全文获取类型
收费全文 | 11601篇 |
免费 | 859篇 |
国内免费 | 3篇 |
出版年
2023年 | 89篇 |
2022年 | 115篇 |
2021年 | 358篇 |
2020年 | 270篇 |
2019年 | 281篇 |
2018年 | 421篇 |
2017年 | 333篇 |
2016年 | 501篇 |
2015年 | 651篇 |
2014年 | 668篇 |
2013年 | 822篇 |
2012年 | 975篇 |
2011年 | 945篇 |
2010年 | 587篇 |
2009年 | 500篇 |
2008年 | 642篇 |
2007年 | 625篇 |
2006年 | 566篇 |
2005年 | 431篇 |
2004年 | 405篇 |
2003年 | 373篇 |
2002年 | 318篇 |
2001年 | 187篇 |
2000年 | 180篇 |
1999年 | 139篇 |
1998年 | 92篇 |
1997年 | 85篇 |
1996年 | 70篇 |
1995年 | 61篇 |
1994年 | 54篇 |
1993年 | 61篇 |
1992年 | 68篇 |
1991年 | 45篇 |
1990年 | 33篇 |
1989年 | 61篇 |
1988年 | 30篇 |
1987年 | 31篇 |
1986年 | 21篇 |
1985年 | 32篇 |
1984年 | 43篇 |
1983年 | 33篇 |
1982年 | 36篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 18篇 |
1978年 | 17篇 |
1976年 | 11篇 |
1975年 | 14篇 |
1974年 | 28篇 |
1971年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Mixing patterns in Amazon lakes 总被引:2,自引:2,他引:0
Jose G. Tundisi Bruce R. Forsberg Allan H. Devol Thomas M. Zaret Takako M. Tundisi Antonio Dos Santos Jorge S. Ribeiro Elsa R. Hardy 《Hydrobiologia》1984,108(1):3-15
The diel mixing patterns of two small floodplain lakes, Lago Jacaretinga in the Amazon drainage, and Lago Cristalino in the Rio Negro system, were investigated during both the high-water and low-water states of the Amazon River hydrograph. Measurements included temperature, oxygen, ammonia, phosphate, and chlorophyll. In both lakes thermal stratification developed during the day and was eroded at night. During the low-water period when the lakes were shallow, nocturnal circulation extended to the lake bottom, whereas when the lakes were deeper (greater than about 5 m), circulation did not reach the bottom and an anoxic hypolimnion developed. During the low-water period, percent of oxygen concentrations were relatively high but always less than saturation. Low oxygen concentrations were observed during the high-water period. At all times nocturnal mixing supplied a significant amount of oxygen to the lake ecosystems. Nighttime upward mixing of recycled nitrogen and phosphorus also appeared to be important nutrient sources for algal productivity. 相似文献
52.
Introduction of T-dependent antigens into the subarachnoid space (isas) resulted in higher systemic antibody responses in mice than injections into the peritoneal cavity (ip) or other sites commonly used for immunization. Antibody production in isas immunized mice was not increased by treatment with cyclophosphamide (Cy) at doses known to abolish T-suppressor-cell activity, but such treatment increased antibody production in ip immunized mice toward the higher level which was observed in the isas immunized animals. Suppressor cell-dependent low zone tolerance (LZT) to TNP-BSA could not be induced by isas injections of deaggregated BSA (d-BSA). Conversely, mice which were unresponsive to ip injected d-BSA showed consistent systemic antibody responses when the antigen was injected isas. These observations indicate that immune responses initiated within the CNS are associated with relatively ineffective induction of systemic suppressor cell activity. 相似文献
53.
54.
Meria E. Penttilä K. M. Helena Nevalainen Alain Raynal Jonathan K. C. Knowles 《Molecular & general genetics : MGG》1984,194(3):494-499
Summary The possibility of cloning filamentous fungal genes by expression in the yeast Saccharomyces cerevisiae has been studied. A genome bank of Aspergillus niger was made in E. coli using a yeast cosmid shuttle vector and over 10,000 different cosmid clones were individually isolated. Yeast transformants carrying Aspergillus DNA were screened for the expression of the genes for fungal secreted glycoproteins, -galactosidase, -glucosidase, and amyloglucosidase, and for the expression of fungal genes complementing yeast ura3 and leu2 mutations.Of the five Aspergillus genes studied, only one, -glucosidase, was found to be expressed in yeast, and this at a low level. This suggests that there are essential differences between the genes of yeast and filamentous fungi. 相似文献
55.
Antigenic structure of simian virus 40 large tumor antigen and association with cellular protein p53 on the surfaces of simian virus 40-infected and -transformed cells. 总被引:12,自引:9,他引:3 下载免费PDF全文
The antigenic structure of simian virus 40 (SV40) large tumor antigen (T-ag) in the plasma membranes of SV40-transformed mouse cells and SV40-infected monkey cells was characterized as a step toward defining possible biological function(s). Wild-type SV40, as well as a deletion mutant of SV40 (dl1263) which codes for a truncated T-ag with an altered carboxy terminus, was used to infect permissive cells. Members of a series of monoclonal antibodies directed against antigenic determinants on either the amino or the carboxy terminus of the T-ag polypeptide were able to precipitate surface T-ag (as well as nuclear T-ag) from both SV40-transformed and SV40-infected cells. Cellular protein p53 was coprecipitated with T-ag by all T-ag-reactive reagents from the surface and nucleus of SV40-transformed cells. In contrast, T-ag, but not T-ag-p53 complex, was recovered from the surface of SV40-infected cells. These results confirm that nuclear T-ag and surface T-ag are highly related molecules and that a complex of SV40 T-ag and p53 is present at the surface of SV40-transformed cells. Detectable levels of such a complex do not appear to be present on SV40-infected cells. Both the carboxy and amino termini of T-ag are exposed on the surfaces of SV40-transformed and -infected cells. The possible relevance of the presence of a T-ag-p53 complex on the surface of SV40-transformed cells and its absence from SV40-infected cells is considered. 相似文献
56.
The title trisaccharide was synthesized using methyl 1-thioglycoside building blocks. An acyclic analogue, methyl 3-O-(α-D-glycopyranosyl-oxyethyl)-α-D-glucopyranoside, which has an ethylene bridge in place of the galactosyl residue, was also synthesized. 相似文献
57.
Helena Kuerová Marie Strnadová Vladimír Vinter Peter Graba Jií Chaloupka 《FEMS microbiology letters》1986,34(1):21-26
Abstract Netropsin stimulated the rate of synthesis of an extracellular metalloproteinase in Bacillus megaterium incubated in a sporulation medium. The antibiotic delayed but did not suppress the decrease in the ability to synthesize the proteinase occurring at later sporulation stages. Netropsin also stimulated the synthesis of the proteinase when added to a growing culture; it inhibited the increase of protein turnover which was switched on between the 2nd and 3rd hour in the sporulating population. No refractile spores were developed during 6 h at 35°C in the antibiotic-treated culture. In the control 60% of sporulating cells were observed under similar conditions. 相似文献
58.
Transient exposure of mycelia from Aspergillus niger and Aspergillus nidulans to the cytidine analog 5-azacytidine, leading to no more than 0.3 to 0.5% substitution for cytosine by 5-azacytosine in A. nidulans DNA, resulted in the conversion of a high fraction of the cell population (more than 20%) to a mitotically and meiotically stable "fluffy" developmental phenotype. The phenotypic variants are characterized by the developmentally timed production of a profuse fluffy network of undifferentiated aerial hyphae that seem to escape signals governing vegetative growth. Genetic analysis with six different fluffy clones reveals that this trait is not cytoplasmically coded, is recessive in heterozygous diploids but codominant in heterokaryons, and exhibits a 1:1 Mendelian segregation pattern upon sexual sporulation of heterozygous diploids. Complementation and mitotic haploidization studies indicated that all variants are affected in the same gene, which can be tentatively located on chromosome VIII of A. nidulans. Molecular analysis to search for modified bases showed that DNA methylation is negligible in in both A. niger and A. nidulans and that no differences could be detected among DNAs from wild-type cells, fluffy clones, or mycelia exposed to 5-azacytidine. It thus appears that high-frequency conversion of fungal mycelia to a stable, variant developmental phenotype by 5-azacytidine is the result of some kind of target action on a single nuclear gene and that this conversion can occur in organisms virtually devoid of DNA methylation. 相似文献
59.
Accumulation of pyrophosphate in Escherichia coli. Relationship to growth and nucleotide synthesis 总被引:1,自引:0,他引:1
Pyrophosphate (PPi) content of Escherichia coli is increased manyfold when the growth of the cells is limited by inhibition of the synthesis of nucleotides. The accumulated PPi is immediately degraded when inhibition is released and growth allowed to resume. Other tested nutritional deficiencies (starvation of carbon source, sulfate or an amino acid) fail to induce PPi accumulation. 相似文献
60.