全文获取类型
收费全文 | 14203篇 |
免费 | 1111篇 |
出版年
2022年 | 67篇 |
2021年 | 176篇 |
2020年 | 122篇 |
2019年 | 156篇 |
2018年 | 222篇 |
2017年 | 166篇 |
2016年 | 338篇 |
2015年 | 557篇 |
2014年 | 557篇 |
2013年 | 816篇 |
2012年 | 993篇 |
2011年 | 952篇 |
2010年 | 668篇 |
2009年 | 522篇 |
2008年 | 786篇 |
2007年 | 856篇 |
2006年 | 856篇 |
2005年 | 821篇 |
2004年 | 800篇 |
2003年 | 734篇 |
2002年 | 747篇 |
2001年 | 165篇 |
2000年 | 115篇 |
1999年 | 163篇 |
1998年 | 218篇 |
1997年 | 151篇 |
1996年 | 134篇 |
1995年 | 141篇 |
1994年 | 122篇 |
1993年 | 119篇 |
1992年 | 139篇 |
1991年 | 96篇 |
1990年 | 81篇 |
1989年 | 99篇 |
1988年 | 91篇 |
1987年 | 80篇 |
1986年 | 83篇 |
1985年 | 98篇 |
1984年 | 130篇 |
1983年 | 102篇 |
1982年 | 114篇 |
1981年 | 109篇 |
1980年 | 98篇 |
1979年 | 66篇 |
1978年 | 68篇 |
1977年 | 62篇 |
1976年 | 58篇 |
1975年 | 50篇 |
1974年 | 57篇 |
1972年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
141.
142.
David J. James Andrew J. Passey Derek J. Barbara Michael Bevan 《Plant cell reports》1989,7(8):658-661
The disamed Ti-binary vector pBIN 6 in Agrobacterium tumefaciens has been used in leaf disc transfomations to produce transgenic apple (Malus pumila Mill.) plants with a nomal phenotype except for a somewhat reduced capacity to root. The presence of the genes for nopaline synthase and neomycin phosphotrans ferase (conferring kanamycin resistance), inserted into the host genome by the vector, was confirmed by Southern blot analysis, the detection of nopaline synthase activity and rooting in the presence of the antibiotic.The nopaline synthase gene continued to be expressed in glasshouse-grown plants several months after removal from in vitro growth conditions. 相似文献
143.
144.
Sandra Youngman Mansoor Sarfarazi Maja Bucan Marcy MacDonald Barbara Smith Michael Zimmer Conrad Gilliam Anna-Maria Frischauf John J. Wasmuth James F. Gusella Hans Lehrach Peter S. Harper Duncan J. Shaw 《Genomics》1989,5(4)
Genetic linkage studies have mapped Huntington's disease (HD) to the distal portion of the short arm of chromosome 4 (4p16.3), 4 cM distal to D4S10 (G8). To date, no definite flanking marker has been identified. A new DNA marker, D4S90 (D5), which maps to the distal region of 4p16.3, is described. The marker was used in a genetic linkage study in the CEPH reference families with seven other markers at 4p16. The study, together with knowledge of the physical map of the region, places D4S90 as the most distal marker, 6 cM from D4S10. A provisional linkage study with HD gave a maximum lod score of 2.14 at a θ of 0.00 and no evidence of linkage disequilibrium. As D4S90 appears to be located terminally, it should play an important role in the accurate mapping and cloning of the HD gene. 相似文献
145.
Summary The purpose of this study was to characterize the basolateral membrane of the S3 segment of the rabbit proximal tubule using conventional and ion-selective microelectrodes. When compared with results from S1 and S2 segments, S3 cells under control conditions have a more negative basolateral membrane potential (V
bl=–69 mV), a higher relative potassium conductance (t
K=0.6), lower intracellular Na+ activity (A
Na=18.4mm), and higher intracellular K+ activity (A
K=67.8mm). No evidence for a conductive sodium-dependent or sodium-independent HCO
3
–
pathway could be demonstrated. The basolateral Na–K pump is inhibited by 10–4
m ouabain and bath perfusion with a potassium-free (0-K) solution. 0-K perfusion results inA
Na=64.8mm,A
K=18.5mm, andV
bl=–28 mV. Basolateral potassium channels are blocked by barium and by acidification of the bathing medium. The relative K+ conductance, as evaluated by increasing bath K+ to 17mm, is dependent upon the restingV
bl in both S2 and S3 cells. In summary, the basolateral membrane of S3 cells contains a pump-leak system with similar properties to S1 and S2 proximal tubule cells. The absence of conductive bicarbonate pathways results in a hyperpolarized cell and larger Na+ and K+ gradients across the cell borders, which will influence the transport properties and intracellular ion activities in this tubule segment. 相似文献
146.
Dr. Bernard R. Glick Barbara J. Butler Colin I. Mayfield J. J. Pasternak 《Current microbiology》1989,19(3):143-146
We previously observed that whenAzotobacter vinelandii was transformed by different broad-host-range plasmids, normal cellular functions such as growth and siderophore production are impaired. In the present work, whenA. vinelandii was transformed with the low copy number plasmid pRK290, the extent of this metabolic impairment was lessened, as evidenced by increased siderophore production and moderate levels of growth on medium that lacks added iron. It is concluded that the severity of the plasmid-induced metabolic load reflects the relative level of expression of plasmid-encoded proteins. 相似文献
147.
Mary Lou Guerinot Barbara Anne Morisseau Taryn Klapatch 《Molecular & general genetics : MGG》1990,221(2):287-290
Summary Electroporation offers a fast, efficient and reproducible way to introduce DNA into bacteria. We have successfully used this technique to transform two commercially important strains of Bradyrhizobium japonicum, the nitrogen-fixing soybean symbiont. Initially, electroporation conditions were optimized using plasmid DNA which had been prepared from the same B. japonicum strain into which the{imDNA was to b}e transformed. Efficiencies of 105-106 transformants/g DNA were obtained for strains USDA 110 and 61A152 with ready-to-use frozen cells. Successful electroporation of B. japonicum with plasmid DNA prepared from Escherichia coli varied with the E. coli strain from which the plasmid was purified. The highest transformation efficiencies (104 transformants/g DNA) were obtained using DNA prepared from a dcm
–
dam
– strain of E. coli. This suggests that routine isolation of DNA from an E. coli strain incapable of DNA modification should help in increasing transformation efficiencies for other strains of bacteria where DNA restriction appears to be a significant obstacle to successful transformation. We have also monitored the rate of spontaneous mutation in electroporated cells and saw no significant difference in the frequency of streptomycin resistance for electroporated cells compared to control cells. 相似文献
148.
Effect of high light on the efficiency of photochemical energy conversion in a variety of lichen species with green and blue-green phycobionts 总被引:13,自引:0,他引:13
Barbara Demmig-Adams Cristina Máguas William W. Adams III Angelika Meyer Ellen Kilian Otto L. Lange 《Planta》1990,180(3):400-409
Exposure to high light induced a quantitatively similar decrease in the rate of photosynthesis at limiting photon flux density (PFD) and of photosystem II (PSII) photochemical efficiency, FV/FM, in both green and blue-green algal lichens which were fully hydrated. Such depressions in the efficiency of photochemical energy conversion were generally reversible in green algal lichens but rather sustained in blue-green algal lichens. This greater susceptibility of blue-green algal lichens to sustained photoinhibition was not related to differences in the capacity to utilize light in photosynthesis, since the light-and CO2-saturated rates of photosynthetic O2 evolution were similar in the two groups. These reductions of PSII photochemical efficiency were, however, largely prevented in lichen thalli which were fully desiccated prior to exposure to high PFD. Thalli of green algal lichens which were allowed to desiccate during the exposure to high light exhibited similar recovery kinetics to those which were kept fully hydrated, whereas bluegreen algal lichens which became desiccated during a similar exposure exhibited greatly accelerated recovery compared to those which were kept fully hydrated. Thus, green algal lichens were able to recover from exposure to excessive PFDs when thalli were in either the hydrated or desiccated state during such an exposure, whereas in blue-green algal lichens the decrease in photochemical efficiency was reversible in thalli illuminated in the desiccated state but rather sustained subsequent to illumination of thalli in the hydrated state.Abbreviations and Symbols Fo
yield of instantaneous fluorescence
- FM
maximum yield of fluorescence induced by pulses of saturating light
- FV
variable yield of fluorescence
- PFD
photon flux density (400–700 nm)
- PSII
photosystem II
This work was supported by the Deutsche Forschungsgeneinschaft (Forscherguppe Ökophysiologic and Sonderforschungsbereich 251 of the University of Würzburg) and the Fonds der Chemischen Industrie. W.W.A. gratefully acknowledges the support of a fellowship from the Alexander von Humboldt Foundation. We thank Professor T.G.A. Green for identifying and supplying all of the New Zealand lichen material and Professor F.-C. Czygan for advice concerning the chlorophyll analyses which were performed by Johanna Leisner. 相似文献
149.
Fred J. Genthner Janeshwar Upadhyay Robert P. Campbell Barbara R. Sharak Genthner 《Microbial ecology》1990,20(1):283-288
Culturable counts of antibiotic resistant, genetically engineeredPseudomonas fluorescens were determined on antibiotic-containing plate count agar during starvation in water. Prior to starvation, colony counts
obtained on all media separated into two groups. The mean of the colony counts on plate count agar with or without tetracycline
(4.9 × 106 ml−1) was significantly higher than the mean colony counts on plate count agar containing either nalidixic acid or nalidixic acid
plus tetraclycline (2.5×106 ml−1). After 20 days of starvation the highest mean colony counts continued to be obtained on plate count agar (7.2 × 106 ml−1) with slightly, but significantly, lower counts obtained on plate count agar containing either nalidixic acid (5.6 × 106 ml−1) or tetraclycline (1.5×106 ml−1). A combination of nalidixic acid and tetracycline in plate count agar, however, dramatically reduced colony counts (8.3
× 102 ml−1) after this starvation period. The addition of catalase to plate count agar containing nalidixic acid and tetracycline negated
the effect caused by this combination of antibiotics. When colony counts obtained over the entire 20 day incubation were considered,
the addition of MgSO4 to plate count agar containing nalidixic acid and tetracycline resulted in a significant increase in colony counts. Other
combinations of antibiotics, nalidixic acid+carbenicillin, nalidixic acid+kanamycin, streptomycin+tetracycline, streptomycin+carbenicillin,
rifampicin+tetracycline, rifampicin+carbenicillin, and rifampicin+kanamycin, did not inhibit colony formation of starved cells.
Antibiotic resistant strains ofP. putida andEscherichia coli also displayed sensitivity to the combination of nalidixic acid and tetracycline in plate count agar after starvation. 相似文献
150.
Patricia A. Fraser Barbara Moore Rosanne Stein Sharon Alosco Armead H. Johnson Deborah Marcus-Bagley Zuhier Awdeh Edmond J. Yunis Chester A. Alper 《Immunogenetics》1990,31(2):89-93
We analyzed the frequency distribution of 106 complotypes [four allele sets of the major histocompatibility complex (MHC) genes for the complement proteins factor B, C2, C4A, and C4B] from 32 Black families residing in Boston and Washington, DC. Twenty-five different complotypes were identified, among which there were four complotypes that had not been previously observed in our large database of complotypes compiled from family studies of Boston Caucasians and that are, presumably, unique to individuals of African origin. These four African-derived complotypes areFC(1,90)0, FC63, S1C2,17, andSC(3,2,90)0. The frequencies of two of these four unique Black complotypes,FC(1,90)0 andFC63, were increased significantly when compared to Caucasians (pcorr <0.00042, pcorr=0.00294, respectively). The complotypeFC(1,90)0 was in positive linkage disequilibrium withHLA-DR3 haplotypes containing theB locus antigens Bw42, Bw52, Bw53, and Bw58, whileFC63 was associated withHLA-Bw70,-DR5. These findings demonstrate the extensive polymorphism of complotypes in Blacks, and also suggest that it may be possible to define unique extended haplotypes of African origin. 相似文献