全文获取类型
收费全文 | 113161篇 |
免费 | 2349篇 |
国内免费 | 3291篇 |
专业分类
118801篇 |
出版年
2024年 | 81篇 |
2023年 | 347篇 |
2022年 | 721篇 |
2021年 | 1164篇 |
2020年 | 814篇 |
2019年 | 1012篇 |
2018年 | 12581篇 |
2017年 | 11214篇 |
2016年 | 8262篇 |
2015年 | 1963篇 |
2014年 | 1830篇 |
2013年 | 1981篇 |
2012年 | 6112篇 |
2011年 | 14468篇 |
2010年 | 13009篇 |
2009年 | 9166篇 |
2008年 | 10888篇 |
2007年 | 12313篇 |
2006年 | 1169篇 |
2005年 | 1327篇 |
2004年 | 1669篇 |
2003年 | 1740篇 |
2002年 | 1387篇 |
2001年 | 669篇 |
2000年 | 487篇 |
1999年 | 318篇 |
1998年 | 189篇 |
1997年 | 174篇 |
1996年 | 139篇 |
1995年 | 105篇 |
1994年 | 138篇 |
1993年 | 107篇 |
1992年 | 112篇 |
1991年 | 101篇 |
1990年 | 68篇 |
1989年 | 65篇 |
1988年 | 66篇 |
1987年 | 50篇 |
1986年 | 37篇 |
1985年 | 52篇 |
1984年 | 27篇 |
1983年 | 40篇 |
1982年 | 24篇 |
1972年 | 247篇 |
1971年 | 276篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
401.
Human seminal plasma prostatic inhibin (HSPI) is a protein isolated from the human prostate gland. Despite its profound biomedical and biotechnological importance, the 3D structure of this protein of 94 amino acids remains undeciphered. The difficulties in extracting it in pure form and crystallizing it have restrained the determination of its structure experimentally. The homology-based computational methods are also not applicable, as HSPI lacks sufficient sequence homology with known structures in the protein data banks. We have predicted the structure of HSPI by a knowledge-based method using nonparametric multivariate statistical techniques. Stereochemical and other standard validation tests confirm this to be a well-refined structure. The biophysical properties exhibited by this structure are in good agreement with the NMR experimental observations. Docking and other computational studies on this structure provide significant explanation and insight into its binding activities and related biological and immunogenic functions and offer new directions for its potential applications. 相似文献
402.
403.
Fabrice Touzain Erick Denamur Claudine Médigue Valérie Barbe Meriem El Karoui Marie-Agnès Petit 《Genome biology》2010,11(4):R45
Background
Analysis of large scale diversity in bacterial genomes has mainly focused on elements such as pathogenicity islands, or more generally, genomic islands. These comprise numerous genes and confer important phenotypes, which are present or absent depending on strains. We report that despite this widely accepted notion, most diversity at the species level is composed of much smaller DNA segments, 20 to 500 bp in size, which we call microdiversity. 相似文献404.
Background
After a volcano erupts, a lake may form in the cooled crater and become an isolated aquatic ecosystem. This makes fishes in crater lakes informative for understanding sympatric evolution and ecological diversification in barren environments. From a geological and limnological perspective, such research offers insight about the process of crater lake ecosystem establishment and speciation. In the present study we use genetic and coalescence approaches to infer the colonization history of Midas cichlid fishes (Amphilophus cf. citrinellus) that inhabit a very young crater lake in Nicaragua-the ca. 1800 year-old Lake Apoyeque. This lake holds two sympatric, endemic morphs of Midas cichlid: one with large, hypertrophied lips (~20% of the total population) and another with thin lips. Here we test the associated ecological, morphological and genetic diversification of these two morphs and their potential to represent incipient speciation. 相似文献405.
406.
Patrizio Dimitri Ruggiero Caizzi Ennio Giordano Maria Carmela Accardo Giovanna Lattanzi Giuseppe Biamonti 《Chromosoma》2009,118(4):419-435
The organization of chromosomes into euchromatin and heterochromatin is amongst the most important and enigmatic aspects of
genome evolution. Constitutive heterochromatin is a basic yet still poorly understood component of eukaryotic chromosomes,
and its molecular characterization by means of standard genomic approaches is intrinsically difficult. Although recent evidence
indicates that the presence of transcribed genes in constitutive heterochromatin is a conserved trait that accompanies the
evolution of eukaryotic genomes, the term heterochromatin is still considered by many as synonymous of gene silencing. In
this paper, we comprehensively review data that provide a clearer picture of transcribed sequences within constitutive heterochromatin,
with a special emphasis on Drosophila and humans. 相似文献
407.
Construction of a recombinant BHV-1 expressing the VP1 gene of foot and mouth disease virus and its immunogenicity in a rabbit model 总被引:1,自引:0,他引:1
Xian-Gang Ren Fei Xue Yuan-Mao Zhu Guang-Zhi Tong Yan-Hui Wang Jun-Ke Feng Hong-Fei Shi Yu-Ran Gao 《Biotechnology letters》2009,31(8):1159-1165
Foot-and-mouth disease (FMD) and infectious bovine rhinotracheitis (IBR) are two important infectious diseases of cattle.
Using bovine herpesvirus type 1 (BHV-1) as a gene delivery vector for development of live-viral vaccines has gained widespread
interest. In this study, a recombinant BHV-1 was constructed by inserting the synthetic FMDV (O/China/99) VP1 gene in the
the gE locus of BHV-1 genome under the control of immediately early gene promoter of human cytomegalovirus (phIE CMV) and
bovine growth hormone polyadenylation (BGH polyA) signal. After homologous recombination and plaque purification, a recombinant
virus named BHV-1/gE−/VP1 was acquired and identified. The immunogenicity was confirmed in a rabbit model by virus neutralization test and enzyme-linked
immunosorbent assay (ELISA). The result indicated that the BHV-1/gE−/VP1 has the potential for being developed as a bivalent vaccine for FMD and IBR. 相似文献
408.
Adrian Ares Constance A. Harrington Thomas A. Terry Joseph M. Kraft 《Trees - Structure and Function》2008,22(5):603-609
The stable carbon (C) composition of tree rings expressed as δ13C, is a measure of intrinsic water-use efficiency and can indicate the occurrence of past water shortages for tree growth.
We examined δ13C in 3- to 5-year-old rings of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) trees to elucidate if decreased water supply or uptake was a critical factor in the observed growth reduction
of trees competing with understory herb and shrub vegetation compared to those growing without competition. We hypothesized
that there would be no differences in δ13C of earlywood in trees growing in plots with competing vegetation and those in plots receiving complete vegetation control
during 5 years because earlywood formed early in the growing season when soil water was ample. We also hypothesized that δ13C in latewood which was formed during the later half of the growing season when precipitation was low, would be greater (less
negative) in trees in plots without vegetation control. We then separated early and latewood from rings for three consecutive
years and analyzed their δ13C composition. No significant differences in earlywood δ13C in years 3–5 were observed for trees in the two vegetation control treatments. δ13C of untreated latewood separated from wood cores was greater in 4- and 5-year-old rings of trees growing with competing vegetation
compared to trees growing without vegetation competition (i.e., −25.5 vs. −26.3‰ for year 4, and −26.1 vs. −26.8‰ for year
5). Results suggest that water shortages occurred in Douglas-fir trees on this coastal Washington site in the latewood-forming
portion of the growing season of years 4 and 5 in the no-vegetation control treatment. We also compared δ13C from untreated wood, crude cellulose extracted with the Diglyme–HCl method, and holocellulose extracted with toluene–ethanol
to see if the extraction method would increase the sensitivity of the analysis. δ13C values from the two extraction methods were highly correlated with those from untreated samples (r
2 = 0.97, 0.98, respectively). Therefore, using untreated wood would be as effective as using crude cellulose or holocellulose
to investigate δ13C patterns in young Douglas-fir. 相似文献
409.
Michael Montague Christian Barnes Hamilton O. Smith Ray-Yuan Chuang Sanjay Vashee 《Journal of molecular evolution》2009,69(4):360-371
The common understanding of the function of RecD, as derived predominantly from studies in Escherichia coli, is that RecD is one of three enzymes in the RecBCD double-stranded break repair DNA recombination complex. However, comparative
genomics has revealed that many organisms possess a recD gene even though the other members of the complex, recB and recC, are not present. Further, bioinformatic analyses have shown that there is substantial sequence dissimilarity between recD genes associated with recB and recC (recD1), and those that are not associated with recBC (recD2). Deinococcus radiodurans, known for its extraordinary DNA repair capability, is one such organism that does not possess either recB or recC, and yet does possess a recD gene. The recD of D. radiodurans was deleted and this mutant was shown to have a capacity to repair double-stranded DNA breaks equivalent to wild-type. The
phylogenetic history of recD was studied using a dataset of 120 recD genes from 91 fully sequenced species. The analysis focused upon the role of gene duplication and functional genomic context
in the evolution of recD2, which appears to have undergone numerous independent events resulting in duplicate recD2 genes. The role of RecD as part of the RecBCD complex appears to have a divergence from an earlier ancestral RecD function
still preserved in many species including D. radiodurans. 相似文献
410.
Amukotuwa S Choong PF Smith PJ Powell GJ Slavin J Schlicht SM 《International Seminars in Surgical Oncology : ISSO》2005,2(1):10
BACKGROUND: Extrapulmonary tuberculosis is one of the great mimickers of medicine, and often masquerades as malignancy. As a result, patients may be referred to oncologists and surgeons for further evaluation and management, delaying the institution of appropriate anti-tuberculous drug therapy. CASE PRESENTATION: We present the case of a 21 year old man with tuberculous osteomyelitis, who was referred to the Bone and Soft Tissue Sarcoma Service at our institution with a provisional diagnosis of malignancy. Further investigation revealed extensive retroperitoneal abdominal and pelvic lymphadenopathy. The recognition of certain patterns on imaging, and finally the isolation of Mycobacterium tuberculosis from tissue samples obtained under image guidance, enabled the correct diagnosis to be made. CONCLUSION: This case highlights the importance of remaining cognisant of the protean manifestations of extrapulmonary tuberculosis, and illustrates the advantage of a clinically directed multi-modality imaging approach to diagnosis. 相似文献