 | Biological Evolution (part 6 of ?) Video Cam Direct Upload Genes don't "make" organisms. http://www.imbf.ku.dk/MolBioPages/abk/PersonalPages/Jesper/Basel.html "The ordinary textbook talk of DNA as governing cellular or even organismic behavior is therefore rather misleading. In fact if any entity should be thought of as a governor of cellular activity this should certainly be the membrane. DNA contains the recipes for constructing the one-dimensional amino acid chains, which form the backbones of enzymes, and among them the enzymes needed for catalyzing the formation of the constituents of lipid bilayers and assembling them. But whether these recipes are actually 'read' and executed by cellular effectors depends on membrane bound activity. All major activities of cells are topologically connected to membranes. In the prokaryotes (bacteria) the plasma membrane (the active membrane inside the cell wall) is itself in charge of molecular and ionic transport, biosynthetic translocations (of proteins, glycosides etc.), assembly of lipids, communication (via receptors), electron transport and coupled phosphorylation, photoreduction photophosphorylation, and anchoring of the chromosome (replication) (de Duve 1991). In eukaryotic cells these tasks has been taken over by specific subcellular membrane structures of mitochondria, chloroplasts, the nuclear envelope, the Golgi apparatus, ribosomes, lysozomes etc. Many - if not all - of these membranes are themselves descendants from once free living prokaryotic membranes which perhaps a billion years ago became integrated into that co-operative or symbiotic complex of prokaryotic membranes which is the eukaryotic cell. Membranes also are the primary organizers of multi-cellular life. The topological specifications necessary for growth and development of a multi-cellular organism cannot be derived from the DNA for the good reason that the DNA cannot 'know' where in the organism it is located. Such 'knowledge' has to be furnished through the communicative surfaces of the cells. Morphogenesis is mostly a result of local cell-cell interactions in which signaling molecules from one cell affect neighboring cells. Animal cells, for instance, are constantly exploring their environments by means of little cytoplasmic feelers called filopedia (filamentous feet) that extend out from the cell. 'These cytoplasmic extensions that drive cell movement and exploration are expressions of the dynamic activity of the cytoskeleton with its microfilaments and microtubules that are constantly forming and collapsing (polymerizing and depolymerizing), contracting and expanding under the action of calcium and stress' writes Brian Goodwin(Goodwin 1995):36). But not only are membranes involved in all the organized activities of the life sphere, the membrane can actually be seen as the principal locus for life itself (Hoffmeyer 1998b; Hoffmeyer 2000b)[8]. It's the membrane that creates the potential inside-outside asymmetry from which the organism-environment asymmetry must have grown out. The origin of life is by necessity also the origin of the environment, and lack of concern for this aspect of the origin problem has seriously hampered much theorizing on prebiotic evolution. Somehow the world became divided into organism and environment, and the formation of a closed membrane must have been part of this process. Here the membrane not only assures the necessary topological closure, but more significantly it takes on the role of an interface facilitating a flow of messages between its interior and exterior domains. Considered from the point of view of the membrane prebiotic evolution is essentially a process of "interiorisation" (Hoffmeyer 2000b). Prebiotic membranes colonized the interior space and thereby scaffolded themselves through the formation of a multitude of autocatalytic metabolic loops and finally of replicative molecules mapping constituents of the internal autocatalytic system. Thus persistent architectures appeared as entities engaged in the trick of conjuring up a virtual reality at the inside for the purpose of coping effectively with the outsides. On the background of this discussion it might be fruitful to introduce the term the extended membrane as the inner locus for life. The extended membrane encompasses the totality of membranes that make up an organism (including its skin, plumage, etc.) and is responsible for the actual execution of life as process, semiotic agency. It is the extended membrane that directs ontogeny in a selforganized process scaffolded by an internal system of 'labels', genes, kept orderly in the genome." |
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