全文获取类型
收费全文 | 53860篇 |
免费 | 3853篇 |
国内免费 | 137篇 |
专业分类
57850篇 |
出版年
2021年 | 389篇 |
2020年 | 337篇 |
2019年 | 371篇 |
2018年 | 875篇 |
2017年 | 835篇 |
2016年 | 1266篇 |
2015年 | 1936篇 |
2014年 | 2108篇 |
2013年 | 2697篇 |
2012年 | 3301篇 |
2011年 | 3084篇 |
2010年 | 1896篇 |
2009年 | 1448篇 |
2008年 | 2552篇 |
2007年 | 2561篇 |
2006年 | 2513篇 |
2005年 | 2361篇 |
2004年 | 2319篇 |
2003年 | 2140篇 |
2002年 | 2143篇 |
2001年 | 1552篇 |
2000年 | 1661篇 |
1999年 | 1311篇 |
1998年 | 497篇 |
1997年 | 401篇 |
1996年 | 382篇 |
1995年 | 361篇 |
1994年 | 377篇 |
1993年 | 344篇 |
1992年 | 808篇 |
1991年 | 787篇 |
1990年 | 754篇 |
1989年 | 710篇 |
1988年 | 655篇 |
1987年 | 592篇 |
1986年 | 575篇 |
1985年 | 538篇 |
1984年 | 480篇 |
1983年 | 442篇 |
1982年 | 389篇 |
1981年 | 378篇 |
1979年 | 491篇 |
1978年 | 364篇 |
1977年 | 311篇 |
1975年 | 370篇 |
1974年 | 355篇 |
1973年 | 331篇 |
1972年 | 361篇 |
1971年 | 321篇 |
1968年 | 311篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Summary The physical localization of sequences homologous to three cloned genes was determined by in situ hybridization to metaphase chromosomes. Previous work had assigned the skeletal myosin heavy chain gene cluster (Myh), the functional locus for the cellular tumor antigen p53 (Trp53-1), and the cellular homologue of the viral erb-B oncogene (Erbb) toMus musculus chromosome 11 (MMU11). Our results provide regional assignments ofMyh andTrp53-1 to chromosome bands B2C, and ofErbb to bands A1A4. Taken together with in situ mapping of three other loci on MMU 11 (Hox-2 homeobox-containing gene cluster, theSparc protein, and theColla-1 collagen gene), which have been reported elsewhere, these data allowed us to construct a physical map of MMU11 and to compare it with the linkage map of this chromosome. The map positions of the homologous genes on human chromosomes suggest evolutionary relationships of distinct regions of MMU11 with six different human chromosome arms: 1p, 5q, 7p, 16p, 17p, and 17q. The delineation of conserved chromosome regions has important implications for the understanding of karyotype evolution in mammalian species and for the development of animal models of human genetic diseases. 相似文献
172.
We have previously described a specific protease in turkey erythrocytes that converts the larger 50-kDa (P50) form of the beta 1-adrenoceptor to a smaller 40-kDa (P40) form [Jürss, R., Hekman, M., & Helmreich, E. J. M. (1985) Biochemistry 24, 3349-3354]. Further functional and structural characterization studies of the two forms are reported here. When purified P50 and P40 receptors were compared with respect to their relative capabilities to couple in lipid vesicles with pure stimulatory G-proteins (Gs-proteins) prepared from turkey erythrocytes or rabbit liver, a faster and larger activation of Gs-proteins was observed in response to l-isoproterenol and guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) with P40 than with P50 receptor. The kon values for P40 were 0.47 min-1 in the case of liver Gs and 0.22 min-1 in the case of erythrocyte Gs, whereas the corresponding values for P50 were 0.34 min-1 and 0.12 min-1, respectively. The binding properties of P50 and P40 forms of the receptor were not different, and desensitization of turkey erythrocytes on exposure to l-isoproterenol did not activate the protease. We furthermore ascertained that only the larger form with a molecular mass of 50 kDa carries the N-linked carbohydrates, which are removed on proteolytic conversion to the 40-kDa form and have either a triantennary or a tetraantennary nonfucosylated complex-type structure containing terminal sialyl residues. 相似文献
173.
B Kurelec S Britvi? S Krca W E Müller R K Zahn 《Comp. Biochem. Physiol. C, Comp. Pharmacol. Toxicol.》1987,86(1):17-22
Postmitochondrial fractions from marine sponges Geodia cydonium, Tethya aurantium, Verongia aerophoba and Pellina semitubulosa activate precarcinogenic aromatic amine 2-aminoanthracene, but not precarcinogenic polycyclic aromatic hydrocarbon benzo(a)pyrene, to Salmonella typhimurium TA 98 mutagens. All four sponge species lack a benzo(a)pyrene monooxygenase activity, but possesses the enzyme activity whose characteristics (selective activation of aromatic amines, NADPH-dependency, pH optimum at 8.4) are similar to FAD-containing monooxygenase. Tethya postmitochondrial fraction possesses an UDP-glucuronyl transferase activity which catalyzes the conjugation of a considerable part of metabolized 2-acetylamino [9-14C]fluorene to water soluble glucuronides. The possible ecological significance of exuded aromatic amine metabolites as well as the significance of the presence of the selective potential for the activation of aromatic amines to mutagens among sponges for our understanding of the fate and effects of carcinogens in the marine environment are discussed. 相似文献
174.
Summary ANP (atrial natriuretic peptide), a peptide found in granules of mammalian atrial cardiac myocytes, has been shown to be active in regulation of blood pressure and body water homeostasis. The existence of ANP in atrium, pituitary, adrenal gland, and kidney of the rat had been immunocytochemically demonstrated with an antibody against rat ANP (102–126). We used the same antibody in immunocytochemical studies for the detection of ANP in peripheral organs of the tree shrew (Tupaia belangeri). The antibody stained granules in myocytes of cardiac atria which indicated that it reacted with tree shrew ANP. In contrast to the rat, no immunoreactive cells were found in pituitaries and adrenal glands. However, in the kidneys distal tubules in outer medulla and cortex were labeled Ascending limbs of distal tubules were intensely stained when either the peroxidase-antiperoxidase (PAP) or the indirect immunofluorescence method were used. Collecting ducts and convoluted distal tubules in the outer cortex showed a granular type of staining when the immunofluorescence method was used. These data indicate that ANP is present in epithelial cells of distal tubules and collecting ducts, where it may be involved in the regulation of renal salt excretion. 相似文献
175.
Summary The receptor lymph of campaniform sensilla on the halteres of the blowfly, Calliphora vicina, was analyzed histochemically. Acid mucopolysaccharides were demonstrated by a test for iron-binding capacity (Hale-reaction). Further characterization by enzyme treatment showed that the receptor lymph contains hyaluronic acid and/or chondroitin sulfate. Ultrahistochemical studies gave evidence for glycoproteins in the inner and outer receptor lymph space. The significance of acid mucopolysaccharides for arthropod sensilla is discussed. 相似文献
176.
Mammalian DNA polymerase alpha: a replication-competent holoenzyme form from calf thymus 总被引:1,自引:0,他引:1
U Hübscher M Gassmann S Spadari N C Brown E Ferrari H J Buhk 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1987,317(1187):421-428
Calf thymus DNA polymerase alpha, like the replication-specific DNA polymerase III holoenzyme of Escherichia coli, can be isolated as a distinct complex. A specific multiprotein form of the polymerase alpha, a form designated replication-competent (RC) holoenzyme, consists of a complex of a polymerase-primase core and at least six other polypeptides. The RC holoenzyme can efficiently replicate several naturally occurring templates, including the genomic DNA of the porcine circovirus (PCV). The DNA of this virion consists of a single-stranded circle with a defined replication origin, and its replication requires the cellular DNA replication machinery. It might therefore provide an invaluable opportunity to investigate chromosomal replication mechanisms, analogous to the way that studies on E. coli bacteriophage DNA replication elucidated host DNA replication mechanisms. Calf RC holoenzyme alpha selectively initiates PCV DNA replication in vitro at a site that possibly represents a consensus sequence of cellular DNA replication origins. The cell-free PCV replication system will be exploited for the in vitro dissection and reconstitution of the RC holoenzyme and the functional analysis of its component polypeptides. 相似文献
177.
The incidental finding of DM's, minutes, HSR's, PCC, and PCD in two completely unrelated cases--one is a prenatal diagnosis in a twin pregnancy complicated by hydramnios and feto-fetal exsanguination, the other is an adult Klinefelter patient--raises the question whether such findings are coincidental or whether there is a common denominator in such cases. Possible relationships between these phenomena and the observed cases are discussed. 相似文献
178.
The K+-ATPase of Streptococcus faecalis is inhibited by incubation with the arginine-modifying reagent 2,3-butanedione. The inactivation proceeds by pseudo - first order kinetics and a double-logarithmic plot of the pseudo - first order rate constants versus reagent concentrations yields a reaction order of 1.14 with respect to butanedione. Partially inactivated ATPase exhibits a decreased maximal velocity but the same affinity for ATP, as compared to the native enzyme. Butanedione modification is inhibited by adenine nucleotides. These results indicate the involvement of most likely one crucial arginyl residue in adenine nucleotide binding by the ATPase. 相似文献
179.
C Lindbladh M Persson L Bülow S Stahl K Mosbach 《Biochemical and biophysical research communications》1987,149(2):607-614
The gene encoding human proinsulin has been fused in-frame with the E. coli alkaline phosphatase gene (pho A) (EC 3.1.3.1). Two constructions are described. One construction consists of the entire proinsulin gene fused to the 5'-terminal end of pho A. In the other construction a 42 base pair DNA fragment has been deleted from the 3'-terminal end of the proinsulin gene. The two purified fusion proteins are enzymatically active showing a specific activity of 10-15 U/mg and 18-25 U/mg, respectively. The first construction exhibited insulin antigenicity and was used to design a simple competitive ELISA for insulin. The lower detection limit was found to be at least 2.5 ng/ml. Both fusion proteins were also shown to have potential for use in a competitive ELISA for proinsulin. 相似文献
180.
A Walln?fer C Cauvin U T Rüegg 《Biochemical and biophysical research communications》1987,148(1):273-278
[Arg8]Vasopressin (AVP)-induced 45Ca2+ influx was examined in vascular smooth muscle cells derived from rat aorta. AVP stimulated the 45Ca2+ influx in a concentration-dependent manner. The effect was abolished in the presence of La3+. The dihydropyridine calcium channel antagonist darodipine did not affect the AVP-induced influx of 45Ca2+. These data suggest that AVP stimulates in these cultured aortic smooth muscle cells a receptor-operated channel (ROC) that is permeable to Ca2+. 相似文献