全文获取类型
收费全文 | 3256篇 |
免费 | 228篇 |
国内免费 | 1篇 |
出版年
2021年 | 27篇 |
2020年 | 23篇 |
2019年 | 27篇 |
2018年 | 58篇 |
2017年 | 52篇 |
2016年 | 76篇 |
2015年 | 112篇 |
2014年 | 148篇 |
2013年 | 169篇 |
2012年 | 206篇 |
2011年 | 202篇 |
2010年 | 139篇 |
2009年 | 89篇 |
2008年 | 149篇 |
2007年 | 130篇 |
2006年 | 123篇 |
2005年 | 137篇 |
2004年 | 116篇 |
2003年 | 127篇 |
2002年 | 104篇 |
2001年 | 93篇 |
2000年 | 89篇 |
1999年 | 78篇 |
1998年 | 32篇 |
1996年 | 27篇 |
1995年 | 25篇 |
1994年 | 24篇 |
1993年 | 25篇 |
1992年 | 55篇 |
1991年 | 44篇 |
1990年 | 53篇 |
1989年 | 39篇 |
1988年 | 49篇 |
1987年 | 35篇 |
1986年 | 37篇 |
1985年 | 32篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 27篇 |
1979年 | 28篇 |
1978年 | 22篇 |
1977年 | 19篇 |
1974年 | 22篇 |
1973年 | 20篇 |
1972年 | 20篇 |
1971年 | 19篇 |
1970年 | 22篇 |
1969年 | 29篇 |
1968年 | 28篇 |
1966年 | 19篇 |
排序方式: 共有3485条查询结果,搜索用时 15 毫秒
41.
5'-Methylbenzimidazolyl-cobamides are the corrinoids from some sulfate-reducing and sulfur-metabolizing bacteria 总被引:3,自引:0,他引:3
The sulfate-reducing bacteria Desulfobacterium autotrophicum, Desulfobulbus propionicus and Archaeoglobus fulgidus (VC-16) and the sulfur-metabolizing archaebacteria Desulfurolobus ambivalens and Thermoplasma acidophilum were found to contain considerable amounts of corrinoids, that were isolated and crystallized in their Co beta-cyano form. In three other sulfur-metabolizing archaebacteria, Thermoproteus neutrophilus, Pyrodictium occultum and Staphylothermus marinus significant amounts of corrinoids were not detected under the isolation methods used. The samples from the three sulfate-reducers were identified with Co alpha-[alpha-(5'-methylbenzimidazolyl)]-Co beta-cyanocobamide. This corrinoid was also obtained from a 5-methylbenzimidazole-supplemented Propionibacterium fermentation and was structurally characterized by ultraviolet/visible, CD, fast-atom-bombardment MS, 1H-and 13C-NMR spectroscopy. Also the major corrinoid from T. acidophilum was (tentatively) analyzed as a 5'-methylbenzimidazolyl-cobamide, whereas the main corrinoid from D. ambivalens was indicated to be vitamin B12 (a 5',6'-dimethylbenzimidazolyl-cobamide). The 5'-methylbenzimidazolylcobamides are found here as the common corrins of some sulfate-reducing and sulfur-metabolizing bacteria. The structural diversity due to the differing nucleotide bases of the corrins examined here and in methanogenic and acetogenic bacteria appears not to correlate to the biological function(s) of the corrins, but rather to be determined by biosynthetic properties of these organisms under natural growth conditions. 相似文献
42.
Prostaglandin-E2 9-ketoreductase from human uterine decidua vera 总被引:1,自引:0,他引:1
Prostaglandin-E2 9-ketoreductase, the enzyme which catalyzes the reaction from prostaglandin E2 (PGE2) to prostaglandin F2 alpha (PGF2 alpha), has been purified 232-fold from human uterine decidua vera. The molecular mass of the enzyme, as estimated by fast protein liquid chromatography, was 29 kDa. Sodium dodecyl sulfate disc gel electrophoresis of the denatured enzyme revealed a molecular mass of 31 kDa. These data suggest that the enzyme consists of a single polypeptide chain. The rate equation of the enzyme reaction for two substrates was used for the determination of five kinetic constants. The equilibrium constant with respect to PGE2 was 83 microM, the Michaelis constant, Km, for PGE2 was 93 microM. For NADPH, the equilibrium constant was 1.0 microM and Km was 1.6 microM. The maximal velocity for the forward reaction was V1 = 217 pmol/min. The inhibition constants for the analgesic agents indomethacin and fentiazac were Ki = 850 microM and Ki = 450 microM and for the steroid progesterone Ki = 1.5 mM, respectively. Prostaglandin-E2 9-ketoreductase might be responsible for the control of the PGE2/PGF2 alpha ratio in human decidua vera. The enzyme, therefore, might be an important factor in the cascade of events leading to uterine contractions and parturition. 相似文献
43.
Increase of circulating beta-endorphin-like immunoreactivity correlates with the change in feeling of pleasantness after running 总被引:1,自引:0,他引:1
Twenty-one male regular long distance runners participated in two 10 km runs one week apart. Their beta-endorphin-like immunoreactivity (beta-EIR) was assayed in plasma before and immediately after running. Mood was monitored by an adjective check list (Eigenschaftsw?rterliste, EWL) pre- and post-run. beta-EIR was significantly elevated post-run. Self-reliance and good mood scored higher after running. Both mood elevation and plasma beta-EIR increase showed a considerable individual variability but there was a significant correlation in the mean values of the two runs between individual beta-EIR increases (delta beta-EIR) and the changes of ratings in feeling of pleasantness (delta FP). High delta beta-EIR corresponded to positive mood change post-run. 相似文献
44.
K Schauenstein G Kr?mer R S Sundick G Wick 《Journal of immunology (Baltimore, Md. : 1950)》1985,134(2):872-879
The mitogenic response to Con A and the production of T cell growth factor or interleukin 2 (IL 2) by splenic and peripheral blood lymphocytes of obese strain (OS) chickens with spontaneous autoimmune thyroiditis have been investigated. By using an optimized method with Con A-coated chicken erythrocytes (MRC), lymphocytes of OS chickens were found to exhibit significantly elevated mitogenic responses as compared with cells from either Normal White Leghorn chickens (NWL) or animals of the Cornell C-Strain (CS), from which the OS has originally been developed. This difference was observed throughout ontogeny up to 15 mo of age, and was associated with increased levels of IL 2 activity in the culture supernatants. The elevated responsiveness of OS T lymphocytes was also found to be manifested in the expression of receptors for IL 2, because Con A-stimulated lymphocytes of OS birds were significantly more effective than those from normal controls in absorbing IL 2 activity from conditioned media (CM) of stimulated spleen cells. High concentrations of CM were suppressive in IL 2 assays, signaling the presence of an inhibitory factor(s) in addition to IL 2. An additional indication for defective immunoregulation was that CM from OS lymphocyte cultures showed significantly less of this suppressive activity in comparison with CM of normal (NWL and CS) lymphocyte cultures. Finally, the spontaneous uptake of 125IUdR of embryonic and early post hatching OS spleen lymphocytes was consistently and significantly enhanced. This difference, however, in contrast to the one observed in Con A responses, was found to decrease with age. The data are discussed in view of the contradictory results concerning T cell functions reported for several autoimmune states in mammals. 相似文献
45.
A plasmodial alpha-tubulin cDNA from Physarum polycephalum. Nucleotide sequence and comparative analysis 总被引:5,自引:0,他引:5
Günter Krämmer Monika Singhofer-Wowra Klaus Seedorf Melvyn Little Tim Schedl 《Journal of molecular biology》1985,183(4):633-638
As the first step towards correlating structure and function of tubulin in the slime mold Physarum polycephalum we have elucidated the nucleotide sequence of a cDNA that appears to code for all but the last 25 to 30 C-terminal amino acids of a plasmodial alpha-tubulin. Differences in amino acid sequence from those of other alpha-tubulins are distributed fairly evenly throughout the sequence, although a relatively extensive conserved region is found in position 396 to 426 near the C terminus. A small region in position 298 to 307 contains a cluster of amino acid residues unique to Physarum alpha-tubulin. The sequence is 70% homologous to two yeast alpha-tubulins and about 83% homologous to five animal alpha-tubulins. A comparison of the homologies of all the known alpha-tubulins indicates that a large decrease in the accepted point mutation rate has occurred during the evolution of the metazoa, suggesting a major functional specialization of microtubules. 相似文献
46.
Edda De Rossi Giovanna Riccardi Anna Maria Sanangelantoni Orio Ciferri 《FEMS microbiology letters》1985,30(1-2):239-244
Abstract A Spirulina platensis gene library has been constructed using cosmid vector pMMB34. The cosmid bank was controlled for its random gene distribution by colony hybridization. Genes were identified using either homologous or heterologous probes of genes involved in photosynthesis (large and small subunit of d -ribulose 1,5-bisphosphate carboxylase, 32 kDa thylakoid protein, α, β subunits of C-phycocyanin) and protein synthesis (elongation factors EF-Tu, EF-G). 相似文献
47.
Structure and genomic organization of a new family of murine retrovirus-related DNA sequences (MuRRS). 总被引:13,自引:2,他引:11 下载免费PDF全文
A new class of murine retrovirus-related sequences (MuRRS) is described. These 5.7 kb long transposon-like DNA-elements start and end with approximately 600 bp long repeats identical to previously identified solitary LTR-like elements (LTR-IS). There are about 50 - 100 5.7 kb elements and about 500 - 1000 solo LTR-IS elements per mouse haploid genome. Sequence analysis of one cloned MuRRS element revealed several possible open reading frames with partial sequence homologies to retroviral gag, pol and env genes. 相似文献
48.
Summary A new form of ectodermal dysplasia was observed in two siblings, offspring of healthy non-consanguineous parents. The main findings in both children are: hypodontia, abnormally shaped teeth, scalp hypotrichosis, pili annulati, follicular hyperkeratosis on the trunk and limbs, intensified delineation and reticular hyperpigmentation of the nape, and hyperopia; one of the siblings also has astigmatism. As both patients have normal nails and are euhidrotic, this is an ectodermal dysplasia of the pilodental subgroup. The cause is probably genetic and autosomal-recessive inheritance is most likely. 相似文献
49.
50.