全文获取类型
收费全文 | 127526篇 |
免费 | 11047篇 |
国内免费 | 148篇 |
专业分类
138721篇 |
出版年
2021年 | 1297篇 |
2019年 | 1096篇 |
2018年 | 1795篇 |
2017年 | 1719篇 |
2016年 | 2343篇 |
2015年 | 2848篇 |
2014年 | 3330篇 |
2013年 | 4343篇 |
2012年 | 5104篇 |
2011年 | 5146篇 |
2010年 | 3712篇 |
2009年 | 3238篇 |
2008年 | 4575篇 |
2007年 | 4622篇 |
2006年 | 4129篇 |
2005年 | 3992篇 |
2004年 | 3982篇 |
2003年 | 3780篇 |
2002年 | 3535篇 |
2001年 | 5803篇 |
2000年 | 5401篇 |
1999年 | 4179篇 |
1998年 | 1506篇 |
1997年 | 1420篇 |
1996年 | 1214篇 |
1995年 | 1100篇 |
1994年 | 1120篇 |
1993年 | 1055篇 |
1992年 | 2991篇 |
1991年 | 2965篇 |
1990年 | 2882篇 |
1989年 | 2569篇 |
1988年 | 2410篇 |
1987年 | 2266篇 |
1986年 | 2109篇 |
1985年 | 2081篇 |
1984年 | 1711篇 |
1983年 | 1474篇 |
1982年 | 1044篇 |
1979年 | 1567篇 |
1978年 | 1253篇 |
1977年 | 1105篇 |
1976年 | 1092篇 |
1975年 | 1250篇 |
1974年 | 1368篇 |
1973年 | 1444篇 |
1972年 | 1303篇 |
1971年 | 1159篇 |
1970年 | 1023篇 |
1969年 | 1032篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
871.
The epsilon-subunit of ATP synthase from bovine heart mitochondria. Complementary DNA sequence, expression in bovine tissues and evidence of homologous sequences in man and rat. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The epsilon-subunit of ATP synthase from bovine heart mitochondria is assembled into the extrinsic membrane sector, F1-ATPase. The mature protein is 50 amino acid residues in length and its function is unknown. It is a nuclear gene product that is imported into the organelle. A mixture of 64 oligonucleotides 17 bases long, designed on the basis of the known protein sequence, was synthesized and used as a hybridization probe to isolate a cognate cDNA clone from a bovine library. The DNA sequence of this clone was determined, and the protein sequence of the epsilon-subunit deduced from it agrees exactly with that determined by direct sequence analysis of the protein isolated from bovine hearts. The bovine cDNA was used as a hybridization probe to examine the expression of the epsilon-subunit in various bovine tissues. mRNAs related to the cDNA are found in all of these tissues, and no evidence was obtained of the presence of mRNAs for the epsilon-subunit with similar coding sequences and dissimilar 3' non-coding regions. By hybridization experiments with digests of DNA from cow, man and rat it has been shown that sequences related to the bovine cDNA are present in the genomes of all three species. More than one related sequence was detected in all cases, indicating the presence in all three genomes of more than one gene and/or pseudogenes. 相似文献
872.
Groundnut cake ('kulikuli') purchased from four major markets in Ibadan, Oyo State, Nigeria were analysed for aflatoxin B, and associated rnycoflora. In all but two of the samples aflatoxin B, concentrations were between 20 pg/kg and 455 pg/ kg. Mould counts were low (1·0 × 102 4·40 × 102 colonies/g). Eight mould species were isolated. Of these Aspergillus niger, Paecilomyces oarioti, Aspergillus flavrs and Fusarium moniliforme dominated in decreasing sequential order. The results obtained show that groundnut cake on sale in Ibadan markets is unacceptable for animal feed rations and human consumption and there is a need for some form of quality control and decontamination before usage. 相似文献
873.
874.
Expression of the neurotransmitter-synthesizing enzyme glutamic acid decarboxylase in male germ cells. 总被引:12,自引:0,他引:12
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)
H Persson M Pelto-Huikko M Metsis O S?der S Brene S Skog T H?kfelt E M Ritzén 《Molecular and cellular biology》1990,10(9):4701-4711
The gene encoding glutamic acid decarboxylase (GAD), the key enzyme in the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid, is shown to be expressed in the testis of several different species. Nucleotide sequence analysis of a cDNA clone isolated from the human testis confirmed the presence of GAD mRNA in the testis. The major GAD mRNA in the testis was 2.5 kilobases. Smaller amounts of a 3.7-kilobase mRNA with the same size as GAD mRNA in the brain was also detected in the testis. In situ hybridization using a GAD-specific probe revealed GAD mRNA expressing spermatocytes and spermatids located in the middle part of rat seminiferous tubules. Studies on the ontogeny of GAD mRNA expression showed low levels of GAD mRNA in testes of prepubertal rats, with increasing levels as sexual maturation is reached, compatible with GAD mRNA expression in germ cells. In agreement with this, fractionation of cells from the rat seminiferous epithelium followed by Northern (RNA) blot analysis showed the highest levels of GAD mRNA associated with spermatocytes and spermatids. Evidence for the presence of GAD protein in the rat testis was obtained from the demonstration of GAD-like immunoreactivity in seminiferous tubules, predominantly at a position where spermatids and spermatozoa are found. Furthermore, GAD-like immunoreactivity was seen in the midpiece of ejaculated human spermatozoa, the part that is responsible for generating energy for spermatozoan motility. 相似文献
875.
Evidence for a stem cell-specific repressor of Moloney murine leukemia virus expression in embryonal carcinoma cells. 总被引:21,自引:5,他引:16
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)
A negative regulatory element (NRE) spanning the tRNA primer-binding site (PBS) of Moloney murine leukemia virus (M-MuLV) mediates repression of M-MuLV expression specifically in embryonal carcinoma (EC) cells. We precisely defined the element by base-pair mutagenesis to an 18-base-pair segment of the tRNA PBS and showed that the element also restricted expression when moved upstream of the long terminal repeat. A DNA-binding activity specific for the M-MuLV NRE was detected in vitro by using crude EC nuclear extracts in exonuclease III protection assays. Binding was strongly correlated with repression in EC cells. Mutations within the NRE that relieved repression disrupted binding activity. Also, nuclear extracts prepared from permissive, differentiated EC cell cultures showed reduced binding activity for the NRE. These results indicate the presence of a stem cell-specific repressor that extinguishes M-MuLV expression via the NRE at the tRNA PBS. 相似文献
876.
Human bisphosphoglycerate mutase expressed in E coli: purification, characterization and structure studies 总被引:1,自引:0,他引:1
Bisphosphoglycerate mutase (EC 5.4.2.4.) is an erythrocyte-specific enzyme whose main function is to synthesize 2,3-diphosphoglycerate (glycerate-2,3-P2) an effector of the delivery of O2 in the tissues. In addition to its main synthase activity the enzyme displays phosphatase and mutase activities both involving 2,3-diphosphoglycerate in their reaction. Using a prokaryotic expression system, we have developed a recombinant system producing human bisphosphoglycerate mutase in E coli. The expressed enzyme has been extracted and purified to homogeneity by 2 chromatographic steps. Purity of this enzyme was checked with sodium dodecyl sulfate polyacrylamide gel and Cellogel electrophoresis and structural studies. The bisphosphoglycerate mutase expressed in E coli was found to be very similar to that of human erythrocytes and showed identical trifunctionality, thermostability, immunological and kinetics' properties. However, the absence of a blocking agent on the N-terminus results in a slight difference of the electrophoretic mobility of the enzyme expressed in E coli compared to that of the erythrocyte. 相似文献
877.
Terry J. Housh Herbert A. deVries Glen O. Johnson Sharon A. Evans Gerald D. Tharp Dona J. Housh Rommie J. Hughes 《European journal of applied physiology and occupational physiology》1990,60(5):391-394
The purpose of this investigation was to determine the effect of glycogen depletion and supercompensation on the physical working capacity at the fatigue threshold (PWCFT). Ten adult males (mean age 23 years, SD 3) volunteered as subjects for this study. During the first laboratory visit the subjects performed a maximal bicycle ergometer test for the determination of maximum oxygen consumption (VO2max). Between 48 and 72 h later, the subjects pedaled to exhaustion at a power output which corresponded to a mean of 76% of VO2max (range, 72-80%) for the purpose of glycogen depletion. For the next 3 days, the subjects were fed a 10.5 MJ.day-1 low carbohydrate diet which consisted of 7.5% carbohydrates, 22.0% protein and 70.5% fat. The subjects then performed an incremental cycle ergometer test to the onset of fatigue or PWCFT, which was estimated from integrated electromyographic voltages of the vastus lateralis muscle. For the next 3 days the subjects were fed a 10.5 MJ high carbohydrate diet which consisted of 72.2% carbohydrates, 12.4% protein and 15.4% fats for the purpose of glycogen supercompensation. The subjects then performed a second PWCFT test. A paired t-test indicated that there was no significant (p greater than 0.05) difference between the means of the PWCFT values (depletion 246 W, SD 30; supercompensation 265 W, SD 28) and they were highly correlated at r = 0.884. The results of this investigation suggested that the methods commonly used to affect glycogen depletion or supercompensation had no effect on PWCFT. 相似文献
878.
Life expectancy curves have a characteristic ominous shape that has fascinated scientists for centuries. Medawar was the first to explain this shape, specifically the steeply rising proneness of an average individual to die as a function of age, in evolutionary terms. The idea was that the "selective value" of the individual decreases as it has triggered other individuals taking its place (and carrying its genes) into existence. We demonstrate that this idea can be turned into a quantitative model. The resulting 4-parameter function reproduces well two well-known life expectancy curves from the first half of this century. Moreover, the easily interpretable parameters (3 of the 4) seem intuitively reasonable. 相似文献
879.
Distamycin inhibition of topoisomerase I-DNA interaction: a mechanistic analysis. 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
U H Mortensen T Stevnsner S Krogh K Olesen O Westergaard B J Bonven 《Nucleic acids research》1990,18(8):1983-1989
Inhibition of eukaryotic DNA topoisomerase I by the minor groove binding ligand, distamycin A, was investigated. Low concentrations of the ligand selectively prevented catalytic action at a high affinity topoisomerase I binding sequence. A restriction enzyme protection assay indicated that the catalytic cycle was blocked at the binding step. Distamycin binding sites on DNA were localized by hydroxyl radical footprinting. A strongly preferred site mapped to a homopolymeric (dA).(dT)-tract partially included in the essential topoisomerase I binding region. Mutational elimination of the stable helix curvature associated with this ligand binding site demonstrated that (i) the intrinsic bend was unessential for efficient binding of topoisomerase I, and (ii) distamycin inhibition did not occur by deformation of a stable band. Alternative modes of inhibition are discussed. 相似文献
880.