全文获取类型
收费全文 | 1313篇 |
免费 | 116篇 |
国内免费 | 40篇 |
出版年
2023年 | 21篇 |
2022年 | 18篇 |
2021年 | 21篇 |
2020年 | 38篇 |
2019年 | 69篇 |
2018年 | 39篇 |
2017年 | 42篇 |
2016年 | 38篇 |
2015年 | 31篇 |
2014年 | 47篇 |
2013年 | 73篇 |
2012年 | 29篇 |
2011年 | 31篇 |
2010年 | 26篇 |
2009年 | 20篇 |
2008年 | 18篇 |
2007年 | 29篇 |
2006年 | 23篇 |
2005年 | 41篇 |
2004年 | 60篇 |
2003年 | 47篇 |
2002年 | 22篇 |
2001年 | 37篇 |
2000年 | 31篇 |
1999年 | 43篇 |
1998年 | 35篇 |
1997年 | 29篇 |
1996年 | 47篇 |
1995年 | 59篇 |
1994年 | 50篇 |
1993年 | 33篇 |
1992年 | 39篇 |
1991年 | 32篇 |
1990年 | 25篇 |
1989年 | 18篇 |
1988年 | 22篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 30篇 |
1984年 | 18篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有1469条查询结果,搜索用时 218 毫秒
991.
A bacterial strain, Curtobacterium sp., isolated from a soil with zinc added possessed -cyclohexyl fatty acids. -Cyclohexyl undecanoic acid made up 47% of the total fatty acids; it was the most abundant fatty acid in the strain grown in tryptone medium. 12-Methyl tetradecanoic acid (23%) and 14-methyl hexadecanoic acid (22%) were also major fatty acids. The proportion of -cyclohexyl undecanoic acid increased as the pH of the medium decreased and as the culture temperature increased.The bacteria grew almost normally in zinc-enriched medium, and -cyclohexyl undecanoic acid increased with zinc concentration. Zinc added to the medium was not abundant in the cell fraction, and the ratio of increase of zinc in the cells was not so high as in the culture medium. These results suggested that -cyclohexyl fatty acids are related to the zinc tolerance of the isolated strain, and that this tolerance depends on low permeability of the membrane to zinc. 相似文献
992.
Procedures were developed for the isolation and culture of an anucleate protoplast system from cotton fibers actively undergoing secondary wall synthesis. Because the fibers at this stage are elongated single cells (30 m × 1–2 cm), most of the cellular vesicles released in the process of isolation are anucleate. After purification, the protoplast population was nuclei-free. When transferred to culture medium, the anucleate protoplasts (cytoplasts) synthesized starch, hydrolyzed fluorescene diacetate for up to 9 days and formed cell wall material for at least 7 days. The composition of the regenerated cell walls was dependent upon the substrate supplied in the medium: -1,3-linked glucans were predominantly synthesized when 1 mM UDP[14C]glucose was supplied; -1,4-linked glucans were predominantly synthesized when 1 mM [14C]-glucose was supplied. Thus the composition of the regenerated cell walls formed by the anucleate protoplasts was similar to the secondary cell wall synthesized by intact cotton fibers under the same culture conditions. 相似文献
993.
1-Bungarotoxin (1-Bgt) fromBungarus multicinctus (Taiwan banded krait) snake venom was subjected to tyrosine modification withp-nitrobenzenesulfonyl fluoride (NBSF) atpH 8.0 and the NBS derivatives were separated by high-performance liquid chromatography. The results of amino acid analysis revealed that only one Tyr residue out of 14 was modified, and the modified residue was identified to be Tyr-68 in the A chain of 1-Bgt. Spectrophotometric titration indicated that the phenolic group of Tyr-68 has apK of 10.1. Modification of Tyr-68 in the A chain caused a selective loss in lethal toxicity, but had no effect on either enzymatic or antigenic activities. The Ca2+-induced difference spectra and fluorescence study indicated that 1-Bgt possesses at least two different types of Ca2+-binding sites. However, modification of Tyr-68 in 1-Bgt did not cause any change of the Ca2+-induced difference spectra and fluorescence spectra in native toxin and the two types of Ca2+-binding sites were retained. Moreover, the affinity of Tyr-68-modified 1-Bgt for 8-anilinonaphthalene sulfonate was also unaffected in both the presence and absence of Ca2+. All of the results indicated that Tyr-68 is not involved in the Ca2+ and substrate bindings in the A chain of 1-Bgt. It is concluded that lethal toxicity is not necessarily associated with enzymatic, antigenic, and Ca2+-binding activities in 1-Bgt. 相似文献
994.
Om Prakash Mishra L. Craig Wagerle Maria Delivoria-Papadopoulos 《Neurochemical research》1988,13(11):1055-1060
The activity of key enzymes of adenosine metabolism was studied in the developing fetal guinea pig brain. The activities of 5-nucleotidase and adenosine deaminase were determined in the brains of fetal guinea pigs at 30, 35, 40, 45, 50, 55, and 60 days of gestation. The level of 5-nucleotidase activity was extremely low at 30 and 35 days of gestation but increased rapidly during the 40 to 60 day period. The enzyme activity increased in the presence of Mg2+ with the Mg2+-dependent activation increasing with the age of gestation. This Mg2+-dependent activity was primarily associated with the membrane fraction. Prenatal hypoxia significantly increased the fetal brain M2+-independent 5-nucleotidase activity at 45 days of gestational age and beyond. Prior to this age, no effect was evident. Furthermore, following hypoxia, the Mg2+-dependent activation of 5-nucleotidase activity was lost. The activity of adenosine deaminase was present at 30 days of gestation and, unlike 5-nucleotidase, it remained at the same level until 60 days. The results indicate that the term fetal guinea pig brain has the enzymatic mechanisms of adenosine metabolism and thus the potential for adenosine-mediated regulation of cerebrovasculature during hypoxia. 相似文献
995.
Pedro E. Maldonado Birgit Rose Werner R. Loewenstein 《The Journal of membrane biology》1988,106(3):203-210
Summary The epidermal growth factor (EGF) and the platelet-derived growth factor (PDGF) inhibit gap junctional communication in the mammalian cell lines NRK and BalbC 3T3: cell-to-cell transfer of a 400-dalton tracer molecule is reduced and junctional conductance is reduced. The inhibition of cell-to-cell transfer is reversible and dose dependent; half-maximal effects are obtained at 10–9 and 10–11
m concentrations of EGF and PDGF, respectively. The response of junctional conductance is detectable within 2 min of EGF application and reaches a maximum within 10 min. It is among the earliest cellular responses to this growth factor and may be significant in the regulation of growth. The response is lacking in EGF receptor-deficient NIH 3T3 cells. The transforming factor (TGF) enhances junctional communication in BalbC 3T3: cell-to-cell transfer is increased over a period of 8 hr. But in NRK cells, where it upregulates EGF receptors, TGF reduces junctional communication synergistically with EGF. 相似文献
996.
Summary MATa cells of Saccharomyces cerevisiae defective in both the SST1 and SST2 gene products exhibit selfarrest when they express the MF1 gene under the control of the GAL1 promoter. This reponse to endogenously produced pheromone can be alleviated by mutations which prevent the production of, or response to, -factor. Suppressors of the self-arrest phenotype include a class of mutants which remain responsive to low levels of pheromone, but are resistant to high levels of -factor. One of these mutants has been mapped to chromosome X, 31 cM distal to SUP4, and defines a new locus designated STE18. 相似文献
997.
Primary structure of hemoglobin of -chain ofColumba livia is presented. The separation of -chain was obtained from globin by ion-exchange chromatography (CMC-52) and reversed-phase HPLC (RP-2 column). Amino acid sequence of intact as well as tryptic digested chain was determined on gas-phase sequencer. Structure is aligned homologously with 21 other species. Among different exchanges, positions 24 (TyrLeu), 26 (AlaGly), 32 (MetLeu), 64 (AspGlu), 113 (LeuPhe), and 129 (LeuVal) are unique to pigeon hemoglobin. The various exchanges in -chain are discussed with reference to evolution and phylogeny. The results show that the order Columbiformes is evolutionarily closer to the order Anseriformes. Since the pigeon is homogeneous, having HbA (A-chain) and lacks D-chain, its phylogenetic placement could be established among birds having single hemoglobin components. 相似文献
998.
Ethylene production in leaf petiole and laminae tissues was stimulated in tomato (Lycopersicon esculentum Mill. cv. UCT5) plants exposed to salinity-stress. At the highest salinity level (250 mM NaCl), rates of ethylene production more than doubled over those observed in non-stressed plants. Correspondingly, petiolar epinasty increased with increasing levels of stress impositions. Both responses were suppressed when either 1 mM -aminooxyacetic acid (AOA), or 100 M Co2+ was simultaneously applied. Co2+, but not AOA, had a pronounced effect on ethylene production resulting from the application of a saturating dose (2 mM) of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. This result suggests that ethylene production is dependent upon the activity of ethylene forming enzyme (EFE). The magnitude of ethylene stimulation in leaf petioles was related to the salinity level imposed and to the induction of petiole epinasty. In the absence of stress impositions, epinastic responsiveness to ethylene or its precursor, ACC, might provide a simple, indirect criteria to adjudge salt-sensitivity among plants.Research supported by AID contract II, NEB-1070-A-00-2074-00. 相似文献
999.
DNA packaging of bacteriophage T4 proheads in vitro. Evidence that prohead expansion is not coupled to DNA packaging 总被引:4,自引:0,他引:4
We developed a system for DNA packaging of isolated bacteriophage T4 proheads in vitro and studied the role of prohead expansion in DNA packaging. Biologically active proheads have been purified from a number of packaging-deficient mutant extracts. The cleaved mature prohead is the active structural precursor for the DNA packaging reaction. Packaging of proheads requires ATP, Mg2+ and spermidine, and is stimulated by polyethylene glycol and dextran. Predominantly expanded proheads (ELPs) are produced at 37 degrees C and predominantly unexpanded proheads (ESPs) are produced at 20 degrees C. Both the expanded and unexpanded proheads are active in DNA packaging in vitro. This is based on the observations that (1) both ESPs and ELPs purified by chromatography on DEAE-Sephacel showed DNA packaging activity; (2) apparently homogeneous ELPs prepared by treatment with sodium dodecyl sulfate (which dissociates ESPs) retained significant biological activity; (3) specific precipitation of ELPs with anti-hoc immunoglobulin G resulted in loss of DNA packaging activity; and (4) ESPs upon expansion in vitro to ELPs retained packaging activity. Therefore, contrary to the models that couple DNA packaging to head expansion, in T4 the expansion and packaging appear to be independent, since the already expanded DNA-free proheads can be packaged in vitro. We therefore propose that the unexpanded to expanded prohead transition has evolved to stabilize the capsid and to reorganize the prohead shell functionally from a core-interacting to a DNA-interacting inner surface. 相似文献
1000.
Construction of a human X-chromosome-enriched phage library which facilitates analysis of specific loci 总被引:7,自引:0,他引:7
A human X-chromosome-enriched MboI-partial-digest recombinant library in phage lambda Charon30 has been constructed. Twelve out of the thirteen X-chromosome DNA sequences that were tested were present in the library. Most regions were covered in overlapping phage inserts; mean insert size was 13.7 kb. One phage from the library allowed detection of a 225-bp insertion of DNA into a region near the Duchenne muscular dystrophy (DMD) locus. Another recombinant phage represents an expansion of a region which exhibits extensive and varying homology with other human chromosomes, including the Y, as well as with rodent DNA. The present library should have widespread use for examining DNA sequences on the human X chromosome. 相似文献