全文获取类型
收费全文 | 34678篇 |
免费 | 2536篇 |
国内免费 | 1595篇 |
出版年
2024年 | 70篇 |
2023年 | 366篇 |
2022年 | 894篇 |
2021年 | 1544篇 |
2020年 | 1051篇 |
2019年 | 1290篇 |
2018年 | 1312篇 |
2017年 | 1007篇 |
2016年 | 1466篇 |
2015年 | 2233篇 |
2014年 | 2506篇 |
2013年 | 2712篇 |
2012年 | 3145篇 |
2011年 | 2804篇 |
2010年 | 1787篇 |
2009年 | 1503篇 |
2008年 | 1878篇 |
2007年 | 1675篇 |
2006年 | 1443篇 |
2005年 | 1230篇 |
2004年 | 1004篇 |
2003年 | 892篇 |
2002年 | 687篇 |
2001年 | 527篇 |
2000年 | 427篇 |
1999年 | 454篇 |
1998年 | 276篇 |
1997年 | 289篇 |
1996年 | 267篇 |
1995年 | 228篇 |
1994年 | 231篇 |
1993年 | 161篇 |
1992年 | 225篇 |
1991年 | 198篇 |
1990年 | 137篇 |
1989年 | 114篇 |
1988年 | 86篇 |
1987年 | 111篇 |
1986年 | 86篇 |
1985年 | 81篇 |
1984年 | 59篇 |
1983年 | 42篇 |
1982年 | 42篇 |
1981年 | 27篇 |
1980年 | 24篇 |
1979年 | 27篇 |
1978年 | 19篇 |
1975年 | 22篇 |
1974年 | 19篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
61.
Basudev Shome Albert F. Parlow Wan-Kyng Liu Hyun S. Nahm Ted Wen Darrell N. Ward 《Journal of Protein Chemistry》1988,7(4):325-339
A collaborative study from two laboratories has been undertaken to re-evaluate the human follitropin -subunit sequence (hFSH), since areas of uncertainty remain in the wake of two earlier reports. The first report was by Shome and Parlow (1974). The second, by Saxena and Rathnam (1976), proposed revisions for sequence not definitively placed in the first study, as well as some differences in other placements. We have re-examined the sequence of the hFSH with more recent methodology. This has led to revision of certain areas of the sequence and resolution of differences between the two earlier proposals. Specifically, an-Ile-Ser- is established at 21–22, Asp at 41, Arg at 44, Lys at 46, and Glu at 111. These were areas of disagreement in the earlier proposals. A definitive placement of the residues around tryptophan-27 has now been obtained by three laboratories. C-terminal heterogeneity was observed with subunits ending at residue 107, 109, or 111. N-terminal heterogeneity has been observed in all preparations examined to date. A significant population of molecules with a proteolytic nick between residues 38–39 is noted. This is very likely an artifact of the collection and processing. The preparations examined in the present studies showed no evidence of residues 112–118 proposed by Saxena and Rathnam. 相似文献
62.
I. L. Sun W. Toole-Simms F. L. Crane D. J. Morré H. Löw J. Y. Chou 《Journal of bioenergetics and biomembranes》1988,20(3):383-391
Retinoic acid inhibits the reduction of diferric transferrin through the transplasma membrane electron transport system on fetal rat liver cells infected with a temperature-sensitive SV40 virus when the cells are in the nontransformed state cultured at 40°C. When the cells are in the transformed state (grown at the permissive 33°C temperature), retinoic acid does not inhibit the diferric transferrin reduction. Inhibition of activity of nontransformed cells is specific for retinoic acid with only slight inhibition by retinol and retinyl acetate at higher concentrations. Isolated rat liver plasma membrane NADH diferric transferrin reductase is also inhibited by retinoic acid. The effect of transformation with SV40 virus to decrease susceptibility to retinoic acid inhibition stands in contrast to much greater adriamycin inhibition of diferric transferrin reduction in the transformed cells than in nontransformed cells. 相似文献
63.
Structure and expression of the cAMP cell-surface receptor 总被引:3,自引:0,他引:3
Using antibodies specific for the 3',5'-cyclic AMP (cAMP) cell surface receptor of Dictyostelium discoideum, we have screened lambda gtll expression libraries and isolated a series of cDNAs derived from cAMP receptor mRNA during early development. The identity of the cDNA clones was verified by multiple criteria: 1) beta-galactosidase fusion proteins synthesized by isolated cDNA clones stain intensely with cAMP receptor directed antiserum, 2) these fusion proteins affinity purify antibodies specific for the cAMP receptor, 3) the cDNA probes hybridize to a 2 kb mRNA whose change in relative level of abundance during development parallels that of receptor mRNA as assayed by in vitro translation, 4) the 2 kb mRNA size equals that of receptor mRNA as determined by in vitro translation of size fractionated poly (A)+ RNA, and 5) RNA transcribed in vitro from cDNAs containing the entire protein-coding region produces a polypeptide by in vitro translation with an apparent molecular weight in close agreement with that of nascent cAMP receptor protein produced by in vitro translation of cellular RNA. The DNA sequence predicts an open reading frame of 392 amino acids. The deduced amino acid sequence contains seven domains enriched in hydrophobic residues. A model is proposed in which the cAMP cell-surface receptor traverses the lipid bilayer seven times in a pattern similar to that of other receptors, such as rhodopsin, which interact with G-proteins. The structural similarities suggest a gene family of related surface receptors from such evolutionarily diverse species as Dictyostelium, yeast, and mammals. 相似文献
64.
Incubation of rat brain synaptosomes prelabeled with [2-3H]inositol resulted in a time-dependent release of labeled inositol 1-phosphate. This process was Ca2+ dependent, and ATP (1 mM) enhanced the inositol 1-phosphate formation three- to fivefold. Using [1-14C]arachidonoyl-phosphatidylinositol which was introduced into saponin-permeabilized synaptosomes, ATP (1 mM) and free Ca2+ (approximately 20 microM) enhanced the phospholipase C hydrolysis of this substrate to form labeled diacylglycerol. When the same permeabilized synaptosomal preparation was incubated with [2-3H]inositol-phosphatidylinositol, ATP not only enhanced the formation of labeled inositol 1-phosphate, but also inhibited the conversion of inositol 1-phosphate to inositol. Furthermore, ATP appeared to reduce the Ca2+ requirement of the phosphatidylinositol-phospholipase C. Inhibition of the conversion of inositol 1-phosphate to inositol could not be overcome by increasing the Mg2+ concentration in the incubation medium. Although the ATP effect is not viewed as a receptor-mediated event, it is possible that such an event may occur in synaptosomes under conditions in which intrasynaptic Ca2+ concentration becomes elevated. 相似文献
65.
66.
67.
Alternate use of divergent forms of an ancient exon in the fructose-1,6-bisphosphate aldolase gene of Drosophila melanogaster. 下载免费PDF全文
The fructose-1,6-bisphosphate aldolase gene of Drosophila melanogaster contains three divergent copies of an evolutionarily conserved 3' exon. Two mRNAs encoding aldolase contain three exons and differ only in the poly(A) site. The first exon is small and noncoding. The second encodes the first 332 amino acids, which form the catalytic domain, and is homologous to exons 2 through 8 of vertebrates. The third exon encodes the last 29 amino acids, thought to control substrate specificity, and is homologous to vertebrate exon 9. A third mRNA substitutes a different 3' exon (4a) for exon 3 and encodes a protein very similar to aldolase. A fourth mRNA begins at a different promoter and shares the second exon with the aldolase messages. However, two exons, 3a and 4a, together substitute for exon 3. Like exon 4a, exon 3a is homologous to terminal aldolase exons. The exon 3a-4a junction is such that exon 4a would be translated in a frame different from that which would produce a protein with similarity to aldolase. The putative proteins encoded by the third and fourth mRNAs are likely to be aldolases with altered substrate specificities, illustrating alternate use of duplicated and diverged exons as an evolutionary mechanism for adaptation of enzymatic activities. 相似文献
68.
A comprehensive classification of nucleic acid structural families based on strand direction and base pairing. 下载免费PDF全文
We propose a classification of DNA structures formed from 1 to 4 strands, based only on relative strand directions, base to strand orientation and base pairing geometries. This classification and its associated notation enable all nucleic acids to be grouped into structural families and bring to light possible structures which have not yet been observed experimentally. It also helps in understanding transitions between families and can assist in the design of multistrand structures. 相似文献
69.
By means of technique of cell culture, 3H-thymidine incorporation and dot blot, it was demonstrated that angiotensin II (AGT II) stimulated proliferation and c-fos oncogene expression in cultured SHR vascular smooth muscle cells (VSMC) in a dose-dependent manner. This effect of AGT II was significantly inhibited by co-incubation with ANP. The results suggest that proliferation of VSMC is regulated by some interaction between AGT II and ANP. 相似文献
70.