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   524881篇
  免费   66564篇
  国内免费   177篇
  591622篇
  2016年   5517篇
  2015年   7766篇
  2014年   9319篇
  2013年   13199篇
  2012年   14671篇
  2011年   14760篇
  2010年   9718篇
  2009年   9216篇
  2008年   13173篇
  2007年   13947篇
  2006年   12943篇
  2005年   12592篇
  2004年   12496篇
  2003年   12083篇
  2002年   11635篇
  2001年   22677篇
  2000年   23187篇
  1999年   18584篇
  1998年   6638篇
  1997年   7034篇
  1996年   6828篇
  1995年   6565篇
  1994年   6546篇
  1993年   6465篇
  1992年   16228篇
  1991年   16114篇
  1990年   15791篇
  1989年   15361篇
  1988年   14351篇
  1987年   13801篇
  1986年   12913篇
  1985年   13055篇
  1984年   10964篇
  1983年   9683篇
  1982年   7468篇
  1981年   7004篇
  1980年   6580篇
  1979年   10958篇
  1978年   8516篇
  1977年   7843篇
  1976年   7452篇
  1975年   8259篇
  1974年   8558篇
  1973年   8399篇
  1972年   7949篇
  1971年   6916篇
  1970年   6050篇
  1969年   5710篇
  1968年   5208篇
  1967年   4523篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Various 2-nitronaphthofuran derivatives (related to each other by simple structural modifications) were tested for 2 different effects in CV-1 monkey kidney cell cultures: the immediate inhibition of normal DNA synthesis and the capacity of pretreated cultures (40 h of contact) to support the replication of UV-damaged Herpes simplex virus (HSV). For all compounds tested, a fair correlation was found between their efficiencies to inhibit cellular DNA synthesis and to provoke an increase in UV-HSV production (virus reactivation). Virus reactivation was due to an increase in both the number of virus-producing cells and the amount of infectious particles produced per cell. The most efficient 2-nitronaphthofurans (particularly 2-nitro-7-methoxy-naphtho[2,1-b]furan-R 7000) were at least as potent as aflatoxin B1 in inducing virus reactivation.  相似文献   
992.
993.
Amyloid fibril proteins were isolated from the spleen of a patient with IgD(lambda)-plasmocytoma by extraction and gel filtration in 5M guanidine hydrochloride. The molecular mass of the predominant polypeptide chain was approximately 5000 Da. Its complete amino-acid sequence was elucidated by stepwise automated degradation of the carboxymethylated polypeptide chain and by structural studies of tryptic and thermolysinolytic cleavage products. The length of the polypeptide chain was 58 to 59 residues and it was homologous to the amino acids in positions 8 through 65 of the variable part of an lambda-type immunoglobulin light chain, which was most closely related to the lambda II subgroup. The N-terminal sequence of this amyloid fibril protein proved to be heterogeneous, indicating cleavage after the amino acids in positions 7 and 8. Peptides from the constant part of the lambda-chain were unexpectedly found in the tryptic digest of the denatured amyloid protein HAR. One polypeptide derived from the constant region was separated from the main component by high performance liquid chromatography. Its amino-acid sequence commenced at position 111 and could be traced in 41 steps. In this case, at least two constant region fragments were shown to be constituents of the amyloid fibril protein. The association of fragments from the variable as well as the constant region is discussed with respect to amyloid formation.  相似文献   
994.
Studies have been initiated to determine the hormonal regulation of glycogen synthase in rabbit skeletal muscle. It was found that glycogen synthase purified from control animals was quite highly phosphorylated (2.35 mol phosphate/mol synthase subunit) with 40% of the phosphate in the trypsin-sensitive or COOH-terminal domain, and 60% in the trypsin-insensitive or NH2-terminal domain. The phosphorylation state of synthase was elevated (3.9 mol/mol) by epinephrine injection and in the diabetic condition. With epinephrine, about 76% of the additional phosphate was incorporated in the trypsin-sensitive domain, which strongly supports the contention that this hormone acts through the cyclic AMP (cAMP)-dependent protein kinase. In the synthase purified from diabetic rabbits, 90% of the additional phosphate was in the trypsin-insensitive domain. Insulin treatment of the diabetics resulted in specific dephosphorylation of the trypsin-insensitive domain. These results indicate that in this system insulin is not acting by inhibition of the cAMP-dependent protein kinase.  相似文献   
995.
The beta subunits of equine lutropin and equine chorionic gonadotropin were incubated in 0.013 N HCl for 30 min at 110 degrees C and separated into two fragments by reverse-phase high performance liquid chromatography. The amino acid and carbohydrate compositions of both fragments from each subunit were analyzed. The results demonstrated that equine lutropin-beta has a glycosylated COOH-terminal extension that differs only in carbohydrate composition from the COOH-terminal portion of equine chorionic gonadotropin-beta. This is the first demonstration of a glycosylated COOH-terminal extension in a pituitary glycoprotein hormone.  相似文献   
996.
From a genomic library of Xenopus laevis, two genes coding for different preprocaeruleins have been isolated and sequenced. These correspond to the type I and type III precursors analyzed previously at the cDNA level [Richter, K., Egger, R. and Kreil, G. (1986) J. Biol. Chem. 261, 3676-3680]. The type III gene comprises eight exons; the type I apparently contains eight exons as well, of which six have been sequenced. The genetic information for the dekapeptide caerulein is present on small exons of 45 base pairs. The two genes are highly homologous in their 5'-flanking region, the exon/intron boundaries, and long stretches of intron sequences. A possible scheme for the evolution of this small family of genes through exon and gene duplications is presented. In the type I gene, in place of one of the caerulein exons, a potential exon with conserved splice sites was discovered. If expressed in some frog cells, this exon would code for a new peptide 60% homologous to caerulein.  相似文献   
997.
998.
999.
1000.
The statistical methods for the analysis of mutagenicity and carcinogenicity underwent considerable theoretical-practical development following the need for assessing the mutagenic and carcinogenic potential of substances. Antimutagenicity is investigated through the analysis of respondents in dose-response assays, when two different molecules are administered separately and as a mixture to a respondent system. When the number of respondent units is high, and doses are orthogonal, it is possible to apply simple models such as analysis of variance. This is not always possible or common, and alternative approaches have been developed, based on multiple regression and on tables of proportions. In this work, some of the most frequently used methods for the assessment of joint responses are reviewed, particularly those based on multiple regression, such as the method of Shaeffer et al. and the method of Hass et al. In order to illustrate these methods, joint responses of perylene and cyclopentapyrene, of N-acetylcysteine and dinitropyrene, and of N-acetylcysteine and extracts from diesel exhausts were analyzed. An antagonistic effect of perylene on the action of CPP was detected by the algorithm of Shaeffer et al. The effect is not multiplicative, i.e., it is not proportional to the product of doses. The antimutagenic effect of N-acetylcysteine on dinitropyrene is multiplicative, as detected by the method of Hass et al. The latter reveals that the inhibition by N-acetylcysteine on the mutagenic effect of extracts from diesel exhausts is also multiplicative.  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