全文获取类型
收费全文 | 2509篇 |
免费 | 188篇 |
国内免费 | 2篇 |
专业分类
2699篇 |
出版年
2023年 | 12篇 |
2022年 | 25篇 |
2021年 | 55篇 |
2020年 | 29篇 |
2019年 | 30篇 |
2018年 | 41篇 |
2017年 | 36篇 |
2016年 | 61篇 |
2015年 | 102篇 |
2014年 | 114篇 |
2013年 | 163篇 |
2012年 | 186篇 |
2011年 | 170篇 |
2010年 | 115篇 |
2009年 | 115篇 |
2008年 | 114篇 |
2007年 | 149篇 |
2006年 | 117篇 |
2005年 | 110篇 |
2004年 | 99篇 |
2003年 | 86篇 |
2002年 | 71篇 |
2001年 | 51篇 |
2000年 | 57篇 |
1999年 | 34篇 |
1998年 | 19篇 |
1997年 | 27篇 |
1996年 | 15篇 |
1995年 | 21篇 |
1994年 | 20篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 40篇 |
1990年 | 16篇 |
1989年 | 18篇 |
1988年 | 22篇 |
1987年 | 23篇 |
1986年 | 13篇 |
1985年 | 22篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1979年 | 17篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1975年 | 16篇 |
1973年 | 24篇 |
1972年 | 13篇 |
1971年 | 16篇 |
1967年 | 14篇 |
1966年 | 12篇 |
排序方式: 共有2699条查询结果,搜索用时 15 毫秒
151.
Holger?J?SchünemannEmail author Brydon?JB?Grant Paola?Muti Susan?E?McCann Deepa?Kudalkar Malathi?Ram Tom?Nochajski Marcia?Russell Maurizio?Trevisan 《BMC pulmonary medicine》2002,2(1):3
Background
Lung function is a strong predictor of cardiovascular and all-cause mortality. Previous studies suggest that alcohol exposure may be linked to impaired pulmonary function through oxidant-antioxidant mechanisms. Alcohol may be an important source of oxidants; however, wine contains several antioxidants. In this study we analyzed the relation of beverage specific alcohol intake with forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) in a random sample of 1555 residents of Western New York, USA. 相似文献152.
Molecular pathways to colorectal cancer involve multiple genetic changes, whereby extensive oxyradical damage causes mutations in cancer-related genes and leads to a cycle of cell death and regeneration. Besides direct oxidative DNA-damage, reactive oxygen and nitrogen species can induce etheno (epsilon)-DNA adducts mainly via trans-4-hydroxy-2-nonenal, generated as the major aldehyde by lipid peroxidation (LPO) of omega-6 PUFAs. Patients with familial adenomatous polyposis (FAP) develop multiple colorectal adenomas. In affected tissues increased LPO could be triggered due to increased arachidonic acid metabolism as a result of elevated cyclooxygenases. Our studies demonstrated an increased epsilon-DNA adduct level in affected colon epithelia of FAP patients. Epsilon-DNA adducts are promutagenic and can cause genomic instability that drives colorectal adenoma to malignancy. We have further investigated the potential chemopreventive properties of olive oil and its polyphenolic components. 'Mediterranean diet', of which olive oil is a major fatty acid source, has protective effects against human breast and colorectal cancers. Olive oil extracts and the newly identified lignan fractions showed high antioxidant capacity in vitro. As epsilon-DNA adducts are biomarkers for oxidative stress and LPO induced DNA damage, they can verify the efficacy of newly identified antioxidants, e.g. from olive oil, as chemopreventive agents against colon carcinogenesis. 相似文献
153.
154.
155.
Escherichia coli has two catalases, HPI and HPII. HPI is induced during logarithmic growth in response to low concentrations of hydrogen peroxide. This induction is OxyR-dependent. On the other hand, HPII is not peroxide-inducible but is induced in entry to the stationary phase. We demonstrate here that E. coli displayed higher HPI catalase activity when compared to the cultures that were grown in a normal medium, if grown in a medium supplemented with iron-citrate. Iron supplementation had no effect on HPII catalase. This increase of HPI activity was OxyR-independent and not observed in a Deltafur mutant. The physiological significance of the increase of HPI activity is unclear, but it appears that the katG gene that codes for HPI catalase is among the genes that are regulated by Fur. 相似文献
156.
Ramamurthy T Yamasaki S Takeda Y Nair GB 《Microbes and infection / Institut Pasteur》2003,5(4):329-344
Vibrio cholerae O139, the new serogroup associated with epidemic cholera, came into being in the second half of the year 1992 in an explosive fashion and was responsible for several outbreaks in India and other neighbouring countries. This was an unprecedented event in the history of cholera and the genesis of the O139 serogroup was, at that time, thought to be the beginning of the next or the eighth pandemic of cholera. However, with the passage of time, the O1 serogroup of the El Tor biotype again reappeared and displaced the O139 serogroup on the Indian subcontinent, and there was a feeling among cholera workers that the appearance of this new serogroup may have been a one-time event. The resurgence of the O139 serogroup in September 1996 in Calcutta and the coexistence of both the O1 and O139 serogroups in much of the cholera endemic areas in India and elsewhere, suggested that the O139 serogroup has come to stay and is a permanent entity to contend with in the coming years. During the past 10 years, intensive work on all aspects of the O139 serogroup was carried out by cholera researchers around the world. The salient findings on this serogroup over the past 10 years pertinent to its prevalence, clinico-epidemiological features, virulence-associated genes, rapid screening and identification, molecular epidemiology, and vaccine developments have been highlighted. 相似文献
157.
A selective sweep driven by pyrimethamine treatment in southeast asian malaria parasites 总被引:20,自引:0,他引:20
Nair S Williams JT Brockman A Paiphun L Mayxay M Newton PN Guthmann JP Smithuis FM Hien TT White NJ Nosten F Anderson TJ 《Molecular biology and evolution》2003,20(9):1526-1536
Malaria parasites (Plasmodium falciparum) provide an excellent system in which to study the genomic effects of strong selection in a recombining eukaryote because the rapid spread of resistance to multiple drugs during the last the past 50 years has been well documented, the full genome sequence and a microsatellite map are now available, and haplotype data can be easily generated. We examined microsatellite variation around the dihydrofolate reductase (dhfr) gene on chromosome 4 of P. falciparum. Point mutations in dhfr are known to be responsible for resistance to the antimalarial drug pyrimethamine, and resistance to this drug has spread rapidly in Southeast (SE) Asia after its introduction in 1970s. We genotyped 33 microsatellite markers distributed across chromosome 4 in 61 parasites from a location on the Thailand/Myanmar border. We observed minimal microsatellite length variation in a 12-kb (0.7-cM) region flanking the dhfr gene and diminished variation for approximately 100 kb (6 cM), indicative of a single origin of resistant alleles. Furthermore, we found the same or similar microsatellite haplotypes flanked resistant dhfr alleles sampled from 11 parasite populations in five SE Asian countries indicating recent invasion of a single lineage of resistant dhfr alleles in locations 2000 km apart. Three features of these data are of especially interest. (1). Pyrimethamine resistance is generally assumed to have evolved multiple times because the genetic basis is simple and resistance can be selected easily in the laboratory. Yet our data clearly indicate a single origin of resistant dhfr alleles sampled over a large region of SE Asia. (2). The wide valley ( approximately 6 cM) of reduced variation around dhfr provides "proof-of-principle" that genome-wide association may be an effective way to locate genes under strong recent selection. (3). The width of the selective valley is consistent with predictions based on independent measures of recombination, mutation, and selection intensity, suggesting that we have reasonable estimates of these parameters. We conclude that scanning the malaria parasite genome for evidence of recent selection may prove an extremely effective way to locate genes underlying recently evolved traits such as drug resistance, as well as providing an opportunity to study the dynamics of selective events that have occurred recently or are currently in progress. 相似文献
158.
159.
Faruque SM Chowdhury N Khan R Hasan MR Nahar J Islam MJ Yamasaki S Ghosh AN Nair GB Sack DA 《Applied and environmental microbiology》2003,69(12):7028-7031
Shigella dysenteriae type 1 is the causative agent of the most severe form of bacillary dysentery, which occurs as epidemics in many developing countries. We isolated a bacteriophage from surface water samples from Bangladesh that specifically lyses strains of S. dysenteriae type 1. This phage, designated SF-9, belongs to the Podoviridae family and has a 41-kb double-stranded DNA genome. Further screening of water samples for the prevalence of the phage revealed 9 of 71 (12.6%) water samples which were positive for the phage. These water samples were also positive in PCR assays for one or more S. dysenteriae type 1-specific genes, including ipaBCD and stx1, and live S. dysenteriae type 1 was isolated from three phage-positive samples. The results of this study suggest that phage SF-9 may have epidemiological applications in tracing the presence of S. dysenteriae type 1 in environmental waters. 相似文献
160.
In a previous work (part 1), nanocomposite materials were obtained using a latex of either unvulcanized or prevulcanized natural rubber as the matrix and a colloidal suspension of crab chitin whiskers as the reinforcing phase. The mechanical behavior of the resulting nanocomposite films was analyzed in both the linear and the nonlinear range in the present study. The effects of the filler and processing technique were evaluated, and the results are discussed based on the knowledge of the structural morphology and swelling behavior reported in our previous work. The reinforcing effect of chitin whiskers strongly depended on their ability to form a rigid three-dimensional network, resulting from strong interactions such as hydrogen bonds between the whiskers. The results emanating from the successive tensile test experiments give clear evidence for the presence of a three-dimensional chitin network within the evaporated samples. Cross-linking of the matrix was found to interfere with the formation of this network. 相似文献