全文获取类型
收费全文 | 8284篇 |
免费 | 863篇 |
国内免费 | 1篇 |
出版年
2021年 | 79篇 |
2020年 | 47篇 |
2019年 | 52篇 |
2018年 | 91篇 |
2017年 | 78篇 |
2016年 | 143篇 |
2015年 | 249篇 |
2014年 | 272篇 |
2013年 | 393篇 |
2012年 | 498篇 |
2011年 | 451篇 |
2010年 | 264篇 |
2009年 | 288篇 |
2008年 | 445篇 |
2007年 | 460篇 |
2006年 | 410篇 |
2005年 | 399篇 |
2004年 | 440篇 |
2003年 | 385篇 |
2002年 | 424篇 |
2001年 | 169篇 |
2000年 | 133篇 |
1999年 | 133篇 |
1998年 | 131篇 |
1997年 | 127篇 |
1996年 | 113篇 |
1995年 | 106篇 |
1994年 | 86篇 |
1993年 | 83篇 |
1992年 | 108篇 |
1991年 | 111篇 |
1990年 | 119篇 |
1989年 | 111篇 |
1988年 | 99篇 |
1987年 | 87篇 |
1986年 | 80篇 |
1985年 | 92篇 |
1984年 | 91篇 |
1983年 | 91篇 |
1982年 | 78篇 |
1981年 | 82篇 |
1980年 | 74篇 |
1979年 | 75篇 |
1978年 | 81篇 |
1977年 | 62篇 |
1976年 | 64篇 |
1975年 | 61篇 |
1974年 | 65篇 |
1973年 | 68篇 |
1972年 | 49篇 |
排序方式: 共有9148条查询结果,搜索用时 15 毫秒
61.
Pseudomonas putida Fl oxidizes toluene through cis-toluene dihydrodiol to 3-methylcatechol. The latter compound is the substrate for “meta” fission of the aromatic nucleus. Kinetic and induction experiments indicate that the genes encoding enzymes for these reactions are part of an operon, designated the tod operon, that is coordinately induced and regulated. Strains unable to utilize toluene as a growth substrate were isolated at high frequencies by using screening procedures that utilize the redox dye, 2,3,5-triphenyl-2H-tetrazolium chloride. Biochemical characterization of strains with mutations in the structural genes of the tod operon showed that toluene induces the first four enzymes in toluene degradation by P. putida Fl. The isolation and characterization of pleiotropicnegative mutants together with mutants altered in terms of their expression of tod genes suggests that the tod operon may be under the control of a positive regulatory element. 相似文献
62.
Incubation of horse-heart oxymyoglobin or metmyoglobin with excess H2O2 causes formation of myoglobin(IV), followed by haem degradation. At the time when haem degradation is observed, hydroxyl radicals (.OH) can be detected in the reaction mixture by their ability to degrade the sugar deoxyribose. Detection of hydroxyl radicals can be decreased by transferrin or by OH scavengers (mannitol, arginine, phenylalanine) but not by urea. Neither transferrin nor any of these scavengers inhibit the haem degradation. It is concluded that intact oxymyoglobin or metmyoglobin molecules do not react with H2O2 to form OH detectable by deoxyribose, but that H2O2 eventually leads to release of iron ions from the proteins. These released iron ions can react to form OH outside the protein or close to its surface. Salicylate and the iron chelator desferrioxamine stabilize myoglobin and prevent haem degradation. The biological importance of OH generated using iron ions released from myoglobin by H2O2 is discussed in relation to myocardial reoxygenation injury. 相似文献
63.
64.
T Barry S Geary S Hannify C MacGearailt M Shalloo D Heery F Gannon R Powell 《Nucleic acids research》1992,20(18):4940
65.
Jacob Gopas Dganit Itzhaky Yael Segev Samuel Salzberg Barry Trink Noah Isakov Bracha Rager-Zisman 《Cancer immunology, immunotherapy : CII》1992,34(5):313-320
The effect of persistent measles virus infection on the expression of major histocompatibility complex (MHC) class I antigens was studied. Mouse neuroblastoma cells C1300, clone NS20Y, were persistently infected with the Edmonston strain of measles virus. The persistently infected cell line, NS20Y/MS, expressed augmented levels of both H-2Kk and H-2Dd MHC class I glycoproteins. Activation of two interferon(IFN)-induced enzymes, known to be part of the IFN system: (2–5)oligoadenylate synthetase and double-stranded-RNA-activated protein kinase, was detected. Measles-virus-infected cells elicited cytotoxic T lymphocytes that recognized and lysed virus-infected and uninfected neuroblastoma cells in an H-2-restricted fashion. Furthermore, immunization of mice with persistently infected cells conferred resistance to tumor growth after challenge with the highly malignant NS20Y cells. The rationale for using measles virus for immunotherapy is that most patients develop lifelong immunity after recovery or vaccination from this infection. Patients developing cancer are likely to have memory cells. A secondary response induced by measles-virus-infected cells may therefore induce an efficient immune response against non-infected tumour cells. 相似文献
66.
S. A. Clulow M. J. Wilkinson R. Waugh E. Baird M. J. DeMaine W. Powell 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1991,82(5):545-551
Summary Seventeen potato dihaploids, produced by pollinating the tetraploid (2n = 48) cv Pentland Crown with pollen from Solanum phureja (2n = 24) dihaploid inducer clones, were studied. Since dihaploids are thought to develop parthenogenetically from unfertilized ovules they were expected to be euploid (2n = 24), but somatic chromosome counts showed that 15 of the 17 dihaploids were aneusomatic. Ten of the clones were predominantly diploid (2n = 24) with a proportion of hyperploid cells that contained 25 or 26 chromosomes. Five of the dihaploids contained variable numbers of triploid cells (2n = 36). RFLP analysis was used to determine whether the additional chromosomes were from S. phureja or S. tuberosum. Unique hybridizing fragments present in S. phureja but not in Pentland Crown were identified. These S. phureja-specific restriction fragments were present in some of the dihaploid offspring of Pentland Crown. Of the 5 clones that contained triploid cells 4 had S. phureja type banding. Four of the 10 aneusomatic clones that contained hyperploid cells had the unique S. phureja hybridizing fragments. We propose that ovules of Pentland Crown were fertilized by pollen from S. phureja and that the aneusomatic clones were derived from triploid zygotes from which some of the S. phureja chromosomes were eliminated. We consider that this is an additional mechanism of dihaploid formation in potato. 相似文献
67.
The isolation and immunolocalization of iron-binding compounds produced by Gloeophyllum trabeum 总被引:2,自引:0,他引:2
Jody Jellison Vikas Chandhoke Barry Goodell Frank A. Fekete 《Applied microbiology and biotechnology》1991,35(6):805-809
Summary Low molecular weight iron-binding compounds are produced by the brown-rot fungus Gloeophyllum trabeum. These chelators may function in scavenging transition metals for fungal metabolism and extracellular enzyme production. Because of the low molecular mass of the chelate-metal complex (below 1000 Da), and the oxidizing potential of the bound transition metals, certain chelating compounds could also play a role in the early stages of cellulose depolymerization by brown-rot fungi. High-affinity iron-binding compounds were isolated and partially purified from both liquid cultures of the brown-rot Gloeophyllum trabeum and from infected wood. Chelating compounds purified by thin-layer chromatography were used to prepare specific antibodies. These antibodies were shown to detect the chelator in infected wood and liquid fungal cultures by enzyme-linked immunosorbent assay and could be used in immunotransmission electron microscopy to visualize the high-affinity iron-binding compounds in situ. Elucidating the physiological roles of fungal chelate-metal complexes and determining their function in lignocellulose depolymerization will help us to better understand the mechanism of wood biodegradation.Publication no. 1549 Maine Agricultural Experiment Station
Offprint requests to: J. Jellison 相似文献
68.
Structure-function analysis of interleukin-5 utilizing mouse/human chimeric molecules. 总被引:10,自引:4,他引:6 下载免费PDF全文
Interleukin-5 (IL5) is a T cell derived glycoprotein that stimulates eosinophil production and activation. In the mouse, but apparently not in the human, it is active on B cells. The murine and human IL5 polypeptides exhibit 70% sequence similarity and yet display distinct species-specific activity. Whilst mouse and human IL5 are equally active in human cell assays, human IL5 is 100-fold less active than murine IL5 in mouse cell assays. Two restriction sites were utilized to divide the human and mouse sequences into three fragments. Hybrid molecules consisting of all combinations of these fragments were constructed and expressed. In the human cell assays [using bone marrow or the erythroleukaemic cell line (TF-1)] all the hybrid proteins generated activity comparable to that of the human and mouse IL5. This implies that replacing different domains does not result in detrimental effects to the tertiary structure of the molecule. In the mouse cell assays [using bone marrow or the pro-B cell line (B13)] the hybrids clearly identified the importance of residues in the C terminus for biological activity. The changing of only eight residues in this region of human IL5, to those of mouse IL5, resulted in the hybrid producing biological activity comparable to mouse IL5. In addition, competition binding assays showed that this region probably interacts with the receptor. 相似文献
69.
70.
Barry A. Trimmer Janis C. Weeks 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(1):27-43
Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations
ACh
acetylcholine
-
AChE
acetylcholine esterase
-
CNS
central nervous system
-
EPSP
excitatory postsynaptic potential
-
I
h
injected hyperpolarizing current
-
LTP
long-term potentiation
-
PPR
principal planta retractor motoneuron
-
PTP
post-tetanic potentiation
-
R
in
input resistance
-
V
h
hyperpolarized potential
-
V
m
membrane potential
-
VN
ventral nerve
-
VNA
anterior branch of the ventral nerve
-
V
r
resting potential. 相似文献