首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3489篇
  免费   316篇
  国内免费   9篇
  3814篇
  2022年   46篇
  2021年   79篇
  2020年   31篇
  2019年   46篇
  2018年   65篇
  2017年   51篇
  2016年   87篇
  2015年   182篇
  2014年   202篇
  2013年   209篇
  2012年   254篇
  2011年   266篇
  2010年   162篇
  2009年   122篇
  2008年   170篇
  2007年   185篇
  2006年   155篇
  2005年   163篇
  2004年   151篇
  2003年   136篇
  2002年   132篇
  2001年   109篇
  2000年   104篇
  1999年   71篇
  1998年   30篇
  1997年   25篇
  1996年   19篇
  1995年   12篇
  1994年   21篇
  1993年   16篇
  1992年   42篇
  1991年   45篇
  1990年   29篇
  1989年   44篇
  1988年   17篇
  1987年   33篇
  1986年   31篇
  1985年   25篇
  1984年   24篇
  1983年   19篇
  1982年   22篇
  1981年   16篇
  1980年   14篇
  1979年   17篇
  1978年   21篇
  1977年   12篇
  1976年   14篇
  1974年   13篇
  1973年   11篇
  1972年   11篇
排序方式: 共有3814条查询结果,搜索用时 15 毫秒
31.
The distribution of Tn5271-related DNA sequences in samples of groundwater and a groundwater bioremediation system at the Hyde Park (Niagara Falls, N.Y.) chemical landfill site was investigated. PCR amplification of target sequences within the cha genes of Tn5271 revealed similar sequences in the groundwater community and in samples from the sequencing batch reactors treating that groundwater. Cell dilution combined with PCR amplification indicated that cha sequences were carried in about 1 of 10 culturable bacteria from the treatment system. Characterization of isolates involved in chlorobenzoate and toluene biodegradation in the treatment system indicated that two phenotypic clusters, Alcaligenes faecalis type 2 and CDC group IVC-2, contained all of the Tn5271 probe-positive isolates from the community. These two groups differed phenotypically from recipient groups isolated following horizontal transfer of pBRC60 (Tn5271) in pristine freshwater microcosms. A genetic rearrangement in Tn5271 attributable to the intramolecular transposition of the flanking element IS1071R was detected in an isolate from the treatment system. Comparison of the structure of the intramolecular transposition derivative from groundwater isolate OCC13(pBRC13) with a laboratory-derived intramolecular transposition derivative of pBRC60 revealed similarities. The rearrangement was shown to increase the stability of the plasmid under starvation conditions.  相似文献   
32.
One of the most extreme freshwater habitats in Peninsular Malaysia is the peat swamp forest, with dark-coloured and highly acidic waters. Surprisingly, little is known about blackwater fishes in Peninsular Malaysia. Until 1968, only 26 fish species were known from blackwaters throughout Peninsular Malaysia, of which only one can be regarded as stenotopic. A recent intensive survey of part of the North Selangor peat swamp forest yielded 47 species, of which 14 are probably stenotopic taxa. These include four undescribed species and several new records for western Peninsular Malaysia. These discoveries are significant in that they include the family Chaudhuriidae which until 1985, was not reported from Sundaic Southeast Asia, and the rare genus Encheloclarias which had not been encountered for over 50 years. The rapid rate of destruction of the peat swamp forest owing to development, forestry and agricultural activities must be halted or slowed significantly to enable the proper zoological surveys and studies to be conducted. Conservation plans and environmental impact assessments based on inadequate sampling and knowledge of species present is acutely dangerous. There are no longer substantial undisturbed blackwater peat swamp forests left in most of Peninsular Malaysia. Conservation of the remaining blackwater biotopes is critically important if extinction of many species, here regarded as economically valuable renewable resources, is to be prevented.  相似文献   
33.
Seven species of snakeheads (Channidae) are known from Peninsular Malaysia and Singapore, viz. Channa bankanensis, C. gachua, C. lucius, C. marulioides, C. melasoma, C. micropeltes and C. striata. Up-to-date distribution maps of each species are presented, including new records. Their systematics is reviewed and partially revised. The taxonomic status of C. marulioides and C. melanoptera is clarified. Specimens from Peninsular Malaysia identified as C. melanoptera sensu Weber & de Beaufort, 1922, proved to be the adult form of C. marulioides s.str. The real C. melanoptera appears to be restricted to Borneo and possibly Sumatra. The life history of a blackwater species C. bankanensis is also documented, with regards to the morphological and colour-pattern changes associated with growth. An updated key to the seven species based on morphometric measurements and meristic counts is presented.  相似文献   
34.
Abstract: We have expressed and biochemically characterized the human D2long (D2L) dopamine receptor isoform using the baculovirus/Sf9 cell system. The expressed receptor bound ligands with a pharmacological profile similar to that reported for neuronal and cloned D2L receptors expressed in mammalian cell lines. Dopamine binding to D2L receptor was sensitive to guanine nucleotides, indicating receptor coupling to endogenous G proteins. A D2L receptor-specific antibody identified two major protein species at ∼44 kDa and at ∼93 kDa in immunoblots, suggesting the presence of D2L receptor monomers and dimers. Both species were purified by immunoprecipitation from digitonin-solubilized preparation of cells expressing D2L receptor prelabeled with 32Pi or [3H]-palmitate. These results constitute the first direct evidence for D2L receptor phosphorylation and palmitoylation.  相似文献   
35.
36.
37.
38.
Ectopic overexpression of v-H-Ras protein in NIH 3T3 cells resulted in cellular transformation and an acceleration of G1 progression of these cells. A shortened G1 phase was found to be associated with an increased level of cyclin D1 but not cyclin E protein. Using an antisense blocking method, reduced synthesis of cyclin D1 in v-H-Ras transformants resulted in a slower G1 progression rate of these cells. Although constitutive overexpression of cyclin D1 in NIH 3T3 cells accelerated G1 progression, cells remained untransformed. Furthermore, inhibition of cyclin D1 synthesis greatly impaired the soft-agar cloning efficiency of v-H-Ras transformants. These results suggest that increased expression of cyclin D1 is necessary but not sufficient for the transforming activity of v-H-Ras. Similar effect on cell cycle progression was also observed in Raf-transformed cells. In addition to cyclin D1, cyclin E protein was found to be elevated in Src transformants. This may account for the further shortening of the G1 phase of these cells. Activation of an additional Ras-independent pathway was suggested to be responsible for the further acceleration of the G1 phase in Src transformants.  相似文献   
39.
Kent-Braun, J. A., A. V. Ng, M. Castro, M. W. Weiner, D. Gelinas, G. A. Dudley, and R. G. Miller. Strength, skeletal musclecomposition and enzyme activity in multiple sclerosis. J. Appl. Physiol. 83(6):1998-2004, 1997.This study examined functional, biochemical, andmorphological characteristics of skeletal muscle in nine multiplesclerosis (MS) patients and eight healthy controls in an effort toascertain whether intramuscular adaptations could account for excessivefatigue in this disease. Analyses of biopsies of the tibialis anteriormuscle showed that there were fewer type I fibers (66 ± 6 vs. 76 ± 6%), and that fibers of all types were smaller (average26%) and had lower succinic dehydrogenase (SDH; average40%) and SDH/-glycerol-phosphate dehydrogenase (GPDH) butnot GPDH activities in MS vs. control subjects, suggesting that musclein this disease is smaller and relies more on anaerobic thanaerobic-oxidative energy supply than does muscle of healthyindividuals. Maximal voluntary isometric force fordorsiflexion was associated with both average fiber cross-sectionalarea (r = 0.71, P = 0.005) and muscle fat-free cross-sectional area by magnetic resonance imaging(r = 0.80, P < 0.001). Physical activity,assessed by accelerometer, was associated with average fiber SDH/GPDH(r = 0.78, P = 0.008). There was a tendency forsymptomatic fatigue to be inversely associated with average fiber SDHactivity (r = 0.57,P = 0.068). The results of thisstudy suggest that the inherent characteristics of skeletal musclefibers per se and of skeletal muscle as a whole are altered in thedirection of disuse in MS. They also suggest that changes in skeletalmuscle in MS may significantly affect function.

  相似文献   
40.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号