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Abstract (ger): Die Flüssigkeitsschichten, die um ein sich bewegendes Molekül herumfließen, haben infolge der endlichen Abmessungen der Flüssigkeitsmolekeln eine endliche Dicke. Die Berücksichtigung dieses Umstandes führt zu einer Modifikation der Stokesschen Gesetze der Kontinuumstheorie für den Zusammenhang zwischen Reibung und Viskosität. Es ergibt sich die richtige Größenordnung und ungefähr die richtige Radienabhängigkeit der beobachteten Mikroreibung, und zwar sowohl für die Rotation als auch für die Translation.
Upon separation of the protein from the nucleic acid component of tobacco mosaic virus by phenol, using a fast and gentle procedure, the nucleic acid is infective in assays on tobacco leaves. A series of qualitative and quantitative control experiments demonstrates that the biological activity cannot depend on residual proteins in the preparation, but is a property of isolated nucleic acid which is thus the genetic material of the virus.
The generation of viral mutants in vitro was demonstrated by treatment of the isolated RNA of Tobacco Mosaic Virus by nitrous acid. This agent causes deaminations converting cytosine into uracil, and adenine into hypoxanthine. Our assay for mutagenesis was the production of local lesions on a tobacco variety on which the untreated strain produces systemic infections only. A variety of different mutants are generated in this way. Quantitative analysis of the kinetics of mutagenesis leads to the conclusion that alteration of a single out of the 6000 nucleotides of the viral RNA is sufficient for causing a mutation.
The paper addresses the formation of striking patterns within originally near-homogenous tissue, the process prototypical for embryology, and represented in particularly puristic form by cut sections of hydra regenerating a complete animal with head and foot. Essential requirements are autocatalytic, self-enhancing activation, combined with inhibitory or depletion effects of wider range - “lateral inhibition”. Not only de-novo-pattern formation, but also well known, striking features of developmental regulation such as induction, inhibition, and proportion regulation can be explained on this basis. The theory provides a mathematical recipe for the construction of molecular models with criteria for the necessary non-linear interactions. It has since been widely applied to different developmental processes.
Aggregates of previously isolated cells of Hydra are capable, under suitable solvant conditions, of regeneration forming complete animals. In a first stage, ecto- and endodermal cells sort out, producing the bilayered hollow structure characteristic of Hydra tissue; thereafter, heads are formed (even if the original cell preparation contained no head cells), eventually leading to the separation of normal animals with head, body column and foot. Hydra appears to be the highest type of organism that allows for regeneration of the entire structure from random cell aggregates. The system is particularly useful for studying cell interactions, tissue polarity, pattern formation, and cell differentiation.
Physical principles underlying biological pattern formation are discussed. In particular, the combination of local self-enhancement and long-range (“lateral”) inhibition (Gierer and Meinhardt, 1972) accounts for de-novo pattern formation, and for striking features of developmental regulation such as induction, spacing and proportion regulation of centers of activation in tissues and cells. Part I explains physical principles of spatial organisation in biological development. Part II demonstrates in mathematical terms that and how short-range activation and long-range inhibition are conditions for the generation of spatial concentration patterns. The conditions can be expressed in terms of ranges, rates and orders of reactions. These conditions, in turn, can also be derived by analysis of dynamic instabilities by means of Fourier waves, showing the neither obvious nor trivial relation between the latter approach and the theory based primarily on autocatalysis and lateral inhibition.
Der Vortrag über den im Titel „Naturwissenschaft und Menschenbild“ umschriebenen Problemkreis, der natur- und kulturwissenschaftliche Aspekte betrifft, bildete den Abschluss des Symposiums über das Thema „Wie entstehen neue Qualitäten in komplexen Systemen“ am 18. Dezember 1998 in Berlin zum 50-jährigen Gründungsjubiläum der Max-Planck-Gesellschaft. Schwerpunkte sind Reichweite und Grenzen naturwissenschaftlicher Erklärung von Bewusstsein, evolutionsbiologische Grundlagen von Kooperativität und Empathie, sowie die kulturellen Verallgemeinerungs- und Gestaltungsmöglichkeiten biologisch angelegter Fähigkeiten, insbesondere was die Aktivierung der fragilen und begrenzten, aber durchaus realen und wichtigen Ressource „Gemeinsinn“ angeht.
Validity of physical laws for any aspect of brain activity and strict correlation of mental to physical states of the brain do not imply, with logical necessity, that a complete algorithmic theory of the mind-body relation is possible. A limit of decodability may be imposed by the finite number of possible analytical operations which is rooted in the finiteness of the world. It is considered as a fundamental intrinsic limitation of the scientific approach comparable to quantum indeterminacy and the theorems of logical undecidability. An analysis of these limits, applied to dispositions of future behaviour, suggests that limits of decodability of the psycho-physic relation may actually exist with respect to brain states with self-referential aspects, as they are involved in mental processes. Limits for an algorithmic theory of the mind-body problem suggested by this study are formally similar to other intrinsic limits of the scientific method such as quantum indeterminacy and mathematical undecidability which are also related to self-referential operations. At the metatheoretical level, hard sciences, despite their reliability, universality and objectivity, depend on metatheoretical presuppositions which allow for multiple philosophical interpretations.
In diesem Buch zeigt der Physiker und Biologe Alfred Gierer - er ist Direktor am Max-Planck- Institut für Entwicklungsbiologie - die Reichweite, aber auch die prinzipiellen Grenzen naturwissenschaftlichen Denkens auf. Beides wird nirgends so deutlich wie im Verhältnis der Biologie zur Physik: Hier stellen sich die Fragen, was Leben ist, wie es entstand und sich bis zur Höhe des Menschen entwickelte, wie der Reichtum der Formen zu verstehen ist und in welcher Beziehung das Bewußtsein, die “Seele”, zu einem wissenschaftlichen Verständnis der Lebensvorgänge steht. “Die Physik, das Leben und die Seele” informiert über diese wichtigen Zusammenhänge in allgemeinverständlicher Form und regt in besonderem Maße die Freude am kritischen Mitdenken an. Das Buch schlägt einen weiten Bogen von der Grundlagen der Physik und Logik über die neuen Erkenntnisse der Biologie bis zu der Frage, was uns die Naturwissenschaften über den Menschen und sein Bewußtsein lehren können - und was nicht.
Aside from the increasing, impressive evidence on chemical identification of graded molecules involved, it is the capability of axons for approaching the target position from different aspects in a two-dimensional field which is per se a strong indication for the involvement of gradients. Targeting requires, in the target field, counter-graded effects, either by antagonistic gradients, or by a single gradient in each dimension exerting attractive effects at low, reverting to inhibitory (repulsive) effects at high concentrations. A further requirement for mapping is the modulation of the counter-graded effects by components of the growth cone itself which depends on the origin of the corresponding axon.Transduction and processing of graded signals in the navigating growth cones are proposed to be strongly enhanced by intra-growth-cone pattern formation. The concept also encompasses regulatory and branching processes including the formation of the terminal arbors.