全文获取类型
收费全文 | 14625篇 |
免费 | 1059篇 |
国内免费 | 108篇 |
专业分类
15792篇 |
出版年
2022年 | 165篇 |
2021年 | 314篇 |
2020年 | 179篇 |
2019年 | 221篇 |
2018年 | 273篇 |
2017年 | 232篇 |
2016年 | 363篇 |
2015年 | 538篇 |
2014年 | 580篇 |
2013年 | 959篇 |
2012年 | 1005篇 |
2011年 | 931篇 |
2010年 | 588篇 |
2009年 | 499篇 |
2008年 | 704篇 |
2007年 | 718篇 |
2006年 | 673篇 |
2005年 | 549篇 |
2004年 | 519篇 |
2003年 | 505篇 |
2002年 | 494篇 |
2001年 | 243篇 |
2000年 | 234篇 |
1999年 | 225篇 |
1998年 | 137篇 |
1997年 | 134篇 |
1996年 | 125篇 |
1995年 | 105篇 |
1994年 | 114篇 |
1993年 | 114篇 |
1992年 | 205篇 |
1991年 | 180篇 |
1990年 | 181篇 |
1989年 | 158篇 |
1988年 | 138篇 |
1987年 | 160篇 |
1986年 | 135篇 |
1985年 | 165篇 |
1984年 | 165篇 |
1983年 | 160篇 |
1982年 | 128篇 |
1981年 | 109篇 |
1979年 | 142篇 |
1978年 | 108篇 |
1977年 | 108篇 |
1976年 | 82篇 |
1975年 | 95篇 |
1974年 | 98篇 |
1973年 | 92篇 |
1972年 | 102篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The known nucleoside triphosphate-dependent restriction enzymes are hetero-oligomeric proteins that behave as molecular machines in response to their target sequences. They translocate DNA in a process dependent on the hydrolysis of a nucleoside triphosphate. For the ATP-dependent type I and type III restriction and modification systems, the collision of translocating complexes triggers hydrolysis of phosphodiester bonds in unmodified DNA to generate double-strand breaks. Type I endonucleases break the DNA at unspecified sequences remote from the target sequence, type III endonucleases at a fixed position close to the target sequence. Type I and type III restriction and modification (R-M) systems are notable for effective post-translational control of their endonuclease activity. For some type I enzymes, this control is mediated by proteolytic degradation of that subunit of the complex which is essential for DNA translocation and breakage. This control, lacking in the well-studied type II R-M systems, provides extraordinarily effective protection of resident DNA should it acquire unmodified target sequences. The only well-documented GTP-dependent restriction enzyme, McrBC, requires methylated target sequences for the initiation of phosphodiester bond cleavage. 相似文献
992.
Murali Krishna PS Venkateswarlu G Venkateswar Rao L 《Letters in applied microbiology》2000,31(1):73-76
The effect of different organic nitrogen compounds on the production of rifamycin SV by Amycolatopsis mediterranei MV35R and their optimum concentrations have been described. Results obtained indicate that rifamycin SV production increased from 4020 mg l-1 to 4575 mg l-1 when organic nitrogen compound uracil was added at 0.2% (w/v) concentration to the fermentation medium by A. mediterranei MV35R. The rifamycin SV yield was enhanced by 505 mg l-1 using uracil (2 g l-1) when compared with barbital. 相似文献
993.
The relevance of Ca2+-calmodulin-mediated processes in channelling acetate for aflatoxin formation was investigated by studying the influence of trifluoperazine (an anticalmodulin agent) on [14C]-acetate incorporation and activity of acetyl-CoA carboxylase in Aspergillus parasiticus NRRL 2999. Culturing the organism in presence of 0.14 mmol l-1 trifluoperazine resulted in 55% decrease of [14C]-acetate incorporation into aflatoxin B1, along with an 80% decrease in acetyl-CoA carboxylase activity at periods corresponding to maximal aflatoxin production. Concomitant decrement (35%) in the activity of glucose-6-phosphate dehydrogenase indicated decreased availability of reduction potential (NADPH) required for aflatoxin biosynthesis. The ability of calmodulin to activate and trifluoperazine to inhibit acetyl-CoA carboxylase activity in a dose-dependent manner was also noted under in vitro conditions. The combined results suggest calmodulin-mediated activation of acetyl-CoA carboxylase as an important event for aflatoxin production. 相似文献
994.
995.
996.
997.
Dehairs F. Rao R. G. Chandra Mohan P. Raman A.V. Marguillier S. Hellings L. 《Hydrobiologia》2000,431(2-3):225-241
Stable carbon isotopic composition and C/N ratio were used to trace the input of carbon associated with mangrove litter into the estuary of the Godavari–Gautami delta system and Kakinada bay (Andhra Pradesh, India). Suspended organic matter in the mangrove channels was more depleted in 13C (average 13C = –24.5) than in Kakinada bay which showed 13C values for suspended matter (average 13C = –22.7) closer to those expected for marine phytoplankton. Suspended organic matter from mangrove channels was enriched in nitrogen (average C/N atom ratio 12.7) and 13C (average 13C = –24.5) relative to mangrove leaf litter, which had a C/N ratio of 75 and a 13C value of –28. Lowest C/N ratios for suspended matter were observed during southwest monsoon when rainfall was highest. Although in general, mangrove litter fall was also lower during this period, no clear correlation was observed between litter fall and C/N ratio of suspended matter. In general, the composition of suspended matter pointed towards phytoplankton as a major component. Isotopic composition of zooplankton suggested selective feeding on 13C-enriched, marine phytoplankton in open Kakinada bay and on 13C-depleted organic matter, such as estuarine phytoplankton and mangrove litter, in the mangrove channels. From the 13C signature, it appeared that mangrove carbon was present to some extent in zooplankton and macrofauna from the mangrove mudflats and channels, but the signal rapidly decreased in Kakinada bay. Nitrogen isotopic composition of zooplankton and macrofauna indicated a progressive enrichment of 15N away from the mangrove forest towards the northern part of Kakinada bay, in approach of Kakinada city. This is thought to reflect input of anthropogenic nitrogen enriched in 15N and subsequent uptake of this enriched nitrogen into the aquatic food chain. 相似文献
998.
Rao AV JayaSree T Ramesh M Pavan U Sadanandam A 《Indian journal of experimental biology》2000,38(6):617-620
High frequency of streptomycin resistant variants of Lycopersicon esculentum were isolated on selective shoot regeneration medium supplemented with IAA (0.5 mg/L), zeatin (1.5 mg/L) and streptomycin sulphate (500 mg/L). Nonmutagenized (controls) and NMU treated cotyledons were placed on shoot regeneration medium supplemented with antibiotic streptomycin. Resistant shoots appeared at a high frequency in mutagenized cotyledons, whereas in controls morphogenesis was suppressed, accompanied by bleaching. Shoot regeneration occurred from the nodular tissues developed at the cut ends of cotyledons. Resistant shoots developed into complete plantlets on rooting medium containing selective concentration of antibiotic. Stability of streptomycin resistance was confirmed by leaf assay and reciprocal crosses between streptomycin-resistant and sensitive plants. 相似文献
999.
1000.
Allen PD St Pierre TG Chua-anusorn W Ström V Rao KV 《Biochimica et biophysica acta》2000,1500(2):186-196
The biologic effects and mechanisms by which bone morphogenetic proteins (BMPs) function in breast cancer cells are not well defined. A member of this family of growth and differentiation factors, BMP-2, inhibited both basal and estradiol-induced growth of MCF-7 breast tumor cells in culture. Flow cytometric analysis showed that in the presence of BMP-2, 62% and 45% of estradiol-stimulated MCF-7 cells progressed to S-phase at 24 h and 48 h, respectively. Estradiol mediates growth of human breast cancer cells by stimulating cyclins and cyclin-dependent kinases (CDKs). BMP-2 significantly increased the level of the cyclin kinase inhibitor, p21, which in turn associated with and inactivated cyclin D1. BMP-2 inhibited estradiol-induced cyclin D1-associated kinase activity. Also estradiol-induced CDK2 activity was inhibited by BMP-2. This inhibition of CDK activity resulted in hypophosphorylation of retinoblastoma protein thus keeping it in its active form. These data provide the first evidence by which BMP-2 inhibits estradiol-induced proliferation of human breast cancer cells. 相似文献