全文获取类型
收费全文 | 11625篇 |
免费 | 1225篇 |
国内免费 | 4篇 |
专业分类
12854篇 |
出版年
2021年 | 121篇 |
2020年 | 78篇 |
2019年 | 120篇 |
2018年 | 115篇 |
2017年 | 106篇 |
2016年 | 203篇 |
2015年 | 308篇 |
2014年 | 351篇 |
2013年 | 481篇 |
2012年 | 600篇 |
2011年 | 576篇 |
2010年 | 389篇 |
2009年 | 361篇 |
2008年 | 508篇 |
2007年 | 557篇 |
2006年 | 543篇 |
2005年 | 542篇 |
2004年 | 539篇 |
2003年 | 493篇 |
2002年 | 526篇 |
2001年 | 290篇 |
2000年 | 258篇 |
1999年 | 271篇 |
1998年 | 173篇 |
1997年 | 181篇 |
1996年 | 171篇 |
1995年 | 142篇 |
1994年 | 156篇 |
1993年 | 150篇 |
1992年 | 182篇 |
1991年 | 161篇 |
1990年 | 195篇 |
1989年 | 173篇 |
1988年 | 149篇 |
1987年 | 165篇 |
1986年 | 141篇 |
1985年 | 161篇 |
1984年 | 143篇 |
1983年 | 121篇 |
1982年 | 127篇 |
1981年 | 116篇 |
1980年 | 111篇 |
1979年 | 122篇 |
1978年 | 107篇 |
1977年 | 102篇 |
1976年 | 112篇 |
1975年 | 100篇 |
1974年 | 101篇 |
1973年 | 115篇 |
1972年 | 100篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Defective transducing phages carrying aroG, the structural gene for phenylalanine (phe)-inhibitable phospho-2-keto-heptonate aldolase (EC 4.1.2.15; previously known as 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase[phe]), have been isolated, and DNA from two of these phages has been used to construct a restriction map of the region from att lambda to aroG. A 7.6-kb PstI-HindIII fragment from one of these phages was cloned into pBR322 and shown to contain aroG. The location of aroG within the 7.6 kb was established by subcloning and Tn3 transpositional mutagenesis. A fragment carrying the aroG promoter and operator has been cloned into a high copy number promoter-cloning vector (pMC489), and the resulting aroGpo-LacZ' (alpha) fusion subcloned in a low copy number vector. Strains with this fusion on the low copy number vector exhibit negative regulation of beta-galactosidase expression by both phenylalanine and tryptophan and positive regulation by tyrosine in a tyrR+ background. 相似文献
2.
3.
Dominant males of nonseasonally breeding species should have more opportunity than seasonal breeders to monopolize access to estrous females and thus enhance their reproductive success. They may also use other strategies for maximizing reproductive success such as producing offspring by relatively high ranking females. To test these hypotheses, the paternity of 35 (80%) of 44 Macaca fascicularis offspring, born over a 28-month period, was established using electrophoretically and serologically defined genetic markers. The social ranks of each of the seven potential fathers and 26 potential mothers were determined by recording outcomes of dyadio agonistic interactions. No evidence could be found to support either hypothesis. Further, in view of previously reported significant, positive correlations between male social rank and sexual activity rates in this study group, sexual activity rates do not appear to predict the number of offspring a male sires in this species. 相似文献
4.
5.
6.
7.
Abstract: Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation. 相似文献
8.
An Ecteola-cellulose chromatography assay for 3′-phosphoadenosine 5′-phosphosulfate: Phenol sulfotransferase 总被引:1,自引:0,他引:1
Ronald T. Borchardt Anna Baranczyk-Kuzma Carol L. Pinnick 《Analytical biochemistry》1983,130(2):334-338
A new assay procedure for phenol sulfotransferase which employs [35S]-3'-phosphoadenosine-5'-phosphosulfate as a sulfate donor and a variety of phenols as sulfate acceptors was developed. The appearance of the 35S-sulfated products or the disappearance of the [35S]-3'-phosphoadenosine-5'-phosphosulfate are determined simultaneously by chromatography of the assay incubation mixtures on Ecteola-cellulose columns, eluting with an NH4HCO3 step gradient. Various acidic, neutral, and basic phenols can be employed as substrates for phenol sulfotransferase using this procedure. 相似文献
9.
10.