全文获取类型
收费全文 | 16439篇 |
免费 | 1721篇 |
国内免费 | 4564篇 |
出版年
2024年 | 121篇 |
2023年 | 375篇 |
2022年 | 725篇 |
2021年 | 1012篇 |
2020年 | 810篇 |
2019年 | 929篇 |
2018年 | 716篇 |
2017年 | 586篇 |
2016年 | 711篇 |
2015年 | 1030篇 |
2014年 | 1354篇 |
2013年 | 1273篇 |
2012年 | 1606篇 |
2011年 | 1478篇 |
2010年 | 1029篇 |
2009年 | 933篇 |
2008年 | 1101篇 |
2007年 | 925篇 |
2006年 | 932篇 |
2005年 | 809篇 |
2004年 | 695篇 |
2003年 | 565篇 |
2002年 | 540篇 |
2001年 | 472篇 |
2000年 | 410篇 |
1999年 | 314篇 |
1998年 | 210篇 |
1997年 | 135篇 |
1996年 | 120篇 |
1995年 | 140篇 |
1994年 | 85篇 |
1993年 | 69篇 |
1992年 | 80篇 |
1991年 | 60篇 |
1990年 | 50篇 |
1989年 | 55篇 |
1988年 | 39篇 |
1987年 | 36篇 |
1986年 | 29篇 |
1985年 | 33篇 |
1984年 | 29篇 |
1983年 | 24篇 |
1982年 | 21篇 |
1981年 | 9篇 |
1979年 | 4篇 |
1977年 | 3篇 |
1957年 | 8篇 |
1954年 | 4篇 |
1951年 | 6篇 |
1950年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Liu R Cai J Hu X Tan M Qi Y German M Rubenstein J Sander M Qiu M 《Development (Cambridge, England)》2003,130(25):6221-6231
During early neural development, the Nkx6.1 homeodomain neural progenitor gene is specifically expressed in the ventral neural tube, and its activity is required for motoneuron generation in the spinal cord. We report that Nkx6.1 also controls oligodendrocyte development in the developing spinal cord, possibly by regulating Olig gene expression in the ventral neuroepithelium. In Nkx6.1 mutant spinal cords, expression of Olig2 in the motoneuron progenitor domain is diminished, and the generation and differentiation of oligodendrocytes are significantly delayed and reduced. The regulation of Olig gene expression by Nkx6.1 is stage dependent, as ectopic expression of Nkx6.1 in embryonic chicken spinal cord results in an induction of Olig2 expression at early stages, but an inhibition at later stages. Moreover, the regulation of Olig gene expression and oligodendrogenesis by Nkx6.1 also appears to be region specific. In the hindbrain, unlike in the spinal cord, Olig1 and Olig2 can be expressed both inside and outside the Nkx6.1-expressing domains and oligodendrogenesis in this region is not dependent on Nkx6.1 activity. 相似文献
962.
Fei X Zheng QH Liu X Wang JQ Sun HB Mock BH Stone KL Miller KD Sledge GW Hutchins GD 《Bioorganic & medicinal chemistry letters》2003,13(13):2217-2222
Novel matrix metalloproteinase (MMP) inhibitor radiotracers, (S)-3-methyl-2-(2',3',4'-methoxybiphenyl-4-sulfonylamino)-butyric acid [(11)C]methyl ester (1a-c), (S)-3-methyl-2-(2',3',4'-fluorobiphenyl-4-sulfonylamino)-butyric acid [(11)C]methyl ester (1d-f), and (S)-3-methyl-2-(4'-nitrobiphenyl-4-sulfonylamino)-butyric acid [(11)C]methyl ester (1g), a series of substituted biphenylsulfonamide derivatives, have been synthesized for evaluation as new potential positron emission tomography (PET) cancer imaging agents. 相似文献
963.
Cheung AW Danho W Swistok J Qi L Kurylko G Rowan K Yeon M Franco L Chu XJ Chen L Yagaloff K 《Bioorganic & medicinal chemistry letters》2003,13(1):133-137
Systematic substitution of His(6) residue using non-selective hMC4R pentapeptide agonist (Bu-His(6)-DPhe(7)-Arg(8)-Trp(9)-Gly(10)-NH(2)) as the template led to the identification of Bu-Atc(6)(2-aminotetraline-2-carboxylic acid)-DPhe(7)-Arg(8)-Trp(9)-Gly(10)-NH(2) which showed moderate selectivity towards hMC4R over hMC1R. Further SAR studies resulted in the discovery of Penta-5-BrAtc(6)-DPhe(7)-Arg(8)-Trp(9)-Gly(10)-NH(2) and Penta-5-Me(2)NAtc(6)-DPhe(7)-Arg(8)-Trp(9)-Gly(10)-NH(2) which are potent hMC4R agonists and are inactive in hMC1R, hMC3R and hMC5R agonist assays. 相似文献
964.
Mice lacking the Nkx6.2 (Gtx) homeodomain transcription factor develop and reproduce normally
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)
The Nkx homeobox genes are expressed in a variety of developing tissues and have been implicated in controlling tissue patterning and cell differentiation. Expression of Nkx6.2 (Gtx) was previously observed in the embryonic neural tube, testis, and differentiating oligodendrocytes. To investigate the role of Nkx6.2 in the control of cell specification and differentiation, we generated mice with null mutations in Nkx6.2 using the standard gene targeting approach. Null mutant mice were viable and fertile without apparent histological and immunohistochemical changes in the central nervous systems and testis. The absence of detectable phenotypes suggests a redundant function of Nkx6.2 in mouse development. 相似文献
965.
966.
鹟科五种鸟线粒体DNA序列变化与亲缘关系的研究 总被引:4,自引:1,他引:3
首次对鹟科(Muscicapidae)画眉亚科三种鸟----画眉(Garrulax canorus)、红嘴相思鸟(Leiothrix lutea)、棕头雅雀(Paradoxornis webbiana);鸫亚科的乌鸫(Turdinae merula)线粒体DNA的12S rRNA基因片段的DNA序列进行了测定,并与北美画眉亚科弯嘴鹛属(Pomatostomus)的灰冠弯嘴鹛(P.temporalis)同源序列进行分析比较并构建分子进化树。实验结果与传统形态学论述存在一定的差异,与同工酶研究结果相同:画眉与红嘴相思鸟的亲缘关系最近,棕头鸦雀较乌鸫与画眉亚科的关系更近,为将其置于画眉亚科提供了分子证据。
Abstract:The classfication on Muscicapinae disturbed us for a long time.The important thing is to find an appropriate character to resolve it.It is the first time that we testified the mitochondrial 12S rRNA gene fragment sequence of four birds (G.canorus,L.lutea,P.webbianus and T.merula-owing to Muscicapinae) and aligned with the same sequence of P.temporalis.Using the mitochondrial 12S rRNA sequences date,we built phylogenetic trees.The results indicated that that the relationship between Garrulax canorus and Leiothrix lutea in the four birds was closer which is congruent with morphology.According to the results of isoenzyme and DNA sequencing data,we can draw a conclusion that Paradoxornis webbianus has closer relationship to Timaliinae than that of Turdus merula,and that putting Paradoxonithinae to Timaliinae was reasonable. 相似文献
967.
Karbownik M Reiter RJ Garcia JJ Tan DX Qi W Manchester LC 《Biochimica et biophysica acta》2000,1523(2-3):140-146
Delta-aminolevulinic acid, precursor of heme, accumulates in a number of organs, especially in the liver, of patients with acute intermittent porphyria. The potential protective effect of melatonin against oxidative damage to nuclear DNA and microsomal and mitochondrial membranes in rat liver, caused by delta-aminolevulinic acid, was examined. Changes in 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels, an index of DNA damage, and alterations in membrane fluidity (the inverse of membrane rigidity) and lipid peroxidation in microsomal and mitochondrial membranes, as indices of damage to lipid and protein molecules in membranes, were estimated. Measurements were made in rat liver after a 2 week treatment with delta-aminolevulinic acid (40 mg/kg b.w., every other day). To test the potential protective effects of melatonin, the indole was injected (i.p. 10 mg/kg b.w.) 3 times daily for 2 weeks. 8-OHdG levels and lipid peroxidation in microsomal membranes increased significantly whereas microsomal and mitochondrial membrane fluidity decreased as a consequence of delta-aminolevulinic acid treatment. Melatonin completely counteracted the effects of delta-aminolevulinic acid. Melatonin was highly effective in protecting against oxidative damage to DNA as well as to microsomal and mitochondrial membranes in rat liver and it may be useful as a cotreatment in patients with acute intermittent porphyria. 相似文献
968.
969.
An isochromosome has identical arms attached to the same centromere. At the pachytene stage of meiosis, it has four isochromatids and recombination can occur either between cis isochromatids (attached to the same half-centromere) or trans isochromatids (attached to different half-centromeres). Normally such recombination cannot be detected because all four chromatids are homogenetic (arose from misdivision of a centromere to which genetically identical sister chromatids were attached). We isolated an isochromosome of wheat that is heterogenetic for the distal 64% of the long arm. The heterogenetic isochromosome was recovered from the progeny of a cross between Triticum aestivum cv. Chinese Spring containing an isochromosome for the long arm of chromosome 5B (i5BL) and a disomic substitution line of Triticum turgidum ssp. dicoccoides chromosome 5B in Chinese Spring wheat. New recombinants were produced when the two arms of i5BLrec paired at metaphase I of meiosis. Only trans isochromatid exchanges led to some homozygous loci in i5BLrec, whereas exchanges between cis isochromatids resulted in heterozygosity at all loci similar to the parental type. There was an average frequency of 0.87 chiasmata per pollen mother cell for the heterogenetic i5BL, which will result in 0.44 cis and 0.44 trans isochromatid exchanges, assuming that both are occurring at the same frequency. The average crossover frequency based on recombination between trans isochromatid exchange detected by restriction fragment length polymorphism analysis in 98 plants was 0.29. This observed value is significantly lower (P<0.01) than the value of 0.44 as expected from chiasmata counts. Our study provides the first experimental evidence that crossovers preferentially occur between cis isochromatids rather than trans isochromatids. 相似文献
970.