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   647篇
  免费   84篇
  2021年   8篇
  2020年   11篇
  2019年   13篇
  2018年   12篇
  2017年   7篇
  2016年   21篇
  2015年   18篇
  2014年   27篇
  2013年   23篇
  2012年   33篇
  2011年   36篇
  2010年   30篇
  2009年   16篇
  2008年   23篇
  2007年   29篇
  2006年   27篇
  2005年   27篇
  2004年   23篇
  2003年   22篇
  2002年   18篇
  2001年   17篇
  2000年   23篇
  1999年   23篇
  1998年   8篇
  1997年   9篇
  1996年   10篇
  1995年   8篇
  1994年   6篇
  1993年   8篇
  1992年   18篇
  1991年   17篇
  1990年   20篇
  1989年   7篇
  1988年   14篇
  1987年   9篇
  1986年   9篇
  1985年   6篇
  1984年   5篇
  1983年   5篇
  1981年   9篇
  1979年   8篇
  1978年   11篇
  1977年   4篇
  1973年   8篇
  1972年   3篇
  1971年   4篇
  1970年   4篇
  1968年   3篇
  1967年   4篇
  1908年   4篇
排序方式: 共有731条查询结果,搜索用时 31 毫秒
1.
So far six susceptibility loci for renal cell carcinoma (RCC) have been discovered by genome-wide association studies (GWAS). To identify additional RCC common risk loci, we performed a meta-analysis of published GWAS (totalling 2,215 cases and 8,566 controls of Western-European background) with imputation using 1000 Genomes Project and UK10K Project data as reference panels and followed up the most significant association signals [22 single nucleotide polymorphisms (SNPs) and 3 indels in eight genomic regions] in 383 cases and 2,189 controls from The Cancer Genome Atlas (TCGA). A combined analysis identified a promising susceptibility locus mapping to 1q24.1 marked by the imputed SNP rs3845536 (P combined =2.30x10-8). Specifically, the signal maps to intron 4 of the ALDH9A1 gene (aldehyde dehydrogenase 9 family, member A1). We further evaluated this potential signal in 2,461 cases and 5,081 controls from the International Agency for Research on Cancer (IARC) GWAS of RCC cases and controls from multiple European regions. In contrast to earlier findings no association was shown in the IARC series (P=0.94; P combined =2.73x10-5). While variation at 1q24.1 represents a potential risk locus for RCC, future replication analyses are required to substantiate our observation.  相似文献   
2.
Larvae of the mayfly (Drunella grandis [Eaton]) from Diamond Fork Creek, Utah, were covered with a heavy growth of the sulfide-oxidizing bacterium Thiothrix. The bacterium did not seem to harm the mayfly, but the Thiothrix trichomes were parasitized by three morphologically distinct bacteria, two of which were cytoplasmic and one of which was probably periplasmic. At least two of the parasites destroyed the cytoplasmic contents of the Thiothrix sp., thus killing the host cell. Attempts to obtain the parasites in pure culture were unsuccessful.  相似文献   
3.
We report an improved method for white clover (Trifolium repens) transformation usingAgrobacterium tumefaciens. High efficiencies of transgenic plant production were achieved using cotyledons of imbibed mature seed. Transgenic plants were recovered routinely from over 50% of treated cotyledons. Thebar gene and phosphinothricin selection was shown to be a more effective selection system thannptII (kanamycin selection) oraadA (spectinomycin selection). White clover was transformed with the soybean auxin responsive promoter, GH3, fused to the GUS gene (-glucuronidase) to study the involvement of auxin in root development. Analysis of 12 independent transgenic plants showed that the location and pattern of GUS expression was consistent but the levels of expression varied. The level of GH3:GUS expression in untreated plants was enhanced specifically by auxin-treatment but the pattern of expression was not altered. Expression of the GH3:GUS fusion was not enhanced by other phytohormones. A consistent GUS expression pattern was evident in untreated plants presumably in response to endogenous auxin or to differences in auxin sensitivity in various clover tissues. In untreated plants, the pattern of GH3:GUS expression was consistent with physiological responses which are regarded as being auxin-mediated. For the first time it is shown that localised spots of GH3:GUS activity occurred in root cortical tissue opposite the sites where lateral roots subsequently were initiated. Newly formed lateral roots grew towards and through these islands of GH3:GUS expression, implying the importance of auxin in controlling lateral root development. Similarly, it is demonstrated for the first time that gravistimulated roots developed a rapid (within 1 h) induction of GH3:GUS activity in tissues on the non-elongating side of the responding root and this induction occurred concurrently with root curvature. These transgenic plants could be useful tools in determining the physiological and biochemical changes that occur during auxin-mediated responses.  相似文献   
4.
Psychrophilic bacteria were not detected in soil, mud, and water in the summer. In winter, they were present in water and mud and constituted a significant portion of the bacterial flora in lake water.  相似文献   
5.
1-Tributylstannyl-D-glucals, prepared from the corresponding 1-phenylsulfonyl-D-glucals, were coupled efficiently to various organic halides in the presence of a palladium(0) catalyst. This mild reaction is specially useful for the preparation of 1-C-aryl-D-glucals and compatible with unprotected hydroxy groups or hindered aromatic bromides. It has been shown that the resulting 1-C-aryl(alkyl)-D-glucals are suited for further synthetic manipulation of the enol ether group, including stereoselective hydrogenation, hydroboration-oxidation, or epoxidation. All compounds formed resulted from the attack of the alpha-face of the glucal derivatives by the reagent. The reaction, extended to 1,3-, 1,4-di-, and 1,3,5-tri-bromobenzenes, leads to the corresponding symmetrical di-(tri)-C-glucosylbenzenes. Finally, a sequential di-C-glucosylation of 1,3-dibromobenzene with two different 1-stannylated glucals was obtained.  相似文献   
6.
稻米直链淀粉是在由蜡质基因Wx编码的颗粒结合淀粉合成酶(GBSS)的催化下合成的。最近,在Wx基因的区段内发现了一段多态性微卫星序列(CT)n。对74个非糯籼稻和粳稻材料的(CT)n多态性进行了分析,并探讨了其与直链淀粉含量之间的关系。在74个品种(系)中共发现7种(CT)n片段(Wx等位基因),即(CT)8,(CF)10,(CT)11,(CT)16,(CT)17,(CT)18,(Ch)19。在籼粳亚种间,不同(CT)n的分布存在差异较大:在籼稻中,以(CT)11和(CT)18为主,占92.6%,另有(CT)10和(CT)8各2份,(CT)17型1份;在粳稻中,以(CT)16、(CT)17为主,共占20份材料中的90.0%。在上述74个品种(系)中,以(CT)n表示的Wx基因型对稻米直链淀粉含量的决定系数R2达0.912,也即Wx基因型差异可解释这些材料直链淀粉含量变异的91.2%。还发现6份籼稻材料Wx座位上为杂合的(CT)18/(CT)11,其中2份为推广早籼优质品种浙9248和舟优903,并对其在遗传和育种研究中的意义作了探讨。  相似文献   
7.
Our laboratory has previously developed scaffoldless engineered bone constructs (EBC). Bone marrow stromal cells (BMSC) were harvested from rat femur and cultured in medium that induced osteogenic differentiation. After reaching confluence, the monolayer of cells contracted around two constraint points forming a cylinder. EBCs were placed in small diameter (0.5905 × 0.0625 in.) or large diameter (0.5905 × 0.125 in.) silicone tubing and implanted intramuscularly in the hind limb of a rat. Bone mineral content (BMC) of the EBC was analyzed before implantation and at 1 and 2 mo following implantation and compared to that of native femur bone at different stages of development. Negligible BMC was observed in E-20 femur or EBCs prior to implantation. One-month implantation in both small and large tubing increased BMC in the EBC. BMC of EBC from large tubing was greater than in 14 d rat neonatal femurs, but was 2% and 3% of BMC content in adult bone after 1 and 2 mo of implantation, respectively. Alizarine Red and osteopontin staining of the EBCs before and after implantation confirmed increased bone mineralization in the implanted EBCs. Implanted EBCs also had extensive vascularization. Our data suggest that BMSC can be successfully used for the generation of scaffoldless EBC, and this model can be potentially used for the generation of autologous bone transplants in humans.  相似文献   
8.
The human type I keratins K16 and K14 are coexpressed in a number of epithelial tissues, including esophagus, tongue, and hair follicles. We determined that two genes encoding K16 and three genes encoding K14 were clustered in two distinct segments of chromosome 17. The genes within each cluster were tightly linked, and large parts of the genome containing these genes have been recently duplicated. The sequences of the two K16 genes showed striking homology not only within the coding sequences, but also within the intron positions and sequences and extending at least 400 base pairs 5' upstream and 850 base pairs 3' downstream from these genes. Despite the strong homologies between these two genes, only one of the genes encoded a protein which assembled into keratin filaments when introduced into simple epithelial cells. While there were no obvious abnormalities in the sequence of the other gene, its promoter seemed to be significantly weaker, and even a hybrid gene with the other gene's promoter gave rise to a much reduced mRNA level after gene transfection. To demonstrate that the functional K16 gene that we identified was in fact responsible for the K16 expressed in human tissues, we made a polyclonal antiserum which recognized our functional K16 gene product in both denatured and filamentous form and which was specific for bona fide human K16.  相似文献   
9.
A number of milk types and milk fractions were investigated as possible substitutes for serum in cell culture media. A filtrate of reconstituted nonfat dry milk showed promise. Culture fluids containing 5% of the nonfat dry milk filtrate were used to propagate primary and continuous cell cultures, and the cell growth from these cultures was compared with that of cells grown in a serum-containing medium. The nonfat dry milk filtrate-supplemented medium supported the growth of all epithelial cells tested, but two fibroblast-type cultures failed to replicate. Cells grown in the medium containing the milk filtrate grew slowly for 2 to 3 days and then propagated to confluency in 6 to 8 days. Viable cell counts of 9 days were comparable to those of serum-grown cells that had been propagated for 7 days. Cells grown in the milk filtrate could be split 1 to 4 when subcultures were prepared. Cell growth could be stimulated by refeeding on days 2 to 3 or by the addition of 30 microM 2-mercaptoethanol to the growth medium. Virus susceptibility and titer comparisons with poliovirus 1, coxsackievirus B2, echovirus 7, and herpes simplex virus indicated that approximately the same data were obtained when either the nonfat dry milk filtrate-treated or the serum-treated cells were studied. The nonfat dry milk filtrate is inexpensive, is easily prepared, and is a substitute for serum in epithelial cell culture media.  相似文献   
10.
Muscle mass is decreased with advancing age, likely due to altered regulation of muscle fiber size. This study was designed to investigate cellular mechanisms contributing to this process. Analysis of male Fischer 344 X Brown Norway rats at 6, 20, and 32 mo of age demonstrated that, even though significant atrophy had occurred in soleus muscle by old age, myofiber nuclear number did not change, resulting in a decreased myonuclear domain. Also, the number of centrally located nuclei was significantly elevated in soleus muscle of 32-mo-old rats, correlating with an increase in gene expression of MyoD and myogenin. Whereas total 5'-bromo-2'deoxyuridine (BrdU)-positive nuclei were decreased at older ages, BrdU-positive myofiber nuclei were increased. These results suggest that, with age, loss of muscle mass is accompanied by increased myofiber nuclear density that involves fusion of proliferative satellite cells, resembling ongoing regeneration. Interestingly, centrally located myofiber nuclei were not BrdU labeled. Rats were subjected to hindlimb suspension (HS) for 7 or 14 days and intermittent reloading during HS for 1 h each day (IR) to investigate how aging affects the response of soleus muscle to disuse and an atrophy-reducing intervention. After 14 days of HS, soleus muscle size was decreased to a similar extent at all three ages. However, myofiber nuclear number and the total number of BrdU-positive nuclei decreased with HS only in the young rats. IR was associated with an attenuation of atrophy in soleus muscles of 6- and 20- but not 32-mo-old rats. Furthermore, IR was associated with an increase in BrdU-positive myofiber nuclei only in young rats. These data indicate that altered satellite cell function with age contributes to the impaired response of soleus muscle to an intervention that attenuates muscle atrophy in young animals during imposed disuse.  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