全文获取类型
收费全文 | 68761篇 |
免费 | 6620篇 |
国内免费 | 22篇 |
出版年
2023年 | 240篇 |
2021年 | 990篇 |
2020年 | 743篇 |
2019年 | 957篇 |
2018年 | 1174篇 |
2017年 | 1021篇 |
2016年 | 1728篇 |
2015年 | 2856篇 |
2014年 | 3123篇 |
2013年 | 3764篇 |
2012年 | 4904篇 |
2011年 | 4721篇 |
2010年 | 3037篇 |
2009年 | 2760篇 |
2008年 | 3938篇 |
2007年 | 4035篇 |
2006年 | 3793篇 |
2005年 | 3637篇 |
2004年 | 3575篇 |
2003年 | 3302篇 |
2002年 | 3311篇 |
2001年 | 986篇 |
2000年 | 792篇 |
1999年 | 920篇 |
1998年 | 935篇 |
1997年 | 674篇 |
1996年 | 582篇 |
1995年 | 577篇 |
1994年 | 595篇 |
1993年 | 593篇 |
1992年 | 724篇 |
1991年 | 575篇 |
1990年 | 555篇 |
1989年 | 552篇 |
1988年 | 489篇 |
1987年 | 456篇 |
1986年 | 442篇 |
1985年 | 426篇 |
1984年 | 498篇 |
1983年 | 456篇 |
1982年 | 511篇 |
1981年 | 463篇 |
1980年 | 455篇 |
1979年 | 364篇 |
1978年 | 350篇 |
1977年 | 313篇 |
1976年 | 308篇 |
1975年 | 236篇 |
1974年 | 260篇 |
1973年 | 247篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Thomas Arn Hansen Helena Fridholm Tobias Guldberg Fr?slev Kristín Rós Kjartansdóttir Eske Willerslev Lars Peter Nielsen Anders Johannes Hansen 《PloS one》2015,10(11)
Rattus norvegicus (R. norvegicus) are ubiquitous and their presence has several effects on the human populations in our urban areas on a global scale. Both historically and presently, this close interaction has facilitated the dissemination of many pathogens to humans, making screening for potentially zoonotic and emerging viruses in rats highly relevant. We have investigated faecal samples from R. norvegicus collected from urban areas using a protocol based on metagenomic enrichment of circular DNA genomes and subsequent sequencing. We found a new type of papillomavirus, with a L1 region 82% identical to that of the known R. norvegicus Papillomavirus 2. Additionally, we found 20 different circular replication associated protein (Rep)-encoding single stranded DNA (CRESS-DNA) virus-like genomes, one of which has homology to the replication-associated gene of Beak and feather disease virus. Papillomaviruses are a group of viruses known for their carcinogenic potential, and although they are known to infect several different vertebrates, they are mainly studied and characterised in humans. CRESS-DNA viruses are found in many different environments and tissue types. Both papillomaviruses and CRESS-DNA viruses are known to have pathogenic potential and screening for novel and known viruses in R. norvegicus could help identify viruses with pathogenic potential. 相似文献
4.
5.
Thomas Caspari Ingrid Robl Jürgen Stolz Widmar Tanner 《The Plant journal : for cell and molecular biology》1996,10(6):1045-1053
A prokaryotic biotin acceptor domain was fused to the carboxy terminal end of the Chlorella hexose—proton sym- porter. The plant symporter is biotinylated in vivo when expressed in Schizosaccharomyces pombe. The extended biotinylated transport protein is fully active, catalyzes accumulation of d -glucose analogs and restores growth of a glucose-uptake-deficient yeast strain. Crude membranes were solubilized with octyl-β-d -glucoside in the presence of Escherichia colil -α-phosphatidylethanolamine. Biotinylated symporter was purified to homogeneity by biotinavidin affinity chromatography. The symporter protein was reconstituted together with cytochrome-c oxidase prepared from beef heart mitochondria into proteo-liposomes. Cytochrome-c oxidase is a redox-driven H+-pump generating a proton motive force (inside negative and alkaline) while transferring electrons from cytochrome-c to oxygen; this energy is used by the symporter to accumulate d -glucose at least 30-fold. In the absence of the driving force the transport protein facilitates diffusion of d -glucose until the concentration equilibrium is reached. It was shown that maximal transport activity depends highly on the amount of co-reconstituted cytochrome-c oxidase and that the symporter possesses 10% of its in vivo turnover number under optimized in vitro transport conditions. 相似文献
6.
Pyruvate decarboxylase from Zymomonas mobilis is inhibited by 3-hydroxypyruvate, which can also act as a poor substrate. While catalysing the decarboxylation of this alternative substrate, the enzyme undergoes a progressive but partial inactivation over several hours. The extent of inactivation depends upon the pH and upon the concentration of 3-hydroxypyruvate. After partial inactivation and removal of unchanged 3-hydroxypyruvate, enzymic activity recovers slowly. We suggest that inactivation results from accumulation of enzyme-bound glycollaldehyde, which is relatively stable, possibly because it is dehydrated to form an acetyl group. 相似文献
7.
Maxence Gérard Baptiste Martinet Manuel Dehon Pierre Rasmont Paul H. Williams Denis Michez 《Systematic Entomology》2020,45(4):849-856
Since the beginning of taxonomy, species have been described based on morphology, but the advent of using semio-chemicals and genetics has led to the discovery of cryptic species (i.e. morphologically similar species). When a new cryptic species is described, earlier type specimens have to be re-evaluated, although this process can be challenging as only nondestructive methods ought to be used in order to preserve the integrity of the type specimens. Methods should allow comparison with recently collected specimens clustered based on chemical, ethological and/or genetic traits with old specimens (i.e. type specimens) where only morphological traits are available. Here we develop a method based on geometric morphometric analyses of wing shape for a taxonomically challenging group of bumblebees, the subgenus Alpinobombus Skorikov. We consider nine monophyletic taxa (including several cryptic species) to assess the accuracy of this method to discriminate the taxa based on their wing shape and then to attribute type specimens using a leave-one-out cross-validation procedure. We show that, for these bees, wing shape is taxon-specific, except for two sister taxa for which the species status is still debated. Moreover, for most of the taxa, type specimens were correctly attributed with high posterior probabilities of attribution, except for a few type specimens corresponding to the same two sister taxa where taxa delimitation based on wing shape was previously the subject of discussion. Our study highlights the potential of geometric morphometric analyses to help in the re-attribution of type specimens when the existence of cryptic species is revealed. 相似文献
8.
Goedele Paternot Johanna Devroe Sophie Debrock Thomas M D'Hooghe Carl Spiessens 《Reproductive biology and endocrinology : RB&E》2009,7(1):105-6
Background
The aim of this study was to determine the intra- and inter-observer variability in the evaluation of embryo quality. Multilevel images of embryos on day 1, day 2 and day 3, were analysed using different morphological parameters. 相似文献9.
Tamar F Brionez Shervin Assassi John D Reveille Thomas J Learch Laura Diekman Michael M Ward John C Davis Jr Michael H Weisman Perry Nicassio 《Arthritis research & therapy》2009,11(6):R182-9
Introduction
Functional status is an integral component of health-related quality of life in patients with ankylosing spondylitis (AS). The purpose of this study was to investigate the role of psychological variables in self-reported functional limitation in patients with AS, while controlling for demographic and medical variables. 相似文献10.