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   20654篇
  免费   1953篇
  国内免费   21篇
  2023年   88篇
  2022年   177篇
  2021年   466篇
  2020年   209篇
  2019年   314篇
  2018年   360篇
  2017年   316篇
  2016年   572篇
  2015年   992篇
  2014年   1090篇
  2013年   1285篇
  2012年   1720篇
  2011年   1675篇
  2010年   1107篇
  2009年   889篇
  2008年   1379篇
  2007年   1349篇
  2006年   1226篇
  2005年   1198篇
  2004年   1158篇
  2003年   1079篇
  2002年   1021篇
  2001年   175篇
  2000年   100篇
  1999年   205篇
  1998年   256篇
  1997年   156篇
  1996年   136篇
  1995年   124篇
  1994年   115篇
  1993年   113篇
  1992年   93篇
  1991年   93篇
  1990年   83篇
  1989年   66篇
  1988年   55篇
  1987年   62篇
  1986年   50篇
  1985年   65篇
  1984年   69篇
  1983年   73篇
  1982年   80篇
  1981年   75篇
  1980年   75篇
  1979年   36篇
  1978年   66篇
  1977年   43篇
  1976年   38篇
  1975年   39篇
  1973年   27篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
42.
43.
44.
Phylogenetic systematic analysis of 24 taxa representing the rhabdocoel platyhelminths, based on a suite of 89 morphological characters, produced two equally parsimonious trees, 181 steps long, with a consistency index (CI) of 0.69 and a rescaled consistency index (RCI) of 0.56, differing only with respect to that portion of the tree containing Umagillidae, Acholadidae, Graffillinae, Pseudograffillinae, Pterastericolidae and Hypoblepharinidae. Our results accommodate all previously proposed sister taxa to the Neodermata in a single clade in which ((Dalyelliidae + Temnocephalida) Typhloplanidae) is the sister group of ((Fecampiidae +  Urastoma ) ( Udonella ((Aspidogastrea + Digenea) (Monogenea (Gyrocotylidea (Amphilinidea + Eucestoda)))))). Bootstrap and jackknife analyses indicate that the groupings of ((Dalyelliidae + Temnocephalida) Typhloplanidae) and of ((Fecampiidae +  Urastoma ) ( Udonella ((Aspidogastrea + Digenea) (Monogenea (Gyrocotylidea (Amphilinidea + Eucestoda)))))) are highly robust, with the latter clade having a CI of 90% and RCI of 82%. Disagreements among previous analyses of these taxa have been due to the influence of missing data for critical characters in key taxa and differences in the taxa analysed, rather than any inherent weakness in the morphological data. Non-phylogenetic systematic approaches to homology assessment and misconceptions regarding phylogenetic systematic methodology are discussed. Recent analyses combining sequence data with a subset of approximately 60% of the morphological characters should be re-assessed using the entire morphological database. Even if Udonella is a monogenean, it is most parsimonious to suggest that the common ancestor of the Neodermata had a vertebrate–arthropod two-host life cycle.  相似文献   
45.
46.
We have studied the torsional elastic constant (alpha) of short DNA (27mer) oligomers of various sequence by fluorescence polarization anysotropy (FPA) measurements. The lowest alpha values were found in samples with sequence rich in AA dinucleotides or containing the alternating d(A-T) x d(A-T) motif. The torsional rigidity of our DNA samples was compared to that calculated according to the current values of twist angle fluctuations derived for ten dinucleotide steps by recent analyses of DNA crystal structure database. The values of torsional rigidity derived from crystals are higher than our experimental ones, obtained by FPA analysis, suggesting that packing force in crystals may notably hinder the dinucleotide twist angle fluctuations that occur in solution. This behaviour is more evident for samples containing AA, TA and AT steps. In all the samples there is about a twofold change of the alpha value in the 10-40 degrees C range. An activation enthalpy (Delta H (#)) of about 17.4 kJ mol(-1), on average, was obtained for the temperature dependence of eight of the ten samples studied. A correlation with the stacking energy is discussed.  相似文献   
47.
Summary.  Saruma henryi Oliver is described and illustrated. The distribution, ecology, taxonomy and cultural requirements of this unusual member of the Aristolochiaceae are discussed.  相似文献   
48.
Summary Q4, a bacteriophage isolated from soil, mediated the transduction of a number of unlinked markers in Rhodococcus erythropolis. Highest numbers of transductants were obtained at multiplicities of infection of over 100, transductants only being obtained because of the temperate nature of the phage. Under optimal conditions, transduction to prototrophy of auxotrophic markers was over 50 times the spontaneous reversion rate and transduction of some antibitic resistance markers was over 10 times the spontaneous mutation rate. Segregation of unselected, but linked, markers was observed and the phage was used to order loci in a three factor cross. The virus required magnesium ions. Highest phage titres and greatest transduction frequency were obtained with stationary phase cultures.  相似文献   
49.
Length variation of the ribosomal gene spacers of Drosophila melanogaster was studied. Analysis of 47 X chromosomal and 47 Y chromosomal linked rDNA arrays collected from five continents indicates that the arrays on the two chromosomes differ qualitatively. The Y-linked arrays from around the world share little or no similarity for either their overall length or the organization of their spacers. Most of the X-linked arrays do, however, share a major length spacer of 5.1 kb. In addition, those X-linked arrays that have a major 5.1-kb band have similar spacer organization as demonstrated by genomic DNA digestions with several restriction enzymes. These data strongly support the hypothesis that spacer length patterns on only X-linked genes are maintained primarily by natural selection.  相似文献   
50.
Species specificity of bacterial palindromic units   总被引:8,自引:0,他引:8  
We described previously a family of dispersed palindromic sequences highly repeated in Escherichia coli and Salmonella typhimurium genomes. These sequences, called PU (palindromic units), are located outside structural genes. We report here observations suggesting that PU may have a role in bacterial speciation.  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