全文获取类型
收费全文 | 12023篇 |
免费 | 1042篇 |
国内免费 | 7篇 |
专业分类
13072篇 |
出版年
2022年 | 138篇 |
2021年 | 227篇 |
2020年 | 154篇 |
2019年 | 161篇 |
2018年 | 258篇 |
2017年 | 198篇 |
2016年 | 322篇 |
2015年 | 546篇 |
2014年 | 595篇 |
2013年 | 772篇 |
2012年 | 900篇 |
2011年 | 844篇 |
2010年 | 546篇 |
2009年 | 426篇 |
2008年 | 646篇 |
2007年 | 647篇 |
2006年 | 581篇 |
2005年 | 537篇 |
2004年 | 528篇 |
2003年 | 453篇 |
2002年 | 457篇 |
2001年 | 278篇 |
2000年 | 202篇 |
1999年 | 203篇 |
1998年 | 125篇 |
1997年 | 83篇 |
1996年 | 101篇 |
1995年 | 93篇 |
1994年 | 86篇 |
1993年 | 77篇 |
1992年 | 112篇 |
1991年 | 74篇 |
1990年 | 85篇 |
1989年 | 81篇 |
1988年 | 70篇 |
1987年 | 81篇 |
1986年 | 62篇 |
1985年 | 73篇 |
1984年 | 81篇 |
1983年 | 55篇 |
1982年 | 75篇 |
1981年 | 62篇 |
1980年 | 75篇 |
1979年 | 61篇 |
1978年 | 60篇 |
1977年 | 57篇 |
1976年 | 54篇 |
1975年 | 42篇 |
1974年 | 47篇 |
1973年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
111.
112.
Louis Faso Richard S. Trowbridge Wei Quan Xiu-Lan Yao Edmund C. Jenkins Alma Maciulis Thomas D. Bunch Henry M. Wisniewski 《In vitro cellular & developmental biology. Animal》1994,30(4):226-235
Summary A strain of cerebral endothelial cells was established from isolated cortical microvessels of caprine brain. These cells,
which are referred to as ECl cells, can be routinely subcultured to 32 passages without the loss of differentiated morphologic
and immunologic traits. The ability to routinely subculture ECl cells is an important asset, given that isolated cerebral
endothelial cells in mammals generally lose their differentiated traits after only 2 to 3 passages.
ECl cells were shown to contain Factor VIII-related antigen, which is a specific marker for cells of endothelial origin. ECl
cells morphologically demonstrated a scarcity of pinocytotic vesicles on their apical surfaces, a lack of trans-cytoplasmic
vesicles, and the ability to form in culture confluent monolayers with tight junctional complexes. Therefore, ECl cells possess
specific antigenic and ultrastructural features which classify them as being small vessel endothelial cells of the blood-brain
barrier type. Cytogenetic evaluation of ECl cells demonstrated a normal female goat 60,XX karyotype and confirmed the apparent
non-transformed nature of ECl cells due to the lack of chromosome abnormalities or rearrangements. Using scanning electron
microscopy, ECl cells were also shown to form confluent monolayers on mixed nitrocellulose filters, a feature that will enable
the development of an in vitro system to study trans-endothelial transport. Given that ECl cells are readily subcultured and
grow well on nitrocellulose filters, and that they resemble cerebral endothelium in vivo, it seems evident that ECl cells
can be used as a versatile model for the study of blood-brain barrier function, regulation, and pathology. 相似文献
113.
The chromosomal DNA of four strains of Gardnerella vaginaliswere digested with rare cutting restriction enzymes and analyzedby pulsed-field gel electrophoresis (PFGE). The four strainsstudied were two clinical isolates (GVP 004 & GVP 007) andtwo American Type Culture Collection strains (ATCC 14018 &ATCC 14019). The restriction enzyme SfiI generated two DNA fragmentsof about 0.6 Mb and 1.1 Mb in all four strains giving a G. vaginalisgenome size of about 1.7 Mb. A similar genome size was calculatedutilizing two more GC-rich sequence specific restriction endonucleases,NotI and AscI. When digested with AscI, the chromosomal DNAof all four strains gave rise to 11 to 12 DNA fragments rangingbetween 0.01 Mb to 0.43 Mb. DNA from the two clinical isolateswere digested by NotI (yielding 7 to 9 fragments), while theDNA from the two ATCC strains were resistant to NotI digestion.In contrast to the clinical isolates, DNA from the two ATCCstrains gave an identical profile for all restriction endonucleasestested. From double digestion experiments, the two SfiI sitescould be localized on two AscI fragments. From these PFGE studies,it is concluded that the G. vaginalis genome is a circular DNAthat ranges between 1.67 Mb and 1.72 Mb in size. 相似文献
114.
DNA-binding domain of bovine papillomavirus type 1 E1 helicase: structural and functional aspects. 总被引:18,自引:13,他引:5 下载免费PDF全文
The E1 protein of bovine papillomavirus type 1 is a multifunctional enzyme required for papillomaviral DNA replication. It assists in the initiation of replication both as a site-specific DNA-binding protein and as a DNA helicase. Previous work has indicated that at limiting E1 concentrations, the E2 protein is required for efficient E1 binding to the replication origin. In this study, we have defined the domain of the E1 protein required for site-specific DNA binding. Experiments with a series of truncated proteins have shown that the first amino-terminal 299 amino acids contain the DNA-binding domain; however, the coterminal M protein, which is homologous to E1 for the first 129 amino acids, does not bind origin DNA. A series of small internal deletions and substitution mutations in the DNA-binding domain of E1 show that specific basic residues in this region of the protein, which are conserved in all E1 proteins of the papillomavirus family, likely play a direct role in binding DNA and that a flanking conserved hydrophobic subdomain is also important for DNA binding. A region of E1 that interacts with E2 for cooperative DNA binding is also retained in carboxy-terminal truncated proteins, and we show that the ability of full-length E1 to complex with E2 is sensitive to cold. The E1 substitution mutant proteins were expressed from mammalian expression vectors to ascertain whether site-specific DNA binding by E1 is required for transient DNA replication in the cell. These E1 proteins display a range of mutant phenotypes, consistent with the suggestion that site-specific binding by E1 is important. Interestingly, one E1 mutant which is defective for origin binding but can be rescued for such activity by E2 supports significant replication in the cell. 相似文献
115.
Pulsed field gel electrophoresis using OFAGE, TAFE, and CHEF systems has been used to more fully characterize karyotypic variation within the two closely related fungal species of Ophiostoma ulmi sensu lato. Twelve wild-type and laboratory strains, representing the less agressive species O. ulmi and both of the biotypes of the more aggressive species O. novo-ulmi were studied and their karyotypes determined. Depending on the strain, a minimum of four to a minimum of eight chromosomal DNA bands were present that fall into three distinct size classes, with one exception. Strain CESSI6K (O. novo-ulmi, North American aggressive subgroup) contains a unique chromosomal DNA band which comigrated near a Saccharomyces cerevisiae chromosome of 0.95 Mb. This unique band was the smallest O. ulmi s. l. chromosomal DNA observed. Seven of the twelve strains shared a common chromosomal DNA banding pattern, whereas each of the other five had a unique karyotype. There was no correlation between chromosome profile and species, as some O. novo-ulmi and O. ulmi strains shared common electrophoretic karyotypes. 相似文献
116.
Application of rRNA-based probes for observing marine nanoplanktonic protists. 总被引:11,自引:5,他引:6 下载免费PDF全文
The use of small-subunit rRNA-based oligonucleotides as probes for detecting marine nanoplanktonic protists was examined with a ciliate (an Uronema sp.), a flagellate (a Cafeteria sp.), and mixed assemblages of protists from enrichment cultures and natural seawater samples. Flow cytometry and epifluorescence microscopy analyses demonstrated that hybridizations employing fluorescein-labeled, eukaryote-specific probes intensely stained logarithmically growing protists, whereas these same protist strains in late stationary growth were barely detectable. The fluorescence intensity due to probe binding was significantly enhanced by the use of probes end labeled with biotin, which were detected by fluorescein-labeled avidin. The degree of signal amplification ranged from two- to fivefold for cultured protists in both logarithmic and stationary growth phases. Mixed assemblages of heterotrophic protists from enrichment cultures were also intensely labeled by rRNA-targeted oligonucleotide probes by the biotin-avidin detection system. Protists in late stationary growth phase and natural assemblages of protists that were otherwise undetectable when hybridized with fluorescein-labeled probes were easily visualized by this approach. In the latter samples, hybridization with multiple, biotin-labeled probes was necessary for detection of naturally occurring marine protists by epifluorescence microscopy. The signal amplification obtained with the biotin-avidin system should increase the utility of rRNA-targeted probes for identifying protists and facilitate characterization of the population structure and distribution of protists in aquatic environments. 相似文献
117.
Inga Sidén-Kiamos Guido Favia Dora Artiaco Giuseppe Saceone Maria Furia Lino C. Bolito Christos Louis 《Genetica》1993,92(1):43-53
The sequence determination of several genomic clones isolated from the Mediterranean fruitfly Ceratitis capitata identified the existence of opa-like repeats, often more than one being clustered in small chromosomal segments. These repeats have previously been shown to consist of stretches of tandemly reiterated glutamine-encoding residues, and they are found in multiple genes of several organisms. Most of the repeats described here are flanked or interrupted by stop codons in all reading frames and, thus, could not possibly be part of protein-coding sequences. Furthermore, these repeats, of which there are several hundred in the genome of the Medfly, can be used effectively for the determination of sequence polymorphisms, providing a convenient approach to obtain additional landmarks for the construction of genomic maps of this economically important insect.This paper is dedicated to the memory of our colleague and friend Dr. Jim Flach who took part in the initial phase of this work and died during the course of the investigation. 相似文献
118.
The susceptibilities of two isolates ofAspergillus flavus, one from a human case of recalcitrant mycotic keratitis, and an environmental isolate ofA. fumigatus, to itraconazole, clotrimazole and amphotericin B were measured. Observations of macroscopic growth and microscopic evaluations of conidia germination both indicated that the two isolates ofA. flavus were markedly more resistant to amphotericin B than to itraconazole and clotrimazole. Itraconazole was more effective than clotrimazole for all isolates. Ourin vitro susceptibility results suggest the use of itraconazole should be a primary consideration in the treatment ofAspergillus keratitis. 相似文献
119.
120.
Raven L. Bier Jennifer J. Mosher Louis A. Kaplan Jinjun Kan 《Environmental microbiology》2023,25(10):1860-1874
A mechanistic understanding of factors that structure spatiotemporal community composition is a major challenge in microbial ecology. Our study of microbial communities in the headwaters of three freshwater stream networks showed significant community changes at the small spatial scale of benthic habitats when compared to changes at mid- and large-spatial scales associated with stream order and catchment. Catchment (which included temperate and tropical catchments) had the strongest influence on community composition followed by habitat type (epipsammon or epilithon) and stream orders. Alpha diversity of benthic microbiomes resulted from interactions between catchment, habitat, and canopy. Epilithon contained relatively more Cyanobacteria and algae while Acidobacteria and Actinobacteria proportions were higher in epipsammic habitats. Turnover from replacement created ~60%–95% of beta diversity differences among habitats, stream orders, and catchments. Turnover within a habitat type generally decreased downstream indicating longitudinal linkages in stream networks while between habitat turnover also shaped benthic microbial community assembly. Our study suggests that factors influencing microbial community composition shift in dominance across spatial scales, with habitat dominating locally and catchment dominating globally. 相似文献