全文获取类型
收费全文 | 153349篇 |
免费 | 5632篇 |
国内免费 | 864篇 |
出版年
2023年 | 186篇 |
2022年 | 542篇 |
2021年 | 1120篇 |
2020年 | 642篇 |
2019年 | 829篇 |
2018年 | 12753篇 |
2017年 | 11364篇 |
2016年 | 8969篇 |
2015年 | 3476篇 |
2014年 | 3477篇 |
2013年 | 4042篇 |
2012年 | 8918篇 |
2011年 | 17038篇 |
2010年 | 14542篇 |
2009年 | 10568篇 |
2008年 | 13259篇 |
2007年 | 14515篇 |
2006年 | 3392篇 |
2005年 | 3365篇 |
2004年 | 3716篇 |
2003年 | 3474篇 |
2002年 | 2913篇 |
2001年 | 2068篇 |
2000年 | 1845篇 |
1999年 | 1366篇 |
1998年 | 610篇 |
1997年 | 555篇 |
1996年 | 461篇 |
1995年 | 458篇 |
1994年 | 356篇 |
1993年 | 374篇 |
1992年 | 739篇 |
1991年 | 624篇 |
1990年 | 559篇 |
1989年 | 558篇 |
1988年 | 496篇 |
1987年 | 476篇 |
1986年 | 380篇 |
1985年 | 401篇 |
1984年 | 332篇 |
1983年 | 284篇 |
1982年 | 226篇 |
1980年 | 193篇 |
1979年 | 267篇 |
1978年 | 241篇 |
1977年 | 208篇 |
1976年 | 211篇 |
1974年 | 227篇 |
1972年 | 424篇 |
1971年 | 411篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Type I hypersensitivity, which functions to protect the organism from parasites, is caused by binding of antigen to IgE antibodies
pre-attached to the cell surface of tissue mast cells and their circulating counterparts, the basophils. In “allergy,” type
I hypersensitivity is inappropriately induced by protein-based foreign substances (such as pollen) or protein components of
insect stings, which in the normal course of events would be cleared from the organism without causing any damage. Paradoxically,
a successful clinical treatment of allergy involves repeated immunization of allergic persons with low doses of the allergen—immunotherapy. Investigation of the available experimental evidence leads to the conclusion that the phenomena of immunotherapy are best
addressed in terms of the interplay among the mechanism(s) of immune memory—Th1/Th2 cross-regulation—and the physical compart-mentalization
of the immune system. These conclusions are illustrated with a numerical simulation. 相似文献
932.
Summary Maternally inherited microorganisms that kill male (but not female) progeny are widespread in nature. Three hypotheses have been proposed for the evolution of male-killing microorganisms: inbreeding reduction, release of resources to remaining females and inoculum for horizontal transmission. The sonkiller bacterium,Arsenophonus nasoniae, is a maternally inherited bacterium that causes lethality of male embryos of infected females in the parasitoid wasp,Nasonia vitripennis. In this paper we describe the geographical distribution and frequency of the son-killer bacterium in North American populations ofN. vitripennis andNasonia longicornis. We tested the resource release hypothesis using the body size measurements of infected and uninfected females from natural populations. No evidence was found for a fitness increase of females infected with the bacterium compared to uninfected females. We propose a modification of the existing models, termed the incremental gain hypothesis. According to this model, the bacteria are maintained in host populations due to horizontal transmission and male killing provides an incremental gain in the fitness of infected females relative to females infected with non-male-killing bacteria. 相似文献
933.
The covalent attachment of thiol-modified DNA oligomers; to self-assembled monolayer silane films on fused silica and oxidized silicon substrates is described. A heterobifunctional crosslinking molecule bearing both thiol- and amino-reactive moieties was used to tether a DNA oligomer (modified at its terminus with a thiol group) to an aminosilane film formed on silica surfaces. A variety of aminosilanes, crosslinkers and treatment conditions have been tested to identify optimal conditions for DNA immobilization using this approach. The DNA films which result have been characterized using UV spectroscopy, water contact angle measurement, radiolabeling and hybridization methods. 相似文献
934.
Segregation analysis of continuous phenotypes by using higher sample moments. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The present article discusses the use of computational methods based on generalized estimating equations (GEE), as a potential alternative to full maximum-likelihood methods, for performing segregation analysis of continuous phenotypes by using randomly selected family data. The method that we propose can estimate effect and degree of dominance of a major gene in the presence of additional nongenetic or polygenetic familial associations, by relating sample moments to their expectations calculated under the genetic model. It is known that all parameters in basic major-gene models cannot be identified, for estimation purposes, solely in terms of the first two sample moments of data from randomly selected families. Thus, we propose the use of higher (third order) sample moments to resolve this identifiability problem, in a pseudo-profile likelihood estimation scheme. In principle, our methods may be applied to fitting genetic models by using complex pedigrees and for estimation in the presence of missing phenotype data for family members. In order to assess its statistical efficiency we compare several variants of the method with each other and with maximum-likelihood estimates provided by the SAGE computer package in a simulation study. 相似文献
935.
S. D. Colman S. A. Rasmussen V. T. Ho C. R. Abernathy M. R. Wallace 《American journal of human genetics》1996,58(3):484-490
Using loss of heterozygosity analysis, a method designed to detect moderate to large gene deletions, we have identified a new-mutation neurofibromatosis type 1 (NF1) patient who is somatically mosaic for a large maternally derived deletion in the NF1 gene region. The deletion extends at least from exon 4 near the 5' end of the gene to intron 39 near the 3' end. The gene-coding region is, therefore, mostly or entirely deleted, encompassing a loss of > or = 100 kb. We hypothesize that the deletion occurred at a relatively early developmental timepoint, since signs of NF1 in this patient are not confined to a specific body region, as seen in "segmental" NF, and since both mesodermally and ectodermally derived cells are affected. This report provides the first molecular evidence of somatic mosaicism in NF1 and, taken together with a recent report of germ-line mosaicism in NF1, adds credence to the concept that mosaicism plays an important role in phenotypic and genetic aspects of NF1 and may even be a relatively common phenomenon. 相似文献
936.
Virulence determinants invA and spvC in salmonellae isolated from poultry products, wastewater, and human sources.
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The presence of two virulence foci, invA and spvC, in Salmonella isolates obtained from poultry, wastewater, and human sources was determined. All isolates (n = 245) were positive for the invA gene sequence. Differences in degree of invasiveness were apparent with the Madin Darby canine kidney cell line, as only 79 of 159 randomly selected isolates (49.7%) tested were invasive at > 0.1% of the inoculum. 25% were invasive between 0.1 and 1.0% of the inoculum, and 24.5% were invasive at > 1.0% of the inoculum. There was a significant correlation between degree of invasion and source from which the isolate was recovered but no correlation between geographic origin of poultry isolates and degree of invasion. Only 37 of 245 isolates (15.1%) hybridized with the spvC DNA probe. All isolates that were recovered from a commercial egg production environment and chicken eggs and whose sequences exhibited homology with the spvC gene sequence were determined to be either Salmonella enteritidis PT 23 or PT 13. The sequences of few isolates from ceca and none from wastewater or humans demonstrated homology with the spvC gene. 相似文献
937.
A Familial Factor Independent of CAG Repeat Length Influences Age at Onset of Machado-Joseph Disease 总被引:3,自引:0,他引:3
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Anita L. DeStefano L. Adrienne Cupples Patricia Maciel Claudia Gaspar Joao Radvany David M. Dawson Lewis Sudarsky Lee Corwin Paula Coutinho Patrick MacLeod Jorge Sequeiros Guy A. Rouleau Lindsay A. Farrer 《American journal of human genetics》1996,59(1):119-127
Machado-Joseph disease (MJD) is a late-onset, progressive, neurodegenerative disorder caused by the expansion of an unstable trinucleotide (CAG) repeat sequence in a novel gene (MJD1) on chromosome 14. Previous studies showed that age at onset is negatively correlated with the number of CAG repeat units, but only part of the variation in onset age is explained by CAG repeat length. Ages at onset and CAG repeat lengths of 136 MJD patients from 23 kindreds of Portuguese descent were analyzed, to determine whether familial factors independent of CAG repeat length modulate age at onset of MJD. Correlation among sibs for onset age adjusted for CAG repeat length was .43, which indicates that an environmental or genetic factor common to sibs influences onset age. Positive correlations were also observed for avuncular (r = .22) and first-cousin pairs (r = .28), which supports the hypothesis that a genetic factor is influencing age at onset. Commingling analysis of onset ages adjusted for CAG repeat length identified three distributions in this population of affected individuals. Further studies of a much larger sample are needed to determine whether these distributions represent the influence of a genetic or environmental factor. 相似文献
938.
Purification and characterization of a fibrinolytic enzyme produced from Bacillus sp. strain CK 11-4 screened from Chungkook-Jang. 总被引:13,自引:0,他引:13
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Bacillus sp. strain CK 11-4, which produces a strongly fibrinolytic enzyme, was screened from Chungkook-Jang, a traditional Korean fermented-soybean sauce. The fibrinolytic enzyme (CK) was purified from supernatant of Bacillus sp. strain CK 11-4 culture broth and showed thermophilic, hydrophilic, and strong fibrinolytic activity. The optimum temperature and pH were 70 degrees C and 10.5, respectively, and the molecular weight was 28,200 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The first 14 amino acids of the N-terminal sequence of CK are Ala-Gin-Thr-Val-Pro-Tyr-Gly-Ile-Pro-Leu-Ile-Lys-Ala-Asp. This sequence is identical to that of subtilisin Carlsberg and different from that of nattokinase, but CK showed a level of fibrinolytic activity that was about eight times higher than that of subtilisin Carlsberg. The amidolytic activity of CK increased about twofold at the initial state of the reaction when CK enzyme was added to a mixture of plasminogen and substrate (H-D-Val-Leu-Lys-pNA). A similar result was also obtained from fibrin plate analysis. 相似文献
939.
Nonradioactive method to study genetic profiles of natural bacterial communities by PCR-single-strand-conformation polymorphism. 总被引:26,自引:2,他引:24
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We describe a new method for studying the structure and diversity of bacterial communities in the natural ecosystem. Our approach is based on single-strand-conformation polymorphism (SSCP) analysis of PCR products of 16S rRNA genes from complex bacterial populations. A pair of eubacterial universal primers for amplification of the variable V3 region were designed from the 16S rRNA sequences of 1,262 bacterial strains. The PCR conditions were optimized by using genomic DNAs from five gram-positive and seven gram-negative strains. The SSCP analysis of the PCR products demonstrated that a bacterial strain generated its characteristic band pattern and that other strains generated other band patterns, so that the relative diversity in bacterial communities could be measured. In addition, this method was sensitive enough to detect a bacterial population that made up less than 1.5% of a bacterial community. The distinctive differences between bacterial populations were observed in an oligotrophic lake and a eutrophic pond in a field study. The method presented here, using combined PCR amplification and SSCP pattern analyses of 16S rRNA genes, provides a useful tool to study bacterial community structures in various ecosystems. 相似文献
940.