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   1902篇
  免费   119篇
  2023年   8篇
  2022年   12篇
  2021年   38篇
  2020年   15篇
  2019年   43篇
  2018年   44篇
  2017年   27篇
  2016年   42篇
  2015年   84篇
  2014年   115篇
  2013年   132篇
  2012年   139篇
  2011年   154篇
  2010年   92篇
  2009年   92篇
  2008年   123篇
  2007年   116篇
  2006年   109篇
  2005年   110篇
  2004年   110篇
  2003年   82篇
  2002年   72篇
  2001年   22篇
  2000年   19篇
  1999年   19篇
  1998年   19篇
  1997年   12篇
  1996年   11篇
  1995年   17篇
  1994年   17篇
  1993年   10篇
  1992年   14篇
  1991年   8篇
  1990年   7篇
  1989年   7篇
  1988年   7篇
  1987年   5篇
  1986年   4篇
  1985年   5篇
  1984年   11篇
  1983年   4篇
  1982年   8篇
  1981年   3篇
  1980年   3篇
  1979年   6篇
  1978年   3篇
  1975年   4篇
  1972年   2篇
  1970年   2篇
  1959年   2篇
排序方式: 共有2021条查询结果,搜索用时 46 毫秒
81.
Invasive animals depend on finding a balanced nutritional intake to colonize, survive, and reproduce in new environments. This can be especially challenging during situations of fluctuating cold temperatures and food scarcity, but phenotypic plasticity may offer an adaptive advantage during these periods. We examined how lifespan, fecundity, pre‐oviposition periods, and body nutrient contents were affected by dietary protein and carbohydrate (P:C) ratios at variable low temperatures in two morphs (winter morphs WM and summer morphs SM) of an invasive fly, Drosophila suzukii. The experimental conditions simulated early spring after overwintering and autumn, crucial periods for survival. At lower temperatures, post‐overwintering WM lived longer on carbohydrate‐only diets and had higher fecundity on low‐protein diets, but there was no difference in lifespan or fecundity among diets for SM. As temperatures increased, low‐protein diets resulted in higher fecundity without compromising lifespan, while high‐protein diets reduced lifespan and fecundity for both WM and SM. Both SM and WM receiving high‐protein diets had lower sugar, lipid, and glycogen (but similar protein) body contents compared to flies receiving low‐protein and carbohydrate‐only diets. This suggests that flies spend energy excreting excess dietary protein, thereby affecting lifespan and fecundity. Despite having to recover from nutrient depletion after an overwintering period, WM exhibited longer lifespan and higher fecundity than SM in favorable diets and temperatures. WM exposed to favorable low‐protein diet had higher body sugar, lipid, and protein body contents than SM, which is possibly linked to better performance. Although protein is essential for oogenesis, WM and SM flies receiving low‐protein diets did not have shorter pre‐oviposition periods compared to flies on carbohydrate‐only diets. Finding adequate carbohydrate sources to compensate protein intake is essential for the successful persistence of D. suzukii WM and SM populations during suboptimal temperatures.  相似文献   
82.
Recently, there are controversial opinions on the presence of Mycoplasmas/Ureaplasmas as colonizers or pathogens, and on the use of a targeted therapy. This study aimed to characterize Mycoplasmas/Ureaplasmas infections in reproductive age women, including the acquisition of sexually transmitted (ST) pathogens and poor birth outcomes. A total of 646 healthy Italian women fulfilled the inclusion criteria including 521 infertile women, 65 pregnant women, and 60 fertile women with identified risk factors and symptomatic for vaginitis/cervicitis. Multiplex and quantitative molecular techniques and direct automatic DNA sequencing were performed to assess the genome structure of Mycoplasma/Ureaplasma species and ST infected pathogens. Ureaplasma parvum serovar 3 represented the predominant colonizer of the urogenital tract of this series and the unique species significantly associated with ST pathogens coinfection (p < 0.01). U. parvum load >104 bacteria/ml, suggestive of active infection, has been measured only in asymptomatic high-risk human papillomavirus infected women (24.3%) and in 40% of women with idiopathic infertility. To note, 16% of the follicular fluid from these idiopathic women resulted infected with U. parvum. In conclusion, the present study focused the attention on U. parvum serovar 3 as emerging microorganism in sexually active women that may have the benefit of targeted therapy.  相似文献   
83.
Plant and Soil - The aims of this research were: i) to compare Cr, Cu, Ni and Pb concentrations in Quercus ilex L. leaves collected at urban/industrial and urban areas; ii) to investigate the main...  相似文献   
84.
Plant Molecular Biology - Overexpression of BoMYB29 gene up-regulates the aliphatic glucosinolate pathway in Brassica oleracea plants increasing the production of the anti-cancer metabolite...  相似文献   
85.
Excitatory amino acids (EAAs) are found present in the nervous and reproductive systems of animals. Numerous studies have demonstrated a regulatory role for Glutamate (Glu), d -aspartate ( d -Asp) and N-methyl- d -aspartate (NMDA) in the control of spermatogenesis. EAAs are able to stimulate the Glutamate receptors, including the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). Here in, we assess expression of the main AMPAR subunits, GluA1 and GluA2/3, in the mouse testis and in spermatogonial GC-1 cells. The results showed that both GluA1 and GluA2/3 were localized in mouse testis prevalently in spermatogonia. The subunit GluA2/3 was more highly expressed compared with GluA1 in both the testis and the GC-1 cells. Subsequently, GC-1 cells were incubated with medium containing l -Glu, d -Glu, d -Asp or NMDA to determine GluA1 and GluA2/3 expressions. At 30 minutes and 2 hours of incubation, EAA-treated GC-1 cells showed significantly higher expression levels of both GluA1 and GluA2/3. Furthermore, p-extracellular signal-regulated kinase (ERK), p-Akt, proliferating cell nuclear antigen (PCNA), and Aurora B expressions were assayed in l -Glu-, d -Glu-, and NMDA-treated GC-1 cells. At 30 minutes and 2 hours of incubation, treated GC-1 cells showed significantly higher expression levels of p-ERK and p-Akt. A consequent increase of PCNA and Aurora B expressions was induced by l -Glu and NMDA, but not by d -Glu. Our study demonstrates a direct effect of the EAAs on spermatogonial activity. In addition, the increased protein expression levels of GluA1 and GluA2/3 in EAA-treated GC-1 cells suggest that EAAs could activate ERK and Akt pathways through the AMPAR. Finally, the increased PCNA and Aurora B levels may imply an enhanced proliferative activity.  相似文献   
86.
Extreme weather events are predicted to increase as a result of climate change, yet amphibian responses to extreme disturbance events remain understudied, especially in the Neotropics. Recently, an unprecedented windstorm within a protected Costa Rican rainforest opened large light gaps in sites where we have studied behavioral responses of diurnal strawberry poison frogs (Oophaga pumilio) to ultraviolet radiation for nearly two decades. Previous studies demonstrate that O. pumilio selects and defends perches where ultraviolet radiation (UV‐B) is relatively low, likely because of the lethal and sublethal effects of UV‐B. In this natural experiment, we quantified disturbance to O. pumilio habitat, surveyed for the presence of O. pumilio in both high‐disturbance and low‐disturbance areas of the forest, and assessed UV‐B levels and perch selection behavior in both disturbance levels. Fewer frogs were detected in high‐disturbance habitat than in low‐disturbance habitat. In general, frogs were found vocalizing at perches in both disturbance levels, and in both cases, in significantly lower UV‐B levels relative to ambient adjacent surroundings. However, frogs at perches in high‐disturbance areas were exposed to UV‐B levels nearly 10 times greater than males at perches in low‐disturbance areas. Thus, behavioral avoidance of UV‐B may not reduce the risks associated with elevated exposure under these novel conditions, and similarly, if future climate and human‐driven land‐use change lead to sustained analogous environments.  相似文献   
87.
KNL1 is a large intrinsically disordered kinetochore (KT) protein that recruits spindle assembly checkpoint (SAC) components to mediate SAC signaling. The N-terminal region (NTR) of KNL1 possesses two activities that have been implicated in SAC silencing: microtubule (MT) binding and protein phosphatase 1 (PP1) recruitment. The NTR of Drosophila melanogaster KNL1 (Spc105) has never been shown to bind MTs or to recruit PP1. Furthermore, the phosphoregulatory mechanisms known to control SAC protein binding to KNL1 orthologues is absent in D. melanogaster. Here, these apparent discrepancies are resolved using in vitro and cell-based assays. A phosphoregulatory circuit that utilizes Aurora B kinase promotes SAC protein binding to the central disordered region of Spc105 while the NTR binds directly to MTs in vitro and recruits PP1-87B to KTs in vivo. Live-cell assays employing an optogenetic oligomerization tag and deletion/chimera mutants are used to define the interplay of MT and PP1 binding by Spc105 and the relative contributions of both activities to the kinetics of SAC satisfaction.  相似文献   
88.
89.
The mouse Adh1 gene exhibits tissue-specific regulation, is developmentally regulated, and is androgen regulated in kidney and adrenal tissue. To study this complex regulation phenotype a transgenic mouse approach has been used to investigate regulatory regions of the gene necessary for proper tissue expression and hormonal control. Transgenic mice have been produced with an Adh1 minigene as a reporter behind either 2.5- or 10 kb of 5'-flanking sequence [1]. Complete androgen regulation in kidney requires a region between -2.5 and -10 kb. A sequence extending to -10 kb does not confer liver expression in this minigene construct. B6.S mice express an electrophoretically variant protein resulting from a known nucleotide substitution resulting in a restriction endonuclease length polymorphism. Transgenic mice harboring B6.S cosmids can be studied for expression analysis at both protein and mRNA levels, identification of transgenic founders and inheritance studies are greatly facilitated by a PCR-restriction endonuclease cleavage approach, the entire mouse gene is used as a reporter, and the formation of heterodimeric enzyme molecules can be used to infer expression of the transgene in the proper cell types within a given tissue. Expression of a B6.S cosmid containing the entire Adh1 gene and 6 kb of 5'- and 21 kb of 3'-flanking region occurs in transgenic mice in a copy number dependent manner in a number of tissues, but expression in liver does not occur. The ability to analyze expression at the protein and mRNA levels has been confirmed using this system. Future directions will involve the use of large BAC clones modified by RARE cleavage to identify the liver specific elements necessary for expression.  相似文献   
90.
In this paper we identify biologically relevant families of models whose structural identifiability analysis could not be performed with available techniques directly. The models considered come from both the immunological and epidemiological literature.  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