全文获取类型
收费全文 | 7640篇 |
免费 | 617篇 |
国内免费 | 2篇 |
出版年
2023年 | 31篇 |
2022年 | 62篇 |
2021年 | 133篇 |
2020年 | 85篇 |
2019年 | 109篇 |
2018年 | 178篇 |
2017年 | 149篇 |
2016年 | 276篇 |
2015年 | 366篇 |
2014年 | 376篇 |
2013年 | 519篇 |
2012年 | 555篇 |
2011年 | 619篇 |
2010年 | 393篇 |
2009年 | 355篇 |
2008年 | 463篇 |
2007年 | 455篇 |
2006年 | 455篇 |
2005年 | 391篇 |
2004年 | 395篇 |
2003年 | 373篇 |
2002年 | 356篇 |
2001年 | 95篇 |
2000年 | 76篇 |
1999年 | 79篇 |
1998年 | 83篇 |
1997年 | 67篇 |
1996年 | 72篇 |
1995年 | 68篇 |
1994年 | 49篇 |
1993年 | 55篇 |
1992年 | 66篇 |
1991年 | 47篇 |
1990年 | 45篇 |
1989年 | 36篇 |
1988年 | 27篇 |
1987年 | 25篇 |
1986年 | 30篇 |
1985年 | 40篇 |
1984年 | 31篇 |
1983年 | 14篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 18篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1974年 | 8篇 |
排序方式: 共有8259条查询结果,搜索用时 15 毫秒
41.
42.
Rita Maria Ulloa Hector Norberto Torres Claudia M. Ochatt Maria Teresa Téllez-Iñón 《Molecular and cellular biochemistry》1991,102(2):155-163
DEAE-cellulose column chromatography of Neurospora crassa soluble mycelial extracts leads to the resolution of three major protein kinase activity peaks designated PKI, PKII, and PKIII.PKII activity is stimulated by Ca2+ and Neurospora or brain calmodulin. Maximal stimulation was observed at 2 µM-free Ca2+ and 1 µg/ml of the modulator. The stimulatory effect of the Ca2+-calmodulin complex was blocked by EGTA and by some calmodulin antagonists such as phenothiazine drugs or compound 48/80.PKII phosphorylates different proteins, among which histone II-A at a low concentration and CDPKS, the synthetic peptide specific for Ca2+-calmodulin dependent protein kinases, are the best substrates. Some phosphorylation can be detected in the absence of any exogenous acceptor. PKII activity assayed in the presence of histone II-A or in the absence of exogenous phosphate acceptor (autophosphorylation) co-elute in a DEAE-cellulose column at 0.28 M NaCl. As result of the autophosphorylation reaction of the purified enzyme a main phosphorylated component of 70 kDa was resolved by SDS-polyacrylamide gel electrophoresis. It is possible that this component is an active part of this enzyme. 相似文献
43.
Kenneth T. Izutsu Mark K. Goddard Jeanne M. Iversen Murray R. Robinovitch Teresa K. Oswald Marie Cantino Dale Johnson 《Cell and tissue research》1991,263(3):535-540
Summary The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P<0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures. 相似文献
44.
Introduction of the Escherichia coli gdhA gene into Rhizobium phaseoli: effect on nitrogen fixation. 总被引:1,自引:1,他引:0
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)
Rhizobium phaseoli lacks glutamate dehydrogenase (GDH) and assimilates ammonium by the glutamine synthetase-glutamate synthase pathway. A strain of R. phaseoli harboring the Escherichia coli GDH structural gene (gdhA) was constructed. GDH activity was expressed in R. phaseoli in the free-living state and in symbiosis. Nodules with bacteroids that expressed GDH activity had severe impairment of nitrogen fixation. Also, R. phaseoli cells that lost GDH activity and assimilated ammonium by the glutamine synthetase-glutamate synthase pathway preferentially nodulated Phaseolus vulgaris. 相似文献
45.
Ammonium assimilation in Rhizobium phaseoli by the glutamine synthetase-glutamate synthase pathway. 总被引:8,自引:7,他引:1
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)
Evidence from in vitro and in vivo studies showed that in Rhizobium phaseoli ammonium is assimilated by the glutamine synthetase (GS)-glutamate synthase NADPH pathway. No glutamate dehydrogenase activity was detected. R. phaseoli has two GS enzymes, as do other rhizobia. The two GS activities are regulated on the basis of the requirement for low (GSI) or high (GSII) ammonium assimilation. When the 2-oxoglutarate/glutamine ratio decreases, GSI is adenylylated. When GSI is inactivated, GSII is induced. However, induction of GSII activity varied depending on the rate of change of this ratio. GSII was inactivated after the addition of high ammonium concentrations, when the 2-oxoglutarate/glutamine ratio decreased rapidly. Ammonium inactivation resulted in alteration of the catalytic and physical properties of GSII. GSII inactivation was not relieved by shifting of the cultures to glutamate. After GSII inactivation, ammonium was excreted into the medium. Glutamate synthase activity was inhibited by some organic acids and repressed when cells were grown with glutamate as the nitrogen source. 相似文献
46.
XAD-4 resin was used to isolate cytokinins present in Baltic sea water. Four compounds were tentatively identified as 6-(3-methylbut-2-enylamino)
purine and zeatin, and their ribosides. 相似文献
47.
Synthesis of the nuclear protein cyclin (PCNA) and its relationship with DNA replication 总被引:38,自引:0,他引:38
Rodrigo Bravo 《Experimental cell research》1986,163(2):287-293
Synthesis of the nuclear protein cyclin (MW 36 000) and DNA in quiescent mouse fibroblasts is coordinately induced by serum and purified growth factors. Inhibition of DNA synthesis by hydroxyurea or aphidicolin in serum-stimulated quiescent cells does not affect the induction of cyclin. The levels of cyclin synthesis decrease rapidly at the end of the S phase. Immunofluorescence studies reveal that there are dramatic changes in the nuclear distribution of cyclin during S phase and that these depend on DNA synthesis or events during S phase. These observations strengthen the notion that cyclin is an important component of the events leading to DNA replication. 相似文献
48.
Specific proteins synthesized during the viral lytic cycle in vaccinia virus-infected HeLa cells: analysis by high-resolution, two-dimensional gel electrophoresis. 总被引:4,自引:3,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The proteins synthesized in vaccinia-infected HeLa cells have been analyzed at different times after infection by using two-dimensional gel electrophoresis. Vaccinia-infected cells present up to 198 polypeptides (138 acidic, isoelectric focusing; 60 basic, nonequilibrium pH gradient electrophoresis) not detected in control cells. Cells infected in the presence of cycloheximide show 81 additional polypeptides after cycloheximide removal, resulting in a total estimate of 279 proteins induced after vaccinia infection. The glycoproteins made at various times postinfection were also analyzed. At least 13 proteins labeled with [3H]glucosamine were detected in vaccinia-infected HeLa cells. 相似文献
49.
Effects of N-fertilizers,straw, and dry fallow on the nitrogen balance of a flooded soil planted with rice 总被引:1,自引:0,他引:1
Teresita Santiago-Ventura Marjorie Bravo C. Daez V. Ventura I. Watanabe A. A. App 《Plant and Soil》1986,93(3):405-411
Summary Nitrogen balance studies were made on rice (Oryza sativa) grown in flooded soil in pots. A low rate of fertilizer (5.64 mg N. kg−1 soil) did not depress the N gain, but a high rate (99.72 mg N. kg−1 soil) elminated the N gain. Soil N loss was negligible since15N applied as ammonium sulfate and thoroughly mixed with the soil was recovered from the soil-plant system after 3 crops. The
observed N gain, therefore, was caused by N2-fixation, not by a reduction of soil N loss.
Straw enhanced N gain at the rate of 2–4 mg per g straw. However, this gain was not observed when soil N availability was
high. Dry fallow between rice crops decreased the N gain. 相似文献
50.
The Molecular through Ecological Genetics of Abnormal Abdomen in DROSOPHILA MERCATORUM. I. Basic Genetics 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The abnormal abdomen syndrome (aa) in Drosophila mercatorum is characterized by the persistence of juvenilized cuticle on the adult abdomen. The aa phenotype is shown to depend on at least two X-linked genetic elements that are about one map unit apart near the centromeric end of the X chromosome. These two genetic elements are necessary for aa expression; one behaves as a dominant element and the other as a recessive. Overlaying these genetic studies upon molecular work reported elsewhere, it is argued that the dominant element is the presence of a 5 kb insertion in a majority of the X-linked repeats coding for the 28S ribosomal RNA. The recessive element appears to be a locus controlling differential replication of noninserted over inserted 28S genes during polytenization. The aa syndrome requires both the presence of the inserted repeats and the failure to preferentially amplify noninserted repeats. Given the necessary X-linked elements for aa, a variety of modifiers are revealed. First, aa expression in males is Y-linked, apparently corresponding to a deletion of the 18S/28S rDNA gene cluster normally found on the Y. Moreover, all major autosomes can modify the penetrance of aa. 相似文献