全文获取类型
收费全文 | 41171篇 |
免费 | 4321篇 |
国内免费 | 11篇 |
出版年
2023年 | 178篇 |
2022年 | 350篇 |
2021年 | 684篇 |
2020年 | 492篇 |
2019年 | 586篇 |
2018年 | 716篇 |
2017年 | 637篇 |
2016年 | 1066篇 |
2015年 | 1758篇 |
2014年 | 1909篇 |
2013年 | 2362篇 |
2012年 | 2924篇 |
2011年 | 2850篇 |
2010年 | 1896篇 |
2009年 | 1647篇 |
2008年 | 2289篇 |
2007年 | 2275篇 |
2006年 | 2164篇 |
2005年 | 1955篇 |
2004年 | 1931篇 |
2003年 | 1730篇 |
2002年 | 1697篇 |
2001年 | 696篇 |
2000年 | 653篇 |
1999年 | 659篇 |
1998年 | 460篇 |
1997年 | 372篇 |
1996年 | 358篇 |
1995年 | 365篇 |
1994年 | 326篇 |
1993年 | 321篇 |
1992年 | 445篇 |
1991年 | 380篇 |
1990年 | 369篇 |
1989年 | 384篇 |
1988年 | 377篇 |
1987年 | 355篇 |
1986年 | 283篇 |
1985年 | 310篇 |
1984年 | 313篇 |
1983年 | 283篇 |
1982年 | 269篇 |
1981年 | 232篇 |
1980年 | 218篇 |
1979年 | 219篇 |
1978年 | 187篇 |
1977年 | 191篇 |
1976年 | 192篇 |
1975年 | 213篇 |
1973年 | 182篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
S J Weller T P Friedlander J A Martin D P Pashley 《Molecular phylogenetics and evolution》1992,1(4):312-337
The selection of exemplars has been shown both theoretically and empirically to affect tree topology, but the importance of the number and nature of taxa used to represent higher taxonomic lineages in molecular studies is rarely stressed. In our rRNA study of higher moths and butterflies (Lepidoptera: Ditrysia), the selection of different exemplars and outgroups caused major tree rearrangements. We also examined the effectiveness with which conserved rRNA regions track the diversification of Lepidoptera. Homoplasy is as prevalent at the few variable sites of conserved regions (18E, 18J, 28F) as at the many variable sites of a more rapidly evolving region (28B). Finally, 28B sequence variation differs qualitatively among lepidopteran superfamilies of presumed comparable age, the Papilionoidea (true butterflies) and Noctuoidea (cutworm moths and relatives). 相似文献
992.
993.
B Haxhiu-Poskurica M A Haxhiu G K Kumar M J Miller R J Martin 《Journal of applied physiology》1992,72(3):1090-1095
The tachykinins substance P (SP) and neurokinin A (NKA) have been shown to induce airway smooth muscle contraction in mature animals, and the enzyme neutral endopeptidase (NEP) modulates this effect. We evaluated maturation of SP- and NKA-induced tracheal smooth muscle contraction and modulation of their effects by NEP in anesthetized, paralyzed, and artificially ventilated piglets less than 4 days, 2-3 wk, and 10 wk of age. Tracheal smooth muscle tension was measured in vivo from an open tracheal segment by use of a force transducer. Intravenous SP caused a dose-dependent increase in tracheal tension in all three age groups; however, the response in less than 4-day-old piglets was significantly weaker than in 2- to 3- and 10-wk-old piglets. NKA caused a dose-dependent increase in tracheal tension only in 2- to 3- and 10-wk-old piglets. The response of tracheal tension to NKA was weaker than the response to SP in all age groups. Atropine (2 mg/kg) significantly diminished the responses of tracheal tension to SP and NKA, indicating a cholinergic contribution to these responses at all ages. Intravenous thiorphan, a known NEP inhibitor, potentiated the effects of SP only in 2- to 3- and 10-wk-old piglets and did not affect the response of tracheal tension to NKA at any age. Biochemical analyses demonstrated a significant increase in tracheal NEP activity in comparably aged piglets over the first 10 wk of life.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
994.
We assessed pulmonary mechanics in six open-chest rabbits (3 young and 3 adult) by the forced oscillation technique between 0.16 and 10.64 Hz. Under control conditions, pulmonary resistance (RL) decreased markedly between 0.16 and 4 Hz, after which it became reasonably constant. Measurements of alveolar pressure from two alveolar capsules in each rabbit showed that the large decrease of RL with increasing frequency below 4 Hz was due to lung tissue rheology and that tissue resistance was close to zero above 4 Hz. Estimates of resistance and elastance, also obtained by fitting tidal ventilation data at 1 Hz to the equation of the linear single-compartment model, gave values for RL motion that were slightly higher than those obtained by forced oscillations at the same frequency, presumably because of the flow dependence of airways resistance. After treatment with increasing doses of aerosolized methacholine, RL and pulmonary elastance between 0.16 and 1.34 Hz progressively increased, as did the point at which the pulmonary reactance crossed zero (the resonant frequency). The alveolar pressure measurements showed the lung to become increasingly inhomogeneously ventilated in all six animals, whereas in the three younger rabbits lobar atelectasis developed at high methacholine concentrations and the alveolar capsules ceased to communicate with the central airways. We conclude that the low-frequency pulmonary impedance of rabbits exhibits the same qualitative features observed in other species and that it is a sensitive indicator of the changes in pulmonary mechanics occurring during bronchoconstriction. 相似文献
995.
Jia-Ling Chou Zhi-Xiang Shen Irene J. Tan Robert L. Stolfi Daniel S. Martin Steven H. Dikman Samuel Waxman 《Experimental cell research》1990,186(2)
Cells from autochthonous mouse mammary carcinomas which display estrogen-independent growth vivo were studied for their hormonal responses in primary culture. A culture system employing insulin-supplemented, serum-free medium and basement membrane Matrigel as a substratum was used to cultivate tumor cells. The cells did not exhibit in vitro estrogenor prolactin-dependent growth. Primary tumors still displayed a constitutional expression of α-, β-, and γ-casein mRNAs. These messages were dramatically reduced during the culture period. However, seven to eightfold increases in α- and β-casein mRNAs were inducible in the 5-day cultures by treatment with prolactin and hydrocortisone. If the hormones were present through a 2-week culture period, the levels of α-, β-, and γ-casein mRNAs in the cells were maintained and displayed in a time-dependent increase with a peak at 10–14 days. The accumulation of β-casein mRNA in vitro did not require DNA synthesis. Administration of prolactin directly into the growing tumors in vivo could also enhance β-casein mRNA levels in the tumor cells. Morphological studies of the cells cultured in the presence of prolactin and hydrocortisone did not reveal visible changes compared with those without hormonal treatment. Transplantation of tumor cells cultured in the presence or absence of hormones resulted in the development of tumors in mice at approximately the same time. The current studies suggest that the autochthonous mammary tumor cells, independent of estrogen for cell growth, were still inducible for casein gene expression in vitro and in vivo by appropriate hormones. The induction and maintenance of casein messages by a single hormonal treatment did not appear to correlate with morphology and DNA synthesis of cells in vitro or with tumor-producing capacities in vivo. 相似文献
996.
Gerry T. Snoek Cees H. Koster Siegfried W. de Laat Martin Heideveld Antony J. Durston Everardus J. J. van Zoelen 《Experimental cell research》1990,187(2)
The Xenopus laevis XTC cell line has been analyzed for the production of polypeptide growth factors and mesoderm-inducing activity. By the use of specific biological assays, it is shown that XTC cells produce a growth factor functionally related to the platelet-derived growth factor (PDGF) and two transforming growth factor (TGF)β-like activities. Mesoderm-inducing activity, as measured on X. laevis ectodermal explants from stage 10 embryos, was found to coelute on a Bio-Gel P-100 column with one of the TGFβ-like activities at an apparent molecular weight of 6–10 kDa. Analysis of the DNA content from XTC cells by flow cytometry demonstrated that the cell line is heterogeneous and consists of both tetraploid and diploid cells. Cloning of the XTC cells and selecting single-cell colonies on the basis of their ability to grow in soft agar resulted in the isolation of several homogeneous, morphologically different clonal derivatives. Analysis of conditioned medium from these clonal derivatives showed that only one of them, the only diploid line among six investigated, produced a strong heat- and acid-stable mesoderm-inducing activity that induced notochord and muscle formation in stage 10 X. laevis ectodermal expiants. The relation between this activity and a recently described TGFβ-like mesoderm-inducing factor obtained from XTC-conditioned medium will be discussed. In conclusion, a clonal cell line derived from X. laevis XTC cells which provides a good source for further characterization of mesoderm-inducing factors has been established. 相似文献
997.
Auer Martin T. Storey Michelle L. Effler Steven W. Auer Nancy A. Sze Philip 《Hydrobiologia》1990,200(1):603-617
The transparency of polluted, hypereutrophic Onondaga Lake, New York, USA has improved substantially in the late 1980's as
a result of reductions in phytoplankton biomass, in the absence of significant reductions in external phosphorus loading.
Much of this improvement has been due to the occurrence of clearing events, e.g. sudden and dramatic increases in transparency.
Field measurements, laboratory experiments, and modelling analyses were utilized to identify processes regulating phytoplankton
standing crop during the spring to fall interval of 1987. Changes in the zooplankton community documented over the past decade
support the conclusion that increased zooplankton grazing has contributed to improvements in transparency. Herbivores now
represent a greater fraction of the zooplankton population and more efficient cladocerans are present in greater numbers.
Biomanipulation practices, e.g. reestablishment of piscivorous species, designed to reduce the abundance of planktivorous
fish species in Onondaga Lake, may serve to reduce pressure on the grazing community and thus result in further improvements
in transparency. 相似文献
998.
Cloning and nucleotide sequence of luxR, a regulatory gene controlling bioluminescence in Vibrio harveyi. 总被引:6,自引:3,他引:3
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Mutagenesis with transposon mini-Mulac was used previously to identify a regulatory locus necessary for expression of bioluminescence genes, lux, in Vibrio harveyi (M. Martin, R. Showalter, and M. Silverman, J. Bacteriol. 171:2406-2414, 1989). Mutants with transposon insertions in this regulatory locus were used to construct a hybridization probe which was used in this study to detect recombinants in a cosmid library containing the homologous DNA. Recombinant cosmids with this DNA stimulated expression of the genes encoding enzymes for luminescence, i.e., the luxCDABE operon, which were positioned in trans on a compatible replicon in Escherichia coli. Transposon mutagenesis and analysis of the DNA sequence of the cloned DNA indicated that regulatory function resided in a single gene of about 0.6-kilobases named luxR. Expression of bioluminescence in V. harveyi and in the fish light-organ symbiont Vibrio fischeri is controlled by density-sensing mechanisms involving the accumulation of small signal molecules called autoinducers, but similarity of the two luminescence systems at the molecular level was not apparent in this study. The amino acid sequence of the LuxR product of V. harveyi, which indicates a structural relationship to some DNA-binding proteins, is not similar to the sequence of the protein that regulates expression of luminescence in V. fischeri. In addition, reconstitution of autoinducer-controlled luminescence in recombinant E. coli, already achieved with lux genes cloned from V. fischeri, was not accomplished with the isolation of luxR from V. harveyi, suggesting a requirement for an additional regulatory component. 相似文献
999.
Protein surface charges and Ca2+ binding to individual sites in calbindin D9k: stopped-flow studies 总被引:3,自引:0,他引:3
The kinetics of calcium dissociation from two groups of site-specific mutants of calbindin D9k--a protein in the calmodulin superfamily with two Ca2+ sites and a tertiary structure closely similar to that of the globular domains of troponin C and calmodulin--have been studied by stopped-flow kinetic methods, using the fluorescent calcium chelator Quin 2, and by 43Ca NMR methods. The first group of mutants comprises all possible single, double, and triple neutralizations of three particular carboxylate groups (Glu-17, Asp-19, and Glu-26) that are located on the surface of the protein. These carboxylates are close to the two EF-hand calcium binding sites, but are not directly liganded to the Ca2+ ions. Conservative modification of these negative carboxylate side chains by conversion to the corresponding amides results in a marked reduction in the Ca2+ binding constants for both sites, as recently reported [Linse et al. (1988) Nature 335, 651-652]. The stopped-flow kinetic results show that this reduction in Ca2+ affinity derives primarily from a reduction in the Ca2+ association rate constant, kon. The estimated maximum value of the association rate constant (kon(max) for Ca2+ binding to the wild-type protein is ca. 10(9) M-1 s-1. In contrast, for the mutant protein with three charges neutralized the maximum association rate constant is estimated to be only 2 X 10(7) M-1 s-1.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
1000.
The feasibility of using saponin as a permeabilization agent to study the effect of free Ca2+ concentration ([Ca2+]f) on prostaglandin I2 (PGI2) synthesis and mobilization of arachidonic acid from membrane phospholipids was investigated in cultured bovine pulmonary artery endothelial cells (BPAEC). Treatment of BPAEC with 20 micrograms/ml saponin caused selective permeabilization of the plasma membrane as determined by measurements of the release of lactate dehydrogenase and beta-hexosaminidase. In cells prelabeled with [3H]arachidonic acid for 22 h, permeabilization with 20 micrograms/ml saponin induced PGI2 synthesis and release of [3H]arachidonic acid from membrane phospholipids. These effects were dependent upon [Ca2+]f in the range 72 nM to 5 microM. Release of [3H]arachidonic acid from phospholipid classes was determined in suspensions of BPAEC prelabeled with [3H]arachidonic acid and permeabilized with 20 micrograms/ml saponin. At [Ca2+]f optimal for PGI2 synthesis, 16.2% of the total incorporated [3H]arachidonic acid was released from phosphatidylinositol (3.4%), phosphatidylethanolamine (3.5%) and phosphatidylcholine (9.3%). The time course and dependence upon [Ca2+]f of [3H]arachidonic acid release from phospholipids correlated with PGI2 synthesis. The amount of PGI2 synthesized in permeabilized BPAEC was similar to that in cell cultures treated with the calcium ionophore A23187. In comparison, however, PGI2 synthesis induced by A23187 was associated with less release of [3H]arachidonic acid from membrane phospholipids, e.g., 2.3% versus 16.2%. The greater loss of [3H]arachidonic acid from phospholipids in saponin-permeabilized BPAEC was most likely due to the loss of cell integrity and/or nonspecific effects of the detergent on phospholipases. Despite these limitations, the Ca2+ dependence observed for PGI2 synthesis and [3H]arachidonic acid mobilization suggest that saponin-permeabilization may provide a useful system for studies of the intracellular events triggered by the rise in intracellular Ca2+ which culminate in PGI2 synthesis. 相似文献