全文获取类型
收费全文 | 11223篇 |
免费 | 874篇 |
国内免费 | 418篇 |
出版年
2023年 | 75篇 |
2022年 | 93篇 |
2021年 | 429篇 |
2020年 | 287篇 |
2019年 | 302篇 |
2018年 | 346篇 |
2017年 | 281篇 |
2016年 | 421篇 |
2015年 | 630篇 |
2014年 | 711篇 |
2013年 | 846篇 |
2012年 | 950篇 |
2011年 | 865篇 |
2010年 | 548篇 |
2009年 | 437篇 |
2008年 | 565篇 |
2007年 | 509篇 |
2006年 | 475篇 |
2005年 | 392篇 |
2004年 | 353篇 |
2003年 | 303篇 |
2002年 | 278篇 |
2001年 | 218篇 |
2000年 | 244篇 |
1999年 | 169篇 |
1998年 | 103篇 |
1997年 | 105篇 |
1996年 | 85篇 |
1995年 | 78篇 |
1994年 | 81篇 |
1993年 | 73篇 |
1992年 | 116篇 |
1991年 | 116篇 |
1990年 | 93篇 |
1989年 | 79篇 |
1988年 | 68篇 |
1987年 | 90篇 |
1986年 | 57篇 |
1985年 | 59篇 |
1984年 | 47篇 |
1983年 | 32篇 |
1982年 | 33篇 |
1980年 | 29篇 |
1979年 | 43篇 |
1978年 | 28篇 |
1977年 | 34篇 |
1975年 | 34篇 |
1973年 | 45篇 |
1972年 | 30篇 |
1971年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 634 毫秒
51.
Mui Keng Tan 《Plant molecular biology》1991,17(2):221-227
A total of 864 bases from 5 regions interspersed in the 18S and 26S rRNA molecules from various clones of Pteridium covering the general geographical distribution of the genus was analysed using a rapid rRNA sequencing technique. No base difference has been detected amongst the three major lineages, two of which apparently separated before the breakup of the ancient supercontinent, Pangaea. These regions of the rRNA sequences have thus been conserved for at least 160 million years and are here compared with other eukaryotic, especially plant rRNAs. 相似文献
52.
The somitogenetic potential of cells in the primitive streak and the tail bud of the organogenesis-stage mouse embryo. 总被引:7,自引:0,他引:7
The developmental potency of cells isolated from the primitive streak and the tail bud of 8.5- to 13.5-day-old mouse embryos was examined by analyzing the pattern of tissue colonization after transplanting these cells to the primitive streak of 8.5-day embryos. Cells derived from these progenitor tissues contributed predominantly to tissues of the paraxial and lateral mesoderm. Cells isolated from older embryos could alter their segmental fate and participated in the formation of anterior somites after transplantation to the primitive streak of 8.5-day host embryo. There was, however, a developmental lag in the recruitment of the transplanted cells to the paraxial mesoderm and this lag increased with the extent of mismatch of developmental ages between donor and host embryos. It is postulated that certain forms of cell-cell or cell-matrix interaction are involved in the specification of segmental units and that there may be age-related variations in the interactive capability of the somitic progenitor cells during development. Tail bud mesenchyme isolated from 13.5-day embryos, in which somite formation will shortly cease, was still capable of somite formation after transplantation to 8.5-day embryos. The cessation of somite formation is therefore likely to result from a change in the tissue environment in the tail bud rather than a loss of cellular somitogenetic potency. 相似文献
53.
Effect of saffron on thymocyte proliferation, intracellular glutathione levels and its antitumor activity. 总被引:1,自引:0,他引:1
S C Nair M J Salomi C D Varghese B Panikkar K R Panikkar 《BioFactors (Oxford, England)》1992,4(1):51-54
Liposome encapsulation of saffron effectively enhanced its antitumor activity towards Sarcoma-180 (S-180) and Ehrlich ascites carcinoma solid tumors in mice. Significant inhibition (P < 0.001) in the growth of these tumors was observed as compared with vehicle (control) mice. In the presence of phytohemagglutinin (PHA), a T cell mitogen, saffron stimulated non-specific proliferation of lymphocytes in vitro. The intracellular reduced glutathione and related enzymes, i.e. glutathione reductase and glutathione-S-transferase, of S-180 tumor cells were significantly elevated when incubated with saffron, possibly acting to maintain functional levels of other antioxidants. Our studies indicate the antioxidant activity of saffron. 相似文献
54.
55.
A sucrose density gradient analysis of3H-uridine pulse-labelled RNA from the first postirradiation mitotic cycle ofPhysarum polycephalum shows that all the density classes of RNA synthesized during this period are resistant to the peptide-antibiotic, actinomycin
D. In fact, the synthesis is found to be greater in the presence of the drug. The heterogenously sedimenting synthetic activity
here may represent a single species of RNA and its precursors or more than one kind of RNA. Further characterization of this
RNA is meaningful in view of the actinomycin insensitivity of the postirradiation mitotic cycle itself to this antibiotic. 相似文献
56.
57.
E Dubnau J Weir G Nair L Carter rd C Moran Jr I Smith 《Journal of bacteriology》1988,170(3):1054-1062
The DNA sequences of the spo0H genes from Bacillus licheniformis and B. subtilis are described, and the predicted open reading frames code for proteins of 26,097 and 25,447 daltons, respectively. The two spo0H gene products are 91% identical to one another and about 25% identical to most of the procaryotic sigma factors. The predicted proteins have a conserved 14-amino-acid sequence at their amino terminal end, typical of sigma factors. Antibodies raised against the spo0H gene product of B. licheniformis specifically react with RNA polymerase sigma factor protein, sigma 30, purified from B. subtilis. We conclude that the spo0H genes of B. licheniformis and B. subtilis code for sigma 30, now known as sigma H. 相似文献
58.
R A Skidgel C D Bennett J W Schilling F L Tan D K Weerasinghe E G Erd?s 《Biochemical and biophysical research communications》1988,154(3):1323-1329
Human plasma carboxypeptidase N was purified to homogeneity and its active and inactive subunits were separated. By introducing a novel technique, both forms of the active subunit (Mr = 55,000 and Mr = 48,000) were isolated. N-terminal sequencing of the active subunit of human carboxypeptidase N revealed significant homology with the N-terminal sequence of bovine carboxypeptidase H (43% identity) and to a lesser extent with carboxypeptidase A (29% identity) or carboxypeptidase B (18% identity). The active subunit of carboxypeptidase N was hydrolyzed with trypsin and 4 of the tryptic peptides were isolated by HPLC and sequenced. The sequences of the four peptides were homologous (39-64% identity) with regions of carboxypeptidase H corresponding to the middle (residues 148-175) and C-terminal portion (residues 321-408). These regions had essentially no homology with carboxypeptidase A or B. These data indicate that carboxypeptidase H and the active subunit of carboxypeptidase N may have diverged from a common ancestral gene. 相似文献
59.
R LoBrutto G W Smithers G H Reed W H Orme-Johnson S L Tan J S Leigh 《Biochemistry》1986,25(19):5654-5660
Pulsed electron paramagnetic resonance spectroscopy has been used to detect Mn(II)-ligand superhyperfine couplings in complexes with creatine kinase and in the Mn(II) metalloprotein concanavalin A. Electron spin-echo envelopes from Mn(II), bound in these complexes, are modulated by superhyperfine interactions between Mn(II) and nearby, weakly coupled nuclear spins. The characteristic frequencies of the modulations were obtained by Fourier transformation of the three-pulse, spin-echo envelopes. In transition-state analogue complexes of creatine kinase (enzyme-MnIIADP-anion-creatine), superhyperfine interactions from the directly coordinated nitrogen of the thiocyanate ligand give envelope modulations. The source of the modulations was confirmed by measurements with the 14N and 15N forms of thiocyanate. On the other hand, the nitrogen of coordinated nitrate, which is two bonds removed from the paramagnetic center, does not produce detectable modulations. In spectra for Mn(II) concanavalin A, envelope modulations are detected due to the nitrogen of the coordinated histidine residue. Complexes prepared in 2H2O give strong signals due to weakly coupled 2H. For Mn(II)-doped single crystals of sodium pyrophosphate, signals are observed in the frequency domain spectra that are due to coupling from 31P. Phosphorus signals from the ADP ligand in complexes with creatine kinase show approximately the same coupling constant but have a much broader line width. 相似文献
60.
Epitopes, structural domains, and asymmetry of amino acid residues in SS-B/La nuclear protein 总被引:21,自引:0,他引:21
E K Chan A M Francoeur E M Tan 《Journal of immunology (Baltimore, Md. : 1950)》1986,136(10):3744-3749
SS-B/La is a conserved cellular phosphoprotein of 46 to 48 KD that is the target antigen of autoantibodies in sera of patients with Sjogren's syndrome and systemic lupus erythematosus. SS-B/La is also known to be associated with certain small cellular and viral RNA, including adenovirus VAI and VAII RNA. Two relatively protease-resistant domains (X and Y) were defined in SS-B from HeLa cells by using human autoantibodies as reagents. Domain X, a methionine-containing nonphosphorylated 28 KD polypeptide, was found to be resistant to partial digestion with six different proteases. Similar domains were also found in calf and rabbit SS-B. Domain Y, a 23 KD polypeptide, was detected after limited digestion with S. aureus V8 and trypsin. This domain contained little if any methionine, but all the detectable phosphorylated amino acids. Among 16 anti-SS-B sera tested by immunoblotting, 11 (69%) were reactive with both domains, three (19%) only with domain X, and two (13%) only with domain Y. These results showed that there are at least two distinct antigenic epitopes on the 46 to 48 KD SS-B/La protein, each located on a separate structural domain. The asymmetric distribution of methionine and phosphorylated amino acid residues in SS-B/La show striking similarity to the two reported domains of the adenovirus 72 KD DNA-binding protein, and raises questions concerning functional similarities that await investigation. 相似文献