全文获取类型
收费全文 | 11642篇 |
免费 | 1060篇 |
国内免费 | 1146篇 |
出版年
2024年 | 38篇 |
2023年 | 177篇 |
2022年 | 402篇 |
2021年 | 669篇 |
2020年 | 474篇 |
2019年 | 556篇 |
2018年 | 523篇 |
2017年 | 398篇 |
2016年 | 524篇 |
2015年 | 766篇 |
2014年 | 921篇 |
2013年 | 919篇 |
2012年 | 1087篇 |
2011年 | 979篇 |
2010年 | 554篇 |
2009年 | 537篇 |
2008年 | 618篇 |
2007年 | 531篇 |
2006年 | 418篇 |
2005年 | 332篇 |
2004年 | 317篇 |
2003年 | 268篇 |
2002年 | 258篇 |
2001年 | 187篇 |
2000年 | 167篇 |
1999年 | 154篇 |
1998年 | 109篇 |
1997年 | 97篇 |
1996年 | 101篇 |
1995年 | 73篇 |
1994年 | 83篇 |
1993年 | 59篇 |
1992年 | 69篇 |
1991年 | 72篇 |
1990年 | 51篇 |
1989年 | 34篇 |
1988年 | 42篇 |
1987年 | 23篇 |
1986年 | 31篇 |
1985年 | 31篇 |
1984年 | 20篇 |
1983年 | 26篇 |
1982年 | 16篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1977年 | 11篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 11篇 |
1970年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
21.
Wei de Zhang Fan Qing Li Xiao Yun Zhang Ying Chen 《Biological trace element research》1988,17(1):81-90
The growth of the protozoanBlepherisma is stimulated by Lanthanum (La) at concentrations as low as 0.32 ppm. In mice Yttrium (Y) and Ytterbium (Yb) are absorbed,
accumulated, and metabolized. Both rare earth elements (RE) exhibit a high affinity for teeth and bones, accumulation occurs
and metabolism is slow. In the livers of RE-exposed mice, concentrations are variable. The liver is apparently capable of
absorbing and discharging RE in a manner depending on metabolic activity. The main route of discharge for ingested REs is
the alimentary canal. Exposure of pregnant mice to RE leads to rapid placental transfer of RE; 14.1% of the total amount of
RE administered was detected in newborn mice. Young, developing organisms appear to be especially susceptible to RE accumulation. 相似文献
22.
Peptidyl-glycine alpha-amidation enzyme activity has been measured in 36 nuclei or areas in the rat CNS and pituitary using D-Tyr-Phe-Gly as the substrate. The distribution of this enzyme is highly uneven, with highest activity levels (greater than 30 pmol/mg of protein/h) in hypothalamic nuclei, substantia grisea centralis, and nucleus ruber; moderate activity levels (10-30 pmol/mg of protein/h) in globus pallidus, septum, midbrain, pons, medulla oblongata, and cervical spinal cord; and low activity levels (1-10 pmol/mg of protein/h) in other telencephalic and thalamic structures. Almost no alpha-amidation activity (less than 0.5 pmol/mg of protein/h) was detected in cerebellar cortex. The Km values in several brain regions are of the same order. 相似文献
23.
B K Brightman K G Chandy R H Spencer S Gupta P K Pattengale H Fan 《Journal of immunology (Baltimore, Md. : 1950)》1988,141(8):2844-2854
Lymphoid tumors induced by a recombinant murine retrovirus carrying the v-myc oncogene of avian MC29 virus were characterized. The Moloney murine leukemia virus myc oncogene (M-MuLV (myc], carried by an amphotropic MuLV helper, induced tumors in NIH Swiss and NFS/N mice after a relatively long latency (8 to 24 wk). Tumor masses appeared in the thymus, spleen, and lymph nodes. Flow cytometry of the tumor cells indicated that approximately 50% were positive for Thy 1.2. Most of these tumors also expressed one or more other cell surface markers of thymocytes and mature T cells (CD4, CD8). Southern blot hybridization revealed genomic rearrangements for the TCR beta genes. The TCR beta analysis suggested that the M-MuLV(myc)-induced Thy 1.2+ tumors were derived from somewhat less mature cells than tumors induced by M-MuLV, which is a classical non-acute retrovirus lacking an oncogene. The remainder of the M-MuLV(myc)-induced tumors were Thy 1.2-, but they were positive for Ly-5 (B220) and also for MAC-2. The Thy 1.2- tumors were characteristically located in the thymus. However, they were negative for TCR beta gene rearrangements. Some, but not all, of the Thy 1.2- tumors contained rearrangements for Ig genes. Additionally, they typically expressed mRNA specific for B but not for T cells. Thus, these thymic tumors had characteristics of the B cell lineage. Tumor transplantation experiments demonstrated that the Thy 1.2- tumor cells could reestablish in the thymus and spleen of irradiated hosts, and low level expression of the Thy 1 molecule was observed in the thymus but not the spleen on the first passage. After serial passage, one Thy 1- tumor altered its cell surface phenotype to Thy 1low B220-. 相似文献
24.
Genetic organization and nucleotide sequence of the stability locus of IncFII plasmid NR1 总被引:14,自引:0,他引:14
A Tabuchi Y N Min C K Kim Y L Fan D D Womble R H Rownd 《Journal of molecular biology》1988,202(3):511-525
The stability (stb) locus of IncFII plasmid NR1 was mapped to a 1700 base-pair NaeI-TaqI restriction fragment. A series of unstable plasmids that contained insertion, deletion, and point mutations that inactivated the stability function was isolated. The unstable point mutants examined were all stabilized (complemented) in trans by a copy of the wild-type stb locus, suggesting that the mutations had inactivated diffusible gene products. The nucleotide sequence of the stb locus contained two tandem open reading frames, designated stbA and stbB, that encoded essential trans-acting protein products with predicted sizes of 36,000 Mr and 13,000 Mr, respectively. A third open reading frame, stbC, that could encode a peptide of 8000 Mr was contained within stbB in the complementary DNA strand. Plasmid-encoded proteins of 36,000 Mr and 13,000 Mr were identified in minicell experiments as the products of stbA and stbB, respectively. Unstable deletion mutants that retained the promoter proximal region of the stb locus upstream from stbA but had deleted both stbA and stbB were stabilized in trans by plasmids that could supply StbA and StbB. In contrast, deletion mutants that had lost the stbAB promoter region were not complemented in trans, indicating that this region contained an essential cis-acting site (or sites). Unlike some other loci that mediate stable plasmid inheritance, cloned copies of the wild-type stb locus of NR1 did not exert strong incompatibility (i.e. trans destabilization) against other stb+ derivatives of plasmid NR1 present in the same cell. 相似文献
25.
26.
鼻蝇亚科(Rhiniinae)是双翅目丽蝇科(Calliphoridae)中比较特殊的一大类群,因此也有人把它从丽蝇科分出,独立成科的。其特征有口上片突出如鼻状,后头上部大半是裸出的,约占整个头的宽度的范围内连粉被也缺如;r脉的上方沿后侧有一列明晰的小刚毛,翅下大结节上无立纤毛,下腋瓣裸。主要分布在东洋区、非洲区、大洋洲区以及古北区南缘。目前全世界已知有22个属,近300种,曾有报道其幼虫寄生于白蚁巢内及蝗 相似文献
27.
Culture and characterization of dental follicle cells from rat molars 总被引:12,自引:0,他引:12
Summary Because the dental follicle is necessary for the eruption of teeth of limited eruption, it was the objective of this study to determine if the cells of the follicle could be cultured in vitro. To achieve this, dental follicles and associated enamel organs were dissected from the first and second mandibular molars of 6–7-day-old rats (secretory stage of amelogenesis), and then cultured in a medium that promotes fibroblast growth — the predominant cell type of the dental follicle. The cultured cells grew to confluency and were kept through 3 passages before experimentation. The cultured cells were fibroblastic in shape, elongate with processes, and transmission electron microscopy revealed that they contained an abundant rough endoplasmic reticulum, but did not form desmosomes. Immunofluorescent staining for anti-vimentin showed that all the cells stained and electron-microscopic immunogold labeling indicated that the antibody was associated with intermediate filaments. As revealed by SDS-polyacrylamide gel electrophoresis and Western blotting, the cultured cells synthesized and secreted the extracellular matrix molecules fibronectin and procollagens. Subsequent immunofluorescence staining of permeabilized and non-permeabilized cells confirmed the presence of fibronectin and type I collagen both intra- and extracellularly. Thus, based on all the above characteristics, the cultured cells appeared to be fibroblasts derived from the dental follicle, although a few of the fibroblasts may be derived from undifferentiated mesenchymal cells interposed between the alveolar bone and follicle. Experiments now can be conducted to determine how these cultured cells respond directly to growth factors that alter the rates of tooth eruption. 相似文献
28.
鹅掌楸种子和胚胎发育的研究 总被引:23,自引:0,他引:23
应用控制授粉、软 X-射线法、常规石蜡制片法和荧光检测等手段,研究了鹅掌楸(Lirio-dendron chinense(Hemsl.)Sarg.胚胎发育和控制授粉与结籽率的相关性。控制授粉后2小时花粉萌发,6小时萌发率最高,柱头可授期持续30小时左右。花粉管借助于柱头毛之间的分泌物进入柱头沟,经花柱沟、珠孔塞和珠心冠原进入胚囊,行珠孔受精。授粉后2周,胚乳为2至3细胞厚的狭组织;第6周,胚乳充满胚囊腔,珠心随之解体殆尽;第7到8周,球形胚、心形胚发生;第14到16周,子叶形成;第22周种子或熟,胚乳丰富。单株自然授粉结籽率不足1%。控制授粉后,单个聚合果的最高结籽率可达39%,9个聚合果的平均结籽率为17.7%。 相似文献
29.
用相关和回归处理方法,研究了8条正常狗咽喉部高频喷射通气时,调节驱动压、呼吸比和频率对喷气量、吸入气氧浓度、动脉血气及气道内压的作用。结果显示,驱动压和呼吸此对各观察指标几乎有同等重要的作用,频率的影响很小,喷气量与吸入气氧浓度、动脉血气、气道内压间存在显著的正相关关系。说明调节参数的意义主要在于改变了喷气量。 相似文献
30.
The rate of nuclear cytoplasmic protein transport is determined by the casein kinase II site flanking the nuclear localization sequence of the SV40 T-antigen. 总被引:58,自引:4,他引:54 下载免费PDF全文
We have previously demonstrated [Rihs, H.-P. and Peters, R. (1989) EMBO J., 8, 1479-1484] that the nuclear transport of recombinant proteins in which short fragments of the SV40 T-antigen are fused to the amino terminus of Escherichia coli beta-galactosidase is dependent on both the nuclear localization sequence (NLS, T-antigen residues 126-132) and a phosphorylation-site-containing sequence (T-antigen residues 111-125). While the NLS determines the specificity, the rate of transport is controlled by the phosphorylation-site-containing sequence. The present study furthers this observation and examines the role of the various phosphorylation sites. Purified, fluorescently labeled recombinant proteins were injected into the cytoplasm of Vero or hepatoma (HTC) cells and the kinetics of nuclear transport measured by laser microfluorimetry. By replacing serine and threonine residues known to be phosphorylated in vivo, we identified the casein kinase II (CK-II) site S111/S112 to be the determining factor in the enhancement of the transport. Either of the residues 111 or 112 was sufficient to elicit the maximum transport enhancement. The other phosphorylation sites (S120, S123, T124) had no influence on the transport rate. Examination of the literature suggested that many proteins harboring a nuclear localization sequence also contain putative CK-II sites at a distance of approximately 10-30 amino acid residues from the NLS. CK-II has been previously implicated in the transmission of growth signals to the nucleus. Our results suggest that CK-II may exert this role by controlling the rate of nuclear protein transport. 相似文献