全文获取类型
收费全文 | 6170篇 |
免费 | 698篇 |
国内免费 | 149篇 |
专业分类
7017篇 |
出版年
2024年 | 8篇 |
2023年 | 159篇 |
2022年 | 104篇 |
2021年 | 186篇 |
2020年 | 248篇 |
2019年 | 299篇 |
2018年 | 271篇 |
2017年 | 269篇 |
2016年 | 267篇 |
2015年 | 266篇 |
2014年 | 330篇 |
2013年 | 442篇 |
2012年 | 258篇 |
2011年 | 293篇 |
2010年 | 244篇 |
2009年 | 306篇 |
2008年 | 342篇 |
2007年 | 359篇 |
2006年 | 257篇 |
2005年 | 242篇 |
2004年 | 220篇 |
2003年 | 165篇 |
2002年 | 188篇 |
2001年 | 157篇 |
2000年 | 124篇 |
1999年 | 122篇 |
1998年 | 94篇 |
1997年 | 69篇 |
1996年 | 72篇 |
1995年 | 61篇 |
1994年 | 58篇 |
1993年 | 47篇 |
1992年 | 46篇 |
1991年 | 65篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 32篇 |
1987年 | 36篇 |
1986年 | 27篇 |
1985年 | 30篇 |
1984年 | 28篇 |
1983年 | 13篇 |
1982年 | 47篇 |
1981年 | 22篇 |
1980年 | 16篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1974年 | 6篇 |
1973年 | 11篇 |
1972年 | 12篇 |
排序方式: 共有7017条查询结果,搜索用时 3 毫秒
81.
82.
Molecular evolution of a portion of the mitochondrial 16S ribosomal gene region in scleractinian corals 总被引:1,自引:0,他引:1
Relationships among families and suborders of scleractinian corals are poorly understood because of difficulties 1) in making
inferences about the evolution of the morphological characters used in coral taxonomy and 2) in interpreting their 240-million-year
fossil record. Here we describe patterns of molecular evolution in a segment of the mitochondrial (mt) 16S ribosomal gene from taxa of 14 families of corals and the use of this gene segment in a phylogenetic analysis of relationships
within the order. We show that sequences obtained from scleractinians are homologous to other metazoan 16S ribosomal sequences
and fall into two distinct clades defined by size of the amplified gene product. Comparisons of sequences from the two clades
demonstrate that both sets of sequences are evolving under similar evolutionary constraints: they do not differ in nucleotide
composition, numbers of transition and transversion substitutions, spatial patterns of substitutions, or in rates of divergence.
The characteristics and patterns observed in these sequences as well as the secondary structures, are similar to those observed
in mt 16S ribosomal DNA sequences from other taxa. Phylogenetic analysis of these sequences shows that they are useful for evaluating
relationships within the order. The hypothesis generated from this analysis differs from traditional hypotheses for evolutionary
relationships among the Scleractinia and suggests that a reevaluation of evolutionary affinities in the order is needed.
Received: 4 September 1996 / Accepted: 7 April 1997 相似文献
83.
Francesco Frati Chris Simon Jack Sullivan David L. Swofford 《Journal of molecular evolution》1997,44(2):145-158
The sequence of the mitochondrial COII gene has been widely used to estimate phylogenetic relationships at different taxomonic
levels across insects. We investigated the molecular evolution of the COII gene and its usefulness for reconstructing phylogenetic
relationships within and among four collembolan families. The collembolan COII gene showed the lowest A + T content of all
insects so far examined, confirming that the well-known A + T bias in insect mitochondrial genes tends to increase from the
basal to apical orders. Fifty-seven percent of all nucleotide positions were variable and most of the third codon positions
appeared free to vary. Values of genetic distance between congeneric species and between families were remarkably high; in
some cases the latter were higher than divergence values between other orders of insects. The remarkably high divergence levels
observed here provide evidence that collembolan taxa are quite old; divergence levels among collembolan families equaled or
exceeded divergences among pterygote insect orders. Once the saturated third-codon positions (which violated stationarity
of base frequencies) were removed, the COII sequences contained phylogenetic information, but the extent of that information
was overestimated by parsimony methods relative to likelihood methods. In the phylogenetic analysis, consistent statistical
support was obtained for the monophyly of all four genera examined, but relationships among genera/families were not well
supported. Within the genus Orchesella, relationships were well resolved and agreed with allozyme data. Within the genus Isotomurus, although three pairs of populations were consistently identified, these appeared to have arisen in a burst of evolution from
an earlier ancestor. Isotomurus italicus always appeared as basal and I. palustris appeared to harbor a cryptic species, corroborating allozyme data.
Received: 12 January 1996 / Accepted: 10 August 1996 相似文献
84.
Louis Prusse Yvon C. Chagnon France T. Dionne Claude Bouchard 《Obesity (Silver Spring, Md.)》1997,5(1):49-61
An update of the human obesity gene map up to October 1996 is presented. Evidence from Mendelian disorders exhibiting obesity as a clinical feature, single-gene mutation rodent models, quantitative trait loci uncovered in crossbreeding experiments with mouse, rat, and pig models, association and case-control studies with candidate genes, and linkage studies with genes and other markers is reviewed. All chromosomal locations of the animal loci are converted into human genome locations based on syntenic relationships between the genomes. A complete listing of all these loci reveals that only 4 of the 24 human chromosomes are not yet represented, i.e., 9, 18, 21, and Y. Several chromosome arms are characterized by the presence of several putative loci. The following arms include at least three such loci: 1p, 1q, 3p, 4q, 6p, 7q, 8p, 8q, 11p, 11q, 15q, 20q, and Xq. Studies with negative association and linkage results are also reviewed. 相似文献
85.
Global stability of an SIR epidemic model with time delays 总被引:11,自引:0,他引:11
An SIR disease transmission model is formulated under the assumption that the force of infection at the present time depends on the number of infectives at the past. It is shown that a disease free equilibrium point is globally stable if no endemic equilibrium point exists. Further the endemic point (if it exists) is globally stable with respect to the whole state space except the neighborhood of the disease free state.Research partly supported by the Ministry of Education, Science and Culture, Japan, Grant 05640256 相似文献
86.
Brief overview of control of genetic expression by antisense oligonucleotides and in vivo applications 总被引:7,自引:0,他引:7
Gerald Zon 《Molecular neurobiology》1995,10(2-3):219-229
Over the past several years, the use of synthetic oligonucleotides and functional analogs thereof as a possibly general means
of controlling genetic expression has received widespread attention. Following a brief overview of some of the basic principles
and strategies for this approach, attention is focused here on summarizing some recent reports of in vitro and, in particular,
in vivo investigations in various animal models using phosphorothioate analogs of 2′-deoxyoligo-nucleotides. In view of these
findings, which include studies related to neurobiology, this field should find significant utility in applications of the
antisense method for controlling genetic expression. 相似文献
87.
R. S. DeWitte S. W. Michnick E. I. Shakhnovich 《Protein science : a publication of the Protein Society》1995,4(9):1780-1791
We present an efficient new algorithm that enumerates all possible conformations of a protein that satisfy a given set of distance restraints. Rapid growth of all possible self-avoiding conformations on the diamond lattice provides construction of alpha-carbon representations of a protein fold. We investigated the dependence of the number of conformations on pairwise distance restraints for the proteins crambin, pancreatic trypsin inhibitor, and ubiquitin. Knowledge of between one and two contacts per monomer is shown to be sufficient to restrict the number of candidate structures to approximately 1,000 conformations. Pairwise RMS deviations of atomic position comparisons between pairs of these 1,000 structures revealed that these conformations can be grouped into about 25 families of structures. These results suggest a new approach to assessing alternative protein folds given a very limited number of distance restraints. Such restraints are available from several experimental techniques such as NMR, NOESY, energy transfer fluorescence spectroscopy, and crosslinking experiments. This work focuses on exhaustive enumeration of protein structures with emphasis on the possible use of NOESY-determined distance restraints. 相似文献
88.
A. Georgallas D. L. Hunter T. Lookman M. J. Zuckermann D. A. Pink 《European biophysics journal : EBJ》1984,11(2):79-86
We have modelled a phospholipid bilayer as two monolayer sheets which interact with each other by a coupling which depends upon the states of the lipid hydrocarbon chains in each sheet. We make use of a model (Georgallas and Pink 1982a) and its parameters, already used to study monolayer phase changes at the LC-LE transition, in order to study the lipid main transition. Although the monolayer coexistence curve can be calculated exactly, we have made use of high-temperature series expansions to calculate the critical point of the bilayer. We also present the results of computer simulations on triangular lattices for the pressure-area isotherms. We find: (i) the interaction between the sheets of a DPPC bilayer is about 1.5–2% of the maximum interaction within the plane of each sheet; (ii) the internal lateral pressure of a DPPC bilayer is about 30.5 dyne/cm; (iii) the bilayer transition enthalpy depends sensitively upon the coupling between the sheets. Should this coupling vary from sample to sample (due, possibly, to its preparation) then very different values of transition enthalpy may be measured. (iv) We present a rough rule-of-thumb for estimating the internal lateral pressure of a bilayer from a knowledge of the corresponding monolayer pressure-area isotherms.Abbreviations LC-LE
liquid condensed — liquid expanded
- DPPC
dipalmitoylphosphatidylcholine
- Q
transition enthalpy
Work supported in part by the Natural Sciences and Engineering Research Council of Canada 相似文献
89.
Diversity concept in ecology 总被引:1,自引:0,他引:1
Hierarchy of systems organization is used as a framework in advancing methodological guidelines for posing correct questions related to ecological diversity.Diversity if defined in general terms as a property of a set of elements dependent on and determines: by the epistemological perspective. Ontological diversity, because it is indefinite, is regarded as unmeasurable.Ecological diversity — be it species, spatial, reproductive or trophic, is a particular case of diversity of matter and must be precisely defined on the studied level of system organization.Diversity measurements combining more than one level of organization are information-void, for data become irretrievable as a result of such a treatment.Hypotheses explaining diversity sources are briefly surveyed. An integrated model interrelating them is constructed as a result of some basic views on the structure of matter coupled with the empirical knowledge of involved factors. The model reflects hierarchies of systems, their dynamics, and the complexities of factors affecting diversity. The model's properties suggest that it is conceptually related to an imaginary model of organization of ecological systems. Parameters recognized as invariant components of this complexity are: habitat differentiation, ecological specialization of species, ambiental changes, and integration of this system.Finally, it is shown that the peculiar shape of the species abundances curve is a necessary consequence of stratification of the system structure, which is an intrinsic attribute of hierarchical organization. 相似文献
90.
A. Gimelfarb 《Journal of mathematical biology》1984,21(2):205-211
The main purpose of this paper is to attract attention to the fact that a mathematical model of quantitative inheritance can lead to qualitatively different results, if distributions and functions which in the model represent biological processes are assumed to take analytically different (even though qualitatively similar) forms. A simple model of the effect of environmental variation on the post-selection genotypic variance in a population under phenotypic stabilizing selection is considered. It is demonstrated that this model leads to qualitatively different conclusions depending on whether the phenotypic fitness function is assumed in a Gaussian or in a quadratic form. 相似文献