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  • 1
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Solid state phenomena Vol. 97-98 (Apr. 2004), p. 27-36 
    ISSN: 1662-9779
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Physics
    Notes: Autocatalytic chemical reactions may lead to spatio-temporal patterns if processed under non-equilibrium conditions. The patterns disappear when the conditions change and information stored in these non-equilibrium structures is lost since precise reconstructions are impossible. Replication of molecules, in particular of polynucleotides RNA or DNA, is an autocatalytic process too. The storage of information in polynucleotide sequences, however, allows for reconstruction ofthe molecules under suitable conditions. Conservation of information in polymer sequences constitutes the basic difference between chemical and biological self-organization. Evolution of RNA molecules is considered as pattern formation in sequence space, which manifests itself as another pattern in the space of minimum-free-energy structures. In addition, optimization of RNA structures and properties is visualized as an evolutionary trial-and-error process. This process can beinterpreted as a simple form of learning at the level of ensembles or populations of molecules. Evolutionary optimization of RNA molecules occurs in steps: Short adaptive periods are interrupted by long epochs of quasi-stationarity during which the mean replication rate of the populations is essentially constant. Understanding of evolution is largely facilitated through consideration of sequence-structure relation as a many-to-one or non-invertible mapping from sequence space into structure space. Neutrality of sequences with respect to structure formation is highly relevant for evolutionary optimization on rugged fitness landscapes
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of mathematical biology 32 (1994), S. 545-562 
    ISSN: 1432-1416
    Keywords: Map dynamics ; Bifurcation theory ; Deterministic chaos ; Poincaré section ; Replicator system ; Autocatalytic networks
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Mathematics
    Notes: Abstract Processes of replication and mutation pivotal to molecular evolution may be modelled by a set of coupled nonlinear differential equations descriptive of autocatalytic networks. Solutions of the four dimensional system reveal aperiodic behaviours and chaos, punctuated by regions of periodic oscillations of the population variables. This complicated dynamics is encapsulated in terms of polynomial mappings which cast the relevant features of these behaviours in compact form and reproduces many of the fine details of the sequences of bifurcations. The equations descriptive of replication are topologically equivalent to generalized Lotka-Volterra equations, and thus the present map dynamics analysis finds a corresponding broader range of potential future application.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Biological cybernetics 40 (1981), S. 9-15 
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract A discussion of the game dynamics for asymmetric contest between two animal populations is presented by means of qualitative analysis.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Biological cybernetics 40 (1981), S. 1-8 
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract The ordinary differential equation which transformes the game theoretical model of Maynard-Smith into a dynamical system is discussed and some important theorems and applications to symmetric contests in animal societies are presented.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    European biophysics journal 23 (1994), S. 29-38 
    ISSN: 1432-1017
    Keywords: Activation energy landscape ; RNA folding ; RNA melting ; RNA secondary structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract We present and study the behavior of a simple kinetic model for the melting of RNA secondary structures, given that those structures are known. The model is then used as a map that. assigns structure dependent overall rate constants of melting (or refolding) to a sequence. This induces a “landscape” of reaction rates, or activation energies, over the space of sequences with fixed length. We study the distribution and the correlation structure of these activation energies.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1432-1017
    Keywords: Key words RNA structural motifs ; GNRA tetraloops ; Conformational search ; Solvent effects ; Electrostatic continuum model ; AbbreviationsMC-SYM Macromolecular conformations by symbolic programming ; JUMNA Junction minimization of nucleic acids ; AMBER Assisted model building with energy refinement ; RF Reaction field ; FIESTA Field integrated electrostatic approach ; FD Finite difference ; sc Synclinal ; ac Anticlinal ; ap Antiperiplanar ; ED Electrostatic damping
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract The protocol of conformational analysis applied here to ribonucleotide oligomers combines conformational search in the space of torsion angles and energy minimization using the AMBER4.1 force field with a continuum treatment of electrostatic solute-solvent interactions. RNA fragments with 5′-GGGCGNNAGCCU-3′ sequences commonly fold into hairpins with four-membered loops. The combinatorial search for acceptable conformations using the MC-SYM program was restricted to loop nucleotides and yielded roughly 1500 structures being compatible with a double-stranded stem. After energy minimization by the JUMNA program (without applying any experimental constraints), these structures converged into an ensemble of 74 different conformers including 26 structures which contained the sheared G-A base pair observed in experimental studies of GNRA tetraloops. Energetic analysis shows that inclusion of solvent electrostatic effects is critically important for the selection of conformers that agree with experimentally determined structures. The continuum model accounts for solvent polarization by means of the electrostatic reaction field. In the case of GNRA loop sequences, the contributions of the reaction field shift relative stabilities towards conformations showing most of the structural features derived from NMR studies. The agreement of computed conformations with the experimental structures of GAAA, GCAA, and GAGA tetraloops suggests that the continuum treatment of the solvent represents a definitive improvement over methods using simple damping models in electrostatic energy calculations. Application of the procedure described here to the evaluation of the relative stabilities of conformers resulting from searching the conformational space of RNA structural motifs provides some progress in (non-homology based) RNA 3D-structure prediction.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Psychophysiology 23 (1986), S. 0 
    ISSN: 1469-8986
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: The P300 ERP was measured in 10 subjects each for 9 days. The selection of instructions for subjects, the recording technique, the elimination of a few single trials significantly contaminated by eye movements, and the use of a correction procedure for ocular artifacts with calculable reliability and validity resulted in a set of data, in which 94% of the single trials were suitable for further analysis. The correction procedure relies on regression analysis. To reduce coherence between eyeblink activity and ongoing EEG, VEOG and EEG are averaged on eyeblinks. This yields a high reliability and validity of regression factors, determined per day, subject, and lead. In addition, this correction procedure allows for an estimation of the maximal error that must be taken into account. The efficiency of the procedure is demonstrated for single trials and averaged potentials.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1833-1844 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structural and electronic properties of homoatomic chains consisting of up to five Br-atoms are studied by means of the linear combination of Gaussian-type orbitals–local spin density method including nonlocal corrections to the exchange and correlation energy. A highly flexible basis set is used and the effects of introducing additional diffuse basis functions are examined. By comparison of the results for atomic Br and Br2 with those from very accurate correlated ab initio calculations the quality of the present method is established. Based on these results neutral and singly charged Br3, Br4 and Br5 are investigated, for which very few accurate data exist in literature. Geometries, harmonic vibrational frequencies, ionization potentials, electron affinities and charge distributions are reported and found in satisfactory agreement with available experimental data. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1360-0443
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: Aims/design. This study reports findings concerning risk factors for first, "repeated" and "regular" use of cannabis in respondents (N = 1228) who were aged 14-17 years at the outset (i.e. at "baseline") as part of a longitudinal prospective community study (EDSP). Risk factors were assessed at baseline or by separate interviews with the respondents' parents. Cumulative life-time cannabis use was the main outcome measure in this study-assessed by information from both the baseline and the follow-up investigation at an average of 19.7 months later. A cumulative logistic regression model was used to estimate associations. Findings. Using seven of a total of 25 variables examined, the final model classified 72.1% of respondents correctly. Family history of substance use disorders, self-esteem and competence, unconditional commitment to not using drugs, immediate availability of drugs, peer group drug use and previous history of nicotine dependence and alcohol use disorders all contributed significantly to the final model, predicting the progression to cannabis use from "no use", to "one time only", "repeated use", and "regular use". Conclusion. In addition to well-documented risk factors such as peer group pressure, availability, low self-esteem and competence, findings suggest that family history and prior experiences with legal drugs play a significant role in the early development of cannabis consumption in teenagers.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1360-0443
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: Aims. To present data on the incidence of cannabis use at follow-up, and its progression, stability, abuse and dependence in a representative sample of adolescents (N = 1228), aged 14-17 years at baseline-the outset of the study-in Munich, Germany. Design. The study employed a prospective longitudinal design using a representative population sample. The mean follow-up time was 19.7 months. The Composite International Diagnostic Interview (M-CIDI) was used to assess patterns of use, abuse and dependence according to DSM-IV criteria. Findings. (1) In our sample, 34.8% of males and 30.2% of females had tried cannabinoids at least once. (2) Of those who used cannabis repeatedly (two or four times) at baseline, 26.1% stopped using by follow-up; the majority went on to regular use in the follow-up period. Similarly, only 17.7% of the regular users at the outset stopped using cannabis completely; 74.2% continued their pattern of regular use. Thus, the higher the baseline use pattern, the higher was the probability of continued or heavier use during follow-up. (3) Although the cumulative life-time incidence for DSM-IV cannabis abuse (3.5%) in this age group was low, it is noteworthy that complete diagnostic remissions were relatively rare (31.7% for dependence, 41.1% for abuse). (4) There was considerable concurrent use of other drugs. Conclusions. Cannabis use was almost as widespread in this sample of adolescents in Germany as in similar age groups in metropolitan areas of North America. There was a relatively low spontaneous remission rate among regular and repeated users. Cannabis use in adolescence appears to be less transient than many people would believe.
    Type of Medium: Electronic Resource
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