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   2759篇
  免费   242篇
  国内免费   1篇
  3002篇
  2024年   3篇
  2023年   22篇
  2022年   40篇
  2021年   85篇
  2020年   60篇
  2019年   51篇
  2018年   87篇
  2017年   66篇
  2016年   140篇
  2015年   166篇
  2014年   171篇
  2013年   223篇
  2012年   250篇
  2011年   226篇
  2010年   143篇
  2009年   111篇
  2008年   178篇
  2007年   176篇
  2006年   131篇
  2005年   130篇
  2004年   110篇
  2003年   104篇
  2002年   79篇
  2001年   19篇
  2000年   13篇
  1999年   20篇
  1998年   16篇
  1997年   16篇
  1996年   16篇
  1995年   15篇
  1994年   7篇
  1993年   10篇
  1992年   14篇
  1991年   6篇
  1990年   10篇
  1989年   8篇
  1988年   7篇
  1987年   5篇
  1986年   9篇
  1985年   5篇
  1984年   5篇
  1982年   4篇
  1979年   3篇
  1974年   2篇
  1973年   5篇
  1970年   4篇
  1962年   3篇
  1958年   3篇
  1944年   3篇
  1942年   2篇
排序方式: 共有3002条查询结果,搜索用时 15 毫秒
1.
First appearing in 2000, and with 20,000 copies sold as it entered its sixth printing in 2004, Richard Trudgen's Why Warriors Lie down and Die is a minor publishing phenomenon. Although scholarly forums have generally ignored the book, Christian media outlets, the popular press and neo‐conservative political activists have enthusiastically received and promoted it. An examination of the merits of Why Warriors Lie down and Die suggests that its popularity is only partly explained by original observations or insights on the part of the author. The most important explanation, and implication, of the book's remarkable take‐up lies in the opportune way in which it corresponds with now ascendant neo‐conservative political perspectives on indigenous policy.  相似文献   
2.
3.
Controlled generation of reactive oxygen species orchestrates numerous physiological signaling events (Finkel, T. (2011) Signal transduction by reactive oxygen species. J. Cell Biol. 194, 7–15). A major cellular target of reactive oxygen species is the thiol side chain (RSH) of Cys, which may assume a wide range of oxidation states (i.e. −2 to +4). Within this context, Cys sulfenic (Cys-SOH) and sulfinic (Cys-SO2H) acids have emerged as important mechanisms for regulation of protein function. Although this area has been under investigation for over a decade, the scope and biological role of sulfenic/sulfinic acid modifications have been recently expanded with the introduction of new tools for monitoring cysteine oxidation in vitro and directly in cells. This minireview discusses selected recent examples of protein sulfenylation and sulfinylation from the literature, highlighting the role of these post-translational modifications in cell signaling.  相似文献   
4.
ABSTRACT.   Latitudinal variation in population structure during the winter has been reported in many migratory birds, but has been documented in few species of waterfowl. Variation in environmental and social conditions at wintering sites can potentially influence the population dynamics of differential migrants. We examined latitudinal variation in sex and age classes of wintering Pacific Black Brant ( Branta bernicla nigricans ). Brant are distributed along a wide latitudinal gradient from Alaska to Mexico during the winter. Accordingly, migration distances for brant using different wintering locations are highly variable and winter settlement patterns are likely associated with a spatially variable food resource. We used resightings of brant banded in southwestern Alaska to examine sex and age ratios of birds wintering at Boundary Bay in British Columbia, and at San Quintin Bay, Ojo de Liebre Lagoon, and San Ignacio Lagoon in Baja California from 1998 to 2000. Sex ratios were similar among wintering locations for adults and were consistent with the mating strategy of geese. The distribution of juveniles varied among wintering areas, with greater proportions of juveniles observed at northern (San Quintin Bay and Ojo de Liebre Lagoon) than at southern (San Ignacio Lagoon) locations in Baja California. We suggest that age-related variation in the winter distribution of Pacific Black Brant is mediated by variation in productivity among individuals at different wintering locations and by social interactions among wintering family groups.  相似文献   
5.
The use of reporter genes to characterise sequence elements that act to regulate gene expression in transgenic plants has been vital to the development of foreign gene expression strategies for use in cereal transformation. ThegusA locus ofEscherichia coli, which encodes the enzyme-glucuronidase (GUS), is by far the most popular reporter gene used in plant transformation. In this paper we extend the utility of the GUS reporter gene system in cereal transformation by describing and evaluating a number of novel constructs suitable for use in direct gene transfer experiments. These plasmids are all available from the Molecular Genetic Resource Service of the Center for the Application of Molecular Biology to International Agriculture.  相似文献   
6.
A simple and versatile method of in vitro site-specific mutagenesis based on polymerase chain reaction (PCR) is described. The complete method required the use of three oligonucleotide primers and two PCRs. The product of the first PCR was used as one of the primers (megaprimer) in the second PCR. Essentially 100% of the final product incorporated the desired mutation. The various aspects of the procedure and its application is described in detail.  相似文献   
7.
Waardenburg syndrome (WS) is a dominantly inherited and clinically variable syndrome of deafness, pigmentary changes, and distinctive facial features. Clinically, WS type I (WS1) is differentiated from WS type II (WS2) by the high frequency of dystopia canthorum in the family. In some families, WS is caused by mutations in the PAX3 gene on chromosome 2q. We have typed microsatellite markers within and flanking PAX3 in 41 WS1 kindreds and 26 WS2 kindreds in order to estimate the proportion of families with probable mutations in PAX3 and to study the relationship between phenotypic and genotypic heterogeneity. Evaluation of heterogeneity in location scores obtained by multilocus analysis indicated that WS is linked to PAX3 in 60% of all WS families and in 100% of WS1 families. None of the WS2 families were linked. In those families in which equivocal lod scores (between −2 and +1) were found, PAX3 mutations have been identified in 5 of the 15 WS1 families but in none of the 4 WS2 families. Although preliminary studies do not suggest any association between the phenotype and the molecular pathology in 20 families with known PAX3 mutations and in four patients with chromosomal abnormalities in the vicinity of PAX3, the presence of dystopia in multiple family members is a reliable indicator for identifying families likely to have a defect in PAX3.  相似文献   
8.
Antibiotic resistance genes can act as either cell autonomous or non-cell autonomous genetic markers with which to monitor the excision of plant transposons. To convert spectinomycin resistance from a noncell autonomous resistance to cell autonomous resistance, a transit peptide for chloroplast localization from a petunia ribulose bisphosphate carboxylase (rbcS) gene was fused in-frame to the aadA gene, which confers spectinomycin and streptomycin resistance. Constructs were generated in which the expression of this chimeric gene was prevented by the presence, in the 5 untranslated leader, of the maize transposons Activator (Ac) or Dissociation (Ds). When progeny of tobacco or tomato plants transformed with these constructs were germinated on spectinomycin-containing medium, germinally revertant and somatically variegated individuals could be distinguished.  相似文献   
9.
The Ac/Ds transposon system of maize shows low activity in Arabidopsis. However, fusion of the CaMV 35S promoter to the transposase gene (35S::TPase) increases the abundance of the single Ac mRNA encoded by Ac and increases the frequency of Ds excision. In the experiments reported here it is examined whether this high excision frequency is associated with efficient re-insertion of the transposon. This was measured by using a Ds that carried a hygromycin resistance gene (HPT) and was inserted within a streptomycin resistance gene (SPT). Excision of Ds therefore gives rise to streptomycin resistance, while hygromycin resistance is associated with the presence of a transposed Ds or with retention of the element at its original location. Self-fertilisation of most individuals heterozygous for Ds and 35S::TPase produced many streptomycin-resistant (strepr) progeny, but in many of these families a small proportion of strepr seedlings were also resistant to hygromycin (hygr). Nevertheless, 70% of families tested did give rise to at least one strepr, hygr seedling, and over 90% of these individuals carried a transposed Ds. In contrast, the Ac promoter fusion to the transposase gene (Ac::TPase) produced fewer streprhygr progeny, and only 53% of these carried a transposed Ds. However, a higher proportion of the strepr seedlings were also hygr than after activation by 35S::TPase. We also examined the genotype of strepr, hygr seedlings and demonstrated that after activation by 35S::TPase many of these were homozygous for the transposed Ds, while this did not occur after activation by Ac::TPase. From these and other data we conclude that excisions driven by 35S::TPase usually occur prior to floral development, and that although a low proportion of strepr progeny plants inherit a transposed Ds, those that do can be efficiently selected with an antibiotic resistance gene contained within the element. Our data have important implications for transposon tagging strategies in transgenic plants and these are discussed.  相似文献   
10.
Many prey species change their antipredator defence during ontogeny, which may be connected to different potential predators over the life cycle of the prey. To test this hypothesis, we compared reactions of two predator taxa – spiders and birds – to larvae and adults of two invasive true bug species, Oxycarenus hyalinipennis and Oxycarenus lavaterae (Heteroptera: Oxycarenidae) with life-stage-specific chemical defence mechanisms. The reactions to larvae and adults of both true bug species strikingly differed between the two predator taxa. The spiders were deterred by the defences of adult bugs, but the larval defences were ineffective against them. By contrast, birds attacked the larvae considerably less often than the adult bugs. The results indicate a predator-specific ontogenetic change in defence effectiveness of both Oxycarenus species. The change in defence is likely linked to the life-stage-specific composition of secretions in both species: whereas secretions of larvae are dominated by unsaturated aldehydes, secretions of adults are rich in terpenoids, which probably serve dual function of defensive chemicals and pheromones. Our results highlight the variation in defence between different life stages and the importance of testing responses of different types of predators.  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