全文获取类型
收费全文 | 40133篇 |
免费 | 3640篇 |
国内免费 | 2460篇 |
出版年
2024年 | 92篇 |
2023年 | 446篇 |
2022年 | 874篇 |
2021年 | 1447篇 |
2020年 | 1028篇 |
2019年 | 1266篇 |
2018年 | 1263篇 |
2017年 | 894篇 |
2016年 | 1355篇 |
2015年 | 2298篇 |
2014年 | 2559篇 |
2013年 | 2891篇 |
2012年 | 3464篇 |
2011年 | 3295篇 |
2010年 | 1967篇 |
2009年 | 1766篇 |
2008年 | 2218篇 |
2007年 | 2016篇 |
2006年 | 1815篇 |
2005年 | 1571篇 |
2004年 | 1460篇 |
2003年 | 1297篇 |
2002年 | 1198篇 |
2001年 | 765篇 |
2000年 | 687篇 |
1999年 | 636篇 |
1998年 | 401篇 |
1997年 | 362篇 |
1996年 | 350篇 |
1995年 | 287篇 |
1994年 | 308篇 |
1993年 | 225篇 |
1992年 | 357篇 |
1991年 | 329篇 |
1990年 | 273篇 |
1989年 | 258篇 |
1988年 | 214篇 |
1987年 | 185篇 |
1986年 | 162篇 |
1985年 | 167篇 |
1984年 | 162篇 |
1983年 | 130篇 |
1982年 | 125篇 |
1981年 | 81篇 |
1980年 | 83篇 |
1979年 | 101篇 |
1978年 | 99篇 |
1976年 | 87篇 |
1974年 | 98篇 |
1972年 | 82篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
191.
本属与多种树种有菌根关系:如Pinus,Larix,Abies和Pseudotsuga,但在Larix林下,本菌往往与土壤中的鞣料相聚集,对周围某些植物根系不利。本属现知15种,本区6种。 相似文献
192.
Evolution of karyotypic abnormalities and C-MYC oncogene amplification in human colonic carcinoma cell lines 总被引:7,自引:0,他引:7
C. C. Lin Kari Alitalo Manfred Schwab Donna George Harold E. Varmus J. Michael Bishop 《Chromosoma》1985,92(1):11-15
Cell lines (COLO 320 DM and COLO 320 HSR), established from a human neuroendocrine tumor, contain an amplified cellular oncogene (c-myc). We have previously shown that the homogeneously staining regions (HSRs) of a marker chromosome in the COLO 320 HSR cells that evolved in culture from COLO 320 DM cells contain amplified c-myc. Molecular hybridization in situ has now been used to demonstrate that the HSRs are on both arms of what was once an X chromosome. We also show that amplified c-myc copies are present in the isolated double minute chromosomes (DMs) of the COLO 320 DM cells that were characteristic of the tumor cells initially established from the patient. The results suggest that the amplified c-myc appeared first as DMs and was subsequently transposed to engender HSRs on an X chromosome. The initial COLO 320 tumor cell may have acquired two early replicating (i.e., active) X chromosomes and lost the late replicating (i.e., inactive) X. 相似文献
193.
194.
Cryptic plasmids in Lactobacillus strains isolated from the murine gastrointestinal tract. 总被引:3,自引:3,他引:0 下载免费PDF全文
Ten of twenty Lactobacillus strains isolated from the gastrointestinal tracts of animals of several species contained plasmids of 80 to 90 megadaltons or less than 2.6 megadaltons in size, as analyzed by agarose gel electrophoresis. The large plasmids were found only in strains originally isolated from the keratinized epithelium of the murine stomach. 相似文献
195.
An improved DNA sequencing strategy 总被引:42,自引:0,他引:42
A modification of Hong's systematic DNA sequencing strategy is described. The original procedure has been simplified and transfectant yield increased. After DNase I limited cleavage in the presence of Mn2+, the single-cut linear DNA does not have to be separated from supercoiled or open circular DNA on an agarose gel. After ligation, the DNA is digested with a second restriction endonuclease for which a unique cleavage site resides between the insert and the first restriction endonuclease cutting site. The original intact DNA is linearized whereas the deleted subclone is not. The background is decreased to an undetectable level. This DNA sequencing strategy was tested on a 1.4-kb DNA fragment containing the araC regulatory gene from Erwinia carotovora. A set of subclones sufficient to sequence the fragment on both strands was produced in 2 days and the yield was at least 60-fold higher than in the original protocol. 相似文献
196.
Regulation of deoxyadenosine and nucleoside analog phosphorylation by human placental adenosine kinase 总被引:2,自引:0,他引:2
The enzymes responsible for the phosphorylation of deoxyadenosine and nucleoside analogs are important in the pathogenesis of adenosine deaminase deficiency and in the activation of specific anticancer and antiviral drugs. We examined the role of adenosine kinase in catalyzing these reactions using an enzyme purified 4000-fold (2.1 mumol/min/mg) from human placenta. The Km values of deoxyadenosine and ATP are 135 and 4 microM, respectively. Potassium and magnesium are absolute requirements for deoxyadenosine phosphorylation, and 150 mM potassium and 5 mM MgCl2 are critical for linear kinetics. With only 0.4 mM MgCl2 in excess of ATP levels, the Km for deoxyadenosine is increased 10-fold. ADP is a competitive inhibitor with a Ki of 13 microM with variable MgATP2-, while it is a mixed inhibitor with a Ki and Ki' of 600 and 92 microM, respectively, when deoxyadenosine is variable. AMP is a mixed inhibitor with Ki and Ki' of 177 and 15 microM, respectively, with variable deoxyadenosine; it is a non-competitive inhibitor with a Ki of 17 microM and Ki' of 27 microM with variable ATP. Adenosine kinase phosphorylates adenine arabinoside with an apparent Km of 1 mM using deoxyadenosine kinase assay conditions. The Km values for 6-methylmercaptopurine riboside and 5-iodotubercidin, substrates for adenosine kinase, are estimated to be 4.5 microM and 2.6 nM, respectively. Other nucleoside analogs are potent inhibitors of deoxyadenosine phosphorylation, but their status as substrates remains unknown. These data indicate that deoxyadenosine phosphorylation by adenosine kinase is primarily regulated by its Km and the concentrations of Mg2+, ADP, and AMP. The high Km values for phosphorylation of deoxyadenosine and adenine arabinoside suggest that adenosine kinase may be less likely to phosphorylate these nucleosides in vivo than other enzymes with lower Km values. Adenosine kinase appears to be important for adenosine analog phosphorylation where the Michaelis constant is in the low micromolar range. 相似文献
197.
Tropomyosin isoforms in chicken embryo fibroblasts: purification, characterization, and changes in Rous sarcoma virus-transformed cells 总被引:16,自引:10,他引:6
Seven polypeptides (a, b, c, 1, 2, 3a, and 3b) have been previously identified as tropomyosin isoforms in chicken embryo fibroblasts (CEF) (Lin, J. J.-C., Matsumura, F., and Yamashiro-Matsumura, S., 1984, J. Cell. Biol., 98:116-127). Spots a and c had identical mobility on two-dimensional gels with the slow-migrating and fast-migrating components, respectively, of chicken gizzard tropomyosin. However, the remaining isoforms of CEF tropomyosin were distinct from chicken skeletal and cardiac tropomyosins on two-dimensional gels. The mixture of CEF tropomyosin has been isolated by the combination of Triton/glycerol extraction of monolayer cells, heat treatment, and ammonium sulfate fractionation. The yield of tropomyosin was estimated to be 1.4% of total CEF proteins. The identical set of tropomyosin isoforms could be found in the antitropomyosin immunoprecipitates after the cell-free translation products of total poly(A)+ RNAs isolated from CEF cells. This suggested that at least seven mRNAs coding for these tropomyosin isoforms existed in the cell. Purified tropomyosins (particularly 1, 2, and 3) showed different actin-binding abilities in the presence of 100 mM KCl and no divalent cation. Under this condition, the binding of tropomyosin 3 (3a + 3b) to actin filaments was significantly weaker than that of tropomyosin 1 or 2. CEF tropomyosin 1, and probably 3, could be cross-linked to form homodimers by treatment with 5,5'-dithiobis-(2-nitrobenzoate), whereas tropomyosin a and c formed a heterodimer. These dimer species may reflect the in vivo assembly of tropomyosin isoforms, since dimer formation occurred not only with purified tropomyosin but also with microfilament-associated tropomyosin. The expression of these tropomyosin isoforms in Rous sarcoma virus-transformed CEF cells has also been investigated. In agreement with the previous report by Hendricks and Weintraub (Proc. Natl. Acad. Sci. USA., 78:5633-5637), we found that major tropomyosin 1 was greatly reduced in transformed cells. We have also found that the relative amounts of tropomyosin 3a and 3b were increased in both the total cell lysate and the microfilament fraction of transformed cells. Because of the different actin-binding properties observed for CEF tropomyosins, changes in the expression of these isoforms may, in part, be responsible for the reduction of actin cables and the alteration of cell shape found in transformed cells. 相似文献
198.
Patricia C. Weber F.R. Salemme Spencer H. Lin Yasuo Konishi H.A. Scheraga 《Journal of molecular biology》1985,181(3):453
A cross-linked derivative of ribonuclease A, Nε,Nε′-(2,4-dinitrophenylene-1,5)-(lysine7-lysine41)-RNase A, has been crystallized by dialysis against 30% () ethanol/water mixtures buffered at high pH. Single crystals belong to the orthorhombic space group P212121, , with one molecule in the Crystallographic asymmetric unit. 相似文献
199.
200.