全文获取类型
收费全文 | 1947篇 |
免费 | 156篇 |
国内免费 | 4篇 |
出版年
2023年 | 30篇 |
2022年 | 16篇 |
2021年 | 29篇 |
2020年 | 19篇 |
2019年 | 16篇 |
2018年 | 63篇 |
2017年 | 54篇 |
2016年 | 80篇 |
2015年 | 107篇 |
2014年 | 107篇 |
2013年 | 134篇 |
2012年 | 156篇 |
2011年 | 126篇 |
2010年 | 104篇 |
2009年 | 73篇 |
2008年 | 107篇 |
2007年 | 83篇 |
2006年 | 75篇 |
2005年 | 71篇 |
2004年 | 84篇 |
2003年 | 69篇 |
2002年 | 53篇 |
2001年 | 54篇 |
2000年 | 53篇 |
1999年 | 44篇 |
1998年 | 25篇 |
1997年 | 16篇 |
1996年 | 10篇 |
1995年 | 16篇 |
1994年 | 5篇 |
1993年 | 11篇 |
1992年 | 15篇 |
1991年 | 18篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1977年 | 6篇 |
1975年 | 7篇 |
1972年 | 8篇 |
1971年 | 6篇 |
1969年 | 6篇 |
1968年 | 4篇 |
排序方式: 共有2107条查询结果,搜索用时 15 毫秒
121.
The 10T1/2-MRF4 fibroblast/myogenic cell system was used to address the following interrelated questions: whether distinct signaling pathways underlie myogenic inhibition by basic fibroblast growth factor (bFGF) and transforming growth factor (TGF)-beta; which of these pathways also up-regulates the fibroblast intermediate conductance calcium-activated potassium channel, FIK, a positive regulator of cell proliferation; and whether FIK up-regulation underlies some or all myogenic inhibitory signaling events. The results show that myogenic inhibition in 10T1/2-MRF4 cells, by both bFGF and TGF-beta, requires activation of the Ras/mitogen-activated protein (MAP) kinase/MAP kinase-ERK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway, and resultant FIK up-regulation. We show that FIK is instrumental in MEK-dependent suppression of acetylcholine receptor channel expression but that MEK activation and FIK up-regulation are not essential to suppression of myosin heavy chain and myotube formation. These data indicate that Ras/MEK/ERK induction of FIK is pivotal to regulation of certain myogenic events by both receptor tyrosine kinases and TGF-beta receptor, and this is also the first demonstration of chronic FIK up-regulation by the TGF-beta receptor family. Furthermore, the results define the physiologic signaling requirements for growth factor-stimulated FIK up-regulation, whereas previous work has concentrated on constitutive FIK up-regulation in cells stably transfected with oncoprotein signaling molecules. This study, together with earlier work showing that FIK positively regulates cell proliferation, establishes this member of the IK channel family as a multifunctional, growth factor-regulated signaling molecule. 相似文献
122.
de la Peña M Elena SF Moya A 《Evolution; international journal of organic evolution》2000,54(2):686-691
Abstract.— RNA viruses show the highest mutation rate in nautre. It has been extensively demonstrated that, in the absence of purifying selection, RNA viruses accumulate deleterious mutations at a high rate. However, the parameters describing this accumulation are, in general, poorly understood. The present study reports evidences for fitness declines by the accumulation of deleterious mutations in the bacteriophage MS2. We estimated the rate of fitness decline to be as high as 16% per bottleneck transfer. In addition, our results agree with an additive model of fitness effects. 相似文献
123.
The stem bark of Lonchocarpus xuul (Leguminosae) has yielded four flavonoids which have been identified by spectroscopic methods as the novel 4beta,5-dimethoxy-6",6"-dimethyl-2H-pyrano-(2",3":7,6)-fl avan (xuulanin), 3beta,4beta,5-trimethoxy-6",6"-dimethyl-2H-pyrano-(2",3":7,6 )-flavan (3beta-methoxyxuulanin). 4beta-ethoxy-5-methoxy-6",6"-dimethyl-2H-pyrano-(2",3":7,6)- flavan (4beta-demethylxuulanin-4beta-ethyl ether), and the known 5,7-dihydroxy-6,8-di(3-methylbut-2-enyl)flavanone (spiniflavanone-B). The ethyl derivative is considered likely to be an artefact. 相似文献
124.
L. Luhová M. Šebela I. Frébort L. Zajoncová H.G. Faulhammer P. Peč 《Biologia Plantarum》1998,41(2):241-254
Aim of this work was to find the best source for obtaining high amount of copper amine oxidase (EC 1.4.3.6) that can be further
used for analytical or industrial applications. The study focused on plant enzymes, because they occur in much higher content
in the starting material than the enzymes from other sources, have higher specific activity and are also more thermostable.
Presence of the amine oxidase was tested in extracts from 4 to 7-d-old seedlings of thirty-four various Fabaceae plants. Amine
oxidases from nine selected plants were purified by general method involving ammonium sulfate fractionation, controlled heat
denaturation, and three chromatographic steps. Kinetic properties of the amine oxidases purified were tested with a wide range
of substrates and inhibitors and were found to be very similar. Best purification yield, and total and specific activities
were obtained for the enzyme from grass pea (Lathyrus sativus) throughout all purification steps. Hence, the grass pea extract
was chosen as a suitable candidate for massive production of the amine oxidase.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
125.
Endogenous oxytocin is released by the sow at the time of mating in response to stimulation by the boar, which may explain, at least partially, the importance of the relationship between the boar's courting activity and the subsequent reproductive performance of the sow. The aim of this study was to determine the effects on reproductive performance of supplementing AI doses with exogenous oxytocin during the low fertility season. At an intensive piggery in northwest Spain 3 experimental groups were randomly formed and observed throughout the year. Group 1 sows were inseminated with semen supplemented with 4 IU oxytocin. Group 2 sows received 4 IU oxytocin injected through the vulvar lips mucosa at the time of insemination. Group 3 sows were inseminated without oxytocin and served as the controls. During the low fertility season the results for each group were as follows: farrowing rate 77.02, 56.25 and 54.39%, and litter size 10.77 +/- 0.28, 10.45 +/- 0.31 and 8.53 +/- 0.34 respectively. It is concluded that the addition of oxytocin to seminal doses just before AI is an easily applicable, effective method for increasing fertility and litter size during the summer months. 相似文献
126.
Identification of a Functional Homolog of the Yeast Copper
Homeostasis Gene ATX1 from
Arabidopsis 总被引:11,自引:0,他引:11 下载免费PDF全文
Edward Himelblau Helena Mira Su-Ju Lin Valeria Cizewski Culotta Lola Pe?arrubia Richard M. Amasino 《Plant physiology》1998,117(4):1227-1234
A cDNA clone encoding a homolog of the yeast (Saccharomyces cerevisiae) gene Anti-oxidant 1 (ATX1) has been identified from Arabidopsis. This gene, referred to as Copper CHaperone (CCH), encodes a protein that is 36% identical to the amino acid sequence of ATX1 and has a 48-amino acid extension at the C-terminal end, which is absent from ATX1 homologs identified in animals. ATX1-deficient yeast (atx1) displayed a loss of high-affinity iron uptake. Expression of CCH in the atx1 strain restored high-affinity iron uptake, demonstrating that CCH is a functional homolog of ATX1. When overexpressed in yeast lacking the superoxide dismutase gene SOD1, both ATX1 and CCH protected the cell from the reactive oxygen toxicity that results from superoxide dismutase deficiency. CCH was unable to rescue the sod1 phenotype in the absence of copper, indicating that CCH function is copper dependent. In Arabidopsis CCH mRNA is present in the root, leaf, and inflorescence and is up-regulated 7-fold in leaves undergoing senescence. In plants treated with 800 nL/L ozone for 30 min, CCH mRNA levels increased by 30%. In excised leaves and whole plants treated with high levels of exogenous CuSO4, CCH mRNA levels decreased, indicating that CCH is regulated differently than characterized metallothionein proteins in Arabidopsis. 相似文献
127.
Rod and cone photoreceptors project from the outer retinal surface into a
carbohydrate-rich interphotoreceptor matrix (IPM). Unique IPM
glycoconjugates are distributed around rods and cones. Wheat germ
agglutinin (WGA) strongly decorates the rod matrix domains and weakly
decorates the cone matrix domains. This study characterizes the major
WGA-binding glycoprotein in the human IPM, which we refer to as SPACR
(sialoprotein associated with cones and rods). SPACR, which has a molecular
weight of 147 kDa, was isolated and purified from the IPM by lectin
affinity chromatography. A polyclonal antibody to SPACR was prepared that
colocalizes in tissue preparations with WGA-binding domains in the IPM.
Sequential digestion of SPACR with N- and O- glycosidases results in a
systematic increase in electrophorectic mobility, indicating the presence
of both N- and O-linked glycoconjugates. Complete deglycosylation results
in a reduction in the relative molecular mass of SPACR by about 30%.
Analysis of lectin binding allowed us to identify some of the structural
characteristics of SPACR glycoconjugates. Treatment with neuraminidase
exposes Galbeta1- 3GalNAc disaccharide as indicated by positive peanut
agglutinin (PNA) staining, accompanied by the loss of WGA staining. Maackia
amurensis agglutinins (MAA-1 and MAA-2), specific for sialic acid in
alpha2-3 linkage to Gal, bind SPACR, while Sambucus nigra agglutinin (SNA),
specific for alpha2-6 linked sialic acid, does not, indicating that the
dominant glycoconjugate determinant on SPACR is the O-linked carbohydrate,
NeuAcalpha2-3Galbeta1-3GalNAc. The abundance of sialic acid in SPACR
suggests that this glycoprotein may contribute substantially to the
polyanionic nature of the IPM. The carbohydrate chains present on SPACR
could also provide sites for extensive crosslinking and participate in the
formation of the ordered IPM lattice that surrounds the elongate
photoreceptors projecting from the outer retinal surface.
相似文献
128.
129.
Gilda J. Erosa-Rejón Alejandro Yam-Puc Manuel J. Chan-Bacab Alberto Giménez-Turbax Efraín Salamanca Luis M. Peña-Rodríguez Olov Sterner 《Phytochemistry letters》2010,3(1):9-12
Two new benzochromenes, (6,6-dimethyl-2-methoxy-6H-benzo[c]chromen-9-yl)methanol (1), and 2-methoxy-6,6-dimethyl-6H-benzo[c]chromen-9-carbaldehyde (2), together with several already known metabolites, were isolated from the root extract of Bourreria pulchra (Boraginaceae). The structures of 1 and 2 were established on the basis of their spectroscopic data. Both were assayed for in vitro antiprotozoan activity, and especially 1 was found to possess significant activity against Leishmania mexicana and Trypanosoma cruzi parasites (IC50 4.6 μg/mL and 7.5 μg/mL, respectively). 相似文献
130.
María Elvira Balcells Patricia García Paulina Meza Carlos Pe?a Marcela Cifuentes David Couvin Nalin Rastogi 《PloS one》2015,10(2)
Tuberculosis (TB) remains a significant public health problem worldwide, but the ecology of the prevalent mycobacterial strains, and their transmission, can vary depending on country and region. Chile is a country with low incidence of TB, that has a geographically isolated location in relation to the rest of South American countries due to the Andes Mountains, but recent migration from neighboring countries has changed this situation. We aimed to assess the genotypic diversity of Mycobacterium tuberculosis complex (MTBC) strains in Santiago, Chile, and compare with reports from other Latin-American countries. We analyzed MTBC isolates from pulmonary tuberculosis cases collected between years 2008 and 2013 in Central Santiago, using two genotyping methods: spoligotyping and 12-loci mycobacterial interspersed repetitive unit-variable number of tandem repeats (MIRU-VNTRs). Data obtained were analyzed and compared to the SITVIT2 database. Mean age of the patients was 47.5 years and 61% were male; 11.6% were migrants. Of 103 strains (1 isolate/patient) included, there were 56 distinct spoligotype patterns. Of these, 16 strains (15.5%) corresponded to orphan strains in the SITVIT2 database, not previously reported. Latin American and Mediterranean (LAM) (34%) and T (33%) lineages were the most prevalent strains, followed by Haarlem lineage (16.5%). Beijing family was scarcely represented with only two cases (1.9%), one of them isolated from a Peruvian migrant. The most frequent clustered spoligotypes were SIT33/LAM3 (10.7%), SIT53/T1 (8.7%), SIT50/H3 (7.8%), and SIT37/T3 (6.8%). We conclude that LAM and T genotypes are the most prevalent genotypes of MTBC in Santiago, Chile, and together correspond to almost two thirds of analyzed strains, which is similar to strain distribution reported from other countries of Latin America. Nevertheless, the high proportion of SIT37/T3, which was rarely found in other Latin American countries, may underline a specific history or demographics of Chile related to probable human migrations and evolutions. 相似文献