Moss - What genes cant do - 2003 (522929), страница 21
Текст из файла (страница 21)
Wieldand, 1996The gene-oriented rhetoric of life, which is broadcast and amplified bymass media, has rendered much of what masquerades as basic and clinical genetics into household vocabulary. And yet, the most basic concepts78Chapter 3of cellular structure remain largely unknown to all but a specialists. Laypeople typically recall, usually from high school biology, something ofan inventory of subcellular organelles, but they do so only as arbitraryfacts lacking the systematic relationships which would allow one to makefurther inferences. Part of this discussion is intended to be basic andremedial; it seeks to provide the reader with at least a baseline understanding of cellular structure and its dynamics.All living organisms are currently grouped into three categories:eukaryotes, eubacteria, and archaea.
The latter two of these are strictlyone-celled [although capable of social aggregation and complex patternformation (Shapiro 1995)], lack a nucleus and other well-defined internal structures, and are thought to predate eukaryotes by 3 billion years.Eukaryotes, which include all multicellular organisms, as well as adiverse array of one-celled organisms (protozoans, yeast, and so forth),are the result of symbiotic “experiments” between eubacterians andarchaeans. The eukaryotic cell has a diameter of about 10 times that ofeubacteria or archaea and also has a complex internal organization.
Partof this complex organization is the inclusion of bodies some of which(mitochondria, plastids, and possibly microtubule organizing centers) arethe descendants of what were once free-living eubacterians.Cellular identity, i.e., the demarcation between the inside and theoutside of a cell, is constituted by an amphipathic boundary known asthe “cell membrane” (or plasma membrane). Life as we know it isever and always a water-based phenomenon.
As in offset printing, liferetains its boundaries on the basis of the immiscibility of water inoil. An “amphipathic” substance is one that has both water-miscible(hydrophilic) and water-immiscible (hydrophobic) components.Cellular membranes are the aggregates of amphipathic molecules. Thestandard membrane-forming molecule is a phospholipid—a moleculewith a hydrophilic phosphodiester head and a hydrophobic lipid tail.Cellular membranes then consist of sheet-like bilayers in which the centerof the sheet is composed of lipid tails, whereas the surface of the sheeton both sides consists of “phospho” heads. The hydrophilic heads thenintervene between the oily, hydrophobic core and the aqueous environments of both the internal milieu of the cell and the extracellular world(whatever that happens to be).A Critique of Pure (Genetic) Information79The force which affects the amphipathic boundary is just the same asthat which causes water to ball up into droplets on the surface of a waterresistent cloth.
Nonpolar, hydrophobic surfaces, like those of a waterresistent material, do not actively repel water, but neither do they providethe opportunity for stabilizing weak bonds.Within the water molecule there is a separation of positive and negative charge. Surfaces in which there is also separation of charge offer theopportunity for transient but stabilizing weak attractive bonds withwater.
In the absence of electrostatic attractions, resistance to the loss ofentropy holds sway. A layer of water one molecule thick, which is therebyall surface and has no interior volume, is considerably more organizedthan a spherical droplet of water, which minimizes its surfaces to volumeratio and maximizes the diffusional mobility of constituent molecules.Within the sphere water molecules can move fully in all three dimensions. On a flat surface diffusion is confined to two dimensions only andis thus far more organized and predictable. The force that results in waterballing up on a hydrophobic surface, then, is the same as that whichallows amphipathic membranes to constitute the principal boundarydefining the “material” of the living world. Phospholipid membranesprovide the principal partitions between cell and outer world butalso serve to partition cells into distinctive intracellular aqueouscompartments.The fact of a plasma membrane that defines the boundary between theinside and the outside of a cell, while wholly indispensable, does not yetundermine Schrödinger’s claim.
A phospholipid bilayer is in itself, by anymeasure, information-poor. It is a fluid structure with little resistance torandom motion in two dimensions and can be readily prepared in thelaboratory. In order to make the case for how membrane-based cellularstructures constitute a system that is independent of, and causally andfunctionally parallel to, as well as an equally basic source of biologicalorder as, the genome, I will have to say more.
I will provide evidenceto show that (1) the membranous organization of the cell is that of ahighly complex structure based on the differentiated inclusion of proteins and (2) that the movement of these proteins in the plane of themembrane is not random but itself a source of biological specificity.Further (3) not only is the orderliness of the membrane structure not80Chapter 3dictated by “genetic information” but membrane and genome organization are complementary and mutually dependent, in effect, coconstitutive sources of cellular information.The principal membranous bodies in a eukaryotic cell can be placedinto two categories. First, there are those fully enclosed spherical-to-ovalbodies: the mitochondria (in all cells) and plastids (in photosyntheticcells), which are the remnants of once free-living prokaryotes.
Second,there is the interdependent network of irregularly shaped membranesextending from the nucleus to the plasma membrane in an essentiallyconcentric fashion, including a variety of associated vesicles. Subsequentdiscussion will focus on the latter system only.The principal membranous system of the cell—and it is very mucha system—consists of something like a series of pancakes bent aroundthe center of the cell and extending one after the other toward the cellsurface. Whether these pancakes form something like concentric rings(albeit with occasional bites taken out of them) depends on the developmental state and tissue type of the cell. A cell which has assumed acertain polarized morphology may have its nucleus down on the basalside of the cell, with a complete nuclear membrane pancake surrounding it but with subsequent “stacks” extending only away from its basalside rather than radially in all directions.
In any event, there is alwaysa vectoral relationship between the membranous stacks, beginning withthe nuclear envelope and ending at the plasma membrane. The nature ofthis vectoral relationship will be described here.The membranous pancakes of the cell partition its contents into“luminal” and “cytosolic” domains.
All the interiors of the pancakes areluminal. All that is not within the membranous pancake is either cytosolor nucleus. The nucleus is not within a membrane-bound pancake butrather is surrounded by the innermost layer of pancakes which wraparound it. The enclosure of the nucleus is not complete, however—itsopenings are referred to as “nuclear pores.” The nuclear pores serve asgatekeepers in regulating which large molecules in the cytoplasm andnucleus can transit between these compartments.
Water-soluble molecules, if sufficiently small, can pass freely through the nuclear pores, butthe passage of larger molecules is regulated by chemically specific criteria. The inner face of the pancake that surrounds (and thus defines) theA Critique of Pure (Genetic) Information81nucleus is referred to as the “nuclear envelope,” as distinct from the outersurface, which is endoplasmic reticulum. Within the pancake is thewater-soluble lumen of the endoplasmic reticulum.
Between the endoplasmic reticulum and the plasma membrane of the cell is situated acomplex stacklike series of pancakes referred to collectively as the Golgicomplex. These are further differentiated into cis, medial, and transgolgi, with cis being close to the endoplasmic reticulum and trans farthest away toward the cell surface. Additional subcategorizations of theGolgi are variably distinguished by different cell biologists and for different cell types.Proteins are universally recognized as the principal determinants ofbiological structure, function, and specificity.
Proteins (1) provide thecatalytic sites of almost all enzymatic reactions, (2) are the principalconstituents of all forms of muscle, (3) are key to most immunologicaland other highly specific receptor-mediated recognition processes, and(4) provide the durability of hair, nails, and skin for protecting thesurface of the body, the microskeletons within cells, the collagen matrixof connective tissue, and much else. It is as a store of template information for synthesizing proteins (and RNA) that DNA is accorded its function and importance. The achievement of the biological function of aprotein is contingent not only upon its correct synthesis but also equallyupon its post-translational modifications as well as its localization withinthe organism.At the most general level of analysis proteins are located in four principal domains. (1) They are embedded within membranes, (2) they areresident in the lumen of various membranous pancakes, (3) they residewithin the cytosolic or nuclear regions of the cell, or (4) they are excretedinto extracellular domains of the organism.
(This would include antibodies, clotting factors, enzymes, peptide hormones, etc., in the bloodand lymph, the fibrillar matrices of connective tissue, and so on.) Thesystem of cellular membranes, ranging from the nuclear envelope to theplasma membrane, is biochemically distinguished first by the composition of the proteins embedded in the membranes and secondarily by thecomposition of the proteins within their respective lumen. Maintenanceof the differentiated identities of components of the cellular membranesystem is requisite to the life of the cell and the life of an organism.
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