全文获取类型
收费全文 | 2146篇 |
免费 | 369篇 |
国内免费 | 1篇 |
专业分类
2516篇 |
出版年
2021年 | 25篇 |
2020年 | 19篇 |
2019年 | 20篇 |
2018年 | 26篇 |
2017年 | 25篇 |
2016年 | 48篇 |
2015年 | 55篇 |
2014年 | 62篇 |
2013年 | 90篇 |
2012年 | 108篇 |
2011年 | 110篇 |
2010年 | 54篇 |
2009年 | 76篇 |
2008年 | 95篇 |
2007年 | 99篇 |
2006年 | 86篇 |
2005年 | 88篇 |
2004年 | 89篇 |
2003年 | 62篇 |
2002年 | 67篇 |
2001年 | 64篇 |
2000年 | 59篇 |
1999年 | 65篇 |
1998年 | 25篇 |
1997年 | 34篇 |
1996年 | 28篇 |
1995年 | 24篇 |
1994年 | 35篇 |
1993年 | 28篇 |
1992年 | 44篇 |
1991年 | 32篇 |
1990年 | 37篇 |
1989年 | 43篇 |
1988年 | 57篇 |
1987年 | 60篇 |
1986年 | 46篇 |
1985年 | 41篇 |
1984年 | 35篇 |
1983年 | 28篇 |
1982年 | 29篇 |
1981年 | 26篇 |
1980年 | 17篇 |
1979年 | 33篇 |
1978年 | 23篇 |
1977年 | 25篇 |
1976年 | 21篇 |
1975年 | 27篇 |
1974年 | 27篇 |
1973年 | 31篇 |
1972年 | 19篇 |
排序方式: 共有2516条查询结果,搜索用时 0 毫秒
61.
Formation of Novel Polysaccharides by Bradyrhizobium japonicum Bacteroids in Soybean Nodules 总被引:2,自引:1,他引:1 下载免费PDF全文
John G. Streeter Seppo O. Salminen Robert E. Whitmoyer Russell W. Carlson 《Applied microbiology》1992,58(2):607-613
Certain strains of Bradyrhizobium japonicum form a previously unknown polysaccharide in the root nodules of soybean plants (Glycine max (L.) Merr.). The polysaccharide accumulates inside of the symbiosome membrane—the plant-derived membrane enclosing the bacteroids. In older nodules (60 days after planting), the polysaccharide occupies most of the symbiosome volume and symbiosomes become enlarged so that there is little host cytoplasm in infected cells. The two different groups of B. japonicum which produce different types of polysaccharide in culture produce polysaccharides of similar composition in nodules. Polysaccharide formed by group I strains (e.g., USDA 5 and USDA 123) is composed of rhamnose, galactose, and 2-O-methylglucuronic acid, while polysaccharide formed by group II strains (e.g., USDA 31 and USDA 39) is composed of rhamnose and 4-O-methylglucuronic acid. That the polysaccharide is a bacterial product is indicated by its composition plus the fact that polysaccharide formation is independent of host genotype but is dependent on the bacterial genotype. Polysaccharide formation in nodules is common among strains in serogroups 123, 127, 129, and 31, with 27 of 39 strains (69%) testing positive. Polysaccharide formation in nodules is uncommon among other B. japonicum serogroups, with only 1 strain in 18 (6%) testing positive. 相似文献
62.
We have examined the binding behavior and fluorescence characteristics of a series of novel ligands for the estrogen receptor (ER). These ligands are derivatives of 5,6,11,12-tetrahydrochrysene (THC), a structure that embodies a stilbene chromophore, found in many nonsteroidal estrogens, within a rigid tetracyclic system where it cannot easily be distorted from planarity, thus providing the conjugation and rigidity required for efficient fluorescence. Additional steric bulk, as trans-disposed ethyl substituents at the internal C-5 and C-11 positions, is required for the highest relative binding affinity (RBA), and the trans-5,11-diethyl-2,8-dihydroxy-THC derivative binds to ER with an affinity greater than that of estradiol. The replacement of one of the phenolic hydroxyl groups of this THC derivative with an electron-withdrawing group (COMe, COOMe, CONH2, CN, or NO2) yields unsymmetrical THCs with binding affinities 15-40% that of estradiol (E2). The fluorescence emission shifts from about 380 nm for the dihydroxy THC to 475-688 nm for the donor-acceptor THCs. The emission of these donor-acceptor THCs is highly solvatochromic and shifts to longer wavelengths as the solvent polarity increases. In ethanol, the fluorescence quantum yield of the first four of these compounds is high (phi f = 0.43-0.69), but the fifth compound, the nitro-THC, is almost nonemissive in protic solvents. When they are incubated with protein solutions containing ER (approximately 10(-9) M), the emission from the donor-acceptor THCs bound specifically to ER is in the 500-570-nm range, whereas fluorescence from non-receptor-bound fluorophores is in the 425-460-nm range. Thus, fluorescence from these probes bound specifically to ER could be measured under equilibrium conditions as well as after the removal of free and non-receptor-bound material by treatment with charcoal-dextran. This is one of the first demonstrations of ligands whose fluorescence is distinctly different when free, when bound to ER, or when bound to non-receptor proteins. It is also the first demonstration of ER assay by fluorescence under equilibrium conditions. 相似文献
63.
The quaternary structure of phosphorylase kinase as influenced by low concentrations of urea. Evidence suggesting a structural role for calmodulin. 总被引:1,自引:0,他引:1 下载免费PDF全文
Skeletal-muscle phosphorylase kinase is a hexadecameric oligomer composed of equivalent amounts of four different subunits, (alpha beta gamma delta)4. The delta-subunit, which is calmodulin, functions as an integral subunit of the oligomer, and the gamma-subunit is catalytic. To learn more about intersubunit contacts within the hexadecamer and about the roles of individual subunits, we induced partial dissociation of the holoenzyme with low concentrations of urea. In the absence of Ca2+ the quaternary structure of phosphorylase kinase is very sensitive to urea over a narrow concentration range. Gel-filtration chromatography in the presence of progressively increasing concentrations of urea indicates that between 1.15 M- and 1.35 M-urea the delta-subunit dissociates, allowing extensive formation of complexes larger than the native enzyme that contain equivalent amounts of alpha-, beta- and gamma-subunits. As the urea concentration is increased to 2 M and 3 M, nearly all of the enzyme aggregates to the heavy species devoid of delta-subunit. Addition of Ca2+, which is known to block dissociation of the delta-subunit [Shenolikar, Cohen, Cohen, Nairn & Perry (1979) Eur. J. Biochem. 100, 329-337], also blocks aggregation of the enzyme induced by the low concentrations of urea. These results suggest that in native phosphorylase kinase the delta-subunit, in addition to activating the catalytic subunit and conferring upon it Ca2(+)-sensitivity, may also serve a structural role in preventing aggregation of the alpha-, beta- and gamma-subunits, thus limiting to four the number of alpha beta gamma delta protomers that associate under standard conditions. In gel-filtration chromatography with urea a protein peak containing equivalent amounts of alpha- and gamma-subunits is also observed, as is a peak containing only beta-subunits. Increasing concentrations of urea have a biphasic effect on the activity of the holoenzyme, being stimulatory up to 1 M and then inhibitory. The concentration-dependence of urea in the inhibitory phase parallels its ability to induce dissociation of the delta-subunit. 相似文献
64.
65.
Plasma membrane fractions were isolated from maize (Zea mays L.) endosperms and etiolated kernels to investigate the possible membrane location of the sucrose synthase (SS) protein. Endosperms from seedlings at both 12 and 21 days after pollination (DAP), representing early and mid-developmental stages, were used, in addition to etiolated leaf and elongation zones from seedlings. Plasma membrane fractions were isolated from this material using differential centrifugation and aqueous two-phase partitioning. The plasma membrane-enriched fraction obtained was then analyzed for the presence of sucrose synthase using protein blots and activity measurements. Both isozymes SS1 and SS2, encoded by the lociSh1 andSus1, respectively, were detected in the plasma membrane-enriched fraction using polyclonal and monoclonal antisera to SS1 and SS2 isozymes. In addition, measurements of sucrose synthase activity in plasma membrane fractions of endosperm revealed high levels of specific activity. The sucrose synthase enzyme is tightly associated with the membrane, as shown by Triton X-100 treatment of the plasma membrane-enriched fraction. It is noteworthy that the gene products of bothSh1 andSus1 were detectable as both soluble and plasma membrane-associated forms. 相似文献
66.
Mutations in the homologous ZDS1 and ZDS2 genes affect cell cycle progression. 总被引:3,自引:2,他引:1 下载免费PDF全文
Y Yu Y W Jiang R J Wellinger K Carlson J M Roberts D J Stillman 《Molecular and cellular biology》1996,16(10):5254-5263
67.
68.
The GLC7 type 1 protein phosphatase is required for glucose repression in Saccharomyces cerevisiae. 总被引:11,自引:4,他引:7 下载免费PDF全文
We cloned the GLC7/DIS2S1 gene by complementation of the cid1-226 mutation, which relieves glucose repression in Saccharomyces cerevisiae. GLC7 encodes the catalytic subunit of type 1 protein phosphatase (PP1). Genetic analysis and sequencing showed that cid1-226 is an allele of GLC7, now designated glc7-T152K, which alters threonine 152 to lysine. We also show that the glc7-1 and glc7-T152K alleles cause distinct phenotypes: glc7-1 causes a severe defect in glycogen accumulation but does not relieve glucose repression, whereas glc7-T152K does not prevent glycogen accumulation. These findings are discussed in light of evidence that interaction with different regulatory or targeting subunits directs the participation of PP1 in diverse cellular regulatory mechanisms. Finally, genetic studies suggest that PP1 functions antagonistically to the SNF1 protein kinase in the regulatory response to glucose. 相似文献
69.
Altered Regulatory Responses to Glucose Are Associated with a Glucose Transport Defect in Grr1 Mutants of Saccharomyces Cerevisiae 总被引:11,自引:1,他引:10 下载免费PDF全文
The GRR1 gene of Saccharomyces cerevisiae affects glucose repression, cell morphology, divalent cation transport and other processes. We present a kinetic analysis showing that the grr1 mutant is also defective in high affinity glucose transport. In combination with a mutation in SNF3, a member of the glucose transporter gene family, grr1 strikingly impairs growth on glucose. These findings suggest that GRR1 and SNF3 affect glucose transport by distinct pathways. The mutation rgt1-1, a suppressor of snf3, restores both glucose transport and glucose repression to a grr1 mutant, but does not remedy the morphological defect. We suggest that GRR1 affects the glucose sensing process and that the association between transport and regulation may reflect the involvement of a transporter in glucose sensing. 相似文献