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   2653篇
  免费   88篇
  国内免费   7篇
  2021年   28篇
  2018年   58篇
  2017年   41篇
  2016年   50篇
  2015年   70篇
  2014年   63篇
  2013年   127篇
  2012年   106篇
  2011年   114篇
  2010年   82篇
  2009年   69篇
  2008年   79篇
  2007年   99篇
  2006年   74篇
  2005年   95篇
  2004年   84篇
  2003年   73篇
  2002年   82篇
  2001年   48篇
  2000年   52篇
  1999年   39篇
  1998年   27篇
  1997年   28篇
  1996年   30篇
  1995年   22篇
  1994年   32篇
  1993年   29篇
  1992年   40篇
  1991年   48篇
  1990年   36篇
  1989年   36篇
  1988年   52篇
  1987年   43篇
  1986年   53篇
  1985年   44篇
  1984年   40篇
  1983年   30篇
  1982年   27篇
  1981年   31篇
  1979年   32篇
  1978年   34篇
  1977年   37篇
  1976年   32篇
  1975年   34篇
  1974年   31篇
  1973年   28篇
  1972年   30篇
  1971年   26篇
  1970年   21篇
  1969年   22篇
排序方式: 共有2748条查询结果,搜索用时 78 毫秒
151.
DNA sequence data of the nuclear-encoded gamma1-gamma2-globin duplication region were used to examine the phylogenetic relationships of 16 cercopithecid (Old World monkey) species representing 12 extant genera. Morphology- and molecular-based hypotheses of Old World monkey branching patterns are generally congruent, except for generic relationships within the subtribe Papionina. The cercopithecids divide into colobines (leaf-eating monkeys) and cercopithecines (cheek-pouched monkeys). The colobines examined by the DNA data divide into an Asian clade (Nasalis, proboscis monkeys; Trachypithecus, langurs) and an African clade (Colobus, colobus monkeys). The cercopithecines divide into tribes Cercopithecini (Erythrocebus, patas monkey; Chlorocebus, green monkeys; Cercopithecus, guenons) and Papionini. Papionins divide into subtribes Macacina (Macaca, macaques) and Papionina (Papio, hamadryas baboons; Mandrillus, drills and mandrills; Theropithecus, gelada baboons; Lophocebus, arboreal mangabeys; Cercocebus, terrestrial mangabeys). In a morphologically based classification, Mandrillus is a subgenus of Papio, whereas Lophocebus is a subgenus of Cercocebus. In contrast, the molecular evidence treats Mandrillus as a subgenus of Cercocebus, and treats both Theropithecus and Lophocebus as subgenera of Papio. Local molecular clock divergence time estimates were used as a yardstick in a "rank equals age" system to propose a reduction in taxonomic rank for most clades within Cercopithecidae.  相似文献   
152.
The populations of Tuvinians (N = 36) and Buryats (N = 105) were characterized by using the data on mitochondrial DNA (mtDNA) polymorphism. The gene pools of both ethnic groups possessed the mtDNA types belonging to the four main haplogroups, A, B, C, and D, found only in the indigenous populations of Asia and America. The total frequencies of the A, B, C, and D haplogroups in Tuvinians and Buryats were 72.3% and 52.4%, respectively. These values, along with the frequency for Altai populations (57.2%), were highest in the Asian populations studied, indicating that the populations Southern and Eastern Siberia can be considered as ancestral relatives to the ethnic groups of the New World. Analysis of the mtDNA region V polymorphism showed the presence of 9-bp deletion and 4-bp insertion in both populations with frequencies respectively of 13.9 and 5.56% in Tuvinians and 4.8 and 1.9% in Buryats. The frequency of the +AvaII/8249 variant was 11.1% in Tuvinians and 3.81% in Buryats. Analysis of the association between the region V deletion-insertion polymorphism and certain restriction haplogroups pointed to repeated and independent emergence of the 4-bp insertion in Siberia.  相似文献   
153.
154.
The expression of the acetyl xylan esterase II (axeII) gene from Penicillium purpurogenum is repressed by glucose and induced by xylan, as well as to a small degree by xylose and xylitol. This gene is expressed at neutral pH, but not under alkaline or acidic conditions, in agreement with previous findings for other xylanolytic genes of this organism. This is the first report showing pH regulation of an axe gene.  相似文献   
155.
A two-dimensional proteome map of maize endosperm   总被引:8,自引:0,他引:8  
We have established a proteome reference map for maize (Zea mays L.) endosperm by means of two-dimensional gel electrophoresis and protein identification with LC-MS/MS analysis. This investigation focussed on proteins in major spots in a 4-7 pI range and 10-100 kDa M(r) range. Among the 632 protein spots processed, 496 were identified by matching against the NCBInr and ZMtuc-tus databases (using the SEQUEST software). Forty-two per cent of the proteins were identified against maize sequences, 23% against rice sequences and 21% against Arabidopsis sequences. Identified proteins were not only cytoplasmic but also nuclear, mitochondrial or amyloplastic. Metabolic processes, protein destination, protein synthesis, cell rescue, defense, cell death and ageing are the most abundant functional categories, comprising almost half of the 632 proteins analyzed in our study. This proteome map constitutes a powerful tool for physiological studies and is the first step for investigating the maize endosperm development.  相似文献   
156.
Dormirator maculatus (n=184) was collected in the Alvarado Lagoon, Mexico during a year period (Oct. 1993-1994). In the helminthologic review, the presence of Clinostomum complanatum (82.3%), Neoechinorhynchus golvani (76.1%), Spiroxys sp. (21.3 %), and Camallanus sp. (6.2%) was registered. Reduction of the hematocrit caused by such infection is significant (t, a= 0.05).  相似文献   
157.
Glucuronic acid n-alkyl esters, a novel class of promising biosurfactants and their corresponding glucose esters with the same side-chain length, were synthesized by direct esterification in a non-aqueous phase (tert-butanol) using an immobilized lipase.  相似文献   
158.
Nitrile hydratase and amidase from Rhodococcus erythropolis CIMB11540 were both cloned and expressed in Escherichia coli. Crude cell free extracts were used for the hydrolysis of different aromatic cyanohydrins. Nitrile hydratase expression was increased up to 5-fold by redesign of the expression cassette. The recombinant enzymes were successfully used for the conversion of several cyanohydrins to the corresponding alpha-hydroxy amides and acids while retaining enantiopurity.  相似文献   
159.
Leaf segments excised from Centella asiatica, a medicinal and neutraceutical plant, produced abundant somaticembryoswhen cultured onMS mediumwith 9.29 Mkinetin in combination with 2.26 M2,4-D. Granular, white,shiny clusters of callus developed after 1 week of culture, and then formed heart and cotyledonary stage embryoson the same medium after 4 weeks. Somatic embryos matured and germinated in the presence of MS mediumcontaining 2.32 M kinetin with (2.89M) GA3. Plantlets were successfully transferred to pots containing amixture of soil and vermiculite (1:1).  相似文献   
160.
Absence of intrinsic electric conductivity in single dsDNA molecules   总被引:3,自引:0,他引:3  
The intrinsic dc conductivity of long, individual lambda phage dsDNA molecules has been investigated by ultrasensitive low current-voltage-spectroscopy (IV) under ambient conditions and controlled low humidity inert gas atmosphere on microfabricated metal-insulator-metal gap structures. We found a strong dependence of the measured conductivity on the apparent humidity, which we attribute to capillary condensation of water to the immobilized DNA molecules, giving rise to additional ionic currents. Additional IV-spectroscopy experiments under controlled argon atmosphere always revealed a significant drop in electrical conductivity to 4 x 10(-15)AV(-1)microm(-1), indicating almost no considerable contribution of electrical long range charge transport.  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