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   1089篇
  免费   106篇
  1195篇
  2023年   9篇
  2022年   9篇
  2021年   16篇
  2020年   11篇
  2019年   12篇
  2018年   15篇
  2017年   7篇
  2016年   27篇
  2015年   39篇
  2014年   37篇
  2013年   53篇
  2012年   59篇
  2011年   77篇
  2010年   40篇
  2009年   38篇
  2008年   52篇
  2007年   57篇
  2006年   39篇
  2005年   51篇
  2004年   49篇
  2003年   38篇
  2002年   38篇
  2001年   26篇
  2000年   9篇
  1999年   21篇
  1998年   16篇
  1997年   17篇
  1996年   14篇
  1995年   13篇
  1992年   14篇
  1991年   15篇
  1990年   6篇
  1989年   15篇
  1988年   19篇
  1987年   19篇
  1985年   12篇
  1984年   11篇
  1983年   13篇
  1982年   8篇
  1981年   6篇
  1979年   10篇
  1978年   16篇
  1977年   7篇
  1975年   16篇
  1974年   13篇
  1973年   12篇
  1972年   14篇
  1971年   16篇
  1969年   9篇
  1966年   8篇
排序方式: 共有1195条查询结果,搜索用时 23 毫秒
101.
Isotopically labeled, 15N and 15N/13C forms of recombinant methionine-r-sulfoxide reductase 1 (MsrB1, SelR) from Mus musculus were produced, in which catalytic selenocysteine was replaced with cysteine. We report here the 1H, 15N and 13C NMR assignment of the reduced form of this mammalian protein.  相似文献   
102.
103.
A Geographic Information System (GIS) is used to model plant distributions The model IS based on annual mean temperature, humidity, and a georeferenced specimen data base and predicts potential and probable distribution for each taxa in the data base Potential distribution range is suitable area determined by climatic constraints Probable distribution range is here defined as the pan of the potential range where a taxon is most likely lo be found based on distance to existing collections Gridded estimates of climatic parameters were calculated based on a Digital Elevation Model and data from ca 100 weather stations The palm family in Ecuador was used to demonstrate the model Potential and probable distribution ranges were calculated for each of the 127 Ecuadorian palm species Maps of potential and probable species richness were calculated by overlaying and accumulating individual distribution maps Potential species richness was highest in the Amazon lowland where it reached 60 species and lowest in the higher parts of the Andes and drier parts of the coastal plain The probable species richness followed a similar pattern but was nowhere higher than 47 species The ratio between potential species richness and probable species richness was used to find areas that are insufficiently sampled  相似文献   
104.
105.
The effect of probiotic bacteria Lactobacillus acidophilus NCFM and Bifidobacterium lactis Bi-07 on the composition of the Lactobacillus group, Bifidobacterium and the total bacterial population in feces from young children with atopic dermatitis was investigated. The study included 50 children randomized to intake of one of the probiotic strain or placebo. Microbial composition was characterized by denaturing gradient gel electrophoresis, quantitative PCR and, in a subset of subjects, by pyrosequencing of the 16S rRNA gene. The core population of the Lactobacillus group was identified as Lactobacillus gasseri, Lactobacillus fermentum, Lactobacillus oris, Leuconostoc mesenteroides, while the bifidobacterial community included Bifidobacterium adolescentis, Bifidobacterium bifidum, Bifidobacterium longum and Bifidobacterium catenulatum. The fecal numbers of L. acidophilus and B. lactis increased significantly after intervention, indicating survival of the ingested bacteria. The levels of Bifidobacterium correlated positively (P=0.03), while the levels of the Lactobacillus group negatively (P=0.01) with improvement of atopic eczema evaluated by the Severity Scoring of Atopic Dermatitis index. This correlation was observed across the whole study cohort and not attributed to the probiotic intake. The main conclusion of the study is that administration of L. acidophilus NCFM and B. lactis Bi-07 does not affect the composition and diversity of the main bacterial populations in feces.  相似文献   
106.
Both ecological and evolutionary timescales are of importance when considering an ecological system; population dynamics affect the evolution of species traits, and vice versa. Recently, these two timescales have been used to explain structural patterns in host-parasite networks, where the evolution of the manner in which species balance the use of their resources in interactions with each other was examined. One of these patterns was nestedness, in which the set of parasite species within a particular host forms a subset of those within a more species-rich host. Patterns of both nestedness and anti-nestedness have been observed significantly more often than expected due to chance in host-parasite networks. In contrast, mutualistic networks tend to display a significant degree of nestedness, but are rarely anti-nested. Within networks with different interaction types, therefore, there appears to be a feature promoting non-random structural patterns, such as nestedness and anti-nestedness, depending on the interaction types involved. Here, we invoke the co-evolution of species trait-values when allocating resources to interactions to explain the structural pattern of nestedness in a mutualistic community. We look at a bipartite, multi-species system, in which the strength of an interaction between two species is determined by the resources that each species invests in that relationship. We then analyze the evolution of these interactions using adaptive dynamics. We found that the evolution of these interactions, reflecting the trade-off of resources, could be used to accurately predict that nestedness occurs significantly more often than expect due to chance alone in a mutualistic network. This complements previous results applying the same concept to an antagonistic network. We conclude that population dynamics and resource trade-offs could be important promoters of structural patterns in ecological networks of different types.  相似文献   
107.
During oocyte maturation in the goldsinny wrasse (Ctenolabrus rupestris) extensive proteolysis of yolk proteins generates a large pool of free amino acids that drive hydration of the pelagic egg. By cloning hepatic vitellogenins (vtg) and using mass spectrometry, N-terminal microsequencing, and Western-immunoblotting to identify the yolk proteins (Yp), we show that multiple forms of vitellogenin mRNAs (vtgAa, vtgAb, and vtgC) are expressed in the liver, but only a single major class of the Yps derived from vtgAa predominates in the oocytes. Some Yps derived from vtgAb and vtgC appear also to be incorporated in the oocytes and eggs, but only at background levels. During oocyte hydration the vtgAa-derived lipovitellin heavy chain (LvH-Aa) and its cleavage variants are completely degraded leaving only a processed lipovitellin light chain (LvL-Aa) fragment as the major yolk protein for embryonic development. The maturational cleavage site of the LvL-Aa is identified as two amino acids downstream from the conserved Tyr(1168) of VtgAa in Atlantic halibut. In addition, although a beta'-component (approximately 18 kDa) is present in the oocytes, it is not fully degraded during the hydration process.  相似文献   
108.
We investigated the effects of updating age-specific social contact matrices to match evolving demography on vaccine impact estimates. We used a dynamic transmission model of tuberculosis in India as a case study. We modelled four incremental methods to update contact matrices over time, where each method incorporated its predecessor: fixed contact matrix (M0), preserved contact reciprocity (M1), preserved contact assortativity (M2), and preserved average contacts per individual (M3). We updated the contact matrices of a deterministic compartmental model of tuberculosis transmission, calibrated to epidemiologic data between 2000 and 2019 derived from India. We additionally calibrated the M0, M2, and M3 models to the 2050 TB incidence rate projected by the calibrated M1 model. We stratified age into three groups, children (<15y), adults (≥15y, <65y), and the elderly (≥65y), using World Population Prospects demographic data, between which we applied POLYMOD-derived social contact matrices. We simulated an M72-AS01E-like tuberculosis vaccine delivered from 2027 and estimated the per cent TB incidence rate reduction (IRR) in 2050 under each update method. We found that vaccine impact estimates in all age groups remained relatively stable between the M0–M3 models, irrespective of vaccine-targeting by age group. The maximum difference in impact, observed following adult-targeted vaccination, was 7% in the elderly, in whom we observed IRRs of 19% (uncertainty range 13–32), 20% (UR 13–31), 22% (UR 14–37), and 26% (UR 18–38) following M0, M1, M2 and M3 updates, respectively. We found that model-based TB vaccine impact estimates were relatively insensitive to demography-matched contact matrix updates in an India-like demographic and epidemiologic scenario. Current model-based TB vaccine impact estimates may be reasonably robust to the lack of contact matrix updates, but further research is needed to confirm and generalise this finding.  相似文献   
109.

Introduction

According to the fetal overnutrition hypothesis, obesity in pregnancy predisposes the offspring to obesity. Previous studies have suggested that after biliopancreatic surgery for obesity, the offspring is less likely to be obese. This study aims to further compare the BMI development of children born before and after maternal surgical weight loss.

Method

Women with at least one child born before and one child born after bariatric surgery were identified by record-linkage. Information about maternal BMI was extracted from medical records, as was information about the children''s BMI from birth to 10 years of age. We retrieved BMI data at four years of age for 340 children, born to 223 women (164 children born before surgery (BS), 176 children born after surgery (AS)). We evaluated prevalence of overweight/obesity and mean BMI in children born BS and AS at the ages of four, six and ten using GEE regression models. For 71 families, where we had complete data on mother and both children, we used a fixed-effects regression model to explore the association between differences in maternal BMI in w10 of the pre- and post-operative pregnancies with siblings'' BMI differences at age four.

Results

In no age group did we see a significantly reduced prevalence of overweight/obesity AS. For 10-year-old girls, the AS group had significantly higher rates of obesity. There was no association between differences in maternal BMI in early pregnancy and differences in siblings'' BMI at four years of age (β = −0.01, CI 95% = −0.11; 0.09).

Conclusions

We have been unable to demonstrate any effect of bariatric surgery on weight development in offspring. It seems unlikely that restrictive bariatric surgery conveys a protective effect in offspring with regards to obesity.  相似文献   
110.
The levels of intramolecular plasmid recombination, following transfection of a plasmid substrate for homologous recombination into normal and immortally transformed cells, have been examined by two independent assays. In the first assay, recovered plasmid was tested for DNA rearrangements which regenerate a functional neomycin resistance gene from two overlapping fragments. Following transformation of bacteria, frequencies of recombinationlike events were determined from the ratio of neomycin-resistant (recombinant) colonies to ampicillin-resistant colonies (indicating total plasmid recovery). Such events, yielding predominantly deletions between the directly repeated sequences, were substantially more frequent in five immortal cell lines than in any of three normal diploid cell strains tested. Effects of plasmid replication or interaction with T antigen and of bacterially mediated rejoining of linear molecules generated in mammalian cells were excluded by appropriate controls. The second assay used limited coamplification of a control segment of plasmid DNA, and of the predicted recombinant DNA region, primed by two sets of flanking oligonucleotides. Each amplified band was quantitated by reference to a near-linear standard curve generated concurrently, and recombination frequencies were determined from the ratio of recombinant/control DNA regions. The results confirmed that recombinant DNA structures were generated within human cells at direct repeats in the transfected plasmid and were markedly more abundant in an immortal cell line than in the diploid normal cells from which that line was derived.  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