全文获取类型
收费全文 | 34429篇 |
免费 | 3100篇 |
国内免费 | 2484篇 |
专业分类
40013篇 |
出版年
2024年 | 89篇 |
2023年 | 422篇 |
2022年 | 817篇 |
2021年 | 1336篇 |
2020年 | 969篇 |
2019年 | 1192篇 |
2018年 | 1171篇 |
2017年 | 808篇 |
2016年 | 1214篇 |
2015年 | 2050篇 |
2014年 | 2268篇 |
2013年 | 2528篇 |
2012年 | 3072篇 |
2011年 | 2852篇 |
2010年 | 1717篇 |
2009年 | 1520篇 |
2008年 | 1853篇 |
2007年 | 1664篇 |
2006年 | 1517篇 |
2005年 | 1254篇 |
2004年 | 1169篇 |
2003年 | 997篇 |
2002年 | 897篇 |
2001年 | 731篇 |
2000年 | 665篇 |
1999年 | 607篇 |
1998年 | 333篇 |
1997年 | 318篇 |
1996年 | 301篇 |
1995年 | 253篇 |
1994年 | 269篇 |
1993年 | 190篇 |
1992年 | 325篇 |
1991年 | 300篇 |
1990年 | 253篇 |
1989年 | 230篇 |
1988年 | 194篇 |
1987年 | 159篇 |
1986年 | 149篇 |
1985年 | 154篇 |
1984年 | 148篇 |
1983年 | 109篇 |
1982年 | 91篇 |
1981年 | 63篇 |
1979年 | 81篇 |
1978年 | 73篇 |
1977年 | 63篇 |
1976年 | 74篇 |
1975年 | 70篇 |
1974年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Summary Microwave irradiation has been shown to decrease the input resistance of snail neurons. In this study, we examined the role of extracellular calcium in triggering the microwave-induced enhancement of membrane conductance. Two sets of experiments were conducted. In the first set, nerve cells were superfused using Ringer solution with added Cd2+ (0.9 mM) which is a known blocker of calcium channels. In the second set, cells were superfused with low Ca2+ (0.7 mM) Ringer solution. Microwave irradiation was conducted at 2,450 MHz for 30 min with a specific absorption rate of 13 mW/g. It was found that 7 mM to 0.7 mM lowering of Ca2+ in bathing solution as well as blocking of calcium channels in neuronal membrane by means of Cd2+ did not influence the fall in membrane resistance induced by microwave radiation. In fact, the observed changed in membrane resistance in these experiments were nearly equal to those observed for neurons superfused by normal Ringer's. Thus, these results rule out the possible contribution of external Ca2+ in the observed microwave effect. Experiments with high Ca2+ solution also support this conclusion. 相似文献
142.
Cloning of the pectate lyase genes from Erwinia carotovora and their expression in Escherichia coli 总被引:11,自引:0,他引:11
A hybrid cosmid coding for pectate lyase (PL) activity was identified from an Erwinia carotovora genomic library by an immunological screening method. A 7-kb DNA fragment was identified which codes for three proteins identical in size to proteins with PL activity purified from E. carotovora culture supernatants. The three proteins had apparent Mrs of 41, 44 and 44 X 10(3) as estimated by SDS-PAGE. None of the PLs were exported from Escherichia coli strain HB101 but all were found in the periplasmic space. Plant tissue was macerated by the PLs made in E. coli. 相似文献
143.
The araBAD operon of Salmonella typhimurium LT2. I. Nucleotide sequence of araB and primary structure of its product, ribulokinase 总被引:6,自引:0,他引:6
Hybrid plasmids containing the araBAD operon of Salmonella typhimurium LT2 were characterized by Southern blot and genetic analyses. The nucleotide sequence of araB was determined. The araB gene product, ribulokinase (EC 2.7.1.16), was purified and the results of amino acid composition analysis and partial amino acid sequence are in agreement with predictions from the DNA sequence. Ribulokinase is 569 amino acid residues long and has a calculated Mr of 61 793. Ribulokinase shares significant homology with xylulose kinase from Escherichia coli. Codon usage in the araB gene does not favor those codons which have intermediate codon-anticodon binding energy. 相似文献
144.
145.
Aerobic and anaerobic L-fucose utilization by Escherichia coli involves an inducible trunk pathway mediated by a permease, an isomerase, a kinase, and an aldolase. Tn5 insertion mutants of a parental strain expressing this pathway constitutively were used to map the positions of the structural genes by transduction. Results from this and previous studies show that all of the structural genes of the L-fucose trunk pathway map between eno and argA at minute 60.2 of the chromosome. 相似文献
146.
Kinetic models for the mode of action of processive and non-processive DNA-helicases are detailed. Fluxes at the steady state are analyzed, and the random walk of the enzymes on the DNA is studied in connection with the rate constants of the chemical reactions involved in the transformation of substrate to products. Finally, the constants of the kinetic model for the processive helicase are related to the parameters of an analogous viscoelastic model. 相似文献
147.
A calcium ionophore-inducible cellular promoter is highly active and has enhancerlike properties. 总被引:16,自引:8,他引:8
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We examined the regulatory/promoter sequence of a calcium ionophore-inducible gene isolated from the rat genome. Whereas the promoter of this ubiquitously expressed gene is active under noninduced conditions, after induction by calcium ionophore A23187 this promoter is 10- to 25-fold more active than the simian virus 40 early promoter, as measured by chloramphenicol acetyltransferase activities. Within this regulatory/promoter region, we have identified a DNA fragment with enhancer-like properties immediately 5' to the TATA sequence. This 291-nucleotide fragment acts in cis to enhance expression of the neomycin phosphotransferase (neo) gene driven by the herpes simplex virus thymidine kinase promoter in an orientation-independent manner. In addition, this fragment can confer A23187 inducibility to the neo gene and effectively compete for positive regulatory factors involved in A23187 induction. Sequence analysis of this promoter reveals homology with viral core enhancer sequences, and the apparent organization of direct repeat domains is similar to those observed in viral enhancers. 相似文献
148.
Tyrosyl kinase activity is inversely related to prostatic acid phosphatase activity in two human prostate carcinoma cell lines. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Alterations in prostatic acid phosphatase (PAcP), a phosphotyrosyl phosphatase, corresponded to changes in overall tyrosyl kinase activity. PAcP added to extracts of prostate carcinoma cells with a low endogenous level of PAcP activity and elevated tyrosyl kinase activity decreased the tyrosyl kinase activity. On the other hand, when PAcP activity was decreased by the addition of androgens to cells, there was a corresponding increase in tyrosyl kinase activity. 相似文献
149.
J S Lin K Sakai M Jouvet 《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》1986,303(11):469-474
We have studied the effects of local injections of histaminergic and antihistaminic drugs on the sleep-waking cycle in the cat. Microinjections of alpha-fluoromethylhistidine (alpha-FMH), a specific inhibitor of histidine decarboxylase, in the ventrolateral posterior hypothalamus, where histamine-immunoreactive neurons have been recently identified, resulted in a significant decrease in wakefulness (W) and increase in deep slow wave sleep (SWS). On the other hand, microinjections of SKF-91488 (Homodimaprit), a specific inhibitor of histamine-N-methyltransferase, increased W and decreased SWS and paradoxical sleep (PS). Microinjections of histamine also produced an increase of W, while this effect was abolished by pretreatment with mepyramine, an H1-histamine receptor antagonist. 相似文献
150.
Amino acid sequence of bovine white matter proteolipid 总被引:13,自引:0,他引:13
M B Lees B H Chao L F Lin M Samiullah R A Laursen 《Archives of biochemistry and biophysics》1983,226(2):643-656
The sequence of the bovine white matter proteolipid has been studied by a combination of proteolytic digestion and chemical cleavage at tryptophan residues. Alignment of peptides obtained by digestion with trypsin, chymotrypsin, clostripain, and Staphylococcus aureus protease gave the sequence of 52 residues at the amino terminus, 96 residues at the carboxyl terminus, and several additional segments. Peptides obtained by treatment of the protein with 2-(2-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine confirmed the alignment and extended the sequence. This information, combined with that of other investigators, permits us to propose the primary structure for the entire protein. On the basis of the sequence determination, the molecular weight of the proteolipid protein is 29,869. 相似文献