全文获取类型
收费全文 | 2045篇 |
免费 | 208篇 |
出版年
2023年 | 14篇 |
2022年 | 53篇 |
2021年 | 87篇 |
2020年 | 51篇 |
2019年 | 75篇 |
2018年 | 78篇 |
2017年 | 67篇 |
2016年 | 92篇 |
2015年 | 130篇 |
2014年 | 125篇 |
2013年 | 164篇 |
2012年 | 177篇 |
2011年 | 176篇 |
2010年 | 87篇 |
2009年 | 70篇 |
2008年 | 123篇 |
2007年 | 96篇 |
2006年 | 69篇 |
2005年 | 73篇 |
2004年 | 67篇 |
2003年 | 56篇 |
2002年 | 45篇 |
2001年 | 22篇 |
2000年 | 24篇 |
1999年 | 21篇 |
1998年 | 13篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 6篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1970年 | 3篇 |
1968年 | 2篇 |
排序方式: 共有2253条查询结果,搜索用时 15 毫秒
1.
S Papa M Vitale G Mazzotti R Rizzoli M Falconi A Bartoletti F A Manzoli 《Histochemistry》1988,89(3):241-245
A number of techniques are currently employed for the fractionation of heterogeneous cell populations or for the separation of cells in different phases of their cycle. With the development of osmotically inert colloidal silica particles media, density gradient centrifugation became an established method for the separation and purification of cells and subcellular particles. We have applied this technique to the separation of cycling from resting Friend erythroleukemia cells, to obtain purified populations for further biological assays. The flow cytometric analysis of DNA content of the different fractions obtained by the gradient and stained with Propidium Iodide (PI), showed the S compartment highly concentrated in the 1.073/77 g/ml interface, while the upper levels of the gradient were highly enriched of cells in G1 phase. Moreover, the dual parameter analysis of DNA content by means of Bromodeoxyuridine (BrdUrd) incorporation and PI staining, showed that part of the cells in the 1.067/73 fraction represented the early S phase even if their DNA level, measured on the basis of PI fluorescence was within the diploid cell cluster. This method seems to be suitable to obtain pure cell fractions even when dealing with numerically large populations. 相似文献
2.
S Papa S Capitani A Matteucci M Vitale P Santi A M Martelli N M Maraldi F A Manzoli 《Cytometry》1987,8(6):595-601
The development of hepatocyte polyploidy in rats aged up to 4 months was analyzed by flow cytometry using both scatter and fluorescent parameters to distinguish DNA diploid and DNA tetraploid populations and to discriminate between parenchymal and non-parenchymal compartments. The precise origin of each class of nuclei was assessed in whole liver homogenate using purified hepatocytes, obtained by liver perfusion followed by separation on Percoll gradient, and identifying the peaks corresponding to parenchymal nuclei. The results indicate that preparative procedures involving homogenization of the rat liver tissue caused loss of the DNA octaploid population. Data on the relative proportion of the different DNA ploidy elements during rat liver development, which are in good agreement with those observed by cell analysis by means of microspectrophotometry, indicate the usefulness of flow cytometry as a choice method for the analysis of ploidy distribution. 相似文献
3.
Summary A trypsin-like proteinase was isolated from Streptomyces rimosus culture filtrates obtained from an oxytetracycline production process. The isolation procedure includes ultrafiltration, chromatography on CM-Sephadex, AH-Sepharose and CM-cellulose and gives a homogeneous protein with 19% yield. The enzyme is an anionic trypsin (Mr 28 000, pI 4.5), is stable from pH 4.5 to 9 and up to 40°C, and contains three disulphide bridges, three histidines and three methionines per molecule. At its pH optimum (pH 8.4–8.8) it splits peptide, ester and arylamide bonds of arginine in the endo-position and, to a smaller extent, in the exo-position. Like other streptomycete trypsins, it is a more efficient catalyst than bovine trypsin and has a relative preference for peptide-arylamides, N-benzyloxycarbonyl-l-norleucyl-l-prolyl-l-arginine-p-nitroanilide being by far its best substrate. 相似文献
4.
S Petrovi? L Vitale 《Comparative biochemistry and physiology. B, Comparative biochemistry》1990,95(3):589-595
Hydrolytic activities characteristic for different aminopeptidases were detected in the egg-white of unfertilized chicken eggs, and one aminopeptidase was isolated in an electrophoretically homogeneous form. The isolated aminopeptidase preferentially hydrolyzed bonds of alpha-glutamyl residue at the NH(2)-end of synthetic substrates and peptides. The enzyme is a dimer with an M(r) of 320,000 and pI of 4.2. Its optimal pH and temperature are 7.6 and 60 degrees C, respectively. EDTA, amastatin, and N-bromosuccinimide are inhibitors, while Ca2++ and Mn2+ are activators of the enzyme Ca2+ also stabilizes the enzyme. According to the observed properties, the isolated chicken egg-white aminopeptidase belongs to the glutamyl aminopeptidases. 相似文献
5.
The present paper describes a simple technique that hardens the shell of nuts and makes the use of a tool to crack them open more compelling. Walnuts were coated with a dough of sawdust and nontoxic white glue in different combinations; they were tested for hardness by using machines normally used to test different kinds of wood. Data on relative hardness for uncoated walnuts and walnuts coated with dough of two different combinations are presented. The coated walnuts were significantly harder to break than the uncoated ones, whereas no significant difference was found when comparing the hardness of two types of coated walnuts. Furthermore, observations on a captive group of tufted capuchins (Cebus apella) are described. The monkeys needed significantly more time to break open the coated walnuts. Early results show that coated walnuts may favor acquistion of tool use skills in a juvenile capuchin. 相似文献
6.
7.
Lj. Vitale M. Renko B. Lenarčič V. Turk M. Pokorny 《Applied microbiology and biotechnology》1986,23(6):449-455
Summary A leucine aminopeptidase was purified to homogeneity fromStreptomyces rimosus culture filtrates, which are waste broth of oxytetracycline bioproduction process. Purification procedure includes ultrafiltration
and chromatography on CM-Sephadex, AH-Sepharose and FPLC Mono S column.
The enzyme is a monomer with molecular weight of 27,500 Daltons and pI of 7.3, stable in broad pH range and up to 70°C. It
is a metallo enzyme dependent on Ca2+ ions for its full activity. By its specificity it is a true aminopeptidase active on amino acid amide, arylamide, peptide
and ester bonds. The hydrolysing activity shows preference for leucine at the N-terminal position of substrates, also acts
on aromatic acids and methionine, but does not release glycine, proline, acidic amino acids orD-amino acid residues. 相似文献
8.
9.
M. Vitale L. M. Neri L. Manzoli A. Galanzi R. Rana A. Antonucci S. Papa 《Histochemistry and cell biology》1989,93(1):9-11
Summary Dynamic cell cycle analysis is based on the incorporation of labelled precursors into DNA. Although antibodies to BrdU are very useful for analysing in flow cells which synthesize DNA, this approach has two main limitations. First, the detection of low incorporating cells is often difficult; second, four parameter flow cytometry is not able to correlate cell cycle to any other cellular marker. We have developed a methodology that, employing an IgGH+L as a second antibody and side scatter instead of propidium iodide fluorescence, allows a better discrimination of BudR+ cells. This approach allows the collection of an extra-fluorescent signal, and the analysis of specific cellular markers within the cell cycle. 相似文献
10.
Radiosensitivity of human natural killer cells: binding and cytotoxic activities of natural killer cell subsets 总被引:1,自引:0,他引:1
The sensitivity of human natural killer (NK) cell activities (both binding and killing) after exposure of peripheral blood mononuclear cells to different doses of gamma radiation was studied. A panel of monoclonal antibodies was used to identify the NK and T-lymphocyte subsets and to evaluate their radiosensitivity. Peripheral blood mononuclear cells were irradiated with low (2-6 Gy) and high (10-30 Gy) doses and NK cell binding and cytotoxic activity against K562 target cells were studied after 3 h and 48 h in culture. The primary damage to NK cell activity was identified at the postbinding level and affected mainly the lytic machinery. After 48 h culture postirradiation, an overall depression of cytotoxic activity was observed, but ionizing radiation produced either a selection of the more cytotoxic NK cell subsets, which therefore might be considered more resistant to radiation damage than the less cytotoxic NK cells, or a long-term stimulation of cytotoxic activity in surviving cells. 相似文献