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   79192篇
  免费   19282篇
  国内免费   4634篇
  2024年   112篇
  2023年   747篇
  2022年   1651篇
  2021年   2981篇
  2020年   3705篇
  2019年   5583篇
  2018年   5535篇
  2017年   5433篇
  2016年   6110篇
  2015年   7066篇
  2014年   7334篇
  2013年   8062篇
  2012年   6572篇
  2011年   5949篇
  2010年   5585篇
  2009年   4163篇
  2008年   3525篇
  2007年   2842篇
  2006年   2495篇
  2005年   2275篇
  2004年   1956篇
  2003年   1746篇
  2002年   1525篇
  2001年   1351篇
  2000年   1134篇
  1999年   1020篇
  1998年   574篇
  1997年   574篇
  1996年   524篇
  1995年   447篇
  1994年   442篇
  1993年   354篇
  1992年   514篇
  1991年   447篇
  1990年   399篇
  1989年   294篇
  1988年   272篇
  1987年   235篇
  1986年   181篇
  1985年   211篇
  1984年   142篇
  1983年   129篇
  1982年   97篇
  1981年   74篇
  1980年   64篇
  1979年   98篇
  1978年   82篇
  1977年   58篇
  1976年   63篇
  1973年   57篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
The differentiation of mammalian urothelium culminates in the formation of asymmetrical unit membrane (AUM). Using gradient centrifugation and detergent wash, we purified milligram quantities of AUMs which, interestingly, contained three major proteins (15, 27, and 47 kDa) that appeared to be identical to the three immunoaffinity purified, putatively AUM-associated proteins that we described earlier (Yu, J., Manabe, M., Wu, X.-R., Xu, C., Surya, B., and Sun, T.-T. (1990) J. Cell Biol., 111, 1207-1216). Peptide mapping and immunoblotting established that these three proteins were distinct molecules. Using monospecific antibodies to these three proteins, we showed that they were all restricted to the superficial urothelial cells and were AUM-associated. The 27- and 15-kDa proteins were detected exclusively on the luminal side of mature, apical AUMs. In contrast, epitopes of the 47-kDa protein were detected on both sides of apical AUMs suggesting a transmembranous configuration. These results (i) provide the strongest evidence thus far that AUM contains three major proteins (the 27-kDa uroplakin I, 15-kDa uroplakin II, and 47-kDa uroplakin III) which form an extremely insoluble complex, (ii) suggest that uroplakin II, like uroplakin I (Yu, J., Manabe, M., Wu, X.-R., Xu, C., Surya, B., and Sun, T.-T. (1990) J. Cell. Biol. 111, 1207-1216), translocates from one side of the membrane to another during AUM maturation, (iii) indicate that uroplakin III may play a different structural role than uroplakins I and II in AUM formation, and (iv) establish the three uroplakins as markers for an advanced stage of urothelial differentiation.  相似文献   
902.
Bacillopeptidase F is an extracellular serine protease that is expressed at the beginning of the stationary phase. To study its structure, regulation of expression, and physiological roles, we have cloned and characterized the structural gene (bpf) encoding this protease from Bacillus subtilis. DNA sequence analysis suggests this protease is synthesized as a preproenzyme (Mr = 92,000). Through processing at both the NH2 and COOH termini, it is gradually converted into various forms with molecular mass ranging from 80 to 48 kDa. Shortening the 3' end of bpf demonstrates that at least 290 amino acid residues from the COOH-terminus of bacillopeptidase F are not required for either catalytic activity or secretion. Bacillopeptidase F exhibits sequence similarity with several serine proteases. Its gene is found immediately downstream from the fts operon which was mapped at 135 degrees on the B. subtilis genetic linkage map. Inactivation of the chromosomal copy of bpf shows no effect on cell growth and sporulation. A triple protease-deficient strain (WB300 with the structural genes for bacillopeptidase F and two other major proteases inactivated) was constructed to serve as a better expression host for the production and secretion of foreign proteins.  相似文献   
903.
Cytostatic and growth-stimulating effects of alveolar macrophages (AM) of rats on tumor cells were studied. The experimental results are summarized as follows. 1. The cytotoxicity of AM activated with BCG to tumor cells was increasing with the increase of effector cells/target cells (E/T) ratio. AM without the treatment with BCG expressed slight cytotoxicity to tumor cells at a high E/T, and growth-stimulating effect on tumor cells, at a low E/T. 2. AM after 24-hour culture had a lower manifestation of cytotoxicity to human lung adenocarcinoma cell line than that of AM without 24-hour culture, and had a growth-stimulating effect on B-16 cell line. 3. Cytostatic and growth-stimulating effects of AM without or with 24-hour culture were decreasing with the increase of irradiation doses.  相似文献   
904.
The dot-blots containing DNA isolated from nonmycobacterial and mycobacterial microorganisms were hybridized with 32P-labeled M. tuberculosis whole chromosomal DNA at the various temperatures. The probe did not cross-hybridize to DNA of nonmycobacterial microorganisms (E. coli, Plasmid pUC19, Nocardia asteriodes), nor with DNA from all mycobacteria tested except M. bovis BCG under the higher temperature conditions. Microorganisms could also be directly spotted and lysed on nitrocellulose filters and used for hybridization thus making this technique suitable for clinical diagnosis.  相似文献   
905.
Multiple endocrine neoplasia type 2A (MEN2A) is a rare cancer syndrome that is inherited in an apparently autosomal dominant fashion. Previous linkage studies had assigned the MEN2A locus to chromosome 10 in the pericentromeric region. We recently have described several new easily scorable RFLPs for the chromosome 10-specific alpha satellite DNA (the D10Z1) locus that is known, on the basis of previous in situ hybridization experiments, to lie at the centromere. We report here tight linkage between MEN2A and D10Z1, as demonstrated by a maximum lod score of 12.02 at the recombination frequency of zero (1-lod-unit support interval 0-4 cM), indicating that the genetic defect in MEN2A lies in the immediate vicinity of the centromere. By means of a set of ordered polymorphic DNA markers from the pericentromeric region, multipoint as well as pairwise linkage analyses place the MEN2A locus at the middle of a small region (approximately 11 cM) bracketing the centromere with FNRB (at 10p11.2) and RBP3 (at 10q11.2) on either side, providing further support for the centromeric location of the MEN2A locus. Marked sex difference in recombination frequencies exists in this pericentromeric region: significantly (P less than .01) more female than male crossovers were observed across all of the adjacent intervals D10S24-FNRB, FNRB-D10Z1, and D10Z1-RBP3. However, a sex difference was not seen in the 7-cM interval from RBP3 to D10S5, suggesting that large variation in the sex difference in recombination can occur over small chromosomal regions.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
906.
1. Pig alpha-fetoprotein (AFP) and albumin were isolated from fetal serum by DEAE-Sephadex ion exchange chromatography combined with Cibacron Blue-Sepharose and trypsin-Sepharose adsorptions. 2. AFP, fetal albumin and adult albumin carried 2.6, 2.4, and 1.9 moles of fatty acids per mole of protein, respectively. 3. Most of fatty acids bound to AFP were polyunsaturated: mainly arachidonic (20:4, n-6) and docosahexaenoic (22:6, n-3) acids, which accounted respectively for 21.7 and 18.8% of the total fatty acids. 4. By contrast, the fatty acids found in the albumins (fetal and adult) were preferentially saturated and monounsaturated. 5. Arachidonic acid was a minor component in both albumins, and no docosahexaenoic acid was detected.  相似文献   
907.
Microbial transformations of 7,2-dimethylbenz[a]anthracene.   总被引:3,自引:2,他引:1       下载免费PDF全文
Microbial transformations of 7,12-dimethylbenz[a]anthracene, a carcinogenic polycyclic aromatic hydrocarbon, in cultures of Pseudomonas aeruginosa and Penicillium notatum were studied by high performance liquid chromatographic separation of metabolic fractions followed by gas chromatographic-mass spectrometric analysis of the metabolites. Two methyl-hydroxylated metabolites were identified in each of the incubations. The metabolic activation of the polycyclic aromatic hydrocarbon suggests a possible involvement of microorganisms in environmental carcinogenesis.  相似文献   
908.
Summary Cultured human pancreatic carcinoma cells (MIA PaCa-2) have been shown previously to be very sensitive toE. coli l-asparaginase (EC II). The present studies have demonstrated that another enzyme,Acinetobacter glutaminase-asparaginase (AGA) is much more effective in inhibiting cell growth. At the concentration of 0.0025 U/ml of AGA activity the enzyme totally inhibited cell growth, whereas the EC II with the same concentration did not show any effect. The inhibition of cell growth correlated well with inhibition of protein and glycoprotein synthesis. The addition ofl-glutamine at the concentration of 1 mM completely reversed the inhibition of protein synthesis. Similarly, the addition ofl-glutamine at the concentration of 3 mM daily on 3 successive days after adding AGA resulted in significant reversal of growth inhibition. The results of this study indicate that the action of AGA on MIA PaCa-2 is, to a great extent, exerted through itsl-glutaminase activity. This work was supported in part by USPHS Grant CA 19182. Dr. Wu is recipient of Research Career Development Award Grant CA00686 and Dr. Yunis is a Howard Hughes Investigator.  相似文献   
909.
1. The levels of AFP and albumin in rat brain at birth were 77 and 340 micrograms per g of tissue, respectively. These levels quickly dropped with age. AFP was undetectable in 20 days brain extracts whereas 30 micrograms of albumin per g of brain were still measured. 2. AFP from brain and serum were identical by immunodiffusion, electrophoresis and immuno-affinoelectrophoresis with free Concanavalin A. 3. No in vitro synthesis of AFP and albumin in the postnatal brain was observed. However, the total amount of AFP in the developing rat brain increases from birth to 4-5 days post-partum.  相似文献   
910.
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