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   303589篇
  免费   26240篇
  国内免费   258篇
  2021年   2644篇
  2018年   4461篇
  2017年   4289篇
  2016年   5658篇
  2015年   6578篇
  2014年   7489篇
  2013年   10473篇
  2012年   11900篇
  2011年   12102篇
  2010年   8018篇
  2009年   6824篇
  2008年   10139篇
  2007年   10131篇
  2006年   9606篇
  2005年   8867篇
  2004年   8702篇
  2003年   8272篇
  2002年   7978篇
  2001年   13291篇
  2000年   13231篇
  1999年   10121篇
  1998年   3097篇
  1997年   3066篇
  1996年   3020篇
  1995年   2787篇
  1994年   2663篇
  1993年   2536篇
  1992年   7671篇
  1991年   7408篇
  1990年   7433篇
  1989年   7214篇
  1988年   6725篇
  1987年   6337篇
  1986年   5670篇
  1985年   5975篇
  1984年   4789篇
  1983年   4080篇
  1982年   2878篇
  1981年   2639篇
  1979年   4307篇
  1978年   3319篇
  1977年   3018篇
  1976年   2931篇
  1975年   3447篇
  1974年   3697篇
  1973年   3734篇
  1972年   3236篇
  1971年   2993篇
  1970年   2688篇
  1969年   2465篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
Cyclic nucleotide phosphodiesterase from wheat sprouts was isolated and partially purified. The molecular weight of the enzyme is about 83 000. The enzyme activity sharply rises as the inhibiting factors present in the homogenate are separated. The pH optimum of the enzymatic reaction is 4,8. Divalent cations (Mg2+, Mn2+, Cu2+) within the concentration range of 1--5 mM and complexons (EDTA, EGTA) at the concentration of 1 mM do not affect the PDE activity. The temperature optimum for the reaction is 60 degrees. The enzyme hydrolyzes 3' : 5'-AMP, 3' : 5'-GMP and 2':3'-AMP. The Km value for cAMP is 4 . 10(-3) M. The enzyme activity is inhibited by chemical agents possessing the fungicide activity, the strongest effect being exerted by anylate.  相似文献   
123.
A method for enzyme immunoassay of thyroid-stimulating hormone (TSH) is described, TSH was conjugated with horseradish peroxidase according to periodate oxidation method. Separation of the bound and free was obtained by double-antibody solid-phase technique using Sepharose 4B-anti-rabbit immunogiobulin G (IgG)-geat IgG. The fluorescence reaction using tyramine and hydrogen peroxide as substrates was used for the determination of enzyme activity in order to increase the sensitivity of enzyme immunoassay. The standard curve for serum TSH was satisfactory to recognize TSH concentrations as 0.06 μU/tube. TSH values obtained by this method correlated well with those obtained by radioimmunoassay (r, 0.96). The coefficients of variation were 1.8 to 5.3% (within assay) and 5.1 to 10.5% (between assay). The method is about equal to radioimmunoassay with respect to sensitivity. Since it requires minimal equipment and is less expensive than radioimmunoassay, it is possible to perform routine assays even in laboratories with limited facilities.  相似文献   
124.
125.
1. A phylogenetic study of oxytocin (OXT)-like immunoreactive cells was performed by the PAP method in the central nervous system of invertebrates. 2. The immunoreactivity was detected in the nerve cells of Hydra magnipapillata of the Coelenterata; Neanthes japonica and Pheretima communissima of the Annelida; Oncidium verrucosum, Limax marginatus and Meretrix lamarckii of the Mollusca; and Baratha brassica of the Arthropoda. 3. No immunoreactive cells were found in Bipalium sp. of the Platyhelminthes; Pomacea canaliculata, Aplysia kurodai, Bradybaena similaris and Achatina fulica of the Mollusca; and Gnorimosphaeroma rayi, Procambarus clarkii, Hemigrapsus sanguineus, Helice tridens and Gryllus bimaculatus of the Arthropoda; Asterina pectinifera of the Echinodermata; and Halocynthia roretzi of the Protochordata. 4. These results demonstrate that an OXT-immunoreactive substance is widely present not only in vertebrates but also in invertebrates. 5. OXT seems to have been introduced into these invertebrates at an early stage of their phylogenetic history.  相似文献   
126.
The true slime mould Physarum polycephalum was treated with various agents by spraying them upon the cell surface 4 hrs before the second synchronous mitosis. The onset of mitosis was considerably approximated after the plasmodium treatment with protein kinases from rat hepatoma or Ph. polycephalum at the late G2 phase. The catalytic and regulatory subunits of cAMP-dependent pig brain protein kinase caused retardation of mitosis, while the holoenzyme, casein kinase and alkaline phosphatase did not affect the timing of mitosis. The cyclic nucleotides and inhibitors of their metabolic enzymes were used to investigate the role of phosphorylation processes in the mitotic cycle.  相似文献   
127.
Summary Protoplasts isolated from mesophyll tissues of cauliflower (Brassica oleraceavar. botrytis) were induced to divide in culture, with 2% of them producing calli. Upon transfer to a regeneration medium containing a low auxin/cytokinin balance (0.02mg/l 1-naphthaleneacetic and 2mg/l 6-benzylaminopurine), they displayed an extensive production of hairy roots before the regeneration of shoots. Negative effects of root differentiation on the subsequent caulogenesis by such calli were not observed, since 97% of the calli regenerated hairy roots and 93% gave shoots.Abbreviations NAA 1-naphthaleneacetic acid - BAP 6-benzylaminopurine - 2,4 D 2,4-dichlorophenoxyacetic acid - GA3 gibberellic acid - MES 2-N-morpholinoethane sulfonate  相似文献   
128.
129.
Production of Enterobifidin includes the stages of preparation of culture media, reparation of lyophilizedBifidobacterium adolescentis MS-42 culture, preparation of starters, cultivation of bacteria in fermenters, biomass conservation, and its biological control. The preparation contains physiologically active bifidobacterium cells with high activities of growth = 0.7 h−1,g = 1.0 h) and acid formation (titratable acidity is ∼120–140°T; acetate concentration, 0.50–0.75%; and lactate concentration, 0.33–0.50%). The antagonistic activity of these bacteria towardsEscherichia coli 08,E. coli 086,E. coli 015,E. coli 0115, andE. coli 0101 amounts to 98.2; toProteus vulgaris 102, to 87.2; andStaphylococcus aureus 209p, to 83.2%. The bifidobacteria (with a titer of ∼109 CFU/ml) remained viable for two to five months.  相似文献   
130.
The gene of Cytotoxic T Lymphocyte-associated Antigen 4 (CTLA4), a negative regulator of T lymphocytes, contains a single-nucleotide polymorphism (SNP) at position +6230A->G (ct60A->G), which has been found associated with several autoimmune diseases and appears to reduce T-cell inhibitory activity. In Ghana, West Africa, we compared the frequencies of CTLA4 +6230 A/G and 6 haplotype-tagging SNPs in 2010 smear-positive, HIV-negative patients with pulmonary tuberculosis (TB) and 2346 controls matched for age, gender and ethnicity. We found no difference in allele frequencies between cases and controls. However, +6230A and a distinct CTLA4 haplotype and a diplotype comprising the +6230A allele were significantly less frequent among cases with large opacities in chest radiographs compared to those with small ones (Pcorrected [cor] = 0.002, Pcor = 0.00045, P = 0.0005, respectively). This finding suggests that an increased T-cell activity associated with the CTLA4 +6230G allele contributes to pathology rather than to protection in pulmonary TB.  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