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   7273篇
  免费   901篇
  国内免费   4篇
  2021年   78篇
  2018年   89篇
  2017年   75篇
  2016年   111篇
  2015年   156篇
  2014年   210篇
  2013年   281篇
  2012年   307篇
  2011年   289篇
  2010年   186篇
  2009年   171篇
  2008年   257篇
  2007年   229篇
  2006年   236篇
  2005年   203篇
  2004年   212篇
  2003年   226篇
  2002年   202篇
  2001年   223篇
  2000年   219篇
  1999年   195篇
  1998年   94篇
  1997年   92篇
  1995年   89篇
  1994年   91篇
  1993年   75篇
  1992年   174篇
  1991年   170篇
  1990年   160篇
  1989年   154篇
  1988年   163篇
  1987年   139篇
  1986年   124篇
  1985年   161篇
  1984年   121篇
  1983年   116篇
  1982年   108篇
  1981年   87篇
  1980年   91篇
  1979年   138篇
  1978年   91篇
  1977年   92篇
  1976年   96篇
  1975年   89篇
  1974年   105篇
  1973年   95篇
  1972年   85篇
  1971年   77篇
  1969年   71篇
  1968年   64篇
排序方式: 共有8178条查询结果,搜索用时 484 毫秒
951.
The regulation of the bioavailability of insulin‐like growth factors (IGFs) is critical for normal mammalian growth and development. The imprinted insulin‐like growth factor 2 receptor gene (IGF2R) encodes a transmembrane protein receptor that acts to sequester and degrade excess circulating insulin‐like growth factor 2 (IGF‐II) – a potent foetal mitogen – and is considered an important inhibitor of growth. Consequently, IGF2R may serve as a candidate gene underlying important growth‐ and body‐related quantitative traits in domestic mammalian livestock. In this study, we have quantified genotype–phenotype associations between three previously validated intronic bovine IGF2R single nucleotide polymorphisms (SNPs) (IGF2R:g.64614T>C, IGF2R:g.65037T>C and IGF2R:g.86262C>T) and a range of performance traits in 848 progeny‐tested Irish Holstein‐Friesian artificial insemination sires. Notably, all three polymorphisms analysed were associated (P ≤ 0.05) with at least one of a number of performance traits related to animal body size: angularity, body depth, chest width, rump width, and animal stature. In addition, the C‐to‐T transition at the IGF2R:g.65037T>C polymorphism was positively associated with cow carcass weight and angularity. Correction for multiple testing resulted in the retention of two genotype–phenotype associations (animal stature and rump width). None of the SNPs analysed were associated with any of the milk traits examined. Analysis of pairwise r2 measures of linkage disequilibrium between all three assayed SNPs ranged between 0.41 and 0.79, suggesting that some of the observed SNP associations with performance may be independent. To our knowledge, this is one of the first studies demonstrating associations between IGF2R polymorphisms and growth‐ and body‐related traits in cattle. These results also support the increasing body of evidence that imprinted genes harbour polymorphisms that contribute to heritable variation in phenotypic traits in domestic livestock species.  相似文献   
952.
The widespread use of artificial insemination (AI) in sheep is currently prevented due to the lack of a cost effective insemination technique utilising frozen-thawed semen. The objective of the present study was to determine if the deposition of frozen-thawed semen in the vaginal fornix would result in a pregnancy rate comparable to that achieved following cervical insemination. Multiparous ewes of various breeds were synchronised and inseminated into either the vaginal fornix (n=78) or the cervix (n=79), at 57 h post sponge removal, with frozen-thawed semen. Information on mucus secretion and the depth to which it was possible to penetrate the cervix at insemination (cervically inseminated ewes only) was recorded at the time of AI. Pregnancy rate was subsequently determined either by return to service (oestrus) or after slaughter 30 days post insemination. Insemination site did not significantly influence pregnancy rate using frozen-thawed semen (36.2% compared to 27.6% for cervical and vaginal fornix insemination, respectively; P=0.26). Whilst depth of cervical penetration was positively associated with pregnancy rate (P<0.05), this association needs to be interpreted with caution as none of the ewes where the cervix could not be penetrated (score=0) was pregnant. In conclusion, pregnancy rate following insemination of frozen-thawed semen into the vaginal fornix was within 10% points of that obtained following cervical AI of frozen-thawed semen. As insemination into the vaginal fornix is technically easier than cervical insemination, it may be more practical for use in large scale applications.  相似文献   
953.
Fungal diversity of communities in several activated sludge plants treating different influent wastes was determined by comparative sequence analyses of their 18S rRNA genes. Methods for DNA extraction and choice of primers for PCR amplification were both optimised using denaturing gradient gel electrophoresis profile patterns. Phylogenetic analysis revealed that the levels of fungal biodiversity in some communities, like those treating paper pulp wastes, were low, and most of the fungi detected in all communities examined were novel uncultured representatives of the major fungal subdivisions, in particular, the newly described clade Cryptomycota. The fungal populations in activated sludge revealed by these culture-independent methods were markedly different to those based on culture-dependent data. Members of the genera Penicillium, Cladosporium, Aspergillus and Mucor, which have been commonly identified in mixed liquor, were not identified in any of these plant communities. Non-fungal eukaryotic 18S rRNA genes were also amplified with the primer sets used. This is the first report where culture-independent methods have been applied to flocculated activated sludge biomass samples to estimate fungal community composition and, as expected, the data obtained gave a markedly different view of their population biodiversity compared to that based on culture-dependent methods.  相似文献   
954.
The "vanishing bone" syndromes represent a group of rare skeletal disorders characterized by osteolysis and joint destruction, which can mimic severe rheumatoid arthritis. Winchester syndrome was one of the first recognized autosomal-recessive, multicentric forms of the disorder. It was originally described nearly 50 years ago in two sisters with a severe crippling osteolysis. Using cultured fibroblasts from the proband, we have now identified homozygous mutations in membrane type-1 metalloproteinase (MT1-MMP or MMP14). We demonstrate that the resulting hydrophobic-region signal-peptide substitution (p.Thr17Arg) decreases MT1-MMP membrane localization with consequent impairment of pro-MMP2 activation, and we propose a structure-based mechanism for this effect.  相似文献   
955.
In the N-terminal domain of thermolysin, two anti-parallel β-strands, Asn112-Ala113-Phe114-Trp115 and Ser118-Gln119-Met120-Val121-Tyr122 are connected by an Asn116-Gly117 turn to form a β-hairpin structure. In this study, we examined the role of Asn116 in the activity and stability of thermolysin by site-directed mutagenesis. Of the 19 Asn116 variants, four (N116A, N116D, N116T and N116Q) were produced in Escherichia coli, by co-expressing the mature and pro domains separately, while the other 15 were not. In the hydrolysis of N-[3-(2-furyl)acryloyl]-glycyl-l-leucine amide (FAGLA) at 25°C, the intrinsic k(cat)/K(m) value of N116D was 320% of that of the wild-type thermolysin (WT), and in the hydrolysis of N-carbobenzoxy-l-aspartyl-l-phenylalanine methyl ester (ZDFM) at pH 7.5 at 25°C, the k(cat)/K(m) value of N116D was 140% of that of WT, indicating that N116D exhibited higher activity than WT. N116Q exhibited similar activity as WT, and N116A and N116T exhibited reduced activities. The first-order rate constants, k(obs), of the thermal inactivation at 80°C were in the order N116A, N116D, N116T > N116Q > WT at all CaCl(2) concentrations examined (1-100 mM), indicating that all variants exhibited reduced stabilities. These results suggest that Asn116 plays an important role in the activity and stability of thermolysin presumably by stabilizing this β-hairpin structure.  相似文献   
956.
In eucalypt plantations managed for solid-wood products, radial trends in wood density, microfibril angle (MFA) and stiffness (modulus of elasticity, MoE) are properties of potential commercial importance that can be affected by competition from neighbouring trees. In this study, wood properties at breast height (1.3?m) were studied on radial strips prepared from 12-mm pith-to-bark wood cores taken from 20 trees in a 22-year-old Eucalyptus nitens (Deane and Maiden) Maiden thinning trial in north-eastern Tasmania, Australia. Thinning treatments were applied at age 6?years. Trees were sampled from each of the 200, 400?stems ha?1 and unthinned control treatments. Intra-specific competition for each sampled tree was estimated using the basal area growth of surrounding trees. SilviScan? technology was used to produce radial profiles of wood density, MFA and MoE. Results indicate a reduction in intra-specific competition through thinning of E. nitens plantations at an early age leads to a transient increase in MFA but has no significant effect on wood density or the intra-annual cycle of wood density. The correlation between the level of intra-specific competition and initial change in MFA following thinning, and a significant relationship between tree shape and mean MFA at breast height suggests the change in MFA is a post-thinning response to increased exposure and wind sway.  相似文献   
957.
Energy drinks are frequently consumed by athletes prior to competition to improve performance. This study examined the effect of Red Bull™ on repeated sprint performance in women athletes. Fifteen collegiate soccer players participated, with mean age, height, and body mass equal to 19.5 ± 1.1 year, 168.4 ± 5.8 cm, and 63.4 ± 6.1 kg, respectively. After performing a familiarization trial, subjects performed three sets of eight bouts of the modified t test after ingestion of 255 mL of placebo or Red Bull 1 h pre-exercise in a randomized, placebo-controlled crossover design. Throughout testing, sprint time, heart rate (HR), and rating of perceived exertion (RPE) were continuously obtained. Repeated measures analysis of variance was used to examine differences in variables between drink conditions. Across athletes, t test time ranged from 10.4 to 12.7 s. Mean sprint time was similar (p > 0.05) between Red Bull (11.31 ± 0.61 s) and placebo (11.35 ± 0.61 s). HR and RPE increased (p < 0.05) during the bouts, but there was no effect (p > 0.05) of Red Bull on either variable versus placebo. Findings indicate that 255 mL of Red Bull containing 1.3 mg/kg of caffeine and 1 g of taurine does not alter repeated sprint performance, RPE, or HR in women athletes versus placebo. One serving of this energy drink provides no ergogenic benefit for women athletes engaging in sprint-based exercise.  相似文献   
958.
959.

Aim

Culture is becoming increasingly important in relation to end of life (EoL) care in a context of globalization, migration and European integration. We explore and compare socio-cultural issues that shape EoL care in seven European countries and critically appraise the existing research evidence on cultural issues in EoL care generated in the different countries.

Methods

We scoped the literature for Germany, Norway, Belgium, the Netherlands, Spain, Italy and Portugal, carrying out electronic searches in 16 international and country-specific databases and handsearches in 17 journals, bibliographies of relevant papers and webpages. We analysed the literature which was unearthed, in its entirety and by type (reviews, original studies, opinion pieces) and conducted quantitative analyses for each country and across countries. Qualitative techniques generated themes and sub-themes.

Results

A total of 868 papers were reviewed. The following themes facilitated cross-country comparison: setting, caregivers, communication, medical EoL decisions, minority ethnic groups, and knowledge, attitudes and values of death and care. The frequencies of themes varied considerably between countries. Sub-themes reflected issues characteristic for specific countries (e.g. culture-specific disclosure in the southern European countries). The work from the seven European countries concentrates on cultural traditions and identities, and there was almost no evidence on ethnic minorities.

Conclusion

This scoping review is the first comparative exploration of the cultural differences in the understanding of EoL care in these countries. The diverse body of evidence that was identified on socio-cultural issues in EoL care, reflects clearly distinguishable national cultures of EoL care, with differences in meaning, priorities, and expertise in each country. The diverse ways that EoL care is understood and practised forms a necessary part of what constitutes best evidence for the improvement of EoL care in the future.  相似文献   
960.
Winter habitat use and the magnitude of migratory connectivity are important parameters when assessing drivers of the marked declines in avian migrants. Such information is unavailable for most species. We use a stable isotope approach to assess these factors for three declining African-Eurasian migrants whose winter ecology is poorly known: wood warbler Phylloscopus sibilatrix, house martin Delichon urbicum and common swift Apus apus. Spatially segregated breeding wood warbler populations (sampled across a 800 km transect), house martins and common swifts (sampled across a 3,500 km transect) exhibited statistically identical intra-specific carbon and nitrogen isotope ratios in winter grown feathers. Such patterns are compatible with a high degree of migratory connectivity, but could arise if species use isotopically similar resources at different locations. Wood warbler carbon isotope ratios are more depleted than typical for African-Eurasian migrants and are compatible with use of moist lowland forest. The very limited variance in these ratios indicates specialisation on isotopically restricted resources, which may drive the similarity in wood warbler populations' stable isotope ratios and increase susceptibility to environmental change within its wintering grounds. House martins were previously considered to primarily use moist montane forest during the winter, but this seems unlikely given the enriched nature of their carbon isotope ratios. House martins use a narrower isotopic range of resources than the common swift, indicative of increased specialisation or a relatively limited wintering range; both factors could increase house martins' vulnerability to environmental change. The marked variance in isotope ratios within each common swift population contributes to the lack of population specific signatures and indicates that the species is less vulnerable to environmental change in sub-Saharan Africa than our other focal species. Our findings demonstrate how stable isotope research can contribute to understanding avian migrants' winter ecology and conservation status.  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