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   16545篇
  免费   890篇
  国内免费   8篇
  2022年   39篇
  2021年   135篇
  2020年   71篇
  2019年   101篇
  2018年   167篇
  2017年   169篇
  2016年   280篇
  2015年   438篇
  2014年   512篇
  2013年   1268篇
  2012年   878篇
  2011年   938篇
  2010年   559篇
  2009年   522篇
  2008年   916篇
  2007年   1009篇
  2006年   1009篇
  2005年   1015篇
  2004年   1095篇
  2003年   1018篇
  2002年   970篇
  2001年   271篇
  2000年   225篇
  1999年   235篇
  1998年   259篇
  1997年   240篇
  1996年   194篇
  1995年   165篇
  1994年   134篇
  1993年   181篇
  1992年   195篇
  1991年   164篇
  1990年   149篇
  1989年   149篇
  1988年   157篇
  1987年   120篇
  1986年   129篇
  1985年   120篇
  1984年   139篇
  1983年   115篇
  1982年   129篇
  1981年   114篇
  1980年   105篇
  1979年   79篇
  1978年   71篇
  1977年   64篇
  1976年   69篇
  1975年   40篇
  1974年   56篇
  1973年   51篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Summary The complete nucleotide sequence of the Escherichia coli cybB gene for diheme cytochrome b 561 and its flanking region was determined. The cybB gene comprises 525 nucleotides and encodes a 175 amino acid polypeptide with a molecular weight of 20160. From its deduced amino acid sequence, cytochrome b 561 is predicted to be very hydrophobic (polarity 33.7%) and to have three membrane spanning regions. Histidines, canonical ligand residues for protohemes, are localized in these regions, and the heme pockets are thought to be in the cytoplasmic membrane. No significant homology of the primary structure of cytochrome b 561 with those of other bacterial b-type cytochromes was observed.  相似文献   
92.
Hormone autonomous callus was institutioniated reproduciblyon MS agar medium with 0.25 mg/liter of BA as the sole planthormone (AI medium) from young leaf explants of sugar beets.When leaf explants were inoculated into AI medium and culturedon a reciprocal shaker, single cells began to be released fromthe cut surfaces of the leaf pieces after 6 days, followed byactive release. When the single cells which had been releasedwere transferred to fresh liquid MS medium without any planthormones, they could divide and grow autonomously, giving riseto hormone-autonomous suspension cultures. The effects of BAon induction of hormone-autonomous cells are discussed. (Received March 12, 1987; Accepted October 13, 1987)  相似文献   
93.
Carrot and tobacco plants were transformed with Agrobacteriumtumefaciens harboring wild-type, aux or cyt Tiplasmids. In tobacco, these wild and mutant Ti plasmids inducedthe formation of non-morphogenic galls, galls with shoots, andgalls with roots, respectively. In carrot, however, transformationwith any of these plasmids resulted in only the formation ofamorphous tumors. Determination of IAA and cytokinin contentsshowed that in tobacco, significantly high amounts of cytokininsor IAA are present in the cells transformed with Ti plasmidspossessing cytokinin or IAA biosynthetic genes, respectively.In carrot, cytokinin contents were also high in the cells transformedwith Ti plasmids having cytokinin biosynthetic genes, whereasIAA contents of the cells were similar regardless of the plasmidsused for transformation. These results suggest that the mechanism regulating IAA metabolismmay be different in tobacco and carrot. (Received June 25, 1987; Accepted February 1, 1988)  相似文献   
94.
IAA biosynthetic activity was examined in cultured carrot tissuestransformed with Agrobacterium tumefaciens harboring wild-type,aux or cyt Ti plasmids. In vitro IAAM hydrolaseactivities in tissues transformed with wild-type, and cytTi plasmids were 3.09 and 19.82 nmol/g proteins/30 min, respectively,but not detectable when aux Ti plasmids were used. Theactivity of IAA biosynthesis, determined by the incorporationof radioactivity into IAA in tissues fed with [14C]-tryptophan,was 34.13, 10.92 and 32.47 pmol/g fr wt/30 min in tissues transformedwith wild type, aux and cyt Ti plasmids, respectively.The incorporation of radioactivity into the IAAM fraction wasdetected only in the tissues transformed with wild type andcyt Ti plasmids. These results showed that the T-DNAencoded pathway of IAA biosynthesis was active in tissues transformedwith wild-type and cyth Ti plasmids, and that the activity ofIAA biosynthesis in those tissues was higher than that in tissuestransformed with the aux Ti plasmid. (Received March 16, 1988; Accepted July 31, 1988)  相似文献   
95.
Cells of the auxotrophic mutant, Ad1, of Datura innoxia requiredadenine, adenosine, or inosine for their growth on solid agarmedium which contained Murashige-Skoog salts, 2,4-dichloro-phenoxyaceticacid, and sucrose. Thirteen purine and pyrimidine nucleotidesin extracts of wild-type and Ad1 cells were separated and quantifiedby HPLC. Levels of ADP-glucose and UMP were significantly higherin Ad1 than in wild-type cells, but those of other nucleotideswas found when Ad1 cells were transferred to fresh medium withoutadenine. The rate of the biosynthesis de novo of purines, asestimated from the rate of incorporation of 14C from [2-14C]-glycine and [14C]formate into adenine nucleotides, was reducedin Ad1 cells to 21 and 13% of the wild-type rate, respectively.The activities involved in the salvage of adenine and adenosinein Ad1 cells were similar to those in wild-type cells. Ad1 cellshad the capability to convert adenine to guanine nucleotidesand guanine to adenine nucleotides. 1 Part 27 of the series, "Metabolic Regulation in Plant CellCulture". (Received March 7, 1988; Accepted August 3, 1988)  相似文献   
96.
97.
Summary We have previously reported hyperpolarizing membrane potential changes in a monkey kidney cell line (JTC-12) which has characteristics resembling proximal tubular cells. These hyperpolarizations could be observed spontaneously or evoked by mechanically touching adjacent cells. In this report, we have shown further evidence that these hyperpolarizations are elicited by an increase in membrane conductance to K+ which is caused by an increase in cytosolic Ca2+ concentration. In addition, we have found another type of hyperpolarization which is evoked by applying flow of extracellular fluid to the cell. Intracellular injection of Ca2+ and Sr2+ evoked hyperpolarizations, while intracellular injection of Mn2+ and Ba2+ did not. Intracellular injection of EGTA suppressed both spontaneous and mechanically evoked hyperpolarizations. In Ca2+-free medium, both spontaneous and flow-evoked hyperpolarizations were not observed, while mechanical stimuli consistently evoked hyperpolarization. In Na+-free medium, the incidence of cells showing the spontaneous or flow-evoked hyperpolarization increased, and the amplitude and the duration of the mechanically evoked hyperpolarization became greater. Quinidine inhibited all types of hyperpolarization. These data suggest that hyperpolarizations in JTC-12 cells are due to an increase in Ca2+-activated K+ conductance.  相似文献   
98.
99.
The leaf ultrastructure of NADP-malic enzyme type C4 species possessing different anatomical features in the Cyperaceae was examined: types were the Rhynchosporoid type, a normal Kranz type in which mesophyll cells are adjacent to Kranz cells, and Fimbristyloid and Chlorocyperoid types, unusual Kranz types in which nonchlorophyllous mestome sheath intervenes between the two types of green cells. They show structural characteristics basically similar to the NADP-malic enzyme group of C4 grasses, that is, centrifugally located chloroplasts with reduced grana and no increase of mitochondrial frequency in the Kranz cells. However, the Kranz cell chloroplasts of the Fimbristyloid and Chlorocyperoid types exhibit convoluted thylakoid systems and a trend of extensive development of peripheral reticulum, although those of the Rhynchosporoid type do not possess such particular membrane systems. The suberized lamella, probably a barrier for CO2 diffusion, is present in the Kranz cell walls of the Rhynchosporoid type and in the mestome sheath cell walls of the other two types, and tightly surrounds the Kranz cells (sheaths) that are the sites of the decarboxylation of C4 acids. These ultrastructural features are discussed in relation to C4 photosynthetic function.  相似文献   
100.
Tobacco mesophyll protoplasts were treated with plasmids, pCT2 (17.1 kbp) or pCT2T3 (18.3 kbp), which contained a chimeric aminoglycoside phosphotransferase II (APH(3′)II) gene and an intact nopaline synthase gene. Expression of two marker enzymes, APH(3′)II and nopaline synthase, were analyzed in transformed plants. Four out of 16 transformants obtained by pCT2T3 possessed both enzymes. Upon self-pollination, the progeny of one of transformants (T2) segregated to 153∶4 in terms of resistant and susceptible character to kanamycin, suggesting insertion of foreign genes into three independent chromosomes. The kanamycin resistant character in the rest of transformants showed 3∶1 segregation. DNA blot analysis of the T2 transformant and progenies indicated the presence of two marker genes.  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