Moss - What genes cant do - 2003 (522929), страница 20
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Consider the following quotefrom Marxism and the Philosophy of Language (Voloshinov 1973),which, if we analogize a gene with a word, as publicists for the HumanGenome Project have done on a regular basis, serves precisely to undermine vectoral unidirectionism (all causality emanating outward from thegenes as the “deep text which underlies all else”):Context of usage for one and the same word often contrast with one another.The classical instance of such contrasting contexts of usage for one and the sameword is found in dialogue.
In the alternating lines of a dialogue, the same wordmay figure in two mutually clashing contexts. Of course, dialogue is only themost graphic and obvious instance of varidirectional contexts. Actually, any realutterance, in one way or another or to one degree or another, makes a statementof agreement with or a negation of something. Contexts do not stand side byside in a row, as if unaware of one another, but are in a state of constant tension,or incessant interaction and conflict.The context, in this perspective, determines the significance of the word,not vice versa.
Contexts, in a biological vein, would be found at the manylevels of structured, dynamic systems that are always in some relationship to other such structured, dynamic systems, and/or a complex environmental ambience.That even the simplest free living cell is capable of considerable adaptive plasticity—i.e., successful participation in highly variant “dialogicalcontexts”—speaks well to the explanatory potential of depicting genesas words whose significance is context-dependent.
The contast betweentextualized genes as deep-seated primordial meaning and textualizedgenes as context-dependent utterances in particular “language games”corresponds well with the distinction between conflationary genesThe Rhetoric of Life and the Life of Rhetoric73(Gene-P/Gene-D) and Gene-D (see table 1.1). The critique of geneconflationism offered at the end of chapter 1 can, and should, be avehicle for shifting rhetorical gears. The understanding of Gene-D as acontext-dependent molecular resource is nicely complemented by the“metaphorics” of the dialogic construction of meaning in context, andsuch metaphorics can indeed be productive in elicting intuitions that biologists will be able to realize in new experimental designs.
Doyle touchedon the idea of giving the language metaphor a dialogic, or at least contextual, turn in his reference to the work of theoretician Howard Pattee,but he left it on the sidelines.What Doyle, it seems, will not do is put forward his own claims basedon an appeal to empirical evidence. Now surely one can and shouldacknowledge that there is no pure or original access to empirical evidence and that our practices—theoretical, experimental, rhetorical, andso on—have an impact on what shows up as “empirical.” Yet, as I hopeto demonstrate, none of these are interpretatively unequivocal. One canenter the fray, albeit critically, at the level of putting forward positionsabout biology in the mode of empirically accountable, problemsolving–oriented claims and still make reflectively explicit the rhetoricalresources that are mobilized in doing so.This page intentionally left blank3A Critique of Pure (Genetic) InformationOmnis cellula a cellula.—Rudolph Virchow, 1855At its heart, the debate centres on the extent to which the sources of order inmolecular biology lie predominantly in the stable bond structures of molecules,Schrödinger’s main claim, or in the collective dynamics of a system of such molecules.
Schrödinger emphasized, correctly, the critical role played by quantummechanics, molecular stability, and the possibility of a microcode directingontogeny. Conversely, I suspect that the ultimate sources of self-reproduction andthe stability requisite for heritable variation, development and evolution, whilerequiring the stability of organic molecules, may also require emergent orderedproperties in the collective behaviour of complex, non-equilibrium chemical reaction system . . . The formation of large aperiodic solids carrying a microcode,order from order, may be neither necessary nor sufficient for the emergence andevolution of life. In contrast, certain kinds of stable collective dynamics may beboth necessary and sufficient for life.—Stuart Kauffman, 1995The phenotype is the result of ontogenetic development. This holds true also atthe molecular level, because molecular biological processes take place within theorganism.
In ontogenesis, genetic and non-genetic factors interact in producingsuccessive states, each of which is the prerequisite, and determines the conditions, for the next one to follow. In this interplay, genes are a necessary, butnot a sufficient, component. The structures already present, gradients, thresholdvalues, positional relationships, and conditions of the internal milieu, are equallyessential.
Thus even monofactorial traits can be considered to be of multfactorial causation, and the varying borderline conditions that arise during development add to the complexity. From this standpoint, it is not expected that amutation has a consistent phenotypic outcome, and the phenotype-genotype relationship may be irregular.—U. Wolf, 199576Chapter 3Taking Schrödinger SeriouslySchrödinger’s hereditary code-script metaphor is as good a place as anyto locate the inception of the gene-as-information formulation, but thepower of Schrödinger’s text to mobilize the group of scientists who wenton to constitute the new science of molecular biology was not based uponthe seductions of Schrödinger’s rhetoric alone.
By approaching thephysical basis of biological inheritance in terms of thermodynamics,Schrödinger elicited the strong interest of physical scientists. The claimthat the kind of stability required of living systems is uniquely securedand maintained as tacit information heterogeneously embedded in thesolid state structure of the aperiodic crystal might have turned out to betrue. If it had been shown to be warranted, then the metaphor of thehereditary code-script could justifiably be celebrated as nothing but salutary. In criticizing the rhetoric of genetic information, it does matterwhether the Schrödinger’s formulation is empirically robust.Picking up, in effect, where Schrödinger left off, the effort here willbe to demonstrate and argue the following: Neither DNA nor any otheraperiodic crystal constitutes a unique repository of heritable stability inthe cell; in addition, the chemistry of the solid state does not constituteeither a unique or even an ontologically or causally privileged basis forexplaining the existence and continuity of order in the living world.I will approach this task through characterizing and discussing severalfundamental, non-solid-state sources of stable and heritable biologicalorder which are not reducible to the dictates of the genome.
Followingthe categorizations of Jablonka and Lamb (1995), these will now begrouped into three “epigenetic inheritance systems” (EISs): first, organizational structure; second, steady-state dynamics; and third, chromosome marking. While it is not my intent to imply that all ofthe “developmental resources” (Oyama 1986, Griffiths & Grey 1994)instrumental in determining the course of an ontogeny are encased in thefertilized egg, for present purposes I will confine my focus to that whichis materially transmitted between generations in the most immediatesense.By “organizational structure” I refer to those membranous partitionsand proteinaceous matrices which constitute and dictate the threedimensional form of the cell. By “steady-state dynamics” I refer to thoseA Critique of Pure (Genetic) Information77self-regulating systems of interacting enzymes whose overall state isdetermined (and transmitted) by the relative concentrations and activation states of its constituents and is itself determined by phosphorylationstate, glycosylation state, proteolytic processing, and the like.
By “chromosome marking” is meant the pattern of modification of DNA throughthe addition of methyl groups to C (cytosine) bases, which affects thetranscriptional activation state of DNA.The treatment of these categories here departs significantly from thatof Jablonka and Lamb in certain ways.
Whereas their emphasis was onthe role of chromosome marking as a kind of “shadow government” (myterm) within the nucleus, the emphasis now will be more on the first twocategories. With respect to organizational structure, I will offer, at least inminiature, a systematic discussion of the nature of membrane-based cellular compartmentalization with an emphasis on it negentropic properties and its fundamental and irreducible role in sustaining biologicalorder. With respect to steady-state dynamics, this discussion will notbe limited to its being only a sub-system; but will also focus on StuartKauffman’s ambitious attempt to account for the emergence of biologicalorder writ large through theoretically modeling, complex-system dynamics.
The purpose and goal of discussing these epigenetic inheritancesystems will be to show that the relationship of each of them to thegenome (and to each other) is that of codependence and causal reciprocity. To the extent that genes can be said to carry information at all, it willbe argued, it is only in the context of those organizational structures anddynamics which are, in all respects, co-original with the genome and forwhich the genome itself can never substitute or prefigure.Slippery Borders, Moving Membranes, and the CompartmentalDivision of LaborEurkaryotic life depends upon the spatial and temporal organization of cellularmembrane systems.—James E. Rothman and Felix T.
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