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   3945篇
  免费   290篇
  国内免费   1篇
  4236篇
  2023年   11篇
  2022年   35篇
  2021年   65篇
  2020年   38篇
  2019年   51篇
  2018年   82篇
  2017年   60篇
  2016年   120篇
  2015年   212篇
  2014年   218篇
  2013年   284篇
  2012年   373篇
  2011年   367篇
  2010年   186篇
  2009年   164篇
  2008年   262篇
  2007年   266篇
  2006年   223篇
  2005年   235篇
  2004年   200篇
  2003年   181篇
  2002年   180篇
  2001年   31篇
  2000年   23篇
  1999年   39篇
  1998年   40篇
  1997年   45篇
  1996年   37篇
  1995年   24篇
  1994年   25篇
  1993年   17篇
  1992年   19篇
  1991年   16篇
  1990年   14篇
  1989年   11篇
  1988年   12篇
  1987年   10篇
  1986年   17篇
  1985年   7篇
  1984年   9篇
  1983年   7篇
  1982年   4篇
  1981年   4篇
  1980年   2篇
  1978年   4篇
  1976年   1篇
  1968年   1篇
  1967年   1篇
  1966年   1篇
  1965年   1篇
排序方式: 共有4236条查询结果,搜索用时 78 毫秒
11.
12.
Pathogen access to host nutrients in infected tissues is fundamental for pathogen growth and virulence, disease progression, and infection control. However, our understanding of this crucial process is still rather limited because of experimental and conceptual challenges. Here, we used proteomics, microbial genetics, competitive infections, and computational approaches to obtain a comprehensive overview of Salmonella nutrition and growth in a mouse typhoid fever model. The data revealed that Salmonella accessed an unexpectedly diverse set of at least 31 different host nutrients in infected tissues but the individual nutrients were available in only scarce amounts. Salmonella adapted to this situation by expressing versatile catabolic pathways to simultaneously exploit multiple host nutrients. A genome-scale computational model of Salmonella in vivo metabolism based on these data was fully consistent with independent large-scale experimental data on Salmonella enzyme quantities, and correctly predicted 92% of 738 reported experimental mutant virulence phenotypes, suggesting that our analysis provided a comprehensive overview of host nutrient supply, Salmonella metabolism, and Salmonella growth during infection. Comparison of metabolic networks of other pathogens suggested that complex host/pathogen nutritional interfaces are a common feature underlying many infectious diseases.  相似文献   
13.
CRISPR-associated nucleases are powerful tools for precise genome editing of model systems, including human organoids. Current methods describing fluorescent gene tagging in organoids rely on the generation of DNA double-strand breaks (DSBs) to stimulate homology-directed repair (HDR) or non-homologous end joining (NHEJ)-mediated integration of the desired knock-in. A major downside associated with DSB-mediated genome editing is the required clonal selection and expansion of candidate organoids to verify the genomic integrity of the targeted locus and to confirm the absence of off-target indels. By contrast, concurrent nicking of the genomic locus and targeting vector, known as in-trans paired nicking (ITPN), stimulates efficient HDR-mediated genome editing to generate large knock-ins without introducing DSBs. Here, we show that ITPN allows for fast, highly efficient, and indel-free fluorescent gene tagging in human normal and cancer organoids. Highlighting the ease and efficiency of ITPN, we generate triple fluorescent knock-in organoids where 3 genomic loci were simultaneously modified in a single round of targeting. In addition, we generated model systems with allele-specific readouts by differentially modifying maternal and paternal alleles in one step. ITPN using our palette of targeting vectors, publicly available from Addgene, is ideally suited for generating error-free heterozygous knock-ins in human organoids.

A major downside of double-strand break-mediated genome editing is the need to verify the genomic integrity of the targeted locus and confirm the absence of off-target indels. This study shows that in-trans paired nicking is a mutation-free CRISPR strategy to introduce precise knock-ins into human organoids; its genomic fidelity allows all knock-in cells to be pooled, accelerating the establishment of new organoid models.  相似文献   
14.
While in fungi iron transport via hydroxamate siderophores has been amply proven, iron transport via enterobactin is largely unknown. Enterobactin is a catecholate-type siderophore produced by several enterobacterial genera grown in severe iron deprivation. By using the KanMX disruption module in vector pUG6 in a fet3 background of Saccharomyces cerevisiae we were able to disrupt the gene YOL158c Sce of the major facilitator super family (MFS) which has been previously described as a gene encoding a membrane transporter of unknown function. Contrary to the parental strain, the disruptant was unable to utilize ferric enterobactin in growth promotion tests and in transport assays using 55Fe-enterobactin. All other siderophore transport properties remained unaffected. The results are evidence that in S. cerevisiae the YOL158c Sce gene of the major facilitator super family, now designated ENB1, encodes a transporter protein (Enb1p), which specifically recognizes and transports enterobactin.  相似文献   
15.
Kin selection can explain the evolution of cooperative breeding and the distribution of relatives within a population may influence the benefits of cooperative behaviour. We provide genetic data on relatedness in the cooperatively breeding cichlid Neolamprologus pulcher. Helper to breeder relatedness decreased steeply with increasing helper age, particularly to the breeding males. Helper to helper relatedness was age‐assortative and also declined with age. These patterns of relatedness could be attributed to territory take‐overs by outsiders when breeders had disappeared (more in breeding males), between‐group dispersal of helpers and reproductive parasitism. In six of 31 groups females inherited the breeding position of their mother or sister. These matrilines were more likely to occur in large groups. We conclude that the relative fitness benefits of helping gained through kin selection vs. those gained through direct selection depend on helper age and sex.  相似文献   
16.
The release of energy from particulate substrates such as dietary fiber and resistant starch (RS) in the human colon may depend on the presence of specialist primary degraders (or ‘keystone species'') within the microbial community. We have explored the roles of four dominant amylolytic bacteria found in the human colon in the degradation and utilization of resistant starches. Eubacterium rectale and Bacteroides thetaiotaomicron showed limited ability to utilize RS2- and RS3-resistant starches by comparison with Bifidobacterium adolescentis and Ruminococcus bromii. In co-culture, however, R. bromii proved unique in stimulating RS2 and RS3 utilization by the other three bacterial species, even in a medium that does not permit growth of R. bromii itself. Having previously demonstrated low RS3 fermentation in vivo in two individuals with undetectable populations of R. bromii-related bacteria, we show here that supplementation of mixed fecal bacteria from one of these volunteers with R. bromii, but not with the other three species, greatly enhanced the extent of RS3 fermentation in vitro. This argues strongly that R. bromii has a pivotal role in fermentation of RS3 in the human large intestine, and that variation in the occurrence of this species and its close relatives may be a primary cause of variable energy recovery from this important component of the diet. This work also indicates that R. bromii possesses an exceptional ability to colonize and degrade starch particles when compared with previously studied amylolytic bacteria from the human colon.  相似文献   
17.
While magnetoreception in birds has been studied intensively, the literature on magnetoreception in bony fish, and particularly in non-migratory fish, is quite scarce. We examined alignment of common carps (Cyprinus carpio) at traditional Christmas sale in the Czech Republic. The sample comprised measurements of the directional bearings in 14,537 individual fish, distributed among 80 large circular plastic tubs, at 25 localities in the Czech Republic, during 817 sampling sessions, on seven subsequent days in December 2011. We found that carps displayed a statistically highly significant spontaneous preference to align their bodies along the North-South axis. In the absence of any other common orientation cues which could explain this directional preference, we attribute the alignment of the fish to the geomagnetic field lines. It is apparent that the display of magnetic alignment is a simple experimental paradigm of great heuristic potential.  相似文献   
18.
19.
20.
A phylogenetic framework is developed for the clubmoss family Selaginellaceae based on maximum parsimony analyses of molecular data. The chloroplast gene rbcL was sequenced for 62 species, which represent nearly 10% of living species diversity in the family. Taxa were chosen to reflect morphological, geographical, and ecological diversity. The analyses provide support for monophyly of subgenera Selaginella and Tetragonostachys. Stachygynandrum and Heterostachys are polyphyletic. Monophyly of Ericetorum is uncertain. Results also indicate a large number of new groupings not previously recognized on morphological grounds. Some of these new groups seem to have corresponding morphological synapomorphies, such as the presence of rhizophores (distinctive root-like structures), aspects of rhizophore development, and leaf and stem morphology. Others share distinctive ecological traits (e.g., xerophytism). For many groups, however, no morphological, ecological, or physiological markers are known. This could reflect patchy sampling and a lack of detailed knowledge about many species. Despite a lengthy fossil record dating from the Carboniferous Period, cladogram topology indicates that most of the living tropical species are probably the products of more recent diversifications. Resurrection plants, extreme xerophytes characterized by aridity-driven inrolling of branches and rapid revival on rehydration, have evolved at least three times in quite different clades.  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