全文获取类型
收费全文 | 5461篇 |
免费 | 287篇 |
国内免费 | 4篇 |
出版年
2022年 | 22篇 |
2021年 | 67篇 |
2020年 | 36篇 |
2019年 | 54篇 |
2018年 | 57篇 |
2017年 | 59篇 |
2016年 | 100篇 |
2015年 | 166篇 |
2014年 | 180篇 |
2013年 | 313篇 |
2012年 | 345篇 |
2011年 | 327篇 |
2010年 | 209篇 |
2009年 | 204篇 |
2008年 | 322篇 |
2007年 | 331篇 |
2006年 | 301篇 |
2005年 | 305篇 |
2004年 | 286篇 |
2003年 | 283篇 |
2002年 | 262篇 |
2001年 | 135篇 |
2000年 | 123篇 |
1999年 | 127篇 |
1998年 | 62篇 |
1997年 | 67篇 |
1996年 | 52篇 |
1995年 | 44篇 |
1994年 | 37篇 |
1993年 | 46篇 |
1992年 | 79篇 |
1991年 | 80篇 |
1990年 | 61篇 |
1989年 | 76篇 |
1988年 | 50篇 |
1987年 | 40篇 |
1986年 | 44篇 |
1985年 | 32篇 |
1984年 | 56篇 |
1983年 | 35篇 |
1982年 | 42篇 |
1981年 | 30篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1975年 | 16篇 |
1974年 | 24篇 |
1973年 | 14篇 |
1972年 | 15篇 |
1971年 | 12篇 |
排序方式: 共有5752条查询结果,搜索用时 31 毫秒
81.
The sulfonamide resistance (SAr) determinant was cloned from a transferable R plasmid of Pasteurella piscicida, pSP9351, and the sequence was determined. The resistance gene (pp-sul) was localized to an approximately 1-kb region that includes the PstI-EcoRI site in the restriction map. An open reading frame coding a sul II-type gene composed of 810 nucleotides was identified. A direct repeat sequence was shown in the 5′ flanking region of pp-sul, and a plasmid recombinational event may have occurred during the construction of pSP9351. In the 3′ flanking region of the gene, a sequence homologous to the 5′ noncoding sequence of the trimethoprim resistance gene, dhfr IX was found. 相似文献
82.
Omirulleh Serik Ismagulova Ainur Karabaev Murat Meshi Tetsno Iwabuchi Masaki 《Plant cell reports》1996,16(3-4):133-136
We have demonstrated that foreign DNA can be delivered into cells of mature embryos of wheat (Triticum aestivum L.) using silicon carbide fibers (SCF). The highest transient expression of thegusA (GUS) gene was detected when dry embryos were vortexed for 10–30 min in a SCF-DNA solution containing 90–120 g/l of sucrose. Up to 100 (on average 20–40) blue expression units per embryo were observed. Scutellum side and epiblast of the intact wheat embryos are preferentially transformed. When embryos with the coleoptilar tip removed were treated and allowed to germinate, GUS staining was observed in emerging leaf tissues. The potential of this new approach for stable transformation of wheat is under investigation. It has been found that callus tissues induced from the SCF treated embryos contain GUS-expressing sectors one month after treatment. 相似文献
83.
Cloning, sequencing, and expression of thermophilic Bacillus sp. strain TB-90 urease gene complex in Escherichia coli. 总被引:14,自引:9,他引:5
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The urease of thermophilic Bacillus sp. strain TB-90 is composed of three subunits with molecular masses of 61, 12, and 11 kDa. By using synthetic oligonucleotide probes based on N-terminal amino acid sequences of each subunit, we cloned a 3.2-kb EcoRI fragment of TB-90 genomic DNA. Moreover, we cloned two other DNA fragments by gene walking starting from this fragment. Finally, we reconstructed in vitro a 6.2-kb DNA fragment which expressed catalytically active urease in Escherichia coli by combining these three DNA fragments. Nucleotide sequencing analysis revealed that the urease gene complex consists of nine genes, which were designed ureA, ureB, ureC, ureE, ureF, ureG, ureD, ureH, and ureI in order of arrangement. The structural genes ureA, ureB, and ureC encode the 11-, 12-, and 61-kDa subunits, respectively. The deduced amino acid sequences of UreD, UreE, UreF, and UreG, the gene products of four accessory genes, are homologous to those of the corresponding Ure proteins of Klebsiella aerogenes. UreD, UreF, and UreG were essential for expression of urease activity in E. coli and are suggested to play important roles in the maturation step of the urease in a co- and/or posttranslational manner. On the other hand, UreH and UreI exhibited no significant similarity to the known accessory proteins of other bacteria. However, UreH showed 23% amino acid identity to the Alcaligenes eutrophus HoxN protein, a high-affinity nickel transporter. 相似文献
84.
A genomic clone for the cyc07 gene, which is expressed specifically at the S phase during the cell cycle in synchronous cultures of periwinkle (Catharanthus roseus) cells, was isolated. Determination of the nucleotide sequence of the clone revealed that the cyc07 gene consists of seven exons separated by six introns. Genomic Southern analysis indicated that the cyc07 gene is present as a single copy per haploid genome in periwinkle. Expression of related genes was detected in a wide range of other plants. Transgenic Arabidopsis plants were generated that expressed the gene for -glucuronidase (GUS) under the control of the promoter of the cyc07 gene. The tissue-specific pattern of expression directed by the promoter was investigated by analysis of GUS activity. Histochemical tests demonstrated that 589 bp of the 5-upstream sequence of the cyc07 gene could direct specifical expression of the GUS reporter gene in meristematic tissues in transgenic plants. The spatial pattern of expression directed by the promoter was closely correlated with meristematic activity and cell proliferation, suggesting an association between the function of the cyc07 gene and cell proliferation. 相似文献
85.
Light-enhanced active pyruvate uptake into mesophyll chloroplastsof C4 plants was reported to be mimicked by either of the twotypes of cation jump: H+-jump in maize and phylogenically relatedspecies (H+-type) and Na+-jump in all the other C4 species tested(Na+-type) [Aoki, N., Ohnishi, J. and Kanai, R. (1992) PlantCell Physiol. 33: 805]. In this study, medium and stromal pH was monitored in the suspensionof C4 mesophyll chloroplasts. Medium alkalization lasting for5 to 10 seconds after pyruvate addition was detected by a pHelectrode and observed only in the light and only in mesophyllchloroplasts from H+-type species, Zea mays L. and Coix lacryma-jobiL., but not in those from Na+-type species Panicum miliaceumL., Setaria italica (L.) Beauv. and Panicum maximum Jacq. Theinitial rate of H+ consumption showed good correlation with[14C]pyruvate uptake measured by silicone oil filtering centrifugation,both being inhibited by N-ethylmaleimide and 7-chloro-4-nitrobenzo-2-oxa-l,3-diazole to the same degree. The ratio of the rate of H+ uptaketo that of pyruvate uptake was always about 1. Pyruvate-inducedacidification of the stroma was observed in maize mesophyllchloroplasts. These results show one to one cotransport of H+and pyruvate anion into mesophyll chloroplasts of H+-type C4species in the light. (Received January 5, 1994; Accepted May 6, 1994) 相似文献
86.
Midura Ronald J.; Salustri Antonietta; Calabro Anthony; Yanagishita Masaki; Hascall Vincent C. 《Glycobiology》1994,4(3):333-342
Recent literature indicates that specific glycosaminoglycanstructures are involved in various biological processes, suchas anticoagulation, growth factor activation and viral infection.The initial step in the structural analysis of glycosaminoglycansis a definitive compositional analysis of its characteristicdisaccharide repeat structures. Current chromatographic or electrophoreticprocedures may have limitations in analysing glycosaminoglycansamples that are in low abundance, contain novel structuresthat need to be further characterized, or are metabolicallylabelled from radioactive precursors as a result of biosyntheticexperiments. This study presents a new methodology for analysingdisaccharides and oligosaccharides derived from chondroitinsulphate, dermatan sulphate and hyaluronan that fulfils theabove criteria. The procedure involves the separation of reducedforms of these glycoconjugates on a CarboPac PA1 column usingalkaline eluants. This study adopted a strategy which uses specificenzymes to release these disaccharides from their glycosaminoglycanforms. A borohydride reduction reaction was modified to be compatiblewith the buffer conditions commonly used with these enzymesin order to quantitatively reduce the disaccharides to theiralditol forms (thereby stabilizing them to alkaline pH). Chromatographyconditions were established which separated all known disaccharidealditol structures from chondroitin sulphate, dermatan sulphateand hyaluronan with extremely high resolution in a single run.Integrated pulsed amperometry was compared to UV absorbancemeasurement at 232 nm as two sensitive methods for detectingthese reduced disaccharides; most of them could be routinelydetected in the range of 50500 ng. Data are presentedapplying this method to quantify hyaluronan in a biologicalsample which contains {small tilde}5000 cells and only {smalltilde}10 ng of hyaluronan. Additional data are presented todemonstrate that this procedure will also separate oligosaccharidealditols derived from hyaluronan. borohydride reduction glycosaminoglycans integrated pulsed amperometry 相似文献
87.
Summary Two strains of osmophilic yeast which were isolated from honey-comb, produced good yields of erythritol as a main product. These strains were identified as Trichosporonoides sp., 150-5 and 331-1.From the fermentation studies with these strains using glucose and sucrose as substrate, strain 331-1 produced more erythritol as the sole polyhydric product,with trace quantities of glycerol, than strain 150-5. 相似文献
88.
Hatanaka Shin-Ichi Furukawa Jun Aoki Toshio Akatsuka Hirokazu Nagasawa Eiji 《Mycoscience》1994,35(4):391-394
Combining different chromatography systems, unusual nonprotein amino acids were isolated and unequivocally identified from a small amount (less than 100 g fresh weight) ofAmanita gymnopus fruit body. Without obtaining crystals of these amino acids, on the basis of1H-NMR determination, high resolution mass spectrometry, chlorine analysis and oxidation with L-amino acid oxidase, one of them proved to be a new chloroamino acid, (2S)-2-amino-5-chloro-4-hydroxy-5-hexenoic acid (G2). The other three were (2S)-2-amino-5-hexenoic acid (G1), (2S)-2-amino-4,5-hexadienoic acid (G3) and (2S)-2-amino-5-hexynoic acid (G4). Amino acid (G1) was also encountered for the first time in natural products. Amino acid (G3) has been reported from several kinds of fungi belonging toAmanita, subgenusLepidella. The occurrence of amino acid (G4) was already reported fromCortinarius claricolor.Part 23 in the series Biochemical studies of nitrogen compounds in fungi. Part 22, Hatanaka, S. I. et al. 1985. Trans. Mycol. Soc. Japan26: 61–68. 相似文献
89.
Evaluation of Superoxide Scavenging Activities of Hamamelis Extract and Hamamelitannin 总被引:2,自引:0,他引:2
Hamamelitannin, which is a component of bark extract of hamamelis (Hamamelis virginior L.), was found to be a potent scavenger of superoxide anion radicals. Superoxide anion scavenging activity of the compound was evaluated by ESR-spin trap method using DMPO (5,5'-dimethyl-1-pyrroline-N-oxide) as a spin trapping agent. The IC50 value (the concentration producing 50% inhibition of superoxide anion radicals) of hamamelitannin was found to be 1.38 ± 0.06 μM much lower than that of ascorbic acid (23.31 ± 2.23 μM). Supporting the superoxide scavenging activity of hamamelitannin, the compound showed both suppresive ability against depolymelization of hyaluronic acid and protective ability against cytotoxicity induced by superoxide anion radicals. Hamamelitannin increased the survival rate of fibroblast to 85.5 ± 3.3%, compared with that of control (27.2 ± 4.3%). 相似文献
90.
Karasawa Takusato; Tabuchi Kouji; Fumoto Masaki; Yasukawa Tamio 《Bioinformatics (Oxford, England)》1993,9(3):243-251
A model system is proposed to simulate the folding processesof proteins during thermal annealing. This system consists offour subsystems: (i) the pearl necklace model with isotropicinter-residue interactions; (ii) the extended pearl necklacemodel with anisotropic interaction potentials; (iii) moltenglobule phase dynamics; and (iv) final generation of the three-dimensionalstructure of a given protein. In this paper results obtainedwith the pearl necklace model are reported. This model consistsof spherical elements and virtual bonds of 3.8 Å in lengthand is intended to sinudate dynamical processes at relativelyhigh temperature where entropic terms play a dominant role.Inter-residue interactions are composed of spherical soft repulsivepotentials and hydrophobic interactions inherent to respectiveresidues. A simulation of folding processes of BPTI startingfrom the fully extended conformation indicated that intermediates,even at early stages of folding, are not randomly coiled butassume organized structures that resemble, to some extent, thenative conformation. 相似文献