全文获取类型
收费全文 | 12602篇 |
免费 | 757篇 |
国内免费 | 41篇 |
出版年
2024年 | 18篇 |
2023年 | 128篇 |
2022年 | 362篇 |
2021年 | 650篇 |
2020年 | 390篇 |
2019年 | 478篇 |
2018年 | 555篇 |
2017年 | 402篇 |
2016年 | 566篇 |
2015年 | 674篇 |
2014年 | 801篇 |
2013年 | 1005篇 |
2012年 | 1036篇 |
2011年 | 906篇 |
2010年 | 553篇 |
2009年 | 458篇 |
2008年 | 559篇 |
2007年 | 547篇 |
2006年 | 495篇 |
2005年 | 455篇 |
2004年 | 355篇 |
2003年 | 296篇 |
2002年 | 260篇 |
2001年 | 132篇 |
2000年 | 108篇 |
1999年 | 93篇 |
1998年 | 76篇 |
1997年 | 48篇 |
1996年 | 50篇 |
1995年 | 49篇 |
1994年 | 35篇 |
1993年 | 44篇 |
1992年 | 53篇 |
1991年 | 94篇 |
1990年 | 63篇 |
1989年 | 55篇 |
1988年 | 49篇 |
1987年 | 43篇 |
1986年 | 34篇 |
1985年 | 45篇 |
1984年 | 44篇 |
1983年 | 32篇 |
1982年 | 23篇 |
1981年 | 32篇 |
1980年 | 20篇 |
1979年 | 19篇 |
1978年 | 27篇 |
1977年 | 23篇 |
1976年 | 26篇 |
1975年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Oxacillin-hydrolysing beta-lactamases. A comparative analysis at nucleotide and amino acid sequence levels 总被引:3,自引:0,他引:3
We have extended the sequence of the OXA-2 beta-lactamase which together with S1 mapping has enabled us to identify the promoter site for this gene. This lies in a region that is found upstream from a variety of resistance genes on different plasmids; each gene appears to have been inserted at the same specific site and to be expressed from the same promoter. The ancestral plasmid thus appears to function as a natural expression vector. The sequence of the recombination site at the 5' end of the OXA-2 gene shows a marked similarity with the attP sequence of lambda. DNA-probe analysis confirmed that the OXA-2 and OXA-3 beta-lactamases are related, and indicated no similarity with other beta-lactamase genes. However, a comparison of amino acid sequences demonstrates that the OXA-2, OXA-1 and PSE-2 beta-lactamases show some similarities to the typical class A enzymes, especially in the central helical domain of the latter, which is largely responsible for forming the active site of the enzyme. The three oxacillinases also show marked amino acid sequence similarity with the product of a regulatory gene, blaR1, required for beta-lactamase induction in Bacillus licheniformis. 相似文献
32.
H R Prasanna K D Nakamura S F Ali M H Lu W Slikker R W Hart 《Biochemical and biophysical research communications》1989,160(1):217-221
The microsomal activation of the potent hepatocarcinogen aflatoxin B1 (AFB1) and the expression of selected protooncogenes were investigated in the livers of rats exposed to delta 9-tetrahydrocannabinol (THC). At equimolar levels of cytochrome P-450, the microsome-mediated binding of AFB1 to DNA was significantly lower (56% of the controls) in preparations from drug exposed rats. Hepatic expression of the c-k-ras protooncogene was 3-fold higher in THC exposed animals. These results suggest the possible occurrence of long lasting residual effects in the rats exposed to THC. 相似文献
33.
Generation of small mutation in large genomic fragments by homologous recombination: description of the technique and examples of its use 总被引:2,自引:0,他引:2
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)
M Tripodi S Perfumo R Ali L Amicone C Abbott R Cortese 《Nucleic acids research》1990,18(21):6247-6251
We have developed a technique of homologous recombination in bacteria which allows the mutagenesis of large genomic fragments cloned in cosmids. The desired mutation is first introduced into a plasmid clone and is then transferred to the appropriate cosmid clone by the means of double antibiotic selection coupled with phenotypic selection. We describe three different types of construct made by this technique. 相似文献
34.
Sarah Perreault M. Ather Ali Skúli Skúlason David L. G. Noakes 《Environmental Biology of Fishes》1990,27(2):131-137
Synopsis At high latitudes, such as in Iceland, the daily photoperiod varies from almost continuous darkness in winter to virtually constant light in summer. Previous studies of detailed retinal structure in vertebrates have shown significant daily and annual effects of photoperiod. We sampled arctic charr in Iceland during the summer, including fish that were both light- and dark-adapted, during both day and night. We observed retinomotor responses characteristic of light- and dark-adaptation, but found no difference in the number of synaptic ribbons in the retina. The morpho-physiological changes, appearing as retinomotor responses, are thus not expressed at the synaptic level. 相似文献
35.
Frank Arnal Lucien J. Coté Sara Ginsburg Glen D. Lawrence Ali Naini Mary Sano 《Neurochemical research》1990,15(6):587-591
We have synthesized the tertiary amines of pyridostigmine and neostigmine, 3-pyridinol dimethylcarbamate (norpyridostigmine) and 3-dimethylaminophenol dimethylcarbamate (norneostigmine) respectively, and we have tested their abilities to cross the blood-brain barrier and inhibit mouse brainAChE activity. The in vivo inhibition of AChE activity by norpyridostigmine reaches 72% at 10 minutes which is comparable to that seen with physostigmine (73% at 10 minutes). Inhibition by norneostigmine is less effective (50% at 10 minutes) and approaches that obtained with tetrahydroaminoacridine (57% at 10 minutes). These data show that both norpyridostigmine and norneostigmine cross the blood-brain barrier and that they are effective inhibitors of mouse brain AChE activity. These drugs could be useful in the treatment of memory, impairment associated with Alzheimer's disease, and other memory disorders. 相似文献
36.
Expression of two forms of prolactin receptor in rat ovary and liver 总被引:12,自引:0,他引:12
37.
Octanoylation of the lipoyl domains of the pyruvate dehydrogenase complex in a lipoyl-deficient strain of Escherichia coli 总被引:5,自引:0,他引:5
S. T. Ali A. J. G. Moir P. R. Ashton P. C. Engel J. R. Guest 《Molecular microbiology》1990,4(6):943-950
The overexpression of a subgene encoding a hybrid lipoyl domain of the dihydrolipoamide acetyltransferase component of the pyruvate dehydrogenase complex of Escherichia coli has previously been shown to result in the formation of lipoylated and unlipoylated products. Overexpression of the same subgene in a lipoic acid biosynthesis mutant growing under lipoate-deficient conditions has now been shown to produce domains modified by octanoylation as well as unmodified domains. It was concluded from the mass of a lipoyl-binding-site peptide that the modification involves N6-octanoylation of the lysine residue (Lys244) that is normally lipoylated, and this was confirmed by the trypsin-insensitivity of the corresponding Lys244-Ala-245 bond, and the absence of modification in a mutant domain in which Lys244 is replaced by Gln. This novel protein modification raises interesting questions concerning the pathway of lipoic acid biosynthesis and the mechanism of enzyme lipoylation. 相似文献
38.
39.
Intrinsic polymorphism of variable number tandem repeat loci in the human genome. 总被引:1,自引:0,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)
In the human genome, short tandem repetitive (STR) DNA sequences often show restriction fragment length polymorphisms (RFLPs) due to variation in the number of copies of the repeat unit. For a subset of these sequences known as minisatellites or variable number tandem repeat loci (VNTR), it has been proposed that a homologous "core" sequence of 10-12 nucleotides is involved in the mechanism(s) generating the polymorphism. In our present study we have prepared oligonucleotide probes complementary to one or two repeat units of several VNTR loci. Under stringent hybridization and wash conditions these probes hybridize locus specifically thus allowing the evaluation of the intrinsic polymorphism of individual loci. Our results indicate that not all of the loci having STR DNA sequences are polymorphic despite the fact that they share the "core" sequence. This suggests that more than the DNA sequence of the locus is involved in the mechanism(s) generating the polymorphism. 相似文献
40.
A Farag S F Bottoms E F Mammen M Hosni A Ali 《Folia haematologica (Leipzig, Germany : 1928)》1988,115(3):340-345
Selected hemostasis parameters were assayed in 131 women ingesting different kinds of oral contraceptives and 36 control women. No differences were noted in the plasma levels of fibrinogen, antithrombin III, prekallikrein, alpha 2 antiplasmin, fibrinopeptide A, platelet factor 4 and beta-thromboglobulin. Protein C and plasminogen levels were significantly higher in pill users (p less than 0.001, p less than 0.01) and fibronectin levels were lower (p less than 0.05). Canonical correlation, performed between clinical parameters and hemostasis data, revealed a negative correlation between antithrombin III levels and family history for thromboembolic diseases. A positive correlation was noted between fibrinogen and fibronectin levels in obese OC users. The data suggest that women taking OCs are not in a state of "hypercoagulability." 相似文献