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   473篇
  免费   48篇
  521篇
  2022年   3篇
  2021年   6篇
  2020年   7篇
  2019年   7篇
  2018年   7篇
  2017年   6篇
  2016年   14篇
  2015年   13篇
  2014年   14篇
  2013年   16篇
  2012年   32篇
  2011年   27篇
  2010年   7篇
  2009年   11篇
  2008年   21篇
  2007年   18篇
  2006年   26篇
  2005年   21篇
  2004年   22篇
  2003年   17篇
  2002年   15篇
  2001年   9篇
  2000年   15篇
  1999年   13篇
  1998年   11篇
  1997年   6篇
  1996年   5篇
  1994年   5篇
  1993年   9篇
  1992年   4篇
  1991年   6篇
  1990年   7篇
  1989年   16篇
  1988年   10篇
  1987年   5篇
  1986年   8篇
  1985年   6篇
  1984年   8篇
  1983年   3篇
  1982年   7篇
  1980年   5篇
  1979年   6篇
  1976年   6篇
  1975年   6篇
  1974年   3篇
  1973年   3篇
  1969年   5篇
  1939年   2篇
  1934年   2篇
  1887年   2篇
排序方式: 共有521条查询结果,搜索用时 15 毫秒
81.
We investigated acute and chronic hypoxic pulmonary pressor responses in two groups of calves, one bred to be susceptible, the other resistant to high-altitude pulmonary hypertension. Twelve 5-mo-old susceptible calves residing at 1,524 m increased their mean pulmonary arterial pressure from 26 +/- 2 (SE) to 55 +/- 4 mmHg during 2 h at a simulated altitude of 4,572 m. In 10 resistant calves pressure increased from 22 +/- 1 to 37 +/- 2 mmHg. Five calves were selected from each group for further study. When 9 mo old, the 5 susceptible calves again showed a greater pressor response to acute hypoxia (27 +/- 1 to 55 +/- 4 mmHg) than did 5 resistant calves (23 +/- 1 to 41 +/- 3 mmHg). When 12 mo old, the 5 susceptible calves also developed a greater increase in pulmonary arterial pressure (21 +/- 2 to 9 +/- 4 mmHg) during 18 days at 4,572 m than did the 5 resistant calves (21 +/- 1 to 64 +/- 4 mmHg). Acute and chronic hypoxic pulmonary pressor responses were highly correlated (r = 0.91; P less than 0.001) indicating that they were probably produced through a common mechanism.  相似文献   
82.
Codling moth female calling and male pheromone responsiveness under the defined conditions of 23°C and light:dark (LD) 16:8 occurred primarily during scotophase. Under either continuous photophase or scotophase females called with periodicities very similar to their periodicity under the LD cycle, indicating that the rhythmicity is circadian. Male response rhythmicity was maintained under continuous photophase. A decrease in the temperature from 23° to 16°C resulted in a reduction in the proportion calling when the decrease in temperature occurred during scotophase and a shift of maximal calling into photophase when the decrease in temperature occurred 3 hr prior to the initiation of scotophase. Decreases of temperature from 23° to 16°C and of light intensity did not produce similar shifts in the periodicity of male upwind orientation. Of 6 pheromone dosages from 10?5 to 102 μg, 10?1 and 100 μg elicited the most male upwind orientation.  相似文献   
83.
As a greater number and diversity of high-quality vertebrate reference genomes become available, it is increasingly feasible to use these references to guide new draft assemblies for related species. Reference-guided assembly approaches may substantially increase the contiguity and completeness of a new genome using only low levels of genome coverage that might otherwise be insufficient for de novo genome assembly. We used low-coverage (∼3.5–5.5x) Illumina paired-end sequencing to assemble draft genomes of two bird species (the Gunnison Sage-Grouse, Centrocercus minimus, and the Clark''s Nutcracker, Nucifraga columbiana). We used these data to estimate de novo genome assemblies and reference-guided assemblies, and compared the information content and completeness of these assemblies by comparing CEGMA gene set representation, repeat element content, simple sequence repeat content, and GC isochore structure among assemblies. Our results demonstrate that even lower-coverage genome sequencing projects are capable of producing informative and useful genomic resources, particularly through the use of reference-guided assemblies.  相似文献   
84.
Abstract

With the aid of molecular modelling both adenosine and adenosine A, receptor antagonists belonging to various chemical classes were compared. A model for the antagonist binding site was developed. As a consequence 1H-imidazo[4, 5-c]-quinolin-4-amines were synthesized, constituting a novel class of potent non-xanthine adenosine receptor antagonists.  相似文献   
85.
Defining the design principles guiding the fabrication of superior biocomposite structures from an assemblage of ordinary molecules is a key goal of biomimetics. Considering their low degree of mineralization, Glycera jaws have been shown to be extraordinarily resistant to abrasion based on the metric hardness3/Young's modulus2. The jaws also exhibit an impressive chemical inertness withstanding boiling concentrated hydrochloric acid as well as boiling concentrated sodium hydroxide. A major organic component largely responsible for the chemical inertness of the jaws has been characterized using a spectrophotometric assay for melanin content, 13C solid state nuclear magnetic resonance, IR spectroscopy, and laser desorption ionization-time of flight mass spectrometry and is identified here as a melanin-like network. Although melanin is widely distributed as a pigment in tissues and other structural biomaterials, to our knowledge, Glycera jaws represent the first known integument to exploit melanin as a cohesive load- and shape-bearing material.  相似文献   
86.
87.
Increased IFN-α signaling is a heritable risk factor for systemic lupus erythematosus (SLE). IFN induced with helicase C domain 1 (IFIH1) is a cytoplasmic dsRNA sensor that activates IFN-α pathway signaling. We studied the impact of the autoimmune-disease-associated IFIH1 rs1990760 (A946T) single nucleotide polymorphism upon IFN-α signaling in SLE patients in vivo. We studied 563 SLE patients (278 African-American, 179 European-American, and 106 Hispanic-American). Logistic regression models were used to detect genetic associations with autoantibody traits, and multiple linear regression was used to analyze IFN-α-induced gene expression in PBMCs in the context of serum IFN-α in the same blood sample. We found that the rs1990760 T allele was associated with anti-dsDNA Abs across all of the studied ancestral backgrounds (meta-analysis odds ratio = 1.34, p = 0.026). This allele also was associated with lower serum IFN-α levels in subjects who had anti-dsDNA Abs (p = 0.0026). When we studied simultaneous serum and PBMC samples from SLE patients, we found that the IFIH1 rs1990760 T allele was associated with increased IFN-induced gene expression in PBMCs in response to a given amount of serum IFN-α in anti-dsDNA-positive patients. This effect was independent of the STAT4 genotype, which modulates sensitivity to IFN-α in a similar way. Thus, the IFIH1 rs1990760 T allele was associated with dsDNA Abs, and in patients with anti-dsDNA Abs this risk allele increased sensitivity to IFN-α signaling. These studies suggest a role for the IFIH1 risk allele in SLE in vivo.  相似文献   
88.
Can a 'flawless' live vector vaccine strain be engineered?   总被引:13,自引:0,他引:13  
The efficiency of any live bacterial vector vaccine hinges on its ability to present sufficient foreign antigen to the human immune system to initiate the desired protective immune response(s). However, synthesis of sufficient levels of heterologous antigen can result in an increase in metabolic burden with an accompanying decrease in the fitness of the live vector, which can ultimately lower desired immune responses to both live vector and heterologous antigen. Here, we explore the underlying mechanisms of metabolic load and propose ways of minimizing such burdens to enhance the fitness and immunogenicity of Salmonella-based live vector vaccines.  相似文献   
89.

Background

To have public health impact, evidence-based interventions (EBIs) must be implemented appropriately at meaningful scale. The Center for Disease Control and Prevention’s Replicating Effective Programs and Diffusion of Effective Behavioral Interventions programs disseminate select EBIs by providing program materials and training health providers on their appropriate use and implementation. Sociometrics’ HIV/AIDS Prevention Program Archive (HAPPA) and Program Archive for Sexuality, Health, and Adolescents (PASHA) are likewise the largest EBI collections targeting sexual risk behaviors in the private sector. This study examined the extent to which organizations that obtain EBIs from HAPPA and PASHA implement, adapt and evaluate them and factors associated with program implementation.

Methods

Survey data were collected from 123 organizations that acquired, and had been in possession for a minimum of six months, at least one EBI from HAPPA or PASHA between January 2009 and June 2011. Data regarding program characteristics and date of acquisition were obtained from Sociometrics’ sales and marketing databases. Logistic regression was used to assess barriers to program implementation.

Results

Among organizations that obtained an EBI from Sociometrics intending to implement it, 53% had implemented the program at least once or were in the process of implementing the program for the first time; another 22% were preparing for implementation. Over the three-year time period assessed, over 11,381 individuals participated in these interventions. Almost two-thirds (65%) of implementers made changes to the original program. Common adaptations included: editing content to be more current and of local relevance (81%); adding, deleting or modifying incentives for participation (50%); changing the location in which the program takes place (44%); and/or changing the number, length and/or frequency of program sessions (42%). In total, 80% of implementers monitored program delivery. Participant outcomes were tracked by 78%; 28% of which used evaluation designs that included a control or comparison group. Lack of adequate resources was significantly associated with decreased likelihood of program implementation (odds ratio?=?0.180, p <0.05).

Conclusions

Findings provide greater understanding of implementation processes, barriers and facilitators that may be used to develop strategies to increase the appropriate use of EBIs.  相似文献   
90.
Environmental adaptation and species divergence often involve suites of co‐evolving traits. Pigmentation in insects presents a variable, adaptive, and well‐characterized class of phenotypes for which correlations with multiple other traits have been demonstrated. In Drosophila, the pigmentation genes ebony and tan have pleiotropic effects on flies'' response to light, creating the potential for correlated evolution of pigmentation and vision. Here, we investigate differences in light preference within and between two sister species, Drosophila americana and D. novamexicana, which differ in pigmentation in part because of evolution at ebony and tan and occupy environments that differ in many variables including solar radiation. We hypothesized that lighter pigmentation would be correlated with a greater preference for environmental light and tested this hypothesis using a habitat choice experiment. In a first set of experiments, using males of D. novamexicana line N14 and D. americana line A00, the light‐bodied D. novamexicana was found slightly but significantly more often than D. americana in the light habitat. A second experiment, which included additional lines and females as well as males, failed to find any significant difference between D. novamexicana‐N14 and D. americana‐A00. Additionally, the other dark line of D. americana (A04) was found in the light habitat more often than the light‐bodied D. novamexicana‐N14, in contrast to our predictions. However, the lightest line of D. americana, A01, was found substantially and significantly more often in the light habitat than the two darker lines of D. americana, thus providing partial support for our hypothesis. Finally, across all four lines, females were found more often in the light habitat than their more darkly pigmented male counterparts. Additional replication is needed to corroborate these findings and evaluate conflicting results, with the consistent effect of sex within and between species providing an especially intriguing avenue for further research.  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