全文获取类型
收费全文 | 4129篇 |
免费 | 457篇 |
国内免费 | 1篇 |
出版年
2023年 | 18篇 |
2021年 | 63篇 |
2020年 | 61篇 |
2019年 | 67篇 |
2018年 | 83篇 |
2017年 | 58篇 |
2016年 | 118篇 |
2015年 | 177篇 |
2014年 | 190篇 |
2013年 | 230篇 |
2012年 | 321篇 |
2011年 | 302篇 |
2010年 | 162篇 |
2009年 | 181篇 |
2008年 | 240篇 |
2007年 | 217篇 |
2006年 | 227篇 |
2005年 | 210篇 |
2004年 | 229篇 |
2003年 | 207篇 |
2002年 | 211篇 |
2001年 | 47篇 |
2000年 | 33篇 |
1999年 | 48篇 |
1998年 | 66篇 |
1997年 | 44篇 |
1996年 | 36篇 |
1995年 | 51篇 |
1994年 | 38篇 |
1993年 | 46篇 |
1992年 | 29篇 |
1991年 | 28篇 |
1990年 | 41篇 |
1989年 | 32篇 |
1988年 | 25篇 |
1987年 | 32篇 |
1986年 | 23篇 |
1985年 | 25篇 |
1984年 | 35篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 32篇 |
1980年 | 16篇 |
1979年 | 16篇 |
1978年 | 16篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1974年 | 17篇 |
1973年 | 11篇 |
排序方式: 共有4587条查询结果,搜索用时 16 毫秒
1.
Harriet M. Jackson Kristen D. Onos Keating W. Pepper Leah C. Graham Ellen C. Akeson Candice Byers Laura G. Reinholdt Wayne N. Frankel Gareth R. Howell 《PloS one》2015,10(5)
Alzheimer’s disease (AD) is a leading cause of dementia in the elderly and is characterized by amyloid plaques, neurofibrillary tangles (NFTs) and neuronal dysfunction. Early onset AD (EOAD) is commonly caused by mutations in amyloid precursor protein (APP) or genes involved in the processing of APP including the presenilins (e.g. PSEN1 or PSEN2). In general, mouse models relevant to EOAD recapitulate amyloidosis, show only limited amounts of NFTs and neuronal cell dysfunction and low but significant levels of seizure susceptibility. To investigate the effect of genetic background on these phenotypes, we generated APPswe and PSEN1de9 transgenic mice on the seizure prone inbred strain background, DBA/2J. Previous studies show that the DBA/2J genetic background modifies plaque deposition in the presence of mutant APP but the impact of PSEN1de9 has not been tested. Our study shows that DBA/2J.APPswePSEN1de9 mice are significantly more prone to premature lethality, likely to due to lethal seizures, compared to B6.APPswePSEN1de9 mice—70% of DBA/2J.APPswePSEN1de9 mice die between 2-3 months of age. Of the DBA/2J.APPswePSEN1de9 mice that survived to 6 months of age, plaque deposition was greatly reduced compared to age-matched B6.APPswePSEN1de9 mice. The reduction in plaque deposition appears to be independent of microglia numbers, reactive astrocytosis and complement C5 activity. 相似文献
2.
3.
Su Hao Lo Ellen Weisberg Lan Bo Chen 《BioEssays : news and reviews in molecular, cellular and developmental biology》1994,16(11):817-823
Cytoskeletal proteins provide the structural foundation that allows cells to exist in a highly organized manner. Recent evidence suggests that certain cytoskeletal proteins not only maintain structural integrity, but might also be associated with signal transduction and suppression of tumorigenesis. Since the time of the discovery of tensin, a fair amount of data has been gathered which supports the notion that tensin is one such protein possessing these characteristics. In this review, we discuss recent studies that: (1) elucidate a role for tensin in maintenance of cellular structure and signal transduction; (2) implicate tensin as the anchor for actin filaments at the focal adhesion; (3) describe the phosphorylation of tensin; (4) describe potential targets for its Src homology region 2 domain; (5) describe the association between tensin and the nuclear protein p130; and (6) demonstrate that increased tensin expression in a cell line appears to reduce its transformation potential. 相似文献
4.
5.
6.
Synopsis Testicular maturation indices (TMI) were determined for wild-caught males ofPoecilia mexicana by quantitative analysis of the frequency of ongoing stages of spermatogenesis and spermatid differentiation in sections of testes stained with the Feulgen reaction for DNA. In nature, males maintain essentially constant levels of sperm production throughout the year and show no significant variations in mean TMI values associated with season, microhabitat, standard body length, or rank in male dominance hierarchies. Winter males or males isolated from females in the laboratory show accumulation of mature spermatophores. These findings suggest that fluctuations in the microstructure of wild populations ofPoecilia from northeastern Mexico may be due primarily to differences in the temporal scheduling of female reproductive cycles and not to limitations imposed by sperm availability or male reproductive competence. The data are discussed in terms of competitive interactions within unisexual-bisexual breeding complexes involving the Amazon mollyP. formosa, its related triploid hybrids, and the bisexual species,P. mexicana. 相似文献
7.
Ellen Cocquyt Gillian H Gile Frederik Leliaert Heroen Verbruggen Patrick J Keeling Olivier De Clerck 《BMC evolutionary biology》2010,10(1):327
Background
A non-canonical nuclear genetic code, in which TAG and TAA have been reassigned from stop codons to glutamine, has evolved independently in several eukaryotic lineages, including the ulvophycean green algal orders Dasycladales and Cladophorales. To study the phylogenetic distribution of the standard and non-canonical genetic codes, we generated sequence data of a representative set of ulvophycean green algae and used a robust green algal phylogeny to evaluate different evolutionary scenarios that may account for the origin of the non-canonical code. 相似文献8.
Theo J. Visser Ellen Van Overmeeren-Kaptein 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,631(2):246-252
A radioimmunoassay for 3′-iodothyronine has been developed. All iodothyronine analogues (except 3,3′-diiodothyronine) showed very little (0.02% at most) cross-reactivity, and the assay was sensitive to 1 pg 3′-iodothyronine/ tube. We have studied the 5′-deiodination of 3′,5′-diiodothyronine by rat liver microsomal fraction in the presence of dithiothreitol. Production of 3′-iodothyronine at 37°C was found to be linear with time of incubation up to 30 min and with concentration of microsomal protein up to 100 μg/ml. The reaction rate reached a limit on increasing 3′,5′-diiodothyronine concentration to 10 μM. The effect of pH on 3′-iodothyronine production was found to depend on 3′,5′-diiodothyronine concentration. Increasing 3′,5′-diiodothyronine concentration from 0.1 to 10 μM resulted in a shift of the pH optimum from 6–6.5 to 7.5. Similar effects on the 5′-deiodination of 3,3′,5′-triiodothyronine were observed, supporting the hypothesis that these reactions are catalysed by a single enzyme (iodothyronine 5′-deiodinase). 相似文献
9.
Charles Hagedorn Stephen B. Weisberg 《Reviews in Environmental Science and Biotechnology》2009,8(3):275-287
Fecal source tracking is a rapidly evolving field for which there have been a number of method evaluation studies, workshops,
review articles and a book that synthesize information about method efficacy. Chemicals that are specific to human wastewater
offer several potential advantages over biologically based methods, but have received less scrutiny. More than 35 chemical
analytes have been found to consistently occur in human waste streams and here we review these potential human-origin indicators
in context of seven evaluation criteria. Some chemical methods offer advantages over microbial methods: they are generally
faster to prepare and analyze, more source-specific because they are not confounded by regrowth in the environment, and some
may be more geographically and temporally stable. However, they often require specialized equipment and are usually more expensive
regarding sample preparation and analysis. Additionally, most chemicals that are specific to human waste-streams occur at
concentrations low enough to be diluted below detection limits once the waste-stream enters the ambient environment. These
two factors will likely result in chemical measures being used more often as cross-validation supplements or initial screening
approaches, rather than replacements for microbial measures. Cross-validation supplements include several chemicals that are
highly specific to human sources and can be important contributors when certainties about human sources are critical, such
as in drinking water applications. At least one set of chemicals, fecal sterols and stanols, may have potential for identification
of other sources in addition to humans. Of all the chemicals examined to date, optical brighteners (OBs) in detergents have
shown considerable promise, especially for screening purposes. Optical brighteners are not as sensitive as most microbial
assessments, but can be measured with a hand-held fluorometer, providing near real-time and relatively inexpensive tracking
of signals in the field, if the human fecal source contains an OB concentration large enough to produce a measurable signal. 相似文献
10.