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   1477篇
  免费   134篇
  国内免费   4篇
  1615篇
  2021年   16篇
  2020年   18篇
  2019年   13篇
  2018年   28篇
  2017年   39篇
  2016年   38篇
  2015年   46篇
  2014年   59篇
  2013年   110篇
  2012年   109篇
  2011年   104篇
  2010年   73篇
  2009年   59篇
  2008年   46篇
  2007年   21篇
  2006年   28篇
  2005年   30篇
  2004年   20篇
  2003年   18篇
  2002年   30篇
  2001年   37篇
  2000年   30篇
  1999年   36篇
  1998年   26篇
  1997年   20篇
  1996年   23篇
  1995年   23篇
  1994年   23篇
  1993年   27篇
  1992年   24篇
  1991年   15篇
  1990年   20篇
  1989年   14篇
  1988年   15篇
  1987年   17篇
  1986年   10篇
  1985年   14篇
  1984年   26篇
  1983年   31篇
  1982年   23篇
  1981年   26篇
  1980年   20篇
  1979年   18篇
  1978年   23篇
  1977年   24篇
  1976年   23篇
  1975年   12篇
  1974年   19篇
  1973年   12篇
  1972年   13篇
排序方式: 共有1615条查询结果,搜索用时 15 毫秒
161.
Abstract: This paper reports on the electrophoretic protein/polypeptide pattern of a rat brain primary culture. For comparison, the polypeptide pattern of neuronal and glial enriched fractions from adult rat brain and cerebral hemispheres from newborn and adult rat have been analysed. Water-soluble and SDS-extractable polypeptide fractions appeared and/or increased in amount in the cultures until confluency. The polypeptide pattern of the cultures most resembled that of the glial cell fractions, showing some of this fraction's specificity. Removal of fetal calf serum and addition of 0.1 mM dibutyryl cyclic adenosine monophosphate (dB-cAMP) produced few changes in the electrophoretic pattern. The study thus provides evidence in favour of the astroglial nature of the brain primary culture. It also shows that the cells undergo some maturation in the culture.  相似文献   
162.
To establish indirect in-situ PCR for the detection of intestinal peptide hormones, rat intestine and a murine intestinal tumor cell line, STC 1, were used. The results exhibited intensive staining of GIP-producing K-cells. Paraformaldehyde-fixed cryostat sections yielded the best results in signal to background ratio with RT-PCR in-situ hybridization. Moreover, it was possible to elevate the positive staining signal and to reduce background staining. Digoxigenin-labeled in-situ hybridization served as a control for specificity and sensitivity of GIP (glucose-dependent insulinotropic peptide) mRNA expression on cryostat as well as paraffin sections. In conclusion, this RT-PCR in-situ hybridization protocol proves to be a specific, sensitive and reliable non-radioactive technique for the detection of intestinal peptide hormone mRNA, especially in tissues or tumor cells where the application of ISH is limited.  相似文献   
163.
The “Geita Gold Mine” (GGM) and “North Mara Gold Mines” (NMGM) have developed large quarries in the middle of fertile agricultural lands. Possible hazardous impact on the natural recourse has warranted a study on the trace element concentrations in soils, sediments, and natural waters. Generally, the study shows a great variation in type of elements and intensity of their spill to soils, sediments, and waters. We found indications of acid mine drainage (AMD) of trace metals near the GGM waste rock tailings, but the data are inconclusive. The environmental impact at NMGM was, on the other hand, more extensive. Severe trace element contamination of sediments and waters near one of the mining facilities managed by NMGM is connected to an accidental acid spill in 2009. However, we found strong indication that, in addition to the accident, leakage of alkaline wastewater into surface soils and free water took place from a large wastewater reservoir. We found very high concentrations, especially of As in sediments and water samples downside this reservoir. Water from several sites contained As concentration in the free water at more than one order of magnitude higher than the WHO drinking water recommendations. The chemical speciation of water samples indicated high fraction of Free Ion Activity (FIA) in several samples and the change in pH had a significant impact on the FIA. Due to extensive alkaline and acidic runoff from these sites, more information about hydrological transport routes and the chemical speciation of the free water is of great importance for assessing potential risk of these mines.  相似文献   
164.
The sequence variability of distinct regions of the proviral env gene of human immunodeficiency virus type 2 strain ben (HIV-2ben) isolated sequentially over 3 to 4 years from six experimentally infected macaques was studied. The regions investigated were homologous to the V1, V2, V3, V4, V5, and V7 hypervariable regions identified in the env genes of HIV-1 and simian immunodeficiency virus SIVmac, respectively. In contrast to findings with HIV-1 and SIVmac, the V1- and V2-homologous regions were found to be highly conserved during the course of the HIV-2ben infection in macaques. The V3-homologous region showed a degree of variation comparable to that of HIV-1 but not of SIV. In the V4-, V5-, and V7-homologous regions, mutation hot spots were detected in most reisolates of the infected monkeys. Most of these mutations occurred during the first 10 weeks after infection. After 50 weeks, new mutations were rarely detected. At most mutation sites, a dynamic equilibrium between the mutated viral isotype and the infecting predominant wild type was present. This equilibrium might prevent an accumulation of mutations in isolates later in the course of infection.  相似文献   
165.
166.
Six major basic cytosolic glutathione transferases from rat liver catalyzed the conversion of leukotriene A4 methyl ester to the corresponding leukotriene C4 monomethyl ester. Glutathione transferase 4-4, the most active among these enzymes, had a Vmax of 615 nmol X min-1 X mg protein-1 at 30 degrees C in the presence of 5 mM glutathione. It was followed in efficiency by transferase 3-4 which had a Vmax of 160 nmol X min-1 X mg-1 under the same conditions. Transferases 1-1, 1-2, 2-2 and 3-3 had at least 30 times lower Vmax values than transferase 4-4.  相似文献   
167.
1. A simple method is described for the determination of small amounts of [(35)S]sulphated polysaccharide with 95-100% recovery in the range from 0.3 to 150mug of polysaccharide. 2. The method is based on precipitation with cetylpyridinium chloride of polysaccharide samples applied to filter paper. It is not significantly disturbed by the presence in the sample of a large excess of inorganic (35)SO(4) (2-). 3. Sulphated glucosamino- and galactosaminoglycans may be determined separately by treatment of the sample with chondroitinase ABC. 4. The method is applicable to the assay of [(35)S]sulphated polysaccharide biosynthesis in cell cultures. A stimulation of sulphate incorporation obeying a linear dose-response curve, was demonstrated in somatomedin-incubated fibroblast and glia cell cultures. 5. The described system provides a new assay method for somatomedin. 6. The stimulatory effect of somatomedin on the synthesis of [(35)S]sulphated polysaccharide appeared to be general, rather than specific, for a particular type of polysaccharide.  相似文献   
168.
The qualitative and quantitative composition of sterols in the free form and esterified to fatty acids was studied in seed oils from Brassica napus, B. campestris, B..iuncea, B. nigra, Sinapis alba and S. aruefisis (Brassica kaber). Sitosterol, followed by campesterol, predominated in both the free and the esterified sterols. The free sterols were richer in brassicasterol (ca 10–20%) than the steryl esters (3–10%). Small amounts of δ5-avenasterol and δ7-stigmastenol were also found in the Brassica oils, often more in the esterified than in the free form. The quantity of sterols was studied only in Brassica campestris, which had ca 0.3 % in the free as well as in the esterified form. In Sinapis alba, ca 10% of the sterols in the free form and 20 % in the esterified sterols were δ5-avenasterol. This compared to only a few per cent in both the free and esterified sterols in the Brassica oils. Similarly, ca 2 % of cholesterol was found among the sterols of Sinapis alba but only traces in the Brassica oils. The similarity of sterol compositions among the cultivated brassicas and wild mustard (Sinapis arvensis), and the specific characteristics of the sterols in white mustard (Sinapis alba) adds further weight to the suggestion that wild mustard should be treated as Brassica kaber and strengthens the generic separation of Sinapis alba.  相似文献   
169.
Ni2+-binding staphylococci were generated through surface display of combinatorially engineered variants of a fungal cellulose-binding domain (CBD) from Trichoderma reesei cellulase Cel7A. Novel CBD variants were generated by combinatorial protein engineering through the randomization of 11 amino acid positions, and eight potentially Ni2+-binding CBDs were selected by phage display technology. These new variants were subsequently genetically introduced into chimeric surface proteins for surface display on Staphylococcus carnosus cells. The expressed chimeric proteins were shown to be properly targeted to the cell wall of S. carnosus cells, since full-length proteins could be extracted and affinity purified. Surface accessibility for the chimeric proteins was demonstrated, and furthermore, the engineered CBDs, now devoid of cellulose-binding capacity, were shown to be functional with regard to metal binding, since the recombinant staphylococci had gained Ni2+-binding capacity. Potential environmental applications for such tailor-made metal-binding bacteria as bioadsorbents in biofilters or biosensors are discussed.  相似文献   
170.
The grey wolf (Canis lupus) is an iconic large carnivore that has increasingly been recognized as an apex predator with intrinsic value and a keystone species. However, wolves have also long represented a primary source of human–carnivore conflict, which has led to long‐term persecution of wolves, resulting in a significant decrease in their numbers, genetic diversity and gene flow between populations. For more effective protection and management of wolf populations in Europe, robust scientific evidence is crucial. This review serves as an analytical summary of the main findings from wolf population genetic studies in Europe, covering major studies from the ‘pre‐genomic era’ and the first insights of the ‘genomics era’. We analyse, summarize and discuss findings derived from analyses of three compartments of the mammalian genome with different inheritance modes: maternal (mitochondrial DNA), paternal (Y chromosome) and biparental [autosomal microsatellites and single nucleotide polymorphisms (SNPs)]. To describe large‐scale trends and patterns of genetic variation in European wolf populations, we conducted a meta‐analysis based on the results of previous microsatellite studies and also included new data, covering all 19 European countries for which wolf genetic information is available: Norway, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, Czech Republic, Slovakia, Germany, Belarus, Russia, Italy, Croatia, Bulgaria, Bosnia and Herzegovina, Greece, Spain and Portugal. We compared different indices of genetic diversity in wolf populations and found a significant spatial trend in heterozygosity across Europe from south‐west (lowest genetic diversity) to north‐east (highest). The range of spatial autocorrelation calculated on the basis of three characteristics of genetic diversity was 650?850 km, suggesting that the genetic diversity of a given wolf population can be influenced by populations up to 850 km away. As an important outcome of this synthesis, we discuss the most pressing issues threatening wolf populations in Europe, highlight important gaps in current knowledge, suggest solutions to overcome these limitations, and provide recommendations for science‐based wolf conservation and management at regional and Europe‐wide scales.  相似文献   
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