全文获取类型
收费全文 | 4203篇 |
免费 | 596篇 |
专业分类
4799篇 |
出版年
2021年 | 67篇 |
2018年 | 39篇 |
2017年 | 44篇 |
2016年 | 58篇 |
2015年 | 108篇 |
2014年 | 155篇 |
2013年 | 148篇 |
2012年 | 236篇 |
2011年 | 234篇 |
2010年 | 135篇 |
2009年 | 131篇 |
2008年 | 183篇 |
2007年 | 186篇 |
2006年 | 162篇 |
2005年 | 166篇 |
2004年 | 164篇 |
2003年 | 161篇 |
2002年 | 167篇 |
2001年 | 161篇 |
2000年 | 126篇 |
1999年 | 105篇 |
1998年 | 65篇 |
1997年 | 58篇 |
1996年 | 38篇 |
1995年 | 34篇 |
1993年 | 42篇 |
1992年 | 80篇 |
1991年 | 102篇 |
1990年 | 82篇 |
1989年 | 82篇 |
1988年 | 75篇 |
1987年 | 61篇 |
1986年 | 83篇 |
1985年 | 64篇 |
1984年 | 56篇 |
1983年 | 60篇 |
1982年 | 41篇 |
1981年 | 37篇 |
1980年 | 41篇 |
1979年 | 56篇 |
1978年 | 51篇 |
1977年 | 36篇 |
1976年 | 47篇 |
1975年 | 48篇 |
1974年 | 44篇 |
1973年 | 49篇 |
1972年 | 58篇 |
1971年 | 33篇 |
1969年 | 35篇 |
1968年 | 34篇 |
排序方式: 共有4799条查询结果,搜索用时 15 毫秒
991.
Mark L. Weiss Morris Goodman William Prychodko G.William Moore Toshio Tanaka 《Journal of human evolution》1973,2(3):213-226
Macaque gene frequencies for seven polymorphic protein systems are employed to generate dendrograms via two algorithms. The frequency data employed are drawn from 14 populations of macaques representing nine species. The two algorithms are the unweighted pair group method and an iterative program based on the additive hypothesis. The topologies generated by the two approaches are quite similar. The one major difference, involving M. fuscata, is investigated in detail.As measured by our index of dissimilarity (ID) conspecific populations of M. nemestrina, M. mulatta and M. fascicularis are closely related. Often the ID values separating conspecific groups are in agreement with expectations based on geographic considerations. Interspecific comparisons involving M. mulatta and M. fascicularis also exhibit clinal variation.M. cyclopis and M. fuscata show a particularly strong relationship to M. mulatta. This cluster of three species is closely related to M. fascicularis. M. speciosa, on the other hand, appears to be the most divergent of the species analyzed. The results are considered in light of previous thoughts about macaque systematics based on morphological characteristics. 相似文献
992.
Gary B. Weiss 《Journal of molecular evolution》1973,2(2-3):199-204
Summary The rate of translation of mRNA into protein is controlled, at least in part, by the concentration of tRNA. However, as one tRNA molecule can recognize more than one codon, different codons translated by the same tRNA may be translated at different rates. The difference in response of a tRNA molecule to different codons could also regulate the rate of protein synthesis. Evidence for this theory is presented, and Crick's wobble hypothesis is extended to include the relative strengths of different codon-anticodon interactions. 相似文献
993.
A congenital defect in platelet prostaglandin production associated with impaired hemostasis in storage pool disease 总被引:5,自引:0,他引:5
Prostaglandin (PG) production was found to be impaired in platelets from patients with 'storage pool disease', a well characterized hemostatic disorder. This finding adds to the growing body of evidence which implicates prostaglandin synthesis or a similar arachidonate-consuming process in second phase aggregation and the accompanying platelet release reaction. Prostaglandin production is not impaired in platelets from patients with thrombasthenia. It is thus unlikely that aggregation 'per se' is a stimulus for PG production, because thrombasthenic platelets are virtually incapable of aggregation. 相似文献
994.
Glutamate Catabolism of Rickettsia rickettsi and Factors Affecting Retention of Metabolic Activity 总被引:6,自引:0,他引:6
Glutamate catabolism and the factors contributing to metabolic stability of purified suspensions of Rickettsia rickettsi were investigated. By use of (14)C-glutamate, it was shown that CO(2) was produced from all carbons of glutamate and that (14)CO(2) production was reduced by the addition of most of the unlabeled intermediates of the citric acid cycle and pyruvate. Oxalacetate, added in various concentrations, did not stimulate glutamate utilization. When the cells were suspended in bovine plasma albumin (BPA), CO(2) production from glutamate proceeded at a nearly uniform rate for 8 hr at 32 C and for 24 hr at 15 C. When BPA was used, the cells retained their metabolic activity at 0 or 30 C regardless of cell concentration, and were not influenced by the addition of varoius metabolites. Without BPA, metabolic stability was directly related to concentration. Of the stabilizers tested on low concentrations of rickettsiae, reduced glutathione was the most effective, provided that the gas phase contained predominantly N(2). Under these conditions of low partial pressure of O(2), glutamate further stabilized metabolic activity and was actively metabolized. The cells were also stabilized by oxidized glutathione in a gas phase of air, but under these conditions glutamate was utilized at a more moderate rate and it impaired metabolic stability. 相似文献
995.
AN ELECTRON MICROSCOPE STUDY OF LYMPHATIC TISSUE IN RUNT DISEASE 总被引:2,自引:0,他引:2
The thymus, spleen, and lymph nodes were studied in runt disease induced by a graft of intravenously injected homologous splenic cells into newborn rats and mice. Adult Long-Evans cells (70 x 106) were injected into Sprague-Dawley rats. Adult DBA cells (7 x 106) were injected into C57BL/6 mice. Runted rats were sacrificed at 14 to 28 days of age; mice at 10 to 20 days. The thymic cortex is depleted of small lymphocytes. Those remaining are severely damaged and phagocytized. Evidence of damage includes swelling of mitochondria, myelin figure formation, margination of chromatin, and sharp angulation in nuclear contour. Large numbers of macrophages are present. Epithelial-reticular cells which envelop small cortical blood vessels are often retracted, with the result that the most peripheral layer in the thymic-blood barrier suffers abnormally large gaps. Lymphocytes of the periarterial lymphatic sheaths of spleen and of the cortex of lymph nodes are reduced in number and damaged. Vast numbers of plasma cells and many lymphocytes are evident throughout lymph nodes, in the periarterial lymphatic sheaths, and in the marginal zone and red pulp of the spleen. Plasma cells are of different sizes, the larger having dilated sacs of endoplasmic reticulum. Lymphocytes are small to medium in size. They contain, in varying quantity, ribosomes and smooth membrane-bounded cytoplasmic vesicles approximately 350 to 500 A in diameter. Most plasma cells and lymphocytes are damaged and many of these are phagocytized. Many lymphocytes in lymph nodes, however, show no evidence of damage. Reticular cells and other fixed cells of the connective tissues seldom appear affected. Thus, the major cell types reacting in runt disease are lymphocytes, plasma cells, and histiocytes or macrophages. It appears, therefore, that both the delayed and immediate types of sensitivity play a part in this disease. 相似文献
996.
The Routine Fitting of Kinetic Data to Models: A Mathematical Formalism for Digital Computers 总被引:20,自引:0,他引:20 下载免费PDF全文
A mathematical formalism is presented for use with digital computers to permit the routine fitting of data to physical and mathematical models. Given a set of data, the mathematical equations describing a model, initial conditions for an experiment, and initial estimates for the values of model parameters, the computer program automatically proceeds to obtain a least squares fit of the data by an iterative adjustment of the values of the parameters. When the experimental measures are linear combinations of functions, the linear coefficients for a least squares fit may also be calculated. The values of both the parameters of the model and the coefficients for the sum of functions may be unknown independent variables, unknown dependent variables, or known constants. In the case of dependence, only linear dependencies are provided for in routine use. The computer program includes a number of subroutines, each one of which performs a special task. This permits flexibility in choosing various types of solutions and procedures. One subroutine, for example, handles linear differential equations, another, special non-linear functions, etc. The use of analytic or numerical solutions of equations is possible. 相似文献
997.
Heppner, Gloria (University of California, Berkeley), and David W. Weiss. High susceptibility of strain A mice to endotoxin and endotoxin-red blood cell mixtures. J. Bacteriol. 90:696-703. 1965.-Strain A mice were shown to be considerably more susceptible to lethal effects of endotoxin lipopolysaccharide (LPS) than mice of several other strains. Complexes of sublethal quantities of LPS and sheep red blood cells were synergistically toxic for strain A mice. Separate administration of sheep red blood cells and heat-killed salmonellae, in either order and as long as 24 hr apart, also proved to be synergistically lethal for strain A mice, but not for R(III) animals studied comparatively. Sheep red blood cell lysates possessed the ability of the intact cells in forming lethal combinations for strain A mice with killed salmonellae. Strain A red blood cell-killed salmonellae complexes were also lethal for strain A mice, but less so then complexes made with sheep red blood cells. A x R(III) F(1) hybrid animals showed the same resistance characteristics as the resistant R(III) parental strain. Possible explanations for these findings are suggested, and their relevance to an immunological mode of action of endotoxin lethality is discussed. 相似文献
998.
Ohne ZusammenfassungEine vorläufige Mitteilung unter gleichem Titel erschien als Nr. 152 aus der Biol. Versuchsanst. (Zool. Abt., Vorstand H.Przibram) im Akad. Anz., Nr 17. Wien 4. VII. 1929. 相似文献
999.
1000.
The Enzymatic Phosphorylation of Nucleic Acids and Its Application to End-Group Analysis 总被引:1,自引:0,他引:1
Polynucleotide kinase catalyzes the transfer of a phosphate group from ATP to the 5'-hydroxyl termini of polynucleotides. Selective labeling of the 5'-hydroxyl termini of DNA with polynucleotide kinase has been used to study the number and the identity of the 5'-terminal residues of bacteriophage DNA's, and to examine the nature of the phosphodiester bond cleavages produced by endonucleases and by sonic irradiation. The intact strands of T7 DNA bear 5'-phosphoryl end-groups; only deoxyadenylate and deoxythymidylate are present as 5'-terminal residues. The intact strands of native λ-DNA bear 5'-hydroxyl end-groups. M13 DNA, a circular molecule, cannot be phosphorylated. End-group labeling of DNA provides a method for determination of molecular weight; calibration against other DNA preparations is not required. The molecular weight of a single strand of T7 DNA, determined by end-group labeling, is 13.1 x 106; the molecular weight of a single strand of λ-DNA is 16.0 x 106. These values are in agreement with molecular weight estimates by sedimentation analysis and electron microscopy. Sonic irradiation of DNA has been shown to favor the production of polynucleotides terminated by 5'-phosphomonoester groups. All four deoxyribonucleotides are present as 5'-terminal residues of sonicated DNA. 相似文献