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   641篇
  免费   42篇
  2021年   9篇
  2020年   4篇
  2018年   9篇
  2017年   6篇
  2016年   15篇
  2015年   14篇
  2014年   19篇
  2013年   35篇
  2012年   34篇
  2011年   27篇
  2010年   15篇
  2009年   17篇
  2008年   25篇
  2007年   25篇
  2006年   25篇
  2005年   32篇
  2004年   26篇
  2003年   37篇
  2002年   30篇
  2001年   26篇
  2000年   29篇
  1999年   17篇
  1998年   8篇
  1997年   10篇
  1996年   4篇
  1995年   7篇
  1994年   9篇
  1993年   6篇
  1992年   17篇
  1991年   15篇
  1990年   8篇
  1989年   12篇
  1988年   11篇
  1987年   5篇
  1986年   6篇
  1985年   9篇
  1983年   9篇
  1982年   3篇
  1980年   6篇
  1979年   7篇
  1978年   7篇
  1975年   6篇
  1974年   3篇
  1973年   3篇
  1972年   4篇
  1971年   4篇
  1970年   3篇
  1969年   3篇
  1967年   3篇
  1965年   4篇
排序方式: 共有683条查询结果,搜索用时 15 毫秒
121.
122.
123.
We report a Japanese patient with spinocerebellar ataxia type 31 (SCA31) and sporadic Creutzfeldt-Jakob disease (sCJD). A 52-year-old man developed progressive cognitive impairment after the appearance of cerebellar symptoms. Brain MR diffusion-weighted imaging (DWI) demonstrated a slowly expanding hyperintense lesion in the cerebral cortex. The patient was finally diagnosed as having both SCA31 and sCJD by identification of genetic mutations and by real-time quaking-induced conversion (RT-QUIC) analysis of the cerebrospinal fluid (CSF), respectively. Here, we report the clinical details of this rare combined case, with particular reference to the association between prion protein and the early onset of SCA31.  相似文献   
124.
Outcrossing and self‐fertilization are fundamental strategies of sexual reproduction, each with different evolutionary costs and benefits. Self‐fertilization is thought to be an evolutionary “dead‐end” strategy, beneficial in the short term but costly in the long term, resulting in self‐fertilizing species that occupy only the tips of phylogenetic trees. Here, we use volvocine green algae to investigate the evolution of self‐fertilization. We use ancestral‐state reconstructions to show that self‐fertilization has repeatedly evolved from outcrossing ancestors and that multiple reversals from selfing to outcrossing have occurred. We use three phylogenetic metrics to show that self‐fertilization is not restricted to the tips of the phylogenetic tree, a finding inconsistent with the view of self‐fertilization as a dead‐end strategy. We also find no evidence for higher extinction rates or lower speciation rates in selfing lineages. We find that self‐fertilizing species have significantly larger colonies than outcrossing species, suggesting the benefits of selfing may counteract the costs of increased size. We speculate that our macroevolutionary results on self‐fertilization (i.e., non‐tippy distribution, no decreased diversification rates) may be explained by the haploid‐dominant life cycle that occurs in volvocine algae, which may alter the costs and benefits of selfing.  相似文献   
125.
An intravenous injection of plasma-derived immunoglobulins is used for the treatment of severe infectious and autoimmune disorders. Despite of its clinical efficacy, precise mechanisms by which intravenous immunoglobulin (IVIg) suppresses proinflammatory immune response are still enigmatic. Here, we provide in vitro evidence that IVIg inhibits homeostatic proliferation of B cells accompanied by induction of their cell aggregation. The IVIg-driven suppression of B cell proliferation and induction of cell aggregation are both unaffected by treatment with a neutralizing antibody against low-affinity Fc receptors for IgG (CD16/FcγRIII and CD32/FcγRII), known cell surface ligands for IVIg. Our observations propose a new immunosuppressive action of IVIg, which directly acts on steady-state B cells to suppress their homeostatic expansion.  相似文献   
126.
Genetic and biochemical studies have provided convincing evidence that the 5' noncoding region (5' NCR) of hepatitis C virus (HCV) is highly conserved among viral isolates worldwide and that translation of HCV is directed by an internal ribosome entry site (IRES) located within the 5' NCR. We have investigated inhibition of HCV gene expression using antisense oligonucleotides complementary to the 5' NCR, translation initiation codon, and core protein coding sequences. Oligonucleotides were evaluated for activity after treatment of a human hepatocyte cell line expressing the HCV 5' NCR, core protein coding sequences, and the majority of the envelope gene (E1). More than 50 oligonucleotides were evaluated for inhibition of HCV RNA and protein expression. Two oligonucleotides, ISIS 6095, targeted to a stem-loop structure within the 5' NCR known to be important for IRES function, and ISIS 6547, targeted to sequences spanning the AUG used for initiation of HCV polyprotein translation, were found to be the most effective at inhibiting HCV gene expression. ISIS 6095 and 6547 caused concentration-dependent reductions in HCV RNA and protein levels, with 50% inhibitory concentrations of 0.1 to 0.2 microM. Reduction of RNA levels, and subsequently protein levels, by these phosphorothioate oligonucleotides was consistent with RNase H cleavage of RNA at the site of oligonucleotide hybridization. Chemically modified HCV antisense phosphodiester oligonucleotides were designed and evaluated for inhibition of core protein expression to identify oligonucleotides and HCV target sequences that do not require RNase H activity to inhibit expression. A uniformly modified 2'-methoxyethoxy phosphodiester antisense oligonucleotide complementary to the initiator AUG reduced HCV core protein levels as effectively as phosphorothioate oligonucleotide ISIS 6095 but without reducing HCV RNA levels. Results of our studies show that HCV gene expression is reduced by antisense oligonucleotides and demonstrate that it is feasible to design antisense oligonucleotide inhibitors of translation that do not require RNase H activation. The data demonstrate that chemically modified antisense oligonucleotides can be used as tools to identify important regulatory sequences and/or structures important for efficient translation of HCV.  相似文献   
127.
Two new mouse genes encoding proteins that belong to the yeast minichromosome maintenance (MCM) protein family, which is involved in the initiation of DNA replication, were isolated and their nucleotide sequence was determined. They were a putative CDC46/MCM5 homolog and a putative cdc21 homolog. About 30% amino acid identity was obtained between members in the family, and > 40% between the putative mouse and yeast homologs. The expression of these genes was cell-cycle specific at the late G1 to S phase. Immunochemical analyses showed the physical interaction between mouse P1MCM3 and CDC46 protein. These results suggest that MCM proteins function in co-ordination for DNA replication.  相似文献   
128.
Four related species in the unicellular volvocalean genus Carteria [C. crucifera Pascher, C. eugametos Mitra, C. inversa (Korshikov) Bourrelly and C. cerasiformis Nozaki et al.] were delineated on the basis of recent comparative light and electron microscopy of a large number of culture strains. However, the species thus delineated may not represent natural or monophyletic entities. In the present study, 1128 base pairs of the chloroplast protein-coding gene (large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase gene) from 12 Carteria strains representing the four species as well as from related volvocalean species were analyzed to elucidate the phylogenetic status of the taxonomic or morphologic species of Carteria. The sequence data showed that the 12 Carteria strains exhibit four robust monophyletic groups which are strictly consistent with the four taxonomic species. These results are discussed in relation to contrasting results found in other microalgal genera. It is concluded that phylogenetic analysis, based on DMA sequence data and comparative morphologic characterization of species and using a large number of culture strains, is essential to a natural system of microalgal species taxonomy.  相似文献   
129.
130.
Chlamydomonas (Cd.) is one of the largest but most polyphyletic genera of freshwater unicellular green algae. It consists of 400–600 morphological species and requires taxonomic revision. Toward reclassification, each morphologically defined classical subgenus (or subgroup) should be examined using culture strains. Chlamydomonas subg. Amphichloris is characterized by a central nucleus between two axial pyrenoids, however, the phylogenetic structure of this subgenus has yet to be examined using molecular data. Here, we examined 12 strains including six newly isolated strains, morphologically identified as Chlamydomonas subg. Amphichloris, using 18S rRNA gene phylogeny, light microscopy, and mitochondria fluorescent microscopy. Molecular phylogenetic analyses revealed three independent lineages of the subgenus, separated from the type species of Chlamydomonas, Cd. reinhardtii. These three lineages were further distinguished from each other by light and fluorescent microscopy—in particular by the morphology of the papillae, chloroplast surface, stigmata, and mitochondria—and are here assigned to three genera: Dangeardinia emend., Ixipapillifera gen. nov., and Rhysamphichloris gen. nov. Based on the molecular and morphological data, two to three species were recognized in each genus, including one new species, I. pauromitos. In addition, Cd. deasonii, which was previously assigned to subgroup “Pleiochloris,” was included in the genus Ixipapillifera as I. deasonii comb. nov.  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