首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7582篇
  免费   536篇
  国内免费   4篇
  2021年   63篇
  2020年   43篇
  2019年   55篇
  2018年   91篇
  2017年   62篇
  2016年   92篇
  2015年   175篇
  2014年   218篇
  2013年   481篇
  2012年   319篇
  2011年   315篇
  2010年   215篇
  2009年   210篇
  2008年   319篇
  2007年   346篇
  2006年   303篇
  2005年   324篇
  2004年   326篇
  2003年   306篇
  2002年   307篇
  2001年   269篇
  2000年   272篇
  1999年   242篇
  1998年   115篇
  1997年   96篇
  1996年   90篇
  1995年   92篇
  1994年   87篇
  1993年   94篇
  1992年   179篇
  1991年   151篇
  1990年   163篇
  1989年   163篇
  1988年   129篇
  1987年   145篇
  1986年   100篇
  1985年   134篇
  1984年   111篇
  1983年   87篇
  1982年   71篇
  1981年   59篇
  1980年   52篇
  1979年   67篇
  1978年   45篇
  1977年   61篇
  1976年   42篇
  1975年   39篇
  1974年   52篇
  1973年   48篇
  1970年   57篇
排序方式: 共有8122条查询结果,搜索用时 750 毫秒
201.
A lipoxygenase L-4 gene was isolated from a soybean genomiclibrary. The amino acid sequence of lipoxygenase L-4 is highlyhomologous with the partial amino acid sequence of the 94-kDavegetative storage protein, vsp94, found in paraveinal mesophyllcells in the leaves of depodded soybean plants. No L-4 expressionwas observed in maturing seeds. The L-4 gene is highly expressedin the vegetative tissues of young seedlings, including cotyledons,hypocotyls, roots and primary leaves. L-4 expression followedthe same pattern as lipoxygenase activity in cotyledons peaking3 to 5 days after germination, and returning to a basal levelby 9 days after germination. L-4 gene expression was low inthe roots, stems and leaves of 10-week-old plants. Exposureof 4-week-old plants to atmospheric methyl jasmonate increasedL-4 mRNA in leaves. Continuous pod removal from 7-week-old plantsover a 2 week period resulted in dramatic accumulation of L-4mRNA in leaves. Accumulation of the L-4 protein and three otherlipoxygenase fractions in the leaves of depodded plants wasdemonstrated by ion exchange chromatography. These results indicatethat lipoxygenase L-4 is a component of vsp94. (Received May 31, 1993; Accepted August 9, 1993)  相似文献   
202.
Serological studies have suggested the presence of a new HLA-B39 subtype (B39.2) in the Japanese population. To identify the new HLA-B39 subtype and compare it with an other HLA-B39 subtype (B39.1), the genes encoding HLA-B39.1 (B * 39013) and B39.2 (B * 3902) have been cloned from Japanese. We have sequenced these genes and completed the sequence of HLA-B39.1 (B *39011 ) gene from a Caucasian that was partially sequenced. Comparison of the sequence data revealed that B * 3902 and B * 39013 differ by three nucleotide substitutions which result in a two amino acids change at residues 63 and 67, while one silent substitution at codon 312 is found between B * 39011 and B * 39013. These results suggest that B * 3902 has evolved from B * 39013 rather than B * 39011.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession numbers M94051 (HLA-B*39013), M94052 (HLA-B*39011), and M94053 (HLA-B*3902).  相似文献   
203.
Twenty-four weanling male Wistar rats were divided into four groups fed diets containing adequate or deficient levels of selenium (0.5 ppm [+ Se] or <0.02 ppm [−Se] and protein (15% [+Pro] or 5% [−Pro]), but adequate levels of all other nutrients for 4 wk to determine the effects of Se deficiency and protein deficiency on tissue Se and glutathione peroxidase (GSHPx) activity in rats. Plasma, heart, liver, and kidney Se and GSHPx were significantly lower in Se-deficient groups in relation to Se-sufficient groups. In Se-deficient groups, Se and GSHPx were significantly higher in −Se−Pro rats in heart, liver, and kidney. Data analysis showed that there were significant interaction effects between dietary Se and protein on Se and GSHPx of rats. It is assumed that under the condition of Se deficiency. a low level of protein may decrease Se and GSHPx utilization, increase GSHPx synthesis, and result in Se redistribution. This could account for high levels of Se and GSHPx in the −Se−Pro rats compared to −Se+Pro rats.  相似文献   
204.
Age-related alterations in major neurotransmitter receptors and voltage dependent calcium channels were analyzed by receptor autoradiography in the gerbil brain. [3H]Quinuclidinyl benzilate (QNB). [3H]cyclohexyladenosine (CHA), [3H]muscimol, [3H]MK-801, [3H]SCH 23390, [3H]naloxone, and [3H]PN200-110 were used to label muscarinic acetylcholine receptors, adenosine A1 receptors, γ-aminobutyric acidA (GABAA) receptors, (NMDA) receptors, dopamine D1 receptors, opioid receptors, and voltage dependent calcium channels, respectively. In middle-aged gerbils (16 months old), the hippocampus exhibited a significant elevation in [3H]QNB, [3H]MK-801, [3H]SCH 23390, [3H]naloxone, and [3H]PN200-110 binding, whereas [3H]CHA and [3H]muscimol binding showed a significant reduction in this area, compared with that of young animals (1 month). On the other hand, the cerebellum showed a significant alteration in [3H]QNB, [3H]CHA, and [3H]naloxone binding and the striatum also exhibited a significant alteration in [3H]SCH 23390 and [3H]CHA binding in middle-aged gerbils. The neocortex showed a significant elevation only in [3H]CHA binding in middle-aged animals. The nucleus accumbens and thalamus also showed a significant alteration only in [3H]muscimol binding. However, the hypothalamus and substantia nigra exhibited no significant alteration in these bindings in middle-aged gerbils. These results demonstrate the age-related alterations of various neurotransmitter receptors and voltage dependent calcium channels in most brain regions. Furthermore, they suggest that the hippocampus is most susceptible to aging processes and is altered at an early stage of senescence.  相似文献   
205.
To sequence a DNA segment inserted into a cosmid vector underthe directed sequencing strategy, we established a simple andrapid method for generating nested deletions which uses thein vitro packaging system of bacteriophage T3 DNA. The principleis based on the previous finding that this system can translocateany linear double-stranded DNA up to 40 kb into the phage capsidin a time-dependent manner and the encapsulated DNA becomesDNase-resistant. For this purpose, we constructed a cosmid vectorthat carries two different antibiotic selection markers at bothsides of the multiple cloning site, and after insertion of aDNA segment, the clone was linearized by -terminase at the cossite. After the packaging reaction in vitro followed by DNasetreatment, the encapsulated DNA was introduced into Escherichiacoli cells to give clones with unidirectional deletions by differentialantibiotic selection. Restriction and sequence analyses of deletionclones demonstrated that an ordered set of clones with nesteddeletions, ranging from less than 1 kb to 25 kb, was createdfrom either the end of the DNA segment. Thus, nested deletionclones that cover the entire region of a 40-kb cosmid insertcan be obtained by a single packaging reaction, and its restrictionmap can be simultaneously obtained.  相似文献   
206.
Protein extractions using aerosol OT (AOT)-isooctane reverse micelle solutions have been studied to explore the potential for separating and enriching proteins with the reversed micellar extraction. The effects of pH, ionic strength, and different cations of chlorides in a bulk aqueous phase and of AOT concentration in an organic phase on the partitioning of lysozyme and myoglobin and the solubilization of water are presented in detail. The extraction of lysozyme was affected by the concentration of potassium or barium but was almost independent of that of sodium or calcium, whose ionic diameter is smaller than that of potassium and barium. For the extraction of myoglobin, however, the effect of barium concentration was not appreciable. Lysozyme could be enriched into the reversed micellar phase up to 30 times the aqueous feed concentration. (c) 1993 John Wiley & Sons, Inc.  相似文献   
207.
S Matuoka  H Yao  S Kato    I Hatta 《Biophysical journal》1993,64(5):1456-1460
In the ripple phase of fully hydrated multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC), two kinds of small-angle x-ray diffraction profiles are observed on cooling through the main transition. One is a seemingly normal profile similar to that observed on heating and the other is the superposition of the diffraction profiles for the primary (normal) and the secondary ripple structures. We found that the profile obtained depended on the cooling rate. Increasing the cooling rate from 0.1 degrees C/min to 1 degrees C/min caused the peaks originating from the secondary ripple structure to diminish. After a cooling scan at 43 degrees C/min, the profile became similar to that of the normal ripple structure, although a trace of the secondary ripple structure remains. The results are interpreted in terms of the rise and fall of three-dimensional correlated domains composed of both primary and secondary ripple structures. At slow cooling rates, correlated domains of both kinds of ripple structures develop. As the cooling rate is increased, the domain of the primary ripple structure remains correlated, while that of the secondary ripple structure becomes less correlated. In addition, the multipeak profile appears even at rapid cooling rates, if the final low temperature lies just below the Tm for the main transition. This results suggests that formation of the correlated domains of the secondary ripple structure requires a certain time interval during which the DPPC vesicles experience the temperature just below the main transition. The secondary ripple structure takes place in phosphatidylcholines having more than 15 carbons in each hydrocarbon chain upon cooling through the main transition.  相似文献   
208.
The effects of captopril, an angiotensin-converting enzyme inhibitor, on congestive heart failure (CHF) were investigated in animal and clinical studies. Congestive heart failure was induced in rats by a combination of pressure and volume overload. Cardiac pressure overload was induced by constricting one renal artery (Goldblatt rat) and volume overload was induced by aorto-caval fistula. Captopril (100 mg/kg/day) was then administered for 14 weeks. Isometric contraction was assessed using isolated left ventricular papillary muscles. The maximum developed tension and the maximum rate of increase in tension (dT/dtmax) were decreased in untreated rats with CHF and improved in captopriltreated rats. The left ventricular myosin isoenzyme pattern was shifted towards V3 in untreated rats with CHF, and was shifted back towards V1 in the captopril-treated rats. In the clinical study, captopril (37.5–75 mg/day) was administered to patients with cardiomyopathy for 12 months. There was no effect on left ventricular mass in hypertrophic cardiomyopathy, although systolic anterior motion of the mitral valve disappeared in one patient. In dilated cardiomyopathy, however, left ventricular mass tended to decrease. These results indicate that captopril has a beneficial effect in congestive heart failure.  相似文献   
209.
ADP/ATP carrier protein (AAC) is located in the mitochondrial inner membrane and has an important function in mitochondrial energy supply. This protein transports ATP to the cytoplasm and counter transports ADP into the mitochondria. J-2-N cardiomyopathic hamsters were investigated to determine the AAC content in cardiac mitochondria. After recording an electrocardiogram and collecting blood, the cardiac mitochondria were isolated. The mitochondrial membranes were labelled with eosin-5-maleimide (EMA) and separated on SDS polyacrylamide gels. The position of the AAC component was identified by exposing the gel under UV light, and the AAC content was determined by densitometry after staining with Coomassie blue. The AAC content ratio was significantly decreased in both 10-week-old and 1-year survived J-2-N hamsters when compared to control Golden hamster. Among 10-week-old J-2-N hamsters, the decrease in the AAC content ratio was more marked for the animals with more severe myocardial damage. The H+-ATPase activities of mitochondrial membrane were higher in 10-week-old J-2-N hamsters than in control hamsters. These results suggest that the decrease of AAC in J-2-N hamster plays an important role in the pathogenesis of cardiomyopathy in J-2-N hamsters.  相似文献   
210.
The permanent pancreas carcinoma cell line, PCI-24, was developed in order to analyse cytokine regulation on pancreas carcinoma and lymphokine-activated killer (LAK) cell interaction. PCI cells expressed ICAM-1 and HLA-ABC, but not HLA-DR antigens. PCI cells showed augmented ICAM-1 and HLA-ABC expression when incubated with interferon (IFN) and tumour necrosis factor . A similar but weak augmentary effect on the HLA-ABC and ICAM-1 surface expression was seen with interleukin-1 treatment. Natural attachment of LAK to PCI cells was augmented by recombinant IFN in close association with ICAM-1 up-regulation on PCI cells. In addition, natural attachment was significantly inhibited by anti-LFA-1 and anti-ICAM-1 antibody treatments. Cytotoxicity of the LAK cells against PCI cells was also significantly inhibited with the same treatment. Thus, the attachment of LAK cells to PCI cells through LFA-1/ICAM-1 molecules appeared to be essential for the cytotoxicity for PCI cells. Pretreatment of PCI cells, but not of LAK cells, with IFN or other cytokines resulted in a decrease of susceptibility for LAK cell cytotoxicity. The decreased susceptibility inversely correlated with HLA-ABC expression on the PCI cells. The collective evidence indicates that, although LAK cell attachment to pancreas carcinoma cells through the LFA-1/ICAM-1 molecule is augmented by IFN, IFN treatment of pancreas carcinoma cells reduces LAK cell cytotoxicity possibly through an increase in HLA-ABC or a regulation of molecules closely associated to HLA-ABC expression.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号