全文获取类型
收费全文 | 15969篇 |
免费 | 731篇 |
国内免费 | 444篇 |
专业分类
17144篇 |
出版年
2023年 | 113篇 |
2022年 | 194篇 |
2021年 | 200篇 |
2020年 | 249篇 |
2019年 | 360篇 |
2018年 | 403篇 |
2017年 | 276篇 |
2016年 | 301篇 |
2015年 | 363篇 |
2014年 | 763篇 |
2013年 | 1075篇 |
2012年 | 546篇 |
2011年 | 820篇 |
2010年 | 632篇 |
2009年 | 780篇 |
2008年 | 859篇 |
2007年 | 899篇 |
2006年 | 780篇 |
2005年 | 793篇 |
2004年 | 690篇 |
2003年 | 660篇 |
2002年 | 529篇 |
2001年 | 329篇 |
2000年 | 290篇 |
1999年 | 327篇 |
1998年 | 325篇 |
1997年 | 265篇 |
1996年 | 222篇 |
1995年 | 264篇 |
1994年 | 223篇 |
1993年 | 219篇 |
1992年 | 204篇 |
1991年 | 157篇 |
1990年 | 135篇 |
1989年 | 123篇 |
1988年 | 140篇 |
1987年 | 139篇 |
1986年 | 91篇 |
1985年 | 149篇 |
1984年 | 201篇 |
1983年 | 156篇 |
1982年 | 184篇 |
1981年 | 138篇 |
1980年 | 146篇 |
1979年 | 123篇 |
1978年 | 57篇 |
1977年 | 58篇 |
1976年 | 53篇 |
1975年 | 40篇 |
1973年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Methodological improvements now allow routine analyses of highly degraded DNA samples as found in museum specimens. Using these methods could be useful in studying such groups as rodents of the genus Gerbillus for which i) the taxonomy is still highly debated, ii) collection of fresh specimens may prove difficult. Here we address precise taxonomic questions using a small portion of the cytochrome b gene obtained from 45 dry skin/skull museum samples (from 1913 to 1974) originating from two African and three Asian countries. The specimens were labelled Gerbillus
gerbillus, Gerbillus
andersoni, Gerbillus
nanus, Gerbillus
amoenus, Gerbillus
perpallidus and Gerbillus
pyramidum, and molecular results mostly confirmed these assignations. The close relationship between Gerbillus
nanus (Asian origin) and Gerbillus
amoenus (African origin) confirmed that they represent vicariant sibling species which differentiated in allopatry on either side of the Red Sea. In the closely related Gerbillus
perpallidus and Gerbillus
pyramidum, specimens considered as belonging to one Gerbillus
pyramidum subspecies (Gerbillus
pyramidum
floweri) appeared closer to Gerbillus
perpallidus suggesting that they (Gerbillus
pyramidum
floweri and Gerbillus
perpallidus) may represent a unique species, distributed on both sides of the Nile River, for which the correct name should be Gerbillus
floweri. Furthermore, the three other Gerbillus
pyramidum subspecies grouped together with no apparent genetic structure suggesting that they may not yet represent genetically differentiated lineages. This study confirms the importance of using these methods on museum samples, which can open new perspectives in this particular group as well as in other groups of interest. 相似文献
143.
5-HT1A receptors are involved in the regulation of various behaviors and the mechanism of action of anxiolytics and antidepressants. It is rather difficult to study the expression of the 5-HT1A receptor gene in the brain because of the low concentration of its mRNA. A method developed for quantitating the level of 5-HT1A receptor gene expression in brain structures involves estimation of the copy number for contaminant genomic DNA, the cDNA of the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene (a housekeeping gene), and the 5-HT1A receptor gene cDNA in a cDNA preparation. To estimate the GAPDH and 5-HT1A receptor cDNA copy numbers, the fluorescent intensity of the corresponding PCR products is calibrated using genomic DNA standards of known concentrations. The expression of the 5-HT1A receptor gene is corrected for the content of contaminant genomic DNA and presented as a 5-HT1A receptor cDNA copy number per 100 copies of the GAPDH cDNA. The method was used to demonstrate for the first time that expression of the 5-HT1A receptor gene is increased in the frontal cortex and the amygdala of mice knocked-out in the monoamine oxidase A gene. 相似文献
144.
This review summarizes data on the properties of L-lysine -oxidase, an enzyme that belongs to the group of oxidases of L-amino acids. This enzyme acts virtually only on L-lysine with a rather low K
m yielding -keto--aminocaproic acid. The decrease in the level of the essential amino acid L-lysine and the formation of hydrogen peroxide during the reaction possibly provide the basis for the unique properties of L-lysine -oxidase: cytotoxic, antitumor, antimetastatic, antiinvasive, antibacterial, and antiviral activities, as well as an immunomodulating effect. Native L-lysine -oxidase and its immobilized forms are promising tools for determination of concentration of L-lysine in various biological materials. 相似文献
145.
A cDNA clone of the geneLhca2 encoding a photosystem I (PSI) type II chlorophylla/b-binding protein was isolated fromArabidopsis thaliana. The isolation of this, the fourth PSI cab gene fromArabidopsis, confirms a previous report [1] that indicatedArabidopsis may contain all four PSI cab genes identified in other plant species.Lhca2 is a single-copy gene as are the other knownArabidopsis PSI cab genes. The patterns of developmental expression and tissue-specific regulation ofLhca2 are similar to those of other PSI and PSII cab genes, but the light induction pattern and the steady-state mRNA level ofLhca2 are distinct. This suggests that a different mechanism may be employed to regulate the expression ofLhca2. 相似文献
146.
147.
A convenient synthesis of [1-14C]-mono-trans fatty acid using olefin inversion as a key-step is described. This methodology allows for a facile synthesis of [1-14C]-labelled mono-trans analogues of oleic, linoleic and linolenic acids. As an example, only eleven steps were necessary to obtain the [1-14C]-mono-E isomers of linolenic acid from its commercial all-Z form. In the first step, Barton's decarboxylation procedure yielded a bromo intermediate. Epoxidation of this compound resulted in the formation of three monoepoxides, which could be separated by HPLC. After identification by 1H NMR and MS, the pure monoepoxides were then subjected to inversion consisting of a stereospecific deoxygenation followed by a beta-elimination step. Finally, the labelling was introduced by substitution of the bromine by a [14C]-cyano group followed by hydrolysis. 相似文献
148.
《MABS-AUSTIN》2013,5(1):108-118
Dendritic cells (DCs) function as professional antigen presenting cells and are critical for linking innate immune responses to the induction of adaptive immunity. Many current cancer DC vaccine strategies rely on differentiating DCs, feeding them tumor antigens ex vivo, and infusing them into patients. Importantly, this strategy relies on prior knowledge of suitable “tumor-specific” antigens to prime an effective anti-tumor response. DCs express a variety of receptors specific for the Fc region of immunoglobulins, and antigen uptake via Fc receptors is highly efficient and facilitates antigen presentation to T cells. Therefore, we hypothesized that expression of the mouse IgG1 Fc region on the surface of tumors would enhance tumor cell uptake by DCs and other myeloid cells and promote the induction of anti-tumor T cell responses. To test this, we engineered a murine lymphoma cell line expressing surface IgG1 Fc and discovered that such tumor cells were taken up rapidly by DCs, leading to enhanced cross-presentation of tumor-derived antigen to CD8+ T cells. IgG1-Fc tumors failed to grow in vivo and prophylactic vaccination of mice with IgG1-Fc tumors resulted in rejection of unmanipulated tumor cells. Furthermore, IgG1-Fc tumor cells were able to slow the growth of an unmanipulated primary tumor when used as a therapeutic tumor vaccine. Our data demonstrate that engagement of Fc receptors by tumors expressing the Fc region of IgG1 is a viable strategy to induce efficient and protective anti-tumor CD8+ T cell responses without prior knowledge of tumor-specific antigens. 相似文献
149.
Haiyan Liu Lihong Li Lixian Zhou Yan Xu 《Preparative biochemistry & biotechnology》2013,43(8):725-733
AbstractThe hydrolysates of soy protein and milk protein are nutritional and functional food ingredients. Aspergillus pseudoglaucus aspergillopepsin I (App) is an acidic protease, including signal peptide, propeptide, and catalytic domain. Here, we cloned the catalytic domain App with or without propeptide in Escherichia coli. The results showed that the App without propeptide was not expressed or did not exhibit activity and App with propeptide (proApp) was highly expressed with a specific activity of 903?U/mg. Moreover, the denaturation temperature of proApp was 4.1?°C higher than App’s. The proApp showed 104?U/mg and 252?U/mg hydrolysis activities towards soy protein and milk protein under acidic conditions. By RP-HPLC analysis, the peptides obtained from the hydrolysates of soy protein and milk protein were hydrophilic peptides. This work first demonstrates efficient proteolysis of soy protein and milk protein through the functional expression of full-length proApp, which will likely have valuable industrial applications. 相似文献
150.
Abstract Marine vegetation of Port-Cros Island (National park). XII: on Acrochaetium molinieri sp. nov. and Lophosiphonia cristata Falkenberg. – Acrochaetium molinieri sp. nov. is characterized by its bicellular base; only one of these cells originates an erect filament. Lophosiphonia cristata, rarely mentioned from the Mediterranean, is richly illustrated and described in detail 相似文献