全文获取类型
收费全文 | 1722篇 |
免费 | 113篇 |
国内免费 | 6篇 |
专业分类
1841篇 |
出版年
2022年 | 25篇 |
2021年 | 38篇 |
2020年 | 27篇 |
2019年 | 26篇 |
2018年 | 47篇 |
2017年 | 39篇 |
2016年 | 77篇 |
2015年 | 61篇 |
2014年 | 93篇 |
2013年 | 118篇 |
2012年 | 123篇 |
2011年 | 95篇 |
2010年 | 61篇 |
2009年 | 56篇 |
2008年 | 83篇 |
2007年 | 60篇 |
2006年 | 65篇 |
2005年 | 66篇 |
2004年 | 50篇 |
2003年 | 39篇 |
2002年 | 41篇 |
2001年 | 36篇 |
2000年 | 32篇 |
1999年 | 29篇 |
1998年 | 10篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 11篇 |
1994年 | 15篇 |
1993年 | 9篇 |
1992年 | 23篇 |
1991年 | 21篇 |
1990年 | 16篇 |
1989年 | 25篇 |
1988年 | 22篇 |
1987年 | 13篇 |
1986年 | 22篇 |
1985年 | 17篇 |
1984年 | 23篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 12篇 |
1974年 | 12篇 |
1973年 | 11篇 |
1972年 | 8篇 |
1971年 | 15篇 |
1970年 | 13篇 |
排序方式: 共有1841条查询结果,搜索用时 15 毫秒
1.
The psbQ gene encoding a 16-kDa polypeptide of the oxygen-evolving complex of photosystem II has been isolated from Arabidopsis thaliana and characterized. The gene consists of a 28 nucleotide long leader sequence, two introns and three exons encoding a 223-amino-acid precursor polypeptide. The first 75 amino acids act as a transit peptide for the translocation of the polypeptide into the thylakoid lumen. Expression studies show that the gene is light-inducible and expresses only in green tissues with high steady-state mRNA levels in leaves. Using this gene as a probe, restriction fragment length polymorphism between two ecotypes, Columbia and Estland, has also been detected. 相似文献
2.
A naphthalene-degrading strain of corynebacteria, Corynebacterium renale, harbors multiple small plasmids designated pCR1, pCR2, pCR3, and pCR4 with sizes of 1.4, 3.2, 4.4, and 5.7 kb, respectively. Plasmid pCR1 of 1.4 kb is the smallest plasmid reported in this group of bacteria and is present in high copy number. Attempts to clone whole pCR1 in Escherichia coli were unsuccessful but two of its fragments (750 and 650 bp) could be separately cloned in it. The 4.4-kb plasmid, pCR3, bears considerable restriction pattern similarity to a 4.4-kb plasmid belonging to the pBL1 group of cryptic plasmid of corynebacteria but has no sequence homology, suggesting that pCR3 represents a new member of the 4.4-kb group of corynebacterial plasmids. 相似文献
3.
4.
5.
Administration of cadmium (2.5 mg/kg, sc on alternate days for 3 weeks) to male albino rats led to significant accumulation of cadmium and metallothionein in the liver and kidneys. The activity of glutathione peroxidase was significantly decreased whereas, the concentration of glutathione was increased in these organs. Glycine-l-14C incorporation studies showed enhanced synthesis of glutathione in kidney but not in the liver. Selenium supplementation (1 mg/kg/day orally) failed to prevent these cadmium-induced changes, although it resulted in very high accumulation of selenium in these organs indicating the formation of cadmium-selenium complex. 相似文献
6.
R K Gaur S Paliwal P Sharma K C Gupta 《Journal of biochemical and biophysical methods》1989,18(4):323-329
A simple and sensitive method for the quantitative determination of free amino groups on solid support is described. This approach is a modification of Ngo's [(1986) J. Biochem. Biophys. Methods 12, 349-354] method reported earlier. The method is based on the reaction of the solid support with an excess of 5'-O-(4,4'-dimethoxytrityl)-thymidine-3'-O-(2,4-dinitrophenyl) succinate (DTDS) in the presence of a catalytic amount of 4-dimethylaminopyridine. After removing the excess reagent, solid support is treated with perchloric acid to release 4,4'-dimethoxytrityl cation into the solution. The released 4,4'-dimethoxytrityl cation, which has a strong absorption at 498 nm (epsilon 498 = 70,000), is then determined spectrophotometrically. A comparative study of DTDS, N-succinimidyl-3-(2-pyridyldithio)propionate and 4,4-dimethoxytrityl chloride is also included. The method was found to be very useful to determine those amino groups which are available for functionalization of solid supports, especially, monitoring the functionalization of solid supports for affinity chromatography and synthesis of biopolymers. 相似文献
7.
Changes in epilithic algal communities colonizing introduced substrata were determined in a stream polluted with oil refinery
effluent at Digboi (Assam, India). The number of algal taxa was reduced but the growth of blue-green algae, particularly two
species ofOscillatoria, was encouraged. Epilithic biomass (as chlorophylla) also declined at polluted stations. The algal community of the upstream station was markedly different from the community
occurring just after the confluence of effluent; however, the differences were gradually reduced downstream, indicating improvement
in water quality. Of the various criteria tested for possible relationships with the level of pollutants, species richness,
Shannon diversity and biomass showed significant relationships. The study demonstrates the usefulness of algal criteria for
monitoring oil pollution in running waters. 相似文献
8.
This study was designed to obtain sister-chromatid exchange (SCE) frequencies in bone marrow and spleen cells of mice and Chinese hamsters under in vivo and in vivo/in vitro systems following treatment of animals with varying doses (15-405 micrograms/kg) of triethylenemelamine (TEM). A dose-related SCE response was found in both species, tissues, and systems analyzed following TEM treatment. In vivo, similar responses were noted for both tissues in both species. However, in vivo/in vitro, the response was lower than in vivo and it varied with the tissue. The spleen cells were more sensitive and gave higher numbers of SCEs than bone marrow of both species at the two highest doses tested (135 and 405 micrograms/kg). These differences may be attributed to cell-culturing effects, type of cells analyzed, species and tissue specificities, and pharmacokinetic properties of the chemical. This study lends support to recently established in vivo/in vitro cell culture methodologies employing mice and Chinese hamsters for comparative cytogenetic analysis. 相似文献
9.
The role of the central histaminergic system in depression was studied by using swimming despair test in mice - a behavioural model of depression. In this test, immobility of mice reflects a state of depression. Intracerebral (ic) injection of histamine (50-200 micrograms) increased significantly the immobility. The H1-receptor blocker mepyramine (2.5-20 mg/kg ip) had no effect while H2-receptor blocker cimetidine (100-200 micrograms ic) caused a significant decrease in immobility. The histamine induced facilitation was blocked completely by cimetidine and antidepressant drugs-imipramine and desipramine, but remained unaffected in mice pretreated with mepyramine or atropine. The H2 agonist impromidine (20-40 micrograms ic) also enhanced significantly, the immobility which was blocked by cimetidine and antidepressant drugs. It has been concluded that central H2-receptors facilitate depression and antidepressant drugs block central H2-receptors. 相似文献
10.
Euphorbia pulcherrima, an ornamental plant, exhibits severe systemic viral infection. It grows vegetatively during summer
and is dormant in winter. During the dormant period the buds on healthy stem cuttings remained dormant or rarely formed cyathia
while the buds on virus-infected stem cuttings grew into leafy shoots and never became dormant. Quantitative estimation has
revealed that breaking of bud dormancy in virus-infected stem cuttings may be regulated by markedly higher GA-like activity
in them throughout the period than in their corresponding healthy stem cuttings. 相似文献