全文获取类型
收费全文 | 57885篇 |
免费 | 4604篇 |
国内免费 | 4484篇 |
专业分类
66973篇 |
出版年
2024年 | 143篇 |
2023年 | 789篇 |
2022年 | 1852篇 |
2021年 | 3046篇 |
2020年 | 2082篇 |
2019年 | 2503篇 |
2018年 | 2346篇 |
2017年 | 1806篇 |
2016年 | 2544篇 |
2015年 | 3629篇 |
2014年 | 4382篇 |
2013年 | 4438篇 |
2012年 | 5289篇 |
2011年 | 4764篇 |
2010年 | 2882篇 |
2009年 | 2599篇 |
2008年 | 2937篇 |
2007年 | 2634篇 |
2006年 | 2261篇 |
2005年 | 1887篇 |
2004年 | 1509篇 |
2003年 | 1423篇 |
2002年 | 1073篇 |
2001年 | 909篇 |
2000年 | 889篇 |
1999年 | 810篇 |
1998年 | 499篇 |
1997年 | 454篇 |
1996年 | 477篇 |
1995年 | 422篇 |
1994年 | 413篇 |
1993年 | 325篇 |
1992年 | 446篇 |
1991年 | 324篇 |
1990年 | 284篇 |
1989年 | 260篇 |
1988年 | 210篇 |
1987年 | 194篇 |
1986年 | 176篇 |
1985年 | 154篇 |
1984年 | 115篇 |
1983年 | 122篇 |
1982年 | 81篇 |
1981年 | 45篇 |
1980年 | 51篇 |
1979年 | 63篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1973年 | 45篇 |
1972年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
Abstract: In a previous study, protein kinase FA /glycogen synthase kinase-3 ( FA /GSK-3 ) was identified as a myelin basic protein (MBP) kinase associated with intact brain myelin. In this report, the phosphorylation sites of MBP by kinase FA /GSk-3 were further determined by two-dimensional electrophoresis/TLC, phosphoamino acid analysis, tryptic peptide mapping, Edman degradation, and direct sequencing. Kinase FA /GSK-3 phosphorylates MBP on both threonine and serine residues. Three tryptic phosphopeptide peaks were resolved by C18 reverse-phase HPLC. Sequential manual Edman degradation together with direct sequence analysis revealed that T(p)PPPSQGK is the phosphorylation site sequence for the first major phosphopeptide peak. When mapping with the bovine brain MBP sequence, we finally demonstrate Thr97 -Pro, one of the in vivo phosphorylation sites in MBP, as the major site phosphorylated by kinase FA /GSK-3, implicating a physiologically relevant role of FA /GSK-3 in the regulation of brain myelin function. By using the same approach, we also identified NIVT94 (p)PR as the phosphorylation site sequence in the second major tryptic phosphopeptide derived from [32 P]MBP phosphorylated by kinase FA /GSK-3, further indicating that kinase FA /GSK-3 represents a Thr-Pro motif-directed MBP kinase involved in the phosphorylation of brain myelin. 相似文献
222.
Odile Ozier-Kalogeropoulos Marie-Thérèse Adeline Weng-Lang Yang George M. Carman François Lacroute 《Molecular & general genetics : MGG》1994,242(4):431-439
In the pyrimidine biosynthetic pathway, CTP synthetase catalyses the conversion of uridine 5-triphosphate (UTP) to cytidine 5-triphosphate (CTP). In the yeast Saccharomyces cerevisiae, the URA7 gene encoding this enzyme was previously shown to be nonessential for cell viability. The present paper describes the selection of synthetic lethal mutants in the CTP biosynthetic pathway that led us to clone a second gene, named URA8, which also encodes a CTP synthetase. Comparison of the predicted amino acid sequences of the products of URA7 and URA8 shows 78% identity. Deletion of the URA8 gene is viable in a haploid strain but simultaneous presence of null alleles both URA7 and URA8 is lethal. Based on the codon bias values for the two genes and the intracellular concentrations of CTP in strains deleted for one of the two genes, relative to the wild-type level, URA7 appears to be the major gene for CTP biosynthesis. Nevertheless, URA8 alone also allows yeast growth, at least under standard laboratory conditions. 相似文献
223.
Novel Methods of Measuring Hydraulic Conductivity of Tree Root Systems and Interpretation Using AMAIZED (A Maize-Root Dynamic Model for Water and Solute Transport) 总被引:3,自引:0,他引:3 下载免费PDF全文
Steady-state and dynamic methods were used to measure the conductivity to water flow in large woody root systems. The methods were destructive in that the root must be excised from the shoot but do not require removal of the root from the soil. The methods involve pushing water from the excised base of the root to the apex, causing flow in a direction opposite to that during normal transpiration. Sample data are given for two tropical (Cecropia obtusifolia and Lacistema aggregatum) and two temperate species (Acer saccharum and Juglans regia cv Lara). A hysteresis was observed in the relationship between applied pressure and resulting flow during dynamic measurements. A mathematical model (AMAIZED) was derived for the dynamics of solute and water flow in roots. The model was used to interpret results obtained from steady-state and dynamic measurements. AMAIZED is mathematically identical with the equations that describe Munch pressure flow of solute and water in the phloem of leaves. Results are discussed in terms of the predictions of AMAIZED, and suggestions for the improvement of methods are made. 相似文献
224.
Summary Production of galacto-oligosaccharide (GO), including trisaccharide and tetrasaccharide, was performed using a -galactosidase in water-hydrophobic solvent mixtures. A maximum GO concentration of 45% (w/w) was attained in a 95% cyclohexane/5% water mixture from a 55% (w/w) of lactose at 60°C and pH 6.0, while a maximum of 38% GO in aqueous media. GO production decreased with an increase in surfactant concentration. The optimum water content for GO production showed a broad range from 2.5 to 10% (v/v). Solvent properties, such as log P and the dipole moment, had no relation to GO production. 相似文献
225.
报道了测定CK-MM亚型的聚焦色谱法,此法简单,快速,结果可靠,线性范围宽,最低检测限(8U/L)较正常参考值低,比国外报道的类似方法高6倍以上,分离度亦有改进.测定了20例健康人血清亚型分布,与文献报道结果相近.该法自动化程度高,已在急性心梗的诊断中实际应用. 相似文献
226.
红毛五加叶的三萜皂甙 总被引:2,自引:0,他引:2
红毛五加(AcanthopanaxgiraldiiHarms)系五加科五加属植物,分布于河北、河南、四川、陕西、甘肃等省,有祛风湿、通关节、强筋骨、治痿痹等功效[1]。其化学成分未见报道。本文报道红毛五加叶的5个三萜皂甙(甙Ⅰ、Ⅱ、Ⅲ、Ⅳ和Ⅴ)的分离鉴定。它们均首次从该属植物发现,其结构如下: R1 R2 Ⅰ rham-(1→2)-ara H Ⅱ H rham-(1→4)-glc-(1→6)-glc … 相似文献
227.
孔药花属(鸭跖草科)的核型研究 总被引:2,自引:0,他引:2
本文首次对鸭跖草科孔药花属Porandra Hong进行染色体研究。孔药花P.ramosa Hong和攀缘孔药花P.scandens Hong在染色体的大小、数目和形态上都十分相似,核型公式为2n=36=4m+26sm+6st(2sat),核型类型属于3B。染色体证据支持孔药花属与穿鞘花属Amischotolype和Coleotrype属相近的观点。 相似文献
228.
刺果番荔枝中的番荔枝内酯 总被引:4,自引:0,他引:4
从刺果番荔枝(Annona m uricata L.)的种子中分离到3 个单四氢呋喃型番荔枝内酯类化合物,用波谱方法鉴定为海南哥纳香甲素(how iicin A, S13)、乙素(how iicin B, S5)和新化合物4-去氧海南哥纳香乙素(4-desoxyhow iicin B, S2)。 相似文献
229.
X-radiography was used to study annual linear skeletal extension rates of the reef-building scleractinian corals Diploria strigosa and Diploria labyrinthiformis from the high-latitude reefs of Bermuda. Coral samples for X-radiography were collected from seven localities of varying biotopes and depths around the Bermuda platform and band couplets were measured. Mean extension rates of both species were highest on inshore and nearshore reefs, gradually decreasing towards the edge of the Bermuda platform and onto the fore-reef slope. Extension rates of D. labyrinthiformis were statistically higher than those of D. strigosa at three localities, while at the other four, the rates of both species were not statistically separable. extension rates of d. labyrinthiformis were statistically higher than D. strigosa within depths of 20 m and 32 m but not statistically separable at 3 m and 6 m depths. Extension rates of both species decreased significantly with increasing depth (r
2=0.92, P<0.03 for D. labyrinthiformis and r
2=0.95, P<0.02 for D. strigosa). Each species showed an inverse curvilinear relationship between extension rate and depth, the rate of change (i.e. slope) being the same for each species. Comparison of extension rates of each species from Berumuda with published rates of these species from lower latitudes showed an inverse relationship between extension rate and latitude. 相似文献
230.
Comparison of models for nucleotide substitution used in maximum- likelihood phylogenetic estimation 总被引:31,自引:15,他引:16
Using real sequence data, we evaluate the adequacy of assumptions made in
evolutionary models of nucleotide substitution and the effects that these
assumptions have on estimation of evolutionary trees. Two aspects of the
assumptions are evaluated. The first concerns the pattern of nucleotide
substitution, including equilibrium base frequencies and the
transition/transversion-rate ratio. The second concerns the variation of
substitution rates over sites. The maximum-likelihood estimate of tree
topology appears quite robust to both these aspects of the assumptions of
the models, but evaluation of the reliability of the estimated tree by
using simpler, less realistic models can be misleading. Branch lengths are
underestimated when simpler models of substitution are used, but the
underestimation caused by ignoring rate variation over nucleotide sites is
much more serious. The goodness of fit of a model is reduced by ignoring
spatial rate variation, but unrealistic assumptions about the pattern of
nucleotide substitution can lead to an extraordinary reduction in the
likelihood. It seems that evolutionary biologists can obtain accurate
estimates of certain evolutionary parameters even with an incorrect
phylogeny, while systematists cannot get the right tree with confidence
even when a realistic, and more complex, model of evolution is assumed.
相似文献