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   5889篇
  免费   453篇
  国内免费   2篇
  6344篇
  2023年   17篇
  2022年   39篇
  2021年   71篇
  2020年   51篇
  2019年   51篇
  2018年   80篇
  2017年   70篇
  2016年   118篇
  2015年   189篇
  2014年   265篇
  2013年   350篇
  2012年   352篇
  2011年   388篇
  2010年   264篇
  2009年   255篇
  2008年   344篇
  2007年   366篇
  2006年   345篇
  2005年   353篇
  2004年   310篇
  2003年   292篇
  2002年   311篇
  2001年   71篇
  2000年   62篇
  1999年   82篇
  1998年   110篇
  1997年   87篇
  1996年   76篇
  1995年   60篇
  1994年   67篇
  1993年   66篇
  1992年   50篇
  1991年   57篇
  1990年   40篇
  1989年   59篇
  1988年   45篇
  1987年   31篇
  1986年   42篇
  1985年   34篇
  1984年   32篇
  1983年   24篇
  1982年   53篇
  1981年   34篇
  1980年   33篇
  1979年   23篇
  1978年   16篇
  1977年   23篇
  1976年   16篇
  1975年   21篇
  1974年   15篇
排序方式: 共有6344条查询结果,搜索用时 0 毫秒
81.
Mitochondria are indispensable for cell viability; however, major mitochondrial functions including citric acid cycle and oxidative phosphorylation are dispensable. Most known essential mitochondrial proteins are involved in preprotein import and assembly, while the only known essential biosynthetic process performed by mitochondria is the biogenesis of iron-sulfur clusters (ISC). The components of the mitochondrial ISC-assembly machinery are derived from the prokaryotic ISC-assembly machinery. We have identified an essential mitochondrial matrix protein, Isd11 (YER048w-a), that is found in eukaryotes only. Isd11 is required for biogenesis of cellular Fe/S proteins and thus is a novel subunit of the mitochondrial ISC-assembly machinery. It forms a complex with the cysteine desulfurase Nfs1 and is required for formation of an Fe/S cluster on the Isu scaffold proteins. We conclude that Isd11 is an indispensable eukaryotic component of the mitochondrial machinery for biogenesis of Fe/S proteins.  相似文献   
82.
There is some evidence that insulin-like growth factor 2 (IGF-2) may intervene in the control of T cell differentiation. To further study the immunoregulatory function of this growth factor, we analyzed the immune system of Igf2-/- mice. Phenotypically, some immunological parameters such as lymphoid organ morphology and cellularity were unaltered in Igf2-/- mice, but an increase of CD8+ cells and a decrease of B220+ cells were observed in spleen. In vitro, the development of bone marrow-derived dendritic cells was affected by the absence of Igf2 expression. After maturation, a higher percentage of immature dendritic cells was observed in Igf2-/- population, together with a secondary decrease in allogenic T cell proliferation. Activation of T cells was also affected by the lack of expression of this growth factor. The profile of B cell response in mutant mice immunized with IGF-2 evidenced a T-dependent profile of anti-IGF-2 Abs that was absent in Igf2+/+ mice. The influence of IGF-2 upon tolerance to insulin was also assessed in this model, and this showed that IGF-2 also intervenes in tolerance to insulin. The presence of a T-dependent response in Igf2-deficient mice should allow cloning of specific "forbidden" T CD4+ lymphocytes directed against IGF-2, as well as further investigation of their possible pathogenic properties against insulin family.  相似文献   
83.
Synopsis Young (7–10 days after hatching) brown trout (Salmo trutta) exposed for 5 days to pH 5 in high calcium water and at 2 temperatures (12°, 4°C) in the laboratory displayed no alterations in growth or in mucous cell concentration and volume, compared to the control group kept at pH 7.2. Contamination of acid-stressed young with 230 μg All-1 resulted in significant growth depression and Al accumulation, but in no changes of mucous cell morphometrics. Field tests in low calcium water produced high mortality at low pH (5.1), but showed consistent effects on mucous cells as in laboratory experiments. Three-month-old juveniles of brown trout, subjected to decreased pH values at 12° and in high calcium water for 8 days exhibited mucous cell hyperplasia (without hypertrophy) within 3 h of the acid addition. After 120 h sloughing of the integument occurred with full recovery not possible within a 4-day-recovery period. Although the results presently apply only to hard water conditions, the differences between juveniles and recently hatched young in tolerance to pH- and Al-mediated stress may also be of importance for soft waters affected by acid rain.  相似文献   
84.
85.
Modification interference is a powerful method to identify important functional groups in RNA molecules. We review here recent developments of techniques to screen for chemical modifications that interfere with (i) binding of(pre-)tRNA to bacterial RNase P RNA or (ii) pre-tRNA cleavage by this ribozyme. For example, two studies have analyzed positions at which a substitution of sulfur for thepro-Rp oxygen affects tRNA binding [1] or catalysis [2]. The results emphasize the functional key role of a central core element present in all known RNase P RNA subunits. The four sulfur substitutions identified in one study [2] to inhibit the catalytic step also interfered with binding of tRNA toE. coli RNase P RNA [1]. This suggests that losses in binding energy due to the modification at these positions affect the enzyme-substrate and the enzyme-transition state complex. In addition, the two studies have revealed, for the first time, sites of direct metal ion coordination in RNase P RNA. The potentials, limitations and interpretational ambiguities of modification interference experiments as well as factors influencing their outcome are discussed.Abbreviations nt nucleotide(s) - PAGE polyacrylamide gel electrophoresis  相似文献   
86.
Deep-sea hydrothermal vents are inhabited by complex communities of microbes and their viruses. Despite the importance of viruses in controlling the diversity, adaptation and evolution of their microbial hosts, to date, only eight bacterial and two archaeal viruses isolated from abyssal ecosystems have been described. Thus, our efforts focused on gaining new insights into viruses associated with deep-sea autotrophic archaea. Here, we provide the first evidence of an infection of hyperthermophilic methanogenic archaea by a head-tailed virus, Methanocaldococcus fervens tailed virus 1 (MFTV1). MFTV1 has an isometric head of 50 nm in diameter and a 150 nm-long non-contractile tail. Virions are released continuously without causing a sudden drop in host growth. MFTV1 infects Methanocaldococcus species and is the first hyperthermophilic head-tailed virus described thus far. The viral genome is a double-stranded linear DNA of 31 kb. Interestingly, our results suggest potential strategies adopted by the plasmid pMEFER01, carried by M. fervens, to spread horizontally in hyperthermophilic methanogens. The data presented here open a new window of understanding on how the abyssal mobilome interacts with hyperthermophilic marine archaea.  相似文献   
87.
Aim  Niche theory emphasizes the importance of environmental conditions for the distribution and abundance of species. Using a macroecological approach our study aimed at identifying the important environmental gradients for spiders. We generated numerical values of niche position and niche width. We also investigated relationships between these niche properties as well as the degree of phylogenetic conservatism in order to draw conclusions about the evolution of the habitat niche.
Location  Central Europe: lowlands of Austria, Belgium, Germany, Luxembourg, the Netherlands and Switzerland.
Methods  We analysed 244 published spider communities from 70 habitat types by correspondence analysis. The resulting community scores were used to test for correlations with habitat characteristics. Species scores were used to derive niche position (mean scores) and niche width (standard deviation of scores). To test for niche conservatism we estimated variance components across the taxonomic hierarchy.
Results  The first two axes of the correspondence analysis were correlated with shading and moisture, respectively. Niche width had a hump-shaped relationship to both environmental gradients. β-diversity was strikingly higher in open habitats than in forests. Habitat niche conservatism was lower than phylogenetic conservatism in body size.
Main conclusions  Environmental factors are important drivers for the β-diversity of spiders, especially across open habitats. This underlines the importance of preserving the whole range of moisture conditions in open habitats. Narrow niches of species occurring at the ends of both environmental gradients indicate that adaptations to extreme habitats lead to constraints in ecological flexibility. Nevertheless, the habitat niche of species seems to evolve much faster than morphological or physiological traits.  相似文献   
88.
Previous work has shown that translation of the encephalomyocarditis (EMC) viral ribonucleic acid (RNA) generates at least three primary products, polypeptides A, F, and C. The A and C polypeptides then undergo post-translational cleavages to complete the production of the stable viral polypeptides (delta, beta, gamma, alpha, G, I, F, H, and E). In this communication we show that A, F, and C are produced in equimolar amounts giving further support to the theory that the RNA of picornaviruses has only a single site for the initiation of protein synthesis. The biosynthesis of viral proteins in EMC virus-infected HeLa cells was studied in the presence of pactamycin at concentrations which preferentially inhibit the initiation of protein synthesis. The amount of each polypeptide formed during the residual period of protein synthesis observed after the addition of pactamycin was used as a criterion for ordering the genes on the viral RNA. The results obtained indicate that the primary gene products are ordered on the EMC viral RNA 5' --> 3' A-F-C and that the stable products are ordered delta-beta-gamma-alpha-G-I-F-H-E. Moreover, the intermediate chains B and epsilon map in the capsid region, whereas the intermediate chain D maps in the E region. This order is largely consistent with previously established relationships of the viral polypeptides and thus indicates that pactamycin is a valid tool for "genetic" mapping of polycistronic RNA molecules with single initiation sites.  相似文献   
89.
90.
Zusammenfassung Im Bereich des Meßtischblattes Bad Wildungen (MTB 4820; grundlegende DatenMann 1983,Lübcke &Mann 1984) bevorzugt der Neuntöter Viehweiden (Abb. 3). Reviere an Viehweiden zeigen eine längere kontinuierliche Besetzungszeit (Abb. 4) im Vergleich zu anderen Nutzungsformen. Die Anzahl flügger Jungvögel ist in Viehweiderevieren größer im Gegensatz zu Revieren an Mähwiesen oder Brachflächen (Abb. 5). Dieser Unterschied des Reproduktionserfolges liegt nicht an unterschiedlichen Nahrungsdichten (Abb. 6), sondern an der besseren Zugänglichkeit.
Habitat selection in the Red-backed ShrikeLanius collurio
Summary In the area of Bad Wildungen (FRG; Hessia; MTB 4820; basic data inMann 1983,Lübcke &Mann 1984) the Red-backed Shrike selects for pastures (Abb. 3). Territories at pastures show a longer continual occupation in comparison to other land use systems (Abb. 4). The number of fledgelings is higher at pastures in comparison to meadows or fallow land (Abb. 5). This difference is not caused by higher insect densities (Abb. 6) but by better accessible prey due to permanent low vegetation at pastures.


Reihenfolge alphabetisch und umgekehrt proportional zur geleisteten Arbeit: R. B. Auswertung, W. L. & W. M. Kartierung, W. M. Brut- und Nahrungsbiologie. Sonderdruckanforderungen an R. B.  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