全文获取类型
收费全文 | 8933篇 |
免费 | 769篇 |
国内免费 | 1篇 |
出版年
2024年 | 12篇 |
2023年 | 39篇 |
2022年 | 109篇 |
2021年 | 228篇 |
2020年 | 113篇 |
2019年 | 152篇 |
2018年 | 188篇 |
2017年 | 161篇 |
2016年 | 291篇 |
2015年 | 469篇 |
2014年 | 508篇 |
2013年 | 545篇 |
2012年 | 775篇 |
2011年 | 736篇 |
2010年 | 459篇 |
2009年 | 423篇 |
2008年 | 568篇 |
2007年 | 569篇 |
2006年 | 532篇 |
2005年 | 535篇 |
2004年 | 491篇 |
2003年 | 458篇 |
2002年 | 429篇 |
2001年 | 73篇 |
2000年 | 50篇 |
1999年 | 104篇 |
1998年 | 111篇 |
1997年 | 71篇 |
1996年 | 57篇 |
1995年 | 58篇 |
1994年 | 48篇 |
1993年 | 51篇 |
1992年 | 40篇 |
1991年 | 31篇 |
1990年 | 38篇 |
1989年 | 26篇 |
1988年 | 28篇 |
1987年 | 18篇 |
1986年 | 16篇 |
1985年 | 15篇 |
1984年 | 18篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 2篇 |
排序方式: 共有9703条查询结果,搜索用时 15 毫秒
91.
Rockford K. Draper Daniel Chin Lisa Stubbs Melvin I. Simon 《Journal of cellular biochemistry》1978,9(1):47-55
Concanavalin A, wheat germ agglutinin and the ovalbumin glycopeptide are all inhibitors of the cytotoxic effect of diphtheria toxin on Chinese hamster cells. Ovalbumin glycopeptide loses its inhibitory property after treatment with β-N-acetylglucosaminidase. This demonstrates the importance of the glycopeptide structure for the mechanism of inhibition. The glycopeptide may be a toxin cell-surface receptor analogue. Diphtheria toxin-resistant mutants were isolated in order to search for cells that might have an altered toxin receptor. One mutant was 10-to 15-fold more resistant to diphtheria toxin than wild-type cells when protein synthesis was measured as a function of toxin concentration. However, when protein synthesis was measured as a function of time at a high toxin concentration, the time before onset of inhibition was identical in the mutant and wild-type cells. We present evidence indicating that the resistance of this mutant can be accounted for by a decreased affinity of toxin for a cell-surface receptor. 相似文献
92.
Lisa Juntti-Berggren Peter Pitkänen Erik Wilander 《Virchows Archiv. B, Cell pathology including molecular pathology》1983,43(1):37-42
A middle-aged woman without any symptoms of ectopic hormone production underwent a right-sided mastectomy for infiltrating ductal carcinoma. She later developed axillary lymph node metastases which were somewhat carcinoid-like. This prompted further investigation, when scattered argyrophilic endocrine cells were found in both the primary tumour and its metastases. The endocrine cells reacted immunocytochemically with antisera against ACTH and HCG. Despite the endocrine activity of the tumour, it was still regarded as a ductal carcinoma since the endocrine cells constituted the minority cell population. The present study indicates strongly that ectopic hormone production in association with carcinoma of the breast is a result of hormone synthesis and release by the tumour cells. 相似文献
93.
Leon Eidels Lisa L. Ross David A. Hart 《Biochemical and biophysical research communications》1982,109(2):493-499
Inositol hexaphosphate, and other polyphosphates, inhibit diphtheria toxin-mediated cytotoxicity by binding to the toxin at a highly cationic site called the P site and preventing toxin binding to cell surface receptors. The binding of diphtheria toxin to a solubilized cell surface glycoprotein (150,000 daltons) is also inhibited by these polyphosphates. Treatment of this 150,000 dalton diphtheria toxin-binding cell surface glycoprotein with papain yielded an 88,000 dalton and a 74,000 dalton diphtheria toxin-binding glycoprotein whose binding to toxin was no longer inhibited by inositol hexaphosphate. This result suggests a model of diphtheria toxin-receptor interaction in which the toxin receptor possesses one binding site which interacts with the P site of the toxin in a fashion, and another binding site (located within the papain-derived 74,000–88,000 dalton glycoproteins) which can interact with the toxin at a site distinct from the P site (the X site) in a fashion. This X site-receptor interaction may be involved in the binding of CRM proteins that bind to the toxin receptor but that do not bind polyphosphates, or it may be involved in the entry process of the toxin. 相似文献
94.
The distribution of dissolved DNA in an oligotrophic and a eutrophic river of Southwest Florida 总被引:2,自引:0,他引:2
John H. Paul Lisa H. Cazares Andrew W. David Mary F. DeFlaun Wade H. Jeffrey 《Hydrobiologia》1991,218(1):53-63
The distribution of dissolved DNA concentrations and some microbial variables were compared in an oligo-mesotrophic river (the Crystal River) and a phosphate-rich eutrophic river (the Alafia River) in Southwest Florida over a 15 month period. Concentrations of phosphate and nitrate in the Alafia River averaged 135 and 18.2 times the respective phosphate and nitrate concentrations of the oligo-mesotrophic Crystal River. The seasonal average dissolved DNA concentration for the Alafia River exceeded that of the Crystal River by a factor of 1.8 (8.2 g 1–1 compared to 4.6 g 1–1, respectively). The greatest concentrations of dissolved DNA in the Alafia River were found in areas that contained the largest populations of phytoplankton and bacteria (a reservoir formed from an abandoned phosphate mining pit and two downstream stations near the mouth of the river). Differences in dissolved DNA concentrations between these environments and more pristine environments (i.e. all Crystal River Stations and upstream Alafia River stations) were of the same order of magnitude (1.8 to 2.2-fold) as the differences in bacterial abundance and activity, but considerably less than differences in phytoplankton abundance and activity between such environments. Seasonal variations in dissolved DNA concentrations in the Crystal River corresponded to seasonal variations in microbial populations, with minimal values in January and greater values in July. In the Alafia River, lowest concentrations for dissolved DNA occurred in July during the wet season, when seasonal flooding of area of leaf litter yielded high levels of dissolved organic carbon (DOC) which were low in dissolved DNA. These results suggest that: 1) in situ planktonic activity is a greater source of dissolved DNA than allochthonous or terrestrial sources of DOC; 2) factors that control the magnitude of heterotrophic bacterial populations are more likely to control dissolved DNA levels than factors regulating autotrophic population activity and abundance; 3) differences in dissolved DNA between eutrophic and oligo-mesotrophic environments are often much smaller than the differences in nutrient concentration between such environments. 相似文献
95.
Michael C. Iannuzzi Robert C. Stern Francis S. Collins Catherine Tom Hon Noriko Hidaka Theresa Strong Lisa Becker Mitchell L. Drumm Marga B. White Bernard Gerrard Michael Dean 《American journal of human genetics》1991,48(2):227-231
Cystic fibrosis (CF) is a recessive disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. We have identified in exon 7 two frameshift mutations, one caused by a two-nucleotide insertion and the other caused by a one-nucleotide deletion; these mutations--CF1154insTC and CF1213delT, respectively, are predicted to shift the reading frame of the protein and to introduce UAA(ochre) termination codons at residues 369 and 368. 相似文献
96.
97.
98.
Edwin H. Bryant Lisa M. Meffert 《Evolution; international journal of organic evolution》1990,44(3):660-668
Multivariate phenotypic differentiation in eight morphometric traits was examined in bottleneck lines of the housefly initiated with one, four, or 16 pairs of flies from a natural outbred population. Differentiation was assessed using a Mahalanobis' distance metric in units of additive genetic variance and covariance estimated from the ancestral population (i.e., generalized genetic distance). This distance metric was partitioned into contributions of size and shape to total distance. Bottleneck lines of all sizes diverged significantly from the ancestral line, but the direction of these shifts differed among the lines of different initial founding size. Those populations founded with single pairs diverged from the ancestral line mostly in shape; the 16-pair lines differentiated almost entirely in size, and the four-pair lines were intermediate in the relative contribution of shape to differentiation from the control. Bottlenecks serve to alter the genetic relationships among traits within the derived populations and in doing so could promote speciation by permitting differentiation of the populations along evolutionary trajectories less accessible to the base population. 相似文献
99.
Occurrence of Nitrate Reductase and Molybdopterin in Xanthomonas maltophilia 总被引:1,自引:1,他引:0
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)
Lisa M. Woodard Anthony R. Bielkie John F. Eisses Paul A. Ketchum 《Applied microbiology》1990,56(12):3766-3771
Fifteen of 23 ATCC strains and 2 of 9 clinical isolates of Xanthomonas maltophilia, all of which grew aerobically on ammonia, but not nitrate, as a sole nitrogen source, reduced nitrate to nitrite. X. maltophilia failed to grow anaerobically on complex medium with or without nitrate, so it is considered an obligate aerobe. Nitrate-reducing strains contained reduced methyl viologen nitrate reductase (MVH-NR) with specific activities ranging from 49.2 to 192 U mg of protein−1. Strain ATCC 17666 doubled its cell mass after 3 h of growth on nitrate broth under low aeration, possessed maximal MVH-NR activity, and converted the added nitrate to nitrite, which accumulated. Dissolved oxygen above 15% saturation greatly suppressed nitrite formation. All strains, except ATCC 14535, possessed between 0.25 and 5.05 pmol of molybdopterin mg of protein−1 as measured by the Neurospora crassa nit-1 assay. The molybdopterin activity in the soluble fraction sedimented as a single symmetrical peak with an s20,w of 5.1. Studies identified MVH-NR in selected strains as a membrane-bound protein. The deoxycholate-solubilized MVH-NR sedimented as a single peak in sucrose density gradients with an s20,w of 8.8. The MVH-NR of X. maltophilia has the physical characteristics of a respiratory nitrate reductase and may enable cells to use nitrate as an electron sink under semiaerobic conditions. 相似文献
100.
The possible contribution of pinon (Pinus edulis) seed to the diet of an Anasazi population is investigated using present data on seed productivity and archaeological data on resource use. Annual estimates of total productivity for a 135.7 sq km study area in Montezuma County, Colorado, vary by a factor of 141 in a sample spanning 5 yr. Trees greater than about 25 cm (basal diameter) are usually monoecious and produce significantly more cones per tree but fewer good seeds per cone than smaller, usually dioecious specimens. Archaeological data suggest that piñon seed was a relatively unimportant food item after the initial period of Anasazi colonization of the Dolores River valley about a.D. 600. Use of piñon seed as food was inversely related to degree of agricultural intensification and use of piñon for fuel. 相似文献