全文获取类型
收费全文 | 7673篇 |
免费 | 572篇 |
国内免费 | 2篇 |
专业分类
8247篇 |
出版年
2023年 | 71篇 |
2022年 | 134篇 |
2021年 | 218篇 |
2020年 | 152篇 |
2019年 | 220篇 |
2018年 | 238篇 |
2017年 | 226篇 |
2016年 | 293篇 |
2015年 | 429篇 |
2014年 | 403篇 |
2013年 | 557篇 |
2012年 | 577篇 |
2011年 | 643篇 |
2010年 | 414篇 |
2009年 | 326篇 |
2008年 | 437篇 |
2007年 | 433篇 |
2006年 | 375篇 |
2005年 | 327篇 |
2004年 | 315篇 |
2003年 | 278篇 |
2002年 | 261篇 |
2001年 | 91篇 |
2000年 | 69篇 |
1999年 | 65篇 |
1998年 | 49篇 |
1997年 | 50篇 |
1996年 | 43篇 |
1995年 | 38篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 45篇 |
1991年 | 35篇 |
1990年 | 37篇 |
1989年 | 25篇 |
1988年 | 40篇 |
1987年 | 21篇 |
1986年 | 28篇 |
1985年 | 17篇 |
1984年 | 22篇 |
1983年 | 23篇 |
1982年 | 14篇 |
1981年 | 23篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 13篇 |
1976年 | 15篇 |
1975年 | 14篇 |
1974年 | 7篇 |
排序方式: 共有8247条查询结果,搜索用时 15 毫秒
41.
Site-specific recombination by mutants of Tn21 resolvase with DNA recognition functions from Tn3 resolvase 总被引:3,自引:0,他引:3
The resolvases from the transposons Tn3 and Tn21 are homologous proteins but they possess distinct specificities for the DNA sequence at their respective res sites. The DNA binding domain of resolvase contains an amino acid sequence that can be aligned with the helix-turn-helix motif of other DNA binding proteins. Mutations in the gene for Tn21 resolvase were made by replacing the section of DNA that codes for the helix-turn-helix with synthetic oligonucleotides. Each mutation substituted one amino acid in Tn21 resolvase with either the corresponding residue from Tn3 resolvase or a residue that lacks hydrogen bonding functions. The ability of these proteins to mediate recombination between res sites from either Tn21 or Tn3 was measured in vivo and in vitro. With one exception, where a glutamate residue had been replaced by leucine, the activity of these mutants was similar to that of wild-type Tn21 resolvase. A further mutation was made in which the complete recognition helix of Tn21 resolvase was replaced with that from Tn3 resolvase. This protein retained activity in recombining Tn21 res sites, though at a reduced level relative to wild-type; the reduction can be assigned entirely to weakened binding to this DNA. Neither this mutant nor any other derivative of Tn21 resolvase had any detectable activity for recombination between res sites from Tn3. The exchange of this section of amino acid sequence between the two resolvases is therefore insufficient to alter the DNA sequence specificity for recombination. 相似文献
42.
43.
Mar Pérez José M. Valpuesta Miguel Medina Esteban Montejo de Garcini Jesús Avila 《Journal of neurochemistry》1996,67(3):1183-1190
Abstract: Paired helical filaments isolated from the brains of patients with Alzheimer's disease are composed of a major protein component, the microtubule-associated protein termed τ, together with other nonprotein components, including heparan, a glycosaminoglycan, the more extensively sulfated form of which is heparin. As some of these nonprotein components may modulate the assembly of τ into filamentous structures, we have analyzed the ability of the whole τ protein or some of its fragments to self-assemble in the presence of heparin. Different τ fragments, all of them containing some sequences of the tubulin-binding motif, can assemble in vitro into filaments. We have also found formation of polymers with the 18-residue-long peptide corresponding to the third tubulin-binding motif of τ. This suggests that the ability of τ for self-assembly could be localized in a short sequence of amino acids present in the tubulin-binding repeats of the τ molecule. 相似文献
44.
The present paper describes growth dynamics in a natural bed of the resource luga negra (Sarcothalia crispata) in Guapilinao, southern Chile (41°57 S, 73°31 W). This resource is currently harvested and exported as raw material for the production of carrageenan. Seasonal variation in biomass, frond size, density and phenology was determined by periodic sampling. Natural recruitment was evaluated on different substrata added to the field; at the same time, substrata were inoculated under greenhouse conditions. Results showed that luga negra has seasonal growth: biomass increased from a minimum in spring to a maximum in mid to late summer. On the other hand, density was minimal in winter (200 ind. m–2) and increased to 2000 ind. m–2 in late spring. Peak abundance of mature tetrasporic fronds occurred in late summer, whereas that of cystocarpic fronds occurred in winter. Recruitment began in summer and extended into winter. Survival on different substrata were compared. Gametophytes had better survival rates on clam shells and 5 mm rope while tetrasporophytes had the best survival rate on clam shells and secondarily on boulders. 相似文献
45.
Guy G. Orgambide Saleela Philip-Hollingsworth Pedro F. Mateos Rawle I. Hollingsworth Frank B. Dazzo 《Plant and Soil》1996,186(1):93-98
Axenic seedling bioassays were performed on white clover, vetch, and alfalfa to assess the variety and dose responses of biological activities exhibited by membrane chitolipooligosaccharides (CLOSs) from wild type Rhizobium leguminosarum bv. trifolii ANU843. Subnanomolar concentrations of CLOSs induced deformation of root hairs (Had) and increased the number of foci of cortical cell divisions (Ccd) in white clover, some of which developed into nodule meristems. In contrast, ANU843 CLOSs were unable to induce Had in alfalfa and required a 104-fold higher threshold concentration to induce this response in vetch. Also, ANU843 CLOSs were not mitogenic on either of these non-host legumes. In addition, CLOS action also increased chitinase activity in white clover root exudate. Thus, the membrane CLOSs from wild type R. leguminosarum bv. trifolii are fully capable of eliciting various symbiosis-related responses in white clover in the same concentration range as extracellular CLOSs of other rhizobia on their respective legume hosts. These results and our earlier studies indicate that membrane CLOSs represent one of many different classes of bioactive metabolites made by R. leguminosarum bv. trifolii which elicit more intense symbiosis-related responses in white clover than in other legumes. Therefore, CLOSs evidently play an important role in symbiotic development, but they may not be the sole determinant of host-range in the Rhizobium-clover symbiosis.Abbreviations Ccd
cortical cell division
- CLOS
chitolipooligosaccharide
- Had
root hair deformation 相似文献
46.
Jose Pontón Fernando L. Hernando Maria Dolores Moragues Pedro L. Barea Mara Gerloni Stefania Conti Paola Fisicaro Cristina Cantelli Luciano Polonelli 《Mycopathologia》1996,133(2):89-94
The presence of heat shock mannoproteins (HSMPs) reactive with sIgA was demonstrated in several C. albicans strains. The subculture of the C. albicans isolated from mucosal surfaces on Sabouraud's dextrose agar at 25 °C switched off the HSMP expression. A re-expression of the HSMPs was obtained in the same medium by shifting the temperature of incubation to 37 °C. However, expression of HSMPs in two strains isolated from deep infections was maintained during several subcultures on Sabouraud's dextrose agar at 25 °C. A glycoprotein of 200 kDa seemed to be the main HSMP reacting with vaginal sIgA. The data presented in this study suggest that factors other than temperature can influence the expression of C. albicans HSMPs and therefore these antigens should be referred as stress mannoproteins.Abbreviations HSMPs
heat shock mannoproteins
- MAb
monoclonal antibody
- sIgA
secretory IgA 相似文献
47.
Organization and complexity of minicircle-encoded guide RNAs in Trypanosoma cruzi. 总被引:2,自引:1,他引:1 下载免费PDF全文
The previously observed extensive sequence heterogeneity of the kinetoplast minicircle DNA in Trypanosoma cruzi, both intra- and interstrain, has raised the question as to how the minicircle DNA in this species can have any guide RNA (gRNA)-coding capacity at all, because there do not appear to be any variable-region sequences conserved between different strains. To address this question, we obtained the complete edited sequence of maxicircle unidentified reading frame 4 mRNA and identified 25 cognate gRNAs from gRNA libraries constructed from two clonal strains of T. cruzi--Sylvio X10/CL1 and CAN III/CL1. Libraries of PCR-amplified minicircle-variable regions were also constructed for both strains. A single gene for each gRNA was identified in the same polarity within specific minicircle-variable regions from both strains, 60-100 nt downstream from the conserved 12mer sequence. GTP-capped total gRNA from one strain failed to cross-hybridize with minicircle DNA from the other strain. The explanation for this proved to be the number of polymorphisms, mainly transitions, within the homologous gRNAs in the two strains. In most cases, these transitions did not destroy the edited mRNA/gRNA base pairing, as a result of the allowed G-U wobble base pairing. The sequences of the variable regions containing homologous gRNAs in the two strains probably derived from an ancestral sequence, and each has accumulated sufficient polymorphisms so as not to allow hybridization. Within a strain, multiple redundant gRNAs were identified that encode identical editing information but have different sequences. 相似文献
48.
49.
50.
Peptidoglycan tripeptide content and cross-linking are altered in Enterobacter cloacae induced to produce AmpC beta-lactamase by glycine and D-amino acids. 下载免费PDF全文
Induction of AmpC beta-lactamase in Enterobacter cloacae ATCC 13047 by D-methionine, glycine, or D-tryptophan was accompanied by alterations in peptidoglycan composition and structure; in the case of D-methionine, it was also accompanied by morphologic changes. A decrease in peptidoglycan tripeptides was seen. With glycine, there was an increase in the proportion of diaminopimelic-diaminopimelic cross-links. The possible implications of these changes for beta-lactamase induction are discussed. 相似文献