首页 | 本学科首页   官方微博 | 高级检索  
  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   10805篇
  国内免费   248篇
  完全免费   1508篇
  2018年   181篇
  2017年   79篇
  2016年   62篇
  2015年   66篇
  2014年   636篇
  2013年   704篇
  2012年   728篇
  2011年   671篇
  2010年   548篇
  2009年   844篇
  2008年   942篇
  2007年   923篇
  2006年   838篇
  2005年   817篇
  2004年   605篇
  2003年   594篇
  2002年   489篇
  2001年   417篇
  2000年   356篇
  1999年   353篇
  1998年   315篇
  1997年   263篇
  1996年   279篇
  1995年   266篇
  1994年   221篇
  1993年   141篇
  1992年   130篇
  1991年   53篇
  1990年   30篇
  1989年   8篇
  1988年   1篇
  1985年   1篇
排序方式: 共有12561条查询结果,搜索用时 62 毫秒
Evolution of the cytochromeb gene of mammals   总被引:98,自引:0,他引:98  
Summary With the polymerase chain reaction (PCR) and versatile primers that amplify the whole cytochromeb gene (∼ 1140 bp), we obtained 17 complete gene sequences representing three orders of hoofed mammals (ungulates) and dolphins (cetaceans). The fossil record of some ungulate lineages allowed estimation of the evolutionary rates for various components of the cytochromeb DNA and amino acid sequences. The relative rates of substitution at first, second, and third positions within codons are in the ratio 10 to 1 to at least 33. For deep divergences (>5 million years) it appears that both replacements and silent transversions in this mitochondrial gene can be used for phylogenetic inference. Phylogenetic findings include the association of (1) cetaceans, artiodactyls, and perissodactyls to the exclusion of elephants and humans, (2) pronghorn and fallow deer to the exclusion of bovids (i. e., cow, sheep, and goat), (3) sheep and goat to the exclusion of other pecorans (i. e., cow, giraffe, deer, and pronghorn), and (4) advanced ruminants to the exclusion of the chevrotain and other artiodactyls. Comparisons of these cytochromeb sequences support current structure-function models for this membrane-spanning protein. That part of the outer surface which includes the Qo redox center is more constrained than the remainder of the molecule, namely, the transmembrane segments and the surface that protrudes into the mitochondrial matrix. Many of the amino acid replacements within the transmembrane segments are exchanges between hydrophobic residues (especially leucine, isoleucine, and valine). Replacement changes at first and second positions of codons approximate a negative binomial distribution, similar to other protein-coding sequences. At four-fold degenerate positions of codons, the nucleotide substitutions approximate a Poisson distribution, implying that the underlying mutational spectrum is random with respect to position.  相似文献
Here, the state of the art of the application of denaturing gradient gel electrophoresis (DGGE) and temperature gradient gel electrophoresis (TGGE) in microbial ecology will be presented. Furthermore, the potentials and limitations of these techniques will be discussed, and it will be indicated why their use in ecological studies has become so important.  相似文献
Development and mapping of 2240 new SSR markers for rice (Oryza sativa L.).   总被引:86,自引:0,他引:86  
A total of 2414 new di-, tri- and tetra-nucleotide non-redundant SSR primer pairs, representing 2240 unique marker loci, have been developed and experimentally validated for rice (Oryza sativa L.). Duplicate primer pairs are reported for 7% (174) of the loci. The majority (92%) of primer pairs were developed in regions flanking perfect repeats > or = 24 bp in length. Using electronic PCR (e-PCR) to align primer pairs against 3284 publicly sequenced rice BAC and PAC clones (representing about 83% of the total rice genome), 65% of the SSR markers hit a BAC or PAC clone containing at least one genetically mapped marker and could be mapped by proxy. Additional information based on genetic mapping and "nearest marker" information provided the basis for locating a total of 1825 (81%) of the newly designed markers along rice chromosomes. Fifty-six SSR markers (2.8%) hit BAC clones on two or more different chromosomes and appeared to be multiple copy. The largest proportion of SSRs in this data set correspond to poly(GA) motifs (36%), followed by poly(AT) (15%) and poly(CCG) (8%) motifs. AT-rich microsatellites had the longest average repeat tracts, while GC-rich motifs were the shortest. In combination with the pool of 500 previously mapped SSR markers, this release makes available a total of 2740 experimentally confirmed SSR markers for rice, or approximately one SSR every 157 kb.  相似文献
Six primers for the amplification of three non-coding regions of chloroplast DNA via the polymerase chain reaction (PCR) have been designed. In order to find out whether these primers were universal, we used them in an attempt to amplify DNA from various plant species. The primers worked for most species tested including algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The fact that they amplify chloroplast DNA non-coding regions over a wide taxonomic range means that these primers may be used to study the population biology (in supplying markers) and evolution (inter- and probably intraspecific phylogenies) of plants.  相似文献
猪雌激素受体(ESR)基因对产仔数性状的影响   总被引:81,自引:4,他引:77  
陈克飞  黄路生  李宁  张勤  罗明  吴常信 《遗传学报》2000,27(10):853-857
猪产仔数是重要的经济性状,产仔数的提高将会大大地增加商品肉猪的产量,给现代养猪生产带来巨大的经济效益。ESR基因是影响猪产仔数的主效基因,而且与猪的生长发育性状及酮体性状之间不存在负的基因多效性影响。采用PCR-RFLPs的方法,通过对5个不同品种的、特别是我国地方品种的262头母猪进行了基因多态性分析,研究表明ESR基因在这种种群对产仔数均有极显著影响,ESR位点BB基因型母猪平均比AA基因型母  相似文献
中国人丙型肝炎病毒基因组的一级结构及其变异   总被引:76,自引:3,他引:73  
毕胜利  白宪鹤 《病毒学报》1993,9(2):114-127
利用mtDNA COⅠ基因序列鉴定我国烟粉虱的生物型   总被引:74,自引:10,他引:64       下载免费PDF全文
 利用mtDNA COⅠ基因片段作标记,采用序列分析的方法,从分子生态学的角度研究了近年来在我国暴发危害的烟粉虱Bemisia tabaci 5个种群(北京一品红种群,广州甘蓝种群,西安一品红种群,北京西红柿种群和新疆吐鲁番棉花种群)的生物型。在5个种群的基因片段上,截取与Texas-B型相应的720bp的序列分析,结果表明所测序列中只有2个碱基与Texas-B型不同,序列相似性为99.7%;在西安一品红种群和新疆吐鲁番棉花种群的原序列中截取与Arizona-B型序列相应的423bp片段,分析表明这两个种群与AZB3型属于同一个单倍型。因此,我国烟粉虱5个实验种群的生物型与Texas-B型和Arizona-B型种群为同一生物型“B”。  相似文献
土壤微生物总DNA的提取和纯化   总被引:73,自引:2,他引:71       下载免费PDF全文
本文建立了从土壤中提取总DNA的方法,并通过改进使适合于对革兰氏阳性菌的提取。用9种性质不同的土壤进行验证,均提取到了DNA,每克干土的DNA提取量从3.30μg~13.41μg,通过透析袋回收进行纯化,纯化回收率达到65.34%,纯化后的土壤DNA可以直接扩增出16S rDNA。9种土壤的提取率从60.51%~93.45%,可以从每g干土添加362个菌体的土壤中扩增到目的条带。  相似文献
微波法快速提取放线菌基因组DNA   总被引:69,自引:1,他引:68  
原有的放线菌基因组DNA提取方法费时、费力、费用高,且对极端环境放线菌成功率较低。利用微波热振惊提取固体培养基表面放线菌菌落基因组DNA,具有快速、简便、费用低廉等优点。所得的基因组DNA可作为PCR反应的模板进行16S rRNA基因有效扩增。这为大量放线菌菌株的快速鉴别和系统分类创造了条件。  相似文献
正交设计优化不结球白菜ISSR反应体系研究   总被引:67,自引:0,他引:67  
利用正交试验设计,从Mg2+、dNTP、引物、甘油4种因素3个水平,对不结球白菜ISSR反应体系进行优化,确立了适合不结球白菜的ISSR反应体系:在20μL反应体系中,含1×PCRbuffer,1.5mmol·L-1MgCl2,200μmol·L-1dNTP,0.5μmol·L-1引物,1%甘油,30ng模板DNA,1UTaqDNA聚合酶.通过梯度PCR测验,确定了适宜的退火温度.  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