全文获取类型
收费全文 | 9134篇 |
免费 | 782篇 |
国内免费 | 1篇 |
出版年
2024年 | 10篇 |
2023年 | 33篇 |
2022年 | 70篇 |
2021年 | 229篇 |
2020年 | 115篇 |
2019年 | 155篇 |
2018年 | 191篇 |
2017年 | 163篇 |
2016年 | 299篇 |
2015年 | 476篇 |
2014年 | 519篇 |
2013年 | 565篇 |
2012年 | 787篇 |
2011年 | 750篇 |
2010年 | 470篇 |
2009年 | 432篇 |
2008年 | 583篇 |
2007年 | 577篇 |
2006年 | 541篇 |
2005年 | 543篇 |
2004年 | 500篇 |
2003年 | 465篇 |
2002年 | 440篇 |
2001年 | 78篇 |
2000年 | 57篇 |
1999年 | 110篇 |
1998年 | 113篇 |
1997年 | 73篇 |
1996年 | 56篇 |
1995年 | 60篇 |
1994年 | 48篇 |
1993年 | 54篇 |
1992年 | 44篇 |
1991年 | 37篇 |
1990年 | 44篇 |
1989年 | 31篇 |
1988年 | 32篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 17篇 |
1984年 | 20篇 |
1983年 | 12篇 |
1982年 | 12篇 |
1981年 | 16篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1968年 | 9篇 |
1967年 | 4篇 |
排序方式: 共有9917条查询结果,搜索用时 15 毫秒
21.
Mitochondrial DNA of the extinct quagga: Relatedness and extent of postmortem change 总被引:4,自引:0,他引:4
Russell G. Higuchi Lisa A. Wrischnik Elizabeth Oakes Matthew George Benton Tong Allan C. Wilson 《Journal of molecular evolution》1987,25(4):283-287
Sequences are reported for portions of two mitochondrial genes from a domestic horse and a plains zebra and compared to those published for a quagga and a mountain zebra. The extinct quagga and plains zebra sequences are identical at all silent sites, whereas the horse sequence differs from both of them by 11 silent substitutions. Postmortem changes in quagga DNA may account for the two coding substitutions between the quagga and plains zebra sequences. The hypothesis that the closest relative of the quagga is the domestic horse receives no support from these data. From the extent of sequence divergence between horse and zebra mitochondrial DNAs (mtDNAs), as well as from information about the fossil record, we estimate that the mean rate of mtDNA divergence in Equus is similar to that in other mammals, i.e., roughly 2% per million years. 相似文献
22.
George Harauz Lisa Borland Gunter F. Bahr Elmar Zeitler Marin van Heel 《Chromosoma》1987,95(5):366-374
A complete human metaphase chromosome has been reconstructed from a series of electron microscopical projections obtained by tilting the specimen stage at 3 degree intervals from –60 to +60 degrees. The reconstructed structure is about 3.0 m long, 1.6 m wide, and 0.8 m thick. The mass distribution was fairly homogeneous within the chromatids and neither a hollow nor a dense core was observed. The distribution and course of fibers observed are most consistent with a looping model of chromosome structure. 相似文献
23.
This is a literature review of 361 opportunistic fungal infections caused by the Zygomycetes. The clinical and laboratory diagnosis, pathogenesis, management, treatment, and outcome of infection are discussed. The Zygomycetes are a group of opportunistic fungi (orders Mucorales and Entomophthorales) which cause severe infections which may be fatal. Early clinical recognition, prompt diagnostic procedures, control of underlying disease and treatment with high doses of amphotericin B and aggressive surgery increases survival in an otherwise lethal infection. 相似文献
24.
The effects of nutritional copper deficiency on carrageenin edema in the rat were investigated with emphasis on studying the
correlation between the degree of copper deficiency and the degree of edema. Carrageenin paw edema in both copper-sufficient
and copper-deficient groups of rats was compared after either 20, 40, or 60 d on respective diets. The degree of copper deficiency
was quantitated by analyzing total copper concentrations in a number of tissues. Other copper dependent parameters were also
determined. Results indicated that: (1) although copper sufficient rats showed relatively little change in the degree of edema,
copper-deficient rats showed a steady and significant increase in edema from d 20 to 40 to 60; (2) paw edema in copper-deficient
animals was highly and negatively correlated to the concentrations of copper in the liver; the correlation with liver Cu,Zn-superoxide
dismutase activity, however, was inconsistent; (3) paw edema was not correlated either to copper concentration in tissues
other than liver or to plasma ceruloplasmin activity; and (4) aggravation of carrageenin edema in copper-deficient animals
seemed to be mediated via an as yet unknown secondary effect of copper deficiency. 相似文献
25.
26.
Turnover of Extracellular DNA in Eutrophic and Oligotrophic Freshwater Environments of Southwest Florida 总被引:14,自引:9,他引:5 下载免费PDF全文
John H. Paul Wade H. Jeffrey Andrew W. David Mary F. DeFlaun Lisa H. Cazares 《Applied microbiology》1989,55(7):1823-1828
The turnover of extracellular DNA was investigated in oligotrophic springs of the Crystal River and the eutrophic Medard Reservoir of southwest Florida. The Medard Reservoir possessed large populations of bacterioplankton and phytoplankton (6.8 × 109 cells per liter and 28.6 μg of chlorophyll a per liter, respectively), while the Crystal River springs only contained a fraction of the microbial biomass found in the Medard Reservoir. Although dissolved DNA values were greater in the Medard Reservoir, higher rates of DNA removal resulted in similar extracellular DNA turnover times in both environments (9.62 ± 3.6 h in the Crystal River and 10.5 ± 2.1 h in the Medard Reservoir). These results indicate that regardless of trophic status or microbial standing stock, extracellular DNA turns over rapidly in subtropical planktonic freshwater environments. Therefore, recombinant DNA sequences from released genetically engineered microorganisms might not be expected to survive for long periods of time in freshwater planktonic environments. 相似文献
27.
It was possible to gauge the inhibition of mouse beta-glucuronidase expression by injecting RNA, made from both strands of subclones of a cosmid containing the complete gene, into mouse blastomeres at the four-cell stage. Although our initial screen did not identify the 5' region, we were able to isolate a subclone containing homology to 20 bp coding for N-terminal amino acids of rat and human beta-glucuronidase structural genes. Antisense RNA prepared from one strand of the 350 bp Pst I subclone inhibited beta-glucuronidase expression by 89% while RNA prepared from the other strand had little effect. The subclone appears to correspond to the 350 bp fragment identified by others as one including the ATG start site of mouse beta-glucuronidase. 相似文献
28.
Dr. John L. Johnson Carol Phelps Lisa Barroso Mary D. Roberts David M. Lyerly Tracy D. Wilkins 《Current microbiology》1990,20(6):397-401
Results from our cloning studies on toxin A indicated that the gene for toxin B resided approximately 1 kb upstream of the toxin A gene. Clone pCD19, which contains the 5-end of the toxin A gene and a small open reading frame, was found to contain 1.2 kb of DNA which, when subcloned, expressed a nontoxic peptide that reacted with toxin B antibodies. The rest of the toxin B gene was located on the 6.8 kb cloned fragment of plasmid pCD19L. The two fragments overlapped 0.8 kb. Lysates containing protein expressed by the 6.8 fragment were cytotoxic and lethal, and were neutralized by toxin B antibody. The two fragments were ligated to give the complete toxin B gene. The protein expressed by the complete gene was cytotoxic and lethal, and showed complete immunological identity with toxin B. Further analysis of the expressed protein and the toxin B gene confirmed our earlier findings showing that toxin B has a molecular weight of 240,000 or greater. 相似文献
29.
Summary Methotrexate (MTX) was conjugated to an anti-carcinoembryonic antigen monoclonal antibody (NP2) by using amino-dextran as an intermediate carrier. The drug was chemically linked to amino-dextran (averageM
r = 40000), and the resulting MTX-dextran was then site-specifically attached to the carbohydrate moiety of the antibody. Athymic nude mice that carried human colonic GW-39 tumors (s. c.) were treated with the immunoconjugate. In this study, the specific conjugate caused a greater inhibition of the tumor growth than either free MTX or its conjugate with dextran and an irrelevant antibody. The intermediate MTX-dextran and the unlinked mixture of MTX-dextran with NP2 were both relatively ineffective in inhibiting tumor growth. The greatly reduced host toxicity permitted the use of the MTX-dextran-NP2 in a high-dose therapy of this tumor system.Supported in part by U.S.P.H.S. grant CA39 841 from the NIH 相似文献
30.
The HOX-5 and surfeit gene clusters are linked in the proximal portion of mouse chromosome 2 总被引:1,自引:0,他引:1
Lisa Stubbs Clare Huxley Brigid Hogan Timothy Evans Mike Fried Denis Duboule Hans Lehrach 《Genomics》1990,6(4)
Using an interspecies backcross, we have mapped the HOX-5 and surfeit (surf) gene clusters within the proximal portion of mouse chromosome 2. While the HOX-5 cluster of homeobox-containing genes has been localized to chromosome 2, bands C3-E1, by in situ hybridization, its more precise position relative to the genes and cloned markers of chromosome 2 was not known. Surfeit, a tight cluster of at least six highly conserved “housekeeping” genes, has not been previously mapped in mouse, but has been localized to human chromosome 9q, a region of the human genome with strong homology to proximal mouse chromosome 2. The data presented here place HOX-5 in the vicinity of the closely linked set of developmental mutations rachiterata, lethargic, and fidget and place surf close to the proto-oncogene Abl, near the centromere of chromosome 2. 相似文献