全文获取类型
收费全文 | 2594篇 |
免费 | 213篇 |
国内免费 | 1篇 |
专业分类
2808篇 |
出版年
2022年 | 26篇 |
2021年 | 52篇 |
2020年 | 27篇 |
2019年 | 39篇 |
2018年 | 53篇 |
2017年 | 44篇 |
2016年 | 76篇 |
2015年 | 88篇 |
2014年 | 114篇 |
2013年 | 145篇 |
2012年 | 178篇 |
2011年 | 158篇 |
2010年 | 95篇 |
2009年 | 97篇 |
2008年 | 118篇 |
2007年 | 104篇 |
2006年 | 93篇 |
2005年 | 83篇 |
2004年 | 89篇 |
2003年 | 63篇 |
2002年 | 72篇 |
2001年 | 63篇 |
2000年 | 58篇 |
1999年 | 47篇 |
1998年 | 18篇 |
1997年 | 28篇 |
1996年 | 20篇 |
1993年 | 18篇 |
1992年 | 54篇 |
1991年 | 27篇 |
1990年 | 30篇 |
1989年 | 29篇 |
1988年 | 40篇 |
1987年 | 31篇 |
1986年 | 27篇 |
1985年 | 48篇 |
1984年 | 27篇 |
1983年 | 34篇 |
1982年 | 19篇 |
1981年 | 30篇 |
1980年 | 20篇 |
1979年 | 30篇 |
1978年 | 30篇 |
1977年 | 21篇 |
1976年 | 19篇 |
1975年 | 29篇 |
1974年 | 16篇 |
1973年 | 23篇 |
1972年 | 22篇 |
1971年 | 19篇 |
排序方式: 共有2808条查询结果,搜索用时 15 毫秒
1.
2.
In Escherichia coli, two enzymes catalyze the synthesis of methionine from homocysteine using methyltetrahydrofolate as the donor of the required methyl group: cobalamin-dependent and cobalamin-independent methionine synthases. Comparison of the mechanisms of these two enzymes offers the opportunity to examine two different solutions to the same chemical problem. We initiated the research described here to determine whether the two enzymes were evolutionarily related by comparing the deduced amino acid sequences of the two proteins. We have determined the nucleotide sequence for the metE gene, encoding the cobalamin-independent methionine synthase. Our results reveal an absence of similarity between the deduced amino acid sequences of the cobalamin-dependent and cobalamin-independent proteins and suggest that the two have arisen by convergent evolution. We have developed a rapid one-step purification of the recombinant cobalamin-independent methionine synthase (MetE) that yields homogeneous protein in high yield for mechanistic and structural studies. In the course of these studies, we identified a highly reactive thiol in MetE that is alkylated by chloromethyl ketones and by iodoacetamide. We demonstrated that alkylation of this residue, shown to be cysteine 726, results in complete loss of activity. While we are unable to deduce the role of cysteine 726 in catalysis at this time, the identification of this reactive residue suggests the possibility that this thiol functions as an intermediate methyl acceptor in catalysis, analogous to the role of cobalamin in the reaction catalyzed by the cobalamin-dependent enzyme. 相似文献
3.
A defective form of Hepatitis B virus (HBV) was identified in an apparently healthy voluntary blood donor, who was positive for the presence of HBV by dot blot hybridization, but did not have any serological markers of HBV infection. Two regions, part of X and part of surface antigen genes, were amplified by polymerase chain reaction, cloned and sequenced by Sanger's dideoxy chain termination method. The base sequence analysis revealed that the HBV mutant belonged to ayw serotype and showed three point mutations, in the form of deletions at nucleotides number 1402, 1438 and 1450. Such mutations in the 'X' region, and their likely presence elsewhere, could explain altered antigenic expression. 相似文献
4.
The effects of some local anesthetics on plasma protein secretion by rat liver slices have been studied and have been compared with those of colchicine. Rat liver slices were pulse-labelled with l-[14C]leucine for 9 min at 37°C, collected on filter paper, washed with non-radioactive leucine and reincubated in the presence or absence of the drug to be tested. The radioactive plasma proteins produced were obtained by immunoprecipitation from either the chase medium or from the washed slices. Chlorpomazine, (3 · 10?5 M), dibucaine (10?5 M), lidocaine (10?3 M) and procaine (5 · 10?5 M) inhibited both the synthesis and secretion of plasma protein but did not affect the uptake of l-leucine into the slices nor the incorporation of phosphate into intracellular nucleotide phosphates or into phopholipids. The inhibition of secretion elicited by these drugs is probably not due to the inhibition of protein synthesis since cycloheximide, when added to the chase medium at a concentration which completely inhibits protein synthesis, did not inhibit plasma protein secretion, while cycloheximide plus procaine did inhibit secretion and also caused a retention of non-secreted plasma proteins within the slices. Unlike colchicine, howover, procaine did not cause the retained plasma proteins to accumulate in Goli-derived secretory vesicles, but showed a more general effect causing a distribution among several cell fractions. 相似文献
5.
6.
7.
A Banerjee M A Jordan R F Luduena 《Biochemical and biophysical research communications》1985,128(2):506-512
Incubation of phosphocellulose-purified tubulin with GSH at 30 degrees C results in an inhibition of colchicine binding activity. GSSG has a protective effect against the GSH-induced loss of colchicine-binding. Incubation of tubulin with GSH at 30 degrees C results in the formation of abnormal tubulin polymers which are insensitive to cold. Such aggregation is insensitive to antimicrotubular drugs. Aggregation is inhibited by GSSG but not by DTT or mercaptoethanol. GSH-induced aggregation is very sensitive to the ionic strength of the assembly medium; both the aggregation and colchicine binding inhibition induced by GSH are inhibited at higher ionic strength. These results indicate a very complex interaction of GSH with tubulin. 相似文献
8.
M. K. Banerjee M. K. Kalloo 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(5):707-710
Summary One hundred and twenty-two varieties, lines and wild accessions of Lycopersicon were screened under three different regimes during the autumn/winter season of 1982–83 and 1983–84 for resistance to tomato leaf curl virus (TLCV). L. hirsutum f. glabratum (B6013) and L. hirsutum f. typicum (A1904) proved to be highly resistant to TLCV in all three environments. Various accessions of L. peruvianum were also highly resistant. L. pimpinellifolium (A1921) exhibited no TLCV symptoms within 90 days. Of the cultivated varieties, Acc 99 exhibited the minimim score for susceptibility; AC 142, Collection No. 2, Kalyanpur Angurlata and HS 101 had a low rating for virus incidence. The inheritance of resistance was studied in the interspecific crosses between a TLCV resistant line of L. pimpinellifolium (A1921) and five (HS 101, HS 102, HS 110, Pusa Ruby and Punjab Chhuhara) susceptible cultivars of L. esculentum. Parents, F1, F2 and backcross progenies were artificially inoculated with local strains of TLCV using vector the viruliferious whitefly, Bemisia tabaci (Genn.). Data indicated that the resistance of L. pimpinellifolium (A 1921) is monogenic and incompletely dominant over susceptibility. 相似文献
9.
Dipak Kumar Banerjee 《Journal of biosciences》1987,11(1-4):311-319
Radio-labelled amphomycin (3H-amphomycin) forms a complex with dolichylmonophosphate in presence of Ca2+. Complex formation has also been documented with retinylmonophosphate and perhydromonoeneretinylmonophosphate. Analysis of
the space-filling model suggested both fatty acylated aspartic acid residue at the N-terminus of the lipopeptide and phosphate
head group of dolichylmonophosphate are necessary for the complex formation. The binding ability of amphomycin is then utilized
to localize dolichylmonophosphate in the microsomal membrane. Studies with microsomal membranes from hen oviduct suggested
that dolichylmonophosphate is located in the cytoplasmic side of the membrane. 相似文献
10.
DMBA induced DNA damage and repair in mammary epithelial cells in vitro measured by a nick translation assay 总被引:2,自引:0,他引:2
A new E. coli DNA polymerase I directed nick translation assay was used for measuring 7,12-dimethylbenz[a]anthracene-induced in situ DNA damage and repair in mouse mammary epithelial cells in monolayer culture. The nick translation assay was capable of detecting a DMBA-dose dependent significant increase of DNA damage, and the same assay also allowed monitoring of the DNA repair activity provoked by DMBA treatment of the epithelial cells. This relatively simple method thus provides a rapid assay for carcinogen-induced in situ DNA damage and repair in an epithelial cell tumorigenic system. 相似文献