全文获取类型
收费全文 | 11441篇 |
免费 | 1547篇 |
国内免费 | 4篇 |
出版年
2021年 | 123篇 |
2018年 | 140篇 |
2017年 | 118篇 |
2016年 | 200篇 |
2015年 | 340篇 |
2014年 | 332篇 |
2013年 | 431篇 |
2012年 | 498篇 |
2011年 | 478篇 |
2010年 | 298篇 |
2009年 | 312篇 |
2008年 | 412篇 |
2007年 | 474篇 |
2006年 | 420篇 |
2005年 | 406篇 |
2004年 | 376篇 |
2003年 | 354篇 |
2002年 | 318篇 |
2001年 | 297篇 |
2000年 | 315篇 |
1999年 | 320篇 |
1998年 | 154篇 |
1997年 | 121篇 |
1996年 | 135篇 |
1995年 | 129篇 |
1994年 | 135篇 |
1993年 | 141篇 |
1992年 | 245篇 |
1991年 | 236篇 |
1990年 | 217篇 |
1989年 | 214篇 |
1988年 | 223篇 |
1987年 | 212篇 |
1986年 | 205篇 |
1985年 | 226篇 |
1984年 | 202篇 |
1983年 | 153篇 |
1982年 | 142篇 |
1981年 | 120篇 |
1980年 | 117篇 |
1979年 | 187篇 |
1978年 | 152篇 |
1977年 | 127篇 |
1976年 | 152篇 |
1975年 | 148篇 |
1974年 | 118篇 |
1973年 | 151篇 |
1972年 | 144篇 |
1971年 | 120篇 |
1970年 | 136篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Anecdotal, historical and critical commentaries on genetics twenty years of illegitimate recombination 总被引:6,自引:0,他引:6
Anderson P 《Genetics》1987,115(4):581-583
62.
Sequence-dependent DNA bending of synthetic and natural molecules was studied by computer analysis. Modelling of synthetic oligonucleotides and of 107 kb of natural sequences gave results which closely resembled published electrophoretic data, demonstrating the powerful predictive capacity of the procedure. The analysis was extended to the study of DNA structures involved in chromosome maintenance. Centromeric DNAs from yeast were found to have sequences in their functional elements which cause them to be unusually straight. Autonomous replicating sequences were found to have two structural domains, one consisting of unusually straight sequences surrounding the consensus and the other of bending elements in flanking DNA. In addition to a structural homology, centromeric and autonomous replicating sequences share common sequence elements. These observations show that computer modelling of natural sequences is a viable approach to the study of the biological implications of alternative DNA structures. 相似文献
63.
Michael J. Huether Paula J. Fedorka-Cray Mary Ann Pfannenstiel Gary A. Anderson 《FEMS microbiology letters》1987,48(1-2):179-182
Abstract This paper describes the plasmid profiles obtained for 73 of 96 field isolates of Haemophilus pleuropneumoniae serotypes 1, 3, 5, and 7. We also characterized the antibiotic susceptibilities of these 96 isolates. Because of the high proportion of isolates resistant to some of the antibiotics, no conclusions can be drawn as to the role of plasmids in antibiotic resistance. 相似文献
64.
65.
Water use efficiency and carbon isotope composition of plants in a cold desert environment 总被引:9,自引:0,他引:9
Summary The effects of the availabilities of water and nitrogen on water use efficiency (WUE) of plants were investigated in a sagebrush steppe. The four species studied wereArtemisia tridentata (shrub),Ceratoides lanata (suffrutescent shrub),Elymus lanceolatus (rhizomatous grass), andElymus elymoides (tussock grass). Water and nitrogen levels were manipulated in a two-by-two factorial design resulting in four treatments: control (no additions), added water, added nitrogen, and added water and nitrogen. One instantaneous and two long-term indicators of WUE were used to testa priori predictions of the ranking of WUE among treatments. The short-term indicator was the instantaneous ratio of assimilation to transpiration (A/E). The long-term measures were 1) the slope of the relationship between conductance to water vapor and maximum assimilation and 2) the carbon isotope composition (13C) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and 13C, the mean for added nitrogen alone was significantly greater than the mean for added water alone, and means for the control and added water and nitrogen fell in between. This ranking of WUE supported the hypothesis that both water and nitrogen limit plant gas exchange in this semiarid environment. The short- and long-term indicators were in agreement, providing evidence in support of theoretical models concerning the water cost of carbon assimilation. 相似文献
66.
Summary The distributions of five Drosophila species and four components of the microflora have been compared across a total of 48 traps baited with four different fruit and vegetable substrates in two domestic compost heaps in Canberra (Australia). Large and consistent differences are found, both among the Drosophila and among the microbial classes, in their distributions across traps baited with different substrates. Moreover the distribution of each Drosophila species shows a unique set of strong associations with the microbial distributions. Thus the distributions of both D. simulans and D. melanogaster are found to be strongly negatively correlated with the abundance of bacteria while D. simulans is also strongly positively correlated with the titre of fermenter yeasts. D. immigrans is strongly positively correlated both with bacteria and with non-fermenter yeasts. D. hydei is positively correlated with nonfermentery yeasts and D. busckii is negatively correlated with fermenter yeasts. Moulds are the only microbial class not consistently associated with the distribution of any of the Drosophila species. The correlations with the other microbial classes are sufficient to explain the majority of the abundance differences of the Drosophila species among the trap types. It is therefore proposed that the clear partitioning of the fruit resources by the Drosophila is due to their differing primary interactions with the microflora. 相似文献
67.
Nitrogen Assimilating Enzyme Activities and Enzyme Protein during Development and Senescence of Effective and Plant Gene-Controlled Ineffective Alfalfa Nodules 总被引:9,自引:4,他引:5 下载免费PDF全文
Effective (N2-fixing) alfalfa (Medicago sativa L.) and plant-controlled ineffective (non-N2-fixing) alfalfa recessive for the in1 gene were compared to determine the effects of the in1 gene on nodule development, acetylene reduction activity (ARA), and nodule enzymes associated with N assimilation and disease resistance. Effective nodule ARA reached a maximum before activities of glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AAT), asparagine synthetase (AS), and phosphoenolpyruvate carboxylase (PEPC) peaked. Ineffective nodule ARA was only 5% of effective nodule ARA. Developmental profiles of GS, GOGAT, AAT, and PEPC activities were similar for effective and ineffective nodules, but activities in ineffective nodules were lower and declined earlier. Little AS activity was detected in developing ineffective nodules. Changes in GS, GOGAT, AAT, and PEPC activities in developing and senescent effective and ineffective nodules generally paralleled amounts of immunologically detectable enzyme polypeptides. Effective nodule GS, GOGAT, AAT, AS, and PEPC activities declined after defoliation. Activities of glutamate dehydrogenase, malate dehydrogenase, phenylalanine ammonia lyase, and caffeic acid-o-methyltransferase were unrelated to nodule effectiveness. Maximum expression of nodule N-assimilating enzymes appeared to require the continued presence of a product associated with effective bacteroids that was lacking in in1 effective nodules. 相似文献
68.
69.
Glutamine-dependent carbamoyl-phosphate synthetase and other enzyme activities related to the pyrimidine pathway in spleen of Squalus acanthias (spiny dogfish). 下载免费PDF全文
P M Anderson 《The Biochemical journal》1989,261(2):523-529
The first two steps of urea synthesis in liver of marine elasmobranchs involve formation of glutamine from ammonia and of carbamoyl phosphate from glutamine, catalysed by glutamine synthetase and carbamoyl-phosphate synthetase, respectively [Anderson & Casey (1984) J. Biol. Chem. 259, 456-462]; both of these enzymes are localized exclusively in the mitochondrial matrix. The objective of this study was to establish the enzymology of carbamoyl phosphate formation and utilization for pyrimidine nucleotide biosynthesis in Squalus acanthias (spiny dogfish), a representative elasmobranch. Aspartate carbamoyltransferase could not be detected in liver of dogfish. Spleen extracts, however, had glutamine-dependent carbamoyl-phosphate synthetase, aspartate carbamoyltransferase, dihydro-orotase, and glutamine synthetase activities, all localized in the cytosol; dihydro-orotate dehydrogenase, orotate phosphoribosyltransferase, and orotidine-5'-decarboxylase activities were also present. Except for glutamine synthetase, the levels of all activities were very low. The carbamoyl-phosphate synthetase activity is inhibited by UTP and is activated by 5-phosphoribosyl 1-pyrophosphate. The first three enzyme activities of the pyrimidine pathway were eluted in distinctly different positions during gel filtration chromatography under a number of different conditions; although complete proteolysis of inter-domain regions of a multifunctional complex during extraction cannot be excluded, the evidence suggests that in dogfish, in contrast to mammalian species, these three enzymes of the pyrimidine pathway exist as individual polypeptide chains. These results: (1) establish that dogfish express two different glutamine-dependent carbamoyl-phosphate synthetase activities, (2) confirm the report [Smith, Ritter & Campbell (1987) J. Biol. Chem. 262, 198-202] that dogfish express two different glutamine synthetases, and (3) provide indirect evidence that glutamine may not be available in liver for biosynthetic reactions other than urea formation. 相似文献
70.
Summary A wide range of bacterial strains were examined for their ability to accumulate polyhydroxyalkanoates (PHA) from various carbon sources. Strains were selected from those reported to accumulate poly-3-hydroxybutyrate (PHB), related organisms and laboratory stocks. Other strains known to utilize n-alkanes, n-alcohols or n-acids were chosen to investigate their ability to produce long-chain PHAs. Five strains accumulated only PHB, 13 accumulated PHAs containing only C4 and C5 units and 7 accumulated PHAs containing 3-hydroxyacid units in the range C5 to C10. 相似文献