ideological baggage, but his assessment of the limitations of genetics is excellent. Even when he's not exactly right, he's close enough to be a stimulating interlocutor.

Lewontin does an excellent job debunking the fallacies that have grown up around DNA. First he targets the language of DNA as somehow autonomous and self-reproducing: "The process of copying a photograph includes the production of a complementary negative which is then printed, but we do not describe the Eastman Kodak factory as a place of self-reproduction." After more superb material along this line, he next moves on to the conception of DNA as an active agent or "maker":

Not only is DNA incapable of making copies of itself, aided or unaided, but it is incapable of "making" anything else. The linear sequence of nucleotides in DNA is used by the machinery of the cell to determine what sequence of amino acids is to be built into a protein, and to determine when and where the protein is to be made. But the proteins of the call are made by other proteins, and without that protein-forming machinery nothing can be made. There is an appearance here of infinite regress (What makes the proteins that are necessary to make the protein?), but this appearance is an artifact of another error of vulgar biology, that it is only the genes that are passed from parent to offspring. In fact, an egg, before fertilization, contains a complete apparatus of production deposited there in the course of its cellular development. We inherit not only genes made of DNA but an intricate structure of cellular machinery made up of proteins.

It is the evangelical enthusiasm of the modern Grail Knights and the innocence of the journalistic acolytes whom they have catechized that have so fetishized DNA. There are, too, ideological predispositions that make themselves felt. The more accurate description of the role of DNA is that it bears information that is read by the cell machinery in the productive process. Subtly, DNA as information bearer is transmogrified into DNA as blueprint, as plan, as master plan, as master molecule. It is the transfer onto biology of the belief in the superiority of mental labor over merely physical, of the planner and designer over the unskilled operative on the assembly line. (143-144)

Lewontin touches on an excellent point in the last sentence.1 There's a strong (Cartesian) dualism at work in the mechanical conception of the cell popular in biology these days. One landmark of genetics is Francis Crick's "Central Dogma" (1958) that postulates that genetic information flows only one direction: from DNA to RNA to proteins, and never in reverse. The only way the DNA can change is through random mutations. Thus we have a Master Molecule, like the Cartesian mind, enthroned in splendid isolation from the cell it governs. The Master Molecule cannot be modified except by Chance, which assumes the role of the machine's extrinsic designer (cf. ID theory). In reality, thanks to the work of Barbara McClintock and others, we know that while this picture of information flow is largely true, there are major exceptions. But the important point is not so much that the unidirectionality of information flow is wrong, but that what is wrong is the conception of this information as the all-important determination of the cell (or organism). Lewontin discusses this in the next selection:

Unfortunately it takes more than DNA to make a living organism. I once heard one of the world's leaders in molecular biology say, in the opening address of a scientific congress, that if he had a large enough computer and the complete DNA sequence of an organism, he could compute the organism, by which he meant totally describe its anatomy, physiology, and behavior. But that is wrong. Even the organism does not compute itself from its DNA. A living organism at any moment in its life is the unique consequence of a developmental history that results from the interaction of and determination by internal and external forces. The external forces, what we usually think of as "environment," are themselves partly a consequence of the activities of the organism itself as it produces and consumes the conditions of its own existence. Organisms do not find the world in which they develop. They make it. Reciprocally, the internal forces are not autonomous, but act in response to the external. Part of the internal chemical machinery of a cell is only manufactured when external conditions demand it. For example, the enzyme that breaks down the sugar lactose to provide energy for bacterial growth is only manufactured by the bacterial cells when they detect the presence of lactose in their environment.

Nor is "internal" identical with "genetic." Fruit flies have long hairs that serve as sensory organs, rather like a cat's whiskers. The number and placement of those hairs differs between the two sides of a fly (as they do between the left and right sides of a cat's muzzle), but not in any systematic way. Some flies have more hairs on the left, some more on the right. Moreover, the variation between the sides of a fly is as great as the average variation from fly to fly. But the two sides of a fly have the same genes and have had the same environment during development. The variation between sides is a consequence of random cellular movements and chance molecular events within cells during development, so-called "developmental noise." It is this same developmental noise that accounts for the fact that identical twins have different fingerprints and that the fingerprints on our left and right hands are different. A desktop computer that was as sensitive to room temperature and as noisy in its internal circuitry as a developing organism could hardly be said to compute at all. (147-148, emphasis added)

The organism cannot be reduced to its DNA. The DNA is not the Master Control Molecule, but is more like a library of recipes for making different things an organism's cells need in order to function. It's not that the DNA runs the cell, but that the cell uses the DNA as conditions (internal and external) demand.

Where Lewontin goes wrong is in his implicit acceptance of the conception of life as a machine (i.e., mechanism). Certainly he is right that the random variation of developmental noise is a sign that the organism isn't determined by its genetics and its environment. But more important than chance is the reality of the internality of the organism with which he begins the paragraph. No matter how good we get at explaining organismic activities with our reductionistic science, it will never do away with the fact that organism is not just a bunch of parts, but is a real self-determining whole. By "self-determining whole," I mean that it seeks to fulfill goals that belong to none of its parts separately. To see this, one has only to reflect on the fact that the parts of an organism are all replaceable: this or that protein or DNA molecule, or organelle can be swapped out for another of the same, but the organism retains its identity (in fact organisms continually renew themselves in this way). The drive to metabolize in order to remain living belongs to the organism as a whole, not to any of its parts (as such), which come and go. Whereas the organism must strive if it is to continue to exist, how it fulfills this demand is freely determined from within: the relative freedom of the internal milieu from external coercion is the source of its freedom.2

Another highlight of the article is that Lewontin makes clear the self-serving nature of geneticists' many outrageous claims. It's a tradition dating back to Galileo: scientists attract attention and funding through self-promotion. This point becomes painfully clear in an Evelyn Fox Keller article that Lewontin repeatedly references, "Nature, Nurture, and the Human Genome Project."


Notes

1. I will refrain from pointing out that the planner is superior to the laborer in a real way. Lewontin may be confusing dignity (in which all honest work is equally good) with value (in which the planner is more indispensable than the laborer). The problem is not the conception of difference in value, but the conceptualization (which he aptly observes is widespread) of the parts' differentiation as analogous to planners and laborers.

2. For more on the subject of this paragraph, see my post Four Levels of Teleology, especially the parts on the work of Hans Jonas and Lenny Moss.


Richard Lewontin, It Ain't Necessarily So: The Dream of the Human Genome and Other Illusions (New York: The New York Review of Books, 2001).

Evelyn Fox Keller, "Nature, Nurture, and the Human Genome Project" in The Code of Codes: Scientific and Social Issues in the Human Genome Project, ed. Daniel J. Kevles, Leroy Hood (Cambridge, MA: Harvard Univ. Press, 1992), 281-299.

Hans Jonas, The Phenomenon of Life: Toward a Philosophical Biology (Evanston, Illinois: Northwestern University Press, 2001/1966), esp. p. 126. See this previous post.

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