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101.
Ramin Amirnovin 《Journal of molecular evolution》1997,44(5):473-476
A computer program was used to test Wong's coevolution theory of the genetic code. The codon correlations between the codons
of biosynthetically related amino acids in the universal genetic code and in randomly generated genetic codes were compared.
It was determined that many codon correlations are also present within random genetic codes and that among the random codes
there are always several which have many more correlations than that found in the universal code. Although the number of correlations
depends on the choice of biosynthetically related amino acids, the probability of choosing a random genetic code with the
same or greater number of codon correlations as the universal genetic code was found to vary from 0.1% to 34% (with respect
to a fairly complete listing of related amino acids). Thus, Wong's theory that the genetic code arose by coevolution with
the biosynthetic pathways of amino acids, based on codon correlations between biosynthetically related amino acids, is statistical
in nature.
Received: 8 August 1996 / Accepted: 26 December 1996 相似文献
102.
Wolfgang Stephan 《Journal of mathematical biology》1995,34(1):95-109
A population genetic two-locus model with additive, directional selection and recombination is considered. It is assumed that recombination is weaker than selection; i.e., the recombination parameter r is smaller than the selection coefficients. This assumption is appropriate for describing the effects of two-locus selection at the molecular level. The model is formulated in terms of ordinary differential equations (ODES) for the gamete frequencies x = (x
1, x
2, x
3, x
4), defined on the simplex S
4. The ODEs are analyzed using first a regular pertubation technique. However, this approach yields satisfactory results only if r is very small relative to the selection coefficients and if the initial values x(0) are in the interior part of S
4. To cope with this problem, a novel two-scale perturbation method is proposed which rests on the theory of averaging of vectorfields. It is demonstrated that the zeroth-order solution of this two-scale approach approximates the numerical solution of the model well, even if recombination rate is on the order of the selection coefficients. 相似文献
103.
The ecology of ruffe,Gymnocephalus cernuus (Pisces: Percidae) introduced to Mildevatn,western Norway 总被引:2,自引:0,他引:2
Steinar Kålås 《Environmental Biology of Fishes》1995,42(3):219-232
Synopsis Diet, habitat use, diel and seasonal activity and a number of population parameters were studied on ruffe,Gymnocephalus cernuus, introduced to Mildevatn, western Norway. This lake is sited outside the natural range of the ruffe and has a lower fish
diversity and a different fish species composition than within its native range. From June through September the ruffe was
planktivorous and mainly caught at 4 to 6 m depth in the benthic zone. At other times of year ruffe was feeding on zoobenthos
and caught deeper in the benthic zone. Ruffe was mainly day active. Zooplankton feeding during summer is the clearest difference
compared to ruffe populations living within its natural range. Presence of large zooplankton organisms available for ruffe
is suggested as the main reason for the difference found in food choice. The availability of large zooplankton is probably
due to community structure caused by a predator and lack of interspecific competition for zooplankton in the deeper parts
of the lake. Piscivorous brown trout.Salmo trutta, restrict the habitat of threespined stickleback,Gasterosteus aculeatus, to the zone of littoral vegetation, allowing high densities of larger zooplankton species likeBythotrephes longimanus to be present in the lake. Brown trout is present only in the upper light and well oxygenated parts of the lake, leaving
a refuge for the ruffe, where they can feed on the rich zooplankton community. 相似文献
104.
Lattice models of proteins were used to examine the role of local propensities in stabilizing the native state of a protein, using techniques drawn from spin-glass theory to characterize the free-energy landscapes. In the strong evolutionary limit, optimal conditions for folding are achieved when the contributions from local interactions to the stability of the native state is small. Further increasing the local interactions rapidly decreases the foldability. © 1995 Wiley-Liss, Inc. 相似文献
105.
高原鼠兔与达乌尔鼠兔食物资源维生态位的研究 总被引:19,自引:5,他引:14
作者于1993年在青海省刚察县年诺索玛地区发现高原鼠兔(Ochotona curzoniae)和达乌尔鼠兔(0.daurica)在布哈河谷二阶同域且重叠分布之现象,并从这两种鼠兔的食物资源利用出发,研究了其生态位关系问题。在食物资源维上的生态位宽度(PS),高原鼠兔为0.4744,达乌尔鼠兔为0.4964,即它们具有相近似的生态位宽度。这两种动物的生态位在食物资源谱维上的重叠值(FT)为0.809 相似文献
106.
Specific absorption rates (SARs) were determined theoretically and experimentally for several spherical models of tissue exposed to electrical fields of TE101 mode in a rectangular cavity of 57.3 MHz resonant frequency. The approximate theoretical SAR can be calculated according to the Mie theory by superposition of four plane waves representing the fields excited in the cavity. The theoretical and thermographically determined SAR patterns in spheres with radii of 5, 7.5, and 10 cm and with conductivities of 0.1, 1, and 10 S/m were compared. For a sphere with radius less than 7.5 cm and conductivity less than 1 S/m, the SAR was quite uniform. When conductivity was increased to 10 S/m, the SAR patterns showed higher absorption in the periphery of the largest sphere (10-cm radius). These characteristics are important in evaluating the scaling technique of exposing a model of a human to very-high-frequency fields to obtain power absorption data in humans exposed to high-frequency or very-low-frequency fields. 相似文献
107.
Kurt A. Santarius 《Planta》1984,161(6):555-561
Freezing of isolated spinach thylakoids in the presence of NaCl uncoupled photophosphorylation from electron flow and increased the permeability of the membranes to protons. Addition of ATP prior to freezing diminished membrane inactivation. On a molar basis, ATP was at least 100 times more effective in protecting thylakoids from freezing damage than low-molecularweight carbohydrates such as sucrose and glucose. The cryoprotective effectiveness of ATP was increased by Mg2+. In the absence of carbohydrates, preservation of thylakoids during freezing in 100 mM NaCl was saturated at about 1–2 mM ATP, but under these conditions membranes were not fully protected. However, in the presence of small amounts of sugars which did not significantly prevent thylakoid inactivation during freezing, ATP concentrations considerably lower than 0.5 mM caused nearly complete membrane protection. Neither ADP nor AMP could substitute for ATP. These findings indicate that cryoprotection by ATP cannot be explained by a colligative mechanism. It is suggested that ATP acts on the chloroplast coupling factor, either by modifying its conformation or by preventing its release from the membranes. The results are discussed in regard to freezing injury and resistance in vivo.Abbreviations CF1
chloroplast coupling factor
- Hepes
4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- PMS
phenazine methosulfate
- Tris
2-amino-2-(hydroxymethyl)-1,3-propandiol 相似文献
108.
G. P. Wagner 《Journal of mathematical biology》1984,21(1):77-95
A quantitative genetic model of random pleiotropy is introduced as reference model for detecting the kind and degree of organization in quantitative genetic variation. In this model the genetic dispersion matrix takes the form of G = BB
T, where B is a general, real, Gaussian random matrix. The eigenvalue density of the corresponding ensemble of random matrices (G) is considered. The first two moments are derived for variance-covariance matrices G as well as for correlation matrices R, and an approximate expression of the density function is given. The eigenvalue distribution of all empirical correlation matrices deviates from that of a random pleiotropy model by a very large leading eigenvalue associated with a size factor. However the frequency-distribution of the remaining eigenvalues shows only minor deviations in mammalian skeletal data. A prevalence of intermediate eigenvalues in insect data may be caused by the inclusion of many functionally unrelated characters. Hence two kinds of deviations from random organization have been found: a mammal like and an insect like organization. It is concluded that functionally related characters are on the average more tightly correlated than by chance (= mammal like organization), while functionally unrelated characters appear to be less correlated than by random pleiotropy (insect like organization). 相似文献
109.
The asymptotic information in censored survival data 总被引:2,自引:0,他引:2
110.