全文获取类型
收费全文 | 5509篇 |
免费 | 527篇 |
国内免费 | 375篇 |
出版年
2022年 | 24篇 |
2021年 | 38篇 |
2020年 | 20篇 |
2019年 | 23篇 |
2018年 | 26篇 |
2017年 | 30篇 |
2016年 | 67篇 |
2015年 | 135篇 |
2014年 | 115篇 |
2013年 | 147篇 |
2012年 | 167篇 |
2011年 | 529篇 |
2010年 | 374篇 |
2009年 | 155篇 |
2008年 | 139篇 |
2007年 | 148篇 |
2006年 | 123篇 |
2005年 | 311篇 |
2004年 | 542篇 |
2003年 | 291篇 |
2002年 | 209篇 |
2001年 | 236篇 |
2000年 | 156篇 |
1999年 | 104篇 |
1998年 | 70篇 |
1997年 | 108篇 |
1996年 | 95篇 |
1995年 | 98篇 |
1994年 | 68篇 |
1993年 | 283篇 |
1992年 | 250篇 |
1991年 | 137篇 |
1990年 | 129篇 |
1989年 | 130篇 |
1988年 | 114篇 |
1987年 | 128篇 |
1986年 | 93篇 |
1985年 | 104篇 |
1984年 | 53篇 |
1983年 | 30篇 |
1982年 | 32篇 |
1980年 | 16篇 |
1979年 | 25篇 |
1978年 | 21篇 |
1973年 | 16篇 |
1971年 | 17篇 |
1970年 | 15篇 |
1957年 | 18篇 |
1956年 | 17篇 |
1955年 | 16篇 |
排序方式: 共有6411条查询结果,搜索用时 359 毫秒
81.
A variety of data support the existence of an opioid receptor complex composed of distinct but interacting mu cx and delta cx binding sites, where "cx" indicates "in the complex." The ability of subantinociceptive doses of [Leu5]enkephalin and [Met5]enkephalin to potentiate and attenuate morphine-induced antinociception, respectively, is thought to be mediated via their binding to the delta cx binding site. [D-Pen2,D-Pen5]Enkephalin also modulates morphine-induced antinociception, but has very low affinity for the delta cx binding site in vitro. In the present study, membranes were depleted of their delta ncx binding sites by pretreatment with the site-directed acylating agent, (3S,4S)-(+)-trans-N-[1-[2-(4-isothiocyanato)phenyl)-ethyl]-3-methy l-4- piperidyl]-N-phenylpropaneamide hydrochloride, which permits selective labeling of the delta cx binding site with [3H][D-Ala2,D-Leu5]enkephalin. The major findings of this study are that with this preparation of rat brain membranes: a) there are striking differences between the delta cx and mu binding sites; and b) both [D-Pen2,D-Pen5]enkephalin and [D-Pen2,L-Pen5]enkephalin exhibit high affinity for the delta cx binding site. 相似文献
82.
G A Rice N A Touchette A N Creager J Goldberg R D Cole 《Experimental cell research》1988,177(1):221-226
Differential scanning microcalorimetry of the nuclei of dividing CHO cells revealed DNA structures that showed structural transitions at 60, 76, 88, and 105 degrees C (transitions I to IV, respectively). In cultures synchronized by isoleucine deprivation the enthalpies of transitions I and II were rather constant throughout the cell cycle. While the sum of the enthalpies of III and IV was nearly constant, the ratio of IV to III varied substantially from one phase of the cycle to another. A high IV:III ratio of 6 characterized G1 while S phase gave a IV:III ratio of about 2. Cells containing metaphase chromosomes also showed a IV:III ratio near 2. The IV:III ratio for CHO cells showed a progressive decrease as the cells were maintained in isoleucine-free medium from 0 to 6 days. 相似文献
83.
DNA polymerase I activity in Escherichia coli is influenced by spot 42 RNA. 总被引:4,自引:2,他引:2
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We have shown that the level of DNA polymerase I (Pol I) activity in Escherichia coli is influenced by the level of a 109-nucleotide RNA, spot 42 RNA. Deletion of the gene for spot 42 RNA results in a 20 to 25% decrease in Pol I activity, as assayed by nucleotide incorporation in cell extracts and a decrease in the ability of cells to grow in the presence of the DNA-alkylating agent methyl methanesulfonate. Also, a physiological reduction of the level of spot 42 RNA, by growth in media containing poor carbon sources, results in a corresponding decrease in Pol I activity. Conversely, overproduction of spot 42 RNA results in a 10 to 15% increase in Pol I activity in vitro. Thus, changes in the amount of spot 42 RNA result in relatively small but significant changes in Pol I activity. 相似文献
84.
M H Schadler G M Butterstein B J Faulkner S C Rice L A Weisinger 《Biology of reproduction》1988,38(4):817-820
The plant metabolite, 6-methoxybenzoxazolinone (6-MBOA), occurring in leaf tissue of rapidly growing monocots, cues reproduction in some mammals. In the pine vole, Microtus pinetorum, peripubertal females respond to this nonestrogenic compound with a 40% increase in serum levels of follicle-stimulating hormone (FSH). In addition, 6-MBOA significantly increases the weight of the ovary and uterus in both peripubertal and mature voles. This study is the first to offer evidence that 6-MBOA interacts with the pituitary to stimulate reproduction in voles. 相似文献
85.
R J Linhardt K G Rice Y S Kim J D Engelken J M Weiler 《The Journal of biological chemistry》1988,263(26):13090-13096
This paper demonstrates that heparin-oligosaccharides with low anticoagulant activity have a high capacity to inhibit activation of the amplification pathway of complement in vitro. We prepared heparin-oligosaccharides by partial depolymerization of heparin using purified flavobacterial heparinase. The resulting oligosaccharide mixture was then fractionated using strong anion exchange-high pressure liquid chromatography to produce individual oligosaccharide components of this mixture, with degree of polymerization ranging from 2 to 16. These heparin-oligosaccharides were examined for both their anticoagulant activity and capacity to inhibit activation of the amplification pathway of complement. Although there was little difference among commercial heparins, a correlation between molecular weight and activity to inhibit convertase generation was clearly established for heparin-oligosaccharides between degree of polymerization 2 through 16. Heparin-oligosaccharides of degree of polymerization 10-16 (Mr 3888-5320) demonstrated up to 54% of heparin's activity on a molar basis (and up to 163% of heparin's activity on a weight basis) in inhibiting the amplification pathway of complement in vitro while showing almost no anticoagulant activity. These studies, for the first time, completely separate heparin's ability to inhibit complement activation from its anticoagulant activity. 相似文献
86.
Monochlorobimane (syn-(ClCH2, CH3)-1,5-diazabicyclo-[3.3.0]-octa-3,6-dione-2,8-dione; mBCl) forms a fluorescent adduct with glutathione (GSH), which has been used as a basis for flow cytometric analysis. While mBCl will react nonspecifically with many different thiols, preferential derivatization of GSH can be achieved by using a low concentration of mBCl, since the reaction with GSH is catalyzed by GSH S-transferase, and the nonenzymatic reaction is very slow (k = 3.3 x 10(-1) M-1 s-1 at 37 degrees C, pH 7.5). The rate of derivatization of cellular GSH can be 1000 times greater than predicted from the nonenzymatic reaction rate, although this factor can vary among cell lines. GSH values obtained by flow cytometry (FCM) agree well with those obtained by an enzymatic assay, over a wide range of GSH values, for EMT6/SF cells treated with L-buthionine sulfoximine to vary GSH content. FCM analysis of the GSH content of cells obtained by disaggregation of EMT6/SF tumors, grown in BALB/c mice, revealed a wide variation in single-cell GSH content. The data suggest that there are distinct subpopulations within these tumors, which can be partially characterized by GSH content, but may also have other distinguishing characteristics, such as enhanced sensitivity or resistance to cytotoxic agents. Heterogeneity in single-cell GSH content was also observed by FCM analysis of cells obtained by disaggregation of a biopsy of a human renal cell carcinoma. This result points to the potential value of FCM analysis of GSH in the identification and characterization of human tumor subpopulations which may be of clinical significance in the treatment of cancer by radiation or chemotherapeutic agents. 相似文献
87.
Heat shock proteins within the mammalian cell cycle: relationship to thermal sensitivity, thermal tolerance, and cell cycle progression 总被引:1,自引:0,他引:1
We have measured endogenous and induced rates of 70-kD, 89-kD, and 110-kD heat shock proteins in highly pure G1-, S-, or G2-M phase fractions of Chinese hamster fibroblasts (CHO) separated by fluorescence-activated cell sorting (FACS). Relative rates of synthesis of all three polypeptides as measured by two-dimensional gel electrophoresis were similar throughout the cell cycle, and therefore, endogenous levels were unlikely to explain the thermal sensitivity of S-phase cells. Distinct heterogeneity in induced rates of these polypeptides was noted in all phase fractions. Enhanced rates of 70-kD polypeptide were measured in S and G2-M as compared to G1 following heat shock. Little increase in either the 89-kD or 110k-kD heat shock proteins was observed in heated G1 cells. This heterogeneity in induced rates of synthesis was in contrast to the similarity in thermal tolerance expression kinetics between each phase. Finally, enhanced synthesis of these polypeptides appeared unrelated to regulation of either heat-induced cell cycle delay or to the resumption of phase-specific progression after heat shock as measured by simultaneous flow cytometric measurement of incorporated BrdUrd and DNA content. 相似文献
88.
Conserved epitopes on the hemagglutinin-neuraminidase proteins of human and bovine parainfluenza type 3 viruses: nucleotide sequence analysis of variants selected with monoclonal antibodies. 总被引:16,自引:10,他引:6
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
K J Coelingh C C Winter B R Murphy J M Rice P C Kimball R A Olmsted P L Collins 《Journal of virology》1986,60(1):90-96
We have previously identified 11 epitopes located in two topologically nonoverlapping antigenic sites (A and B) and a third bridging site (C) on the human type 3 parainfluenza virus (PIV3) hemagglutinin-neuraminidase (HN) glycoprotein by using monoclonal antibodies (MAbs) which inhibit hemagglutination and virus infectivity (K. L. Coelingh, C. C. Winter, and B. R. Murphy, Virology 143:569-582, 1985). We have identified three additional antigenic sites (D, E, and F) on the HN molecule by competitive-binding assays of anti-HN MAbs which have no known biological activity. Epitopes in sites A, D, and F are conserved on the bovine PIV3 HN glycoprotein and also among a wide range of human isolates. The dideoxy method was used to identify nucleotide substitutions in the HN genes of antigenic variants selected with neutralizing MAbs representing epitopes in site A which are shared by human and bovine PIV3. The deduced amino acid substitutions in the variants were located in separate hydrophilic stretches of HN residues which are conserved in the primary structures of the HN proteins of both human and bovine PIV3 strains. 相似文献
89.
Effects of NaCl stress on proline and cation accumulation in salt sensitive and tolerant turfgrasses
Summary Concentrations of proline, sodium and potassium in shoot tissues of five turfgrass species were measured following exposure
to 170 mM NaCl salinity stress. Salt tolerant ‘Fults’ alkaligrass and ‘Dawson’ red fescue restricted the accumulation of Na-ions to
significatnly low levels compared to the salt sensitive Kentucky bluegrasses (‘Adelphi’ and ‘Ram I’) and ‘Jamestown’ red fescue.
Accumulation of proline began in all species within 24 h of initiation of salt stress but at a more rapid rate and higher
overall concentration for ‘Fults’ alkaligrass. Proline levels were variable and too low in relation to sodium accumulations
to have any significant osmoregulatory role in salt tolerance among all cultivars tested with the possible exception of alkaligrass. 相似文献
90.
Comparison of arginase activity in red blood cells of lower mammals, primates, and man: evolution to high activity in primates.
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
E B Spector S C Rice R M Kern R Hendrickson S D Cederbaum 《American journal of human genetics》1985,37(6):1138-1145
Arginase activity in red blood cells (RBC) of various mammalian species including man was determined. In nonprimate species, the activity generally fell below the level of detectability of the assay: less than 1.0 mumol urea/g hemoglobin per hr. Activities in higher nonhuman primates were equal to or of the same order of magnitude as those in man (approximately 950 mumol/g hemoglobin per hr). RBC arginase deficiency with normal liver arginase activity has been shown to segregate as an autosomal codominant trait in Macaca fascicularis established and bred in captivity. This study confirms the presence of this polymorphism in wild populations trapped in several geographic areas and demonstrates the absence of immunologically cross-reactive material in the RBC of RBC arginase-deficient animals. These data when taken together suggest that the expression of arginase in RBC is the result of a regulatory alteration, has evolved under positive selective pressure, and is not an example of the vestigial persistence of an arcane function. The expression of arginase in the RBC results in a marked drop in the arginine content of these cells. 相似文献