全文获取类型
收费全文 | 60120篇 |
免费 | 5115篇 |
国内免费 | 4642篇 |
出版年
2024年 | 113篇 |
2023年 | 746篇 |
2022年 | 1651篇 |
2021年 | 2786篇 |
2020年 | 1879篇 |
2019年 | 2332篇 |
2018年 | 2237篇 |
2017年 | 1581篇 |
2016年 | 2364篇 |
2015年 | 3585篇 |
2014年 | 4250篇 |
2013年 | 4544篇 |
2012年 | 5513篇 |
2011年 | 4966篇 |
2010年 | 3081篇 |
2009年 | 2668篇 |
2008年 | 3133篇 |
2007年 | 2857篇 |
2006年 | 2503篇 |
2005年 | 2099篇 |
2004年 | 1893篇 |
2003年 | 1609篇 |
2002年 | 1451篇 |
2001年 | 1161篇 |
2000年 | 1091篇 |
1999年 | 1023篇 |
1998年 | 579篇 |
1997年 | 564篇 |
1996年 | 552篇 |
1995年 | 443篇 |
1994年 | 458篇 |
1993年 | 323篇 |
1992年 | 529篇 |
1991年 | 462篇 |
1990年 | 379篇 |
1989年 | 351篇 |
1988年 | 267篇 |
1987年 | 218篇 |
1986年 | 193篇 |
1985年 | 212篇 |
1984年 | 165篇 |
1983年 | 135篇 |
1982年 | 105篇 |
1981年 | 64篇 |
1980年 | 67篇 |
1979年 | 85篇 |
1978年 | 70篇 |
1976年 | 67篇 |
1975年 | 63篇 |
1974年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
The effect of microelements on the induction of embryogenic callus from epiblast and scutellum of wheat (Triticum aestivum L.) embryos was studied by the sequential omission of each of the microelements from Murashige & Skoog medium. Omission of iron caused a marked decrease in yield and poor shoot formation from embryogenic callus. The yield of embryogenic callus on medium without added manganese was also reduced. Omission of boron, copper-cobalt, iodine, and molybdenum had little effect on the induction of embryogenic epiblast callus. By contrast there was a marked increase in the formation of white structures on the medium without any microelements or, specifically without addition of zinc. Since the formation of typical embryoids of wheat is associated with the formation of white structures, our result highlights the importance of certain microelements on somatic embryogenesis of wheat.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- MS medium
Murashige & Skoog medium 相似文献
192.
D. -L. Wei S. -C. Chang Y. -H. Wei Y. -W. Lin C. -L. Chuang S. -C. Jong 《World journal of microbiology & biotechnology》1992,8(2):141-146
Eighteen strains of xylariaceous fungi have been screened for higher activities of cellulolytic enzymes,Trichoderma reesei QM 9414 was also examined for comparison. Strains ofXylaria anisopleura andX. regalis had higher endocellulase (CMCase) and exocellulase (Avicelase) activities after 2 weeks' incubation.Hypoxylon stygium produced the highest activity of -glucosidase 3 days after inoculation. The optimum pH for these cellulolytic enzymes was approx. 5.0 and the optimum temperatures ranged from 37 to 50°C. A mixed culture process usingT. reesei QM 9414 andH. stygium was developed to obtain enhanced synthesis of cellulase. -Glucosidase activities in the mixed culture increased within 48h whenH. stygium was introduced after 24h. 相似文献
193.
J L Mountain A A Lin A M Bowcock L L Cavalli-Sforza 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1992,337(1280):159-165
Previously we have described studies of the evolution of modern humans based upon data for classical genetic markers and for nuclear DNA polymorphisms. Such polymorphisms provide a different point of view regarding human evolution than do mitochondrial DNA sequences. Here we compare revised dates for major migrations of anatomically modern humans, estimated from archaeological data, with separations suggested by a genetic tree constructed from classical marker allele frequencies. Analyses of DNA polymorphisms have now been extended and compared with those of classical markers; genetic trees continue to support the hypothesis of an initial African and non-African divergence for modern humans. We have also begun testing non-human primates for a set of human DNA polymorphisms. For most polymorphisms tested so far, humans share a single allele with other primates; such shared alleles are likely to be ancestral. Populations living in humid tropical environments have significantly higher frequencies of ancestral alleles than do other populations, supporting the hypothesis that natural selection acts to maintain high frequencies of particular alleles in some environments. 相似文献
194.
Construction and expression of nonsense suppressor tRNAs which function in plant cells 总被引:3,自引:0,他引:3
Scott Franklin Tsai Yun Lin William R. Folk 《The Plant journal : for cell and molecular biology》1992,2(4):583-588
An Arabidopsis thaliana L. DNA containing the tRNA(TrpUGG) gene was isolated and altered to encode the amber suppressor tRNA(TrpUAG) or the ochre suppressor tRNA(TrpUAA). These DNAs were electroporated into carrot protoplasts and tRNA expression was demonstrated by the translational suppression of amber and ochre nonsense mutations in the chloramphenicol acetyltransferase (CAT) reporter gene. DNAs encoding tRNA(TrpUAG) and tRNA(TrpUAA) nonsense suppressor tRNAs caused suppression of their cognate nonsense codons in CAT mRNAs, with the tRNA(TrpUAG) gene exhibiting the greater suppression under optimal conditions for expression of CAT. The development of these translational suppressors which function in plant cells facilitates the study of plant tRNA gene expression and will make possible the manipulation of plant protein structure and function. 相似文献
195.
William C. Groutas He Huang Jeffrey B. Epp Michael J. Brubaker Charles E. Keller Jerald J. McClenahan 《Bioorganic & medicinal chemistry letters》1992,2(12):1565-1570
A general approach toward the rational design of potential inhibitors of serine proteinases is described. The approach is exemplified and validated through the use of appropriate heterocyclic systems in inhibiting human leukocytes elastase (HLE). 相似文献
196.
Identification of the guanine binding domain peptide of the GTP-binding site of glucagon. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Protein science : a publication of the Protein Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M. Shoemaker P. C. Lin B. Haley 《Protein science : a publication of the Protein Society》1992,1(7):884-891
Glucagon, a peptide hormone synthesized and secreted by alpha islet cells, regulates glucose homeostasis by several mechanisms. Using [gamma 32P]8N3GTP, a proven photoaffinity probe for GTP, a specific nucleotide binding site on human glucagon was detected that showed preference for GTP. Half-maximal saturation of photoinsertion into the polypeptide hormone was at 8-12 microM with either [alpha 32P]8N3GTP or [gamma 32P]8N3GTP. GTP protected photolabeling with an apparent kd of 15 microM, whereas ATP was less effective as a protector, exhibiting an apparent kd of about 30 microM. Maximal protection by GTP and ATP was over 90%. UTP, CTP, GDP, ADP, GMP, AMP, guanosine, adenosine, guanine, and adenine were much less effective protectors, indicating that binding is specific for purine nucleoside triphosphates, particularly GTP. Mg2+ at 150 microM enhanced photoinsertion (twofold), whereas at 2-10 mM, it inhibited photoinsertion. Both Ca2+ and Zn2+ at 0.2 mM decreased photoinsertion about 45%. Purification of chymotryptic and tryptic digests of photolabeled glucagon by reverse-phase high performance liquid chromatography (HPLC) revealed that the N-terminal peptide, HSQGTF, was the only peptide region covalently photomodified by [32P]8N3GTP. GTP, if present during photolysis, greatly reduced both photoinsertion into glucagon and the amount of radiolabeled peptide recovered on HPLC. The specificity of binding to the N-terminal region is suggestive of a physiological role for a glucagon-GTP complex in the mechanism of action of this hormone. 相似文献
197.
松辽盆地阿尔必期微体浮游植物新属种 总被引:4,自引:3,他引:1
该文描述了松辽盆地中白垩世阿尔必期泉头组三段的微体浮游植物化石1新属7新种,隶属于微咸水沟鞭藻类2属5新种(含4新亚种),淡水绿藻1属1种和疑源类1新属1新种。并对沟鞭藻 Ngktericysta Bint,1986进行了修订。 相似文献
198.
松辽盆地白垩纪微体浮游植物群及其环境讨论 总被引:9,自引:2,他引:7
该文报道了松辽盆地白垩纪丰富的非海相微体浮游植物群,主要是沟鞭藻类及一些绿藻和疑源类;论述了藻类的生物地层特征,自下而上初步划分出10个组合带;结合微量元素和古地磁等资料,较详细地讨论了含微体浮游植物组段的沉积环境,认为松辽盆地在白垩纪至少遭受过两次重要的海侵(分别在青山口组一段及嫩江组一、二段沉积时期),导致古松辽湖泊五种不同水体环境的演替,指出微体浮游植物组合的变化是受古盐度、古温度和古水深等因素控制的。此外,对有关组段的地质时代也进行了讨论,进一步补充了新的浮游植物化石证据。 相似文献
199.
200.