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   2193篇
  免费   934篇
  国内免费   525篇
  3652篇
  2025年   34篇
  2024年   76篇
  2023年   70篇
  2022年   101篇
  2021年   92篇
  2020年   91篇
  2019年   92篇
  2018年   73篇
  2017年   99篇
  2016年   99篇
  2015年   106篇
  2014年   163篇
  2013年   150篇
  2012年   213篇
  2011年   178篇
  2010年   141篇
  2009年   181篇
  2008年   200篇
  2007年   201篇
  2006年   164篇
  2005年   135篇
  2004年   112篇
  2003年   130篇
  2002年   118篇
  2001年   118篇
  2000年   89篇
  1999年   73篇
  1998年   42篇
  1997年   39篇
  1996年   30篇
  1995年   20篇
  1994年   21篇
  1993年   16篇
  1992年   24篇
  1991年   13篇
  1990年   16篇
  1989年   12篇
  1988年   8篇
  1987年   13篇
  1986年   13篇
  1985年   8篇
  1984年   12篇
  1983年   9篇
  1982年   6篇
  1981年   6篇
  1980年   6篇
  1977年   4篇
  1974年   5篇
  1973年   4篇
  1972年   4篇
排序方式: 共有3652条查询结果,搜索用时 0 毫秒
1.
The aging of T cell memory is often considered in terms of senescence, a process viewed as decay and loss of memory T cells. How senescence would affect memory is a function of the initial structure of the memory repertoire and whether the clonotypes that make up the repertoire decay at random. We examine this issue using the T cell memory generated to the conserved influenza A epitope M1(58-66), which induces a strong, focused, but polyclonal CD8 T cell response in HLA-A2 individuals. We analyzed the CD8 T cell memory repertoires in eight healthy middle-aged and eight healthy older blood donors representing an average age difference of ~ 27 y. Although the repertoires show broadly similar clonotype distributions, the number of observable clonotypes decreases significantly. This decrease disproportionally affects low-frequency clonotypes. Rank frequency analysis shows the same two-component clonotype distribution described earlier for these repertoires. The first component includes lower frequency clonotypes for which distribution can be described by a power law. The slope of this first component is significantly steeper in the older cohort. Generating a representative repertoire for each healthy cohort allowed agent-based modeling of the aging process. Interestingly, simple senescence of middle-aged repertoires is insufficient to describe the older clonotype distribution. Rather, a selective clonotype expansion must be included to achieve the best fit. We propose that responses to periodic virus exposure may drive such expansion, ensuring that the remaining clonotypes are optimized for continued protection.  相似文献   
2.
3.
Microsporidia is a large group of fungi-related unicellular eukaryotes with obligate intracellular lifestyle infecting a wide range of invertebrate and vertebrate hosts. Long adaptation of the parasites to intracellular development resulted in extraordinary minimization of their metabolic system. The paper summarizes the original results and literature data on the study of microsporidian carbohydrate and energy metabolism. On the basis of the material, it is concluded that minimization of microsporidian cell machinery was accompanied by the acquisition of a number of unique characteristics, which were not found in other eukaryotes.  相似文献   
4.
5.
Peculiarities of forming emission flows of carbon dioxide and methane in forest-steppe raised Sphagnum bogs have been studied in view of coenotic structure and damage to the vegetation as a result of fire. The reasons of relatively low rate of methane emission in pine-dwarf shrub Sphagnum bogs under the marginal conditions have been analyzed. The concentrations of CH4 and CO2 in bog waters have been presented.  相似文献   
6.
Kinetic and steady-state characteristics of sodium channels of the Ranvier node membrane, modified by aconitine, were investigated by the voltage clamp method. Channels modified by aconitine were shown to be only partially inactivated. Dependence of the fraction of uninactivated channels on voltage can be described by a model of a channel with three states: closed, open, and inactivated. It is suggested that aconitine does not significantly change the parameters of the inactivated state. Repolarization of the membrane to between –70 and –110 mV after a long (10 msec) test shift of potential induces initially a rapid decline of the current ("tail"), followed by a slow rise to the steady-state value. To explain the kinetics of this current it is necessary to postulate two or more open states of the channel.Institute of Cytology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 12, No. 6, pp. 612–618, November–December, 1980.  相似文献   
7.
A comparative study of Lachancea kluyveri strains isolated in Europe, North America, Japan, and the Russian Far East was performed using restriction analysis, sequencing of non-coding rDNA regions, molecular karyotyping, and the phylogenetic analysis of the α-galactosidase MEL genes. This study showed a close genetic relatedness of these L. kluyveri strains. The chromosomal DNAs of the L. kluyveri strains were found to range in size from 980 to 3100 kb. The haploid number of chromosomes is equal to eight. The IGS2 restriction patterns and single nucleotide substitutions in the ITS1/ITS2 rDNA region correlate neither with geographic origin nor with the source of the strains. The L. kluyveri strains isolated from different sources have a high degree of homology (79–100%) of their MEL genes. The phylogenetic analysis of all of the known α-galactosidases in the “Saccharomyces” clade showed that the MEL genes of the yeasts L. kluyveri, L. cidri, Saccharomyces cerevisiae, S. paradoxus, S. bayanus, and S. mikatae are species specific.  相似文献   
8.
9.
    
During August 2019, Paleolithic archaeological survey was carried out in the Lushi Basin in the middle of South Luohe River, central China, 27 new Paleolithic open-air sites were identified and 123 lithic artifacts were collected. These sites are distributed on the first to fourth terrace of the South Luohe River, in situ lithic artifacts that buried in loess deposits have been found in some sites.  相似文献   
10.
Amyloid oligomers, protofibrils, and fibrils of various amyloidogenic proteins are known to induce cell death. Tetracycline prevents the formation of fibrils of Aβ peptide and other amyloidogenic proteins and decomposes mature fibrils. It was previously shown that sarcomeric cytoskeletal proteins of the titin family (protein X, protein C, and protein H) in vitro form amyloid fibrils and tetracycline decomposes them. In this work, the concentration and time dependence of the survival of polymorphonuclear leukocytes in the presence of protein X amyloid fibrils is demonstrated. It is also shown that the survival rate increases as fibrils are decomposed by tetracycline. The antibiotic itself is found to be nontoxic. The results obtained show that this approach can be used to evaluate the efficiency of drugs that prevent or rectify amyloidoses.  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