全文获取类型
收费全文 | 72527篇 |
免费 | 5043篇 |
国内免费 | 2638篇 |
专业分类
80208篇 |
出版年
2024年 | 138篇 |
2023年 | 1219篇 |
2022年 | 1810篇 |
2021年 | 2414篇 |
2020年 | 2351篇 |
2019年 | 3266篇 |
2018年 | 2840篇 |
2017年 | 2034篇 |
2016年 | 2013篇 |
2015年 | 2515篇 |
2014年 | 4742篇 |
2013年 | 5896篇 |
2012年 | 3653篇 |
2011年 | 4691篇 |
2010年 | 3576篇 |
2009年 | 3868篇 |
2008年 | 3941篇 |
2007年 | 3968篇 |
2006年 | 3521篇 |
2005年 | 3053篇 |
2004年 | 2704篇 |
2003年 | 2146篇 |
2002年 | 1927篇 |
2001年 | 1228篇 |
2000年 | 952篇 |
1999年 | 972篇 |
1998年 | 976篇 |
1997年 | 764篇 |
1996年 | 684篇 |
1995年 | 613篇 |
1994年 | 565篇 |
1993年 | 431篇 |
1992年 | 432篇 |
1991年 | 356篇 |
1990年 | 293篇 |
1989年 | 241篇 |
1988年 | 211篇 |
1987年 | 184篇 |
1986年 | 161篇 |
1985年 | 272篇 |
1984年 | 455篇 |
1983年 | 336篇 |
1982年 | 347篇 |
1981年 | 264篇 |
1980年 | 201篇 |
1979年 | 194篇 |
1978年 | 172篇 |
1977年 | 143篇 |
1976年 | 115篇 |
1975年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
921.
An electrophoretic variant of the X-linked enzyme phosphoglycerate kinase (PGK-1) has been used to study regulation of X chromosome expression in the diploid derivatives of the trophectoderm at 8–8.5 days post coitum in the mouse. These derivatives included the chorionic ectoderm and the polar trophoblast. The biochemical analysis suggests that only the maternally derived X chromosome (Xm) is expressed in the diploid trophectoderm derivatives. Cell selection and maternal tissue contamination were ruled out as possible causes of the observed Xm expression. From these and other results, we conclude that all derivatives of the trophectoderm, along with the primitive endoderm, express only Xm, whereas derivatives of the primitive ectoderm show random X chromosome expression. 相似文献
922.
Short illumination with white light of dark-maintained Anacystis nidulans prior to immersion in liquid nitrogen resulted in a marked change of fluorescence emission characteristics at 77 K. The fluorescence of Photosystem II-associated membrane bound pigments increases, while the emission due to phycobilins decreases. This effect seems to be due to a light-dependent alteration in the extent of contact between phycobilisomes and thylakoids, since the effect is reversible in the dark and is abolished by short glutaraldehyde fixation. The preillumination effect is not inhibited by DCMU. Emission spectra obtained with actively growing and CO2-starved cells indicate that the light-dependent increase in energy transfer from phycobilins to chlorophyll depends upon the physiological state of the cells. 相似文献
923.
924.
925.
Tatsuro Koike 《生物化学与生物物理学报:生物膜》1978,509(3):429-439
A clonal cell line of mouse neuroblastoma cells was found to undergo morphological differentiation in the presence of a K+ ionophore, valinomycin, in the assay medium. This effect was blocked by increasing the concentration of KCl of the medium, suggesting that the changes in resting membrane potential and ion fluxes may be involved in the mechanism of the formation of neurites. No enhancement of the neurite formation was observed in salines containing high concentrations of KCl in the absence of valinomycin. Depolarizing agents including veratridine, gramicidin and ouabain did not stimulate the outgrowth of neurites. Neither electrophoretic mobility of the cells nor molecular anisotropy of fluorescence probes in the membranes was modified by the treatment of valinomycin. Instead, it modified the slow binding phase in kinetics of the interaction of 1-anilinonaphthalene-8-sulfonate (ANS) with the cells, which is related to the penetration process of the probe into membranes. Valinomycin also enhanced the fluorescence intensity of ANS by increasing the binding sites in neuroblastoma cells. 相似文献
926.
E De Clercq A Billiau V G Edy K L Kirk L A Cohen 《Biochemical and biophysical research communications》1978,82(3):840-846
2-Fluoro-L-Histidine inhibits protein synthesis in various cell cultures, as measured by 3H-leucine incorporation. This histidine analog also inhibits the cytopathogenicity of a number of RNA and DNA viruses in primary and continuous cell cultures; it blocks the transformation of normal mouse (MO) cells by murine sarcoma virus, and partially suppresses the release of murine leukemia virus by a continuously infected mouse cell line (JLSV5). In human skin fibroblasts, it reduces the interferon-inducing capacity of poly(I)·poly(C). Inhibition of cell protein synthesis may be the common cause of the various effects. 4-Fluoro-L-histidine is essentially inert in all of the test systems examined. 相似文献
927.
In previous studies on the quaternary structure of Na+,K+-ATPase, cupric-phenanthroline complex (CP) has been used for the cross-linking of the enzyme subunits. Here we show that the same products obtained in the presence of CP (α,α-dimer, α,β-dimer, and products of higher molecular weight) are also obtained when the enzyme is exposed to Cu2+ without o-phenanthroline. The α,β-dimer (but not the α,α-dimer) is dissociated in the presence of EDTA; indicating that this product is not the result of the covalent cross-linking of the subunits through a disulfide bond. The nature of the α,α-dimer remains to be determined. The findings suggest that the results of “cross-linking” experiments with CP should be interpreted with caution until the products are more clearly identified. 相似文献
928.
A spectrofluorimetric investigation of calf thymus DNA modified by BP diolepoxide and 1-pyrenyloxirane 总被引:1,自引:0,他引:1
N C Yang L K Ng S B Neoh D Leonov 《Biochemical and biophysical research communications》1978,82(3):929-934
7β,8α-Dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BP diolepoxide, ) and 1-pyrenyloxirane () bind chemically to calf thymus DNA. The fluorescence efficiency of pyrenyl groups in mutagen modified DNA varies appreciably with its conformation and decreases in the order: pyrenees, modified denatured DNA and modified native DNA. A particularly interesting observation is that the fluorescence efficiency of mutagen modified DNA intensifies substantially upon denaturation. Our results suggest that the pyrenyl groups in mutagen modified DNA are intercalated between the base pairs of DNA. Since both and are powerful frame-shifting mutagens for S. typhimurium TA-98, the intercalative covalent binding of these compounds to DNA may provide a molecular basis for their mutagenic activity. 相似文献
929.
930.
H. Scherrer N.G. Seidah S. Benjannet P. Crine M. Lis M. Chrétien 《Biochemical and biophysical research communications》1978,84(4):874-885
A biosynthetic labeled peptide structurally related to the thymic peptide ubiquitin was first identified fortuitously in bovine pars intermedia cells in regard to its partial NH2 terminal amino acid sequence (Met 1, Leu 8, 15 and Lys 6, 11, 27, 29, 33) after a protein segment data bank search. A peptide with the same behavior on carboxymethylcellulose chromatography and polyacrylamide gel electrophoresis has been purified after labeling experiments in two areas of rat brain, hypothalamus and striatum, and in a mouse and a human ACTH-secreting pituitary tumors. It represents about 1 to 10% of the total labeled proteins in the various experiments. Its identity with the above mentioned bovine pituitary peptide was confirmed by microsequence analysis with respect to Met 1, Lys 6, 11 in hypothalmus, Met 1 in striatum, and Lys 6, 11, 27, 29, 33 in the two pituitary tumors. The availability of standard purified ubiquitin allowed us to show that labeled and cold peptides have the same electrophoretic mobility and elution volume on Sephadex G-50 chromatography this further confirms their identity. Possible interests of such a biosynthetic characterization of a ubiquitin-related peptide are discussed, particularly in view of the structural relationship of ubiquitin to the non histone component of nuclear protein A-24, and as a test of tissue viability and biosynthetic efficiency in our in vitro biosynthetic systems. 相似文献