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   820篇
  免费   108篇
  928篇
  2022年   5篇
  2020年   5篇
  2019年   5篇
  2017年   9篇
  2016年   5篇
  2015年   22篇
  2014年   26篇
  2013年   34篇
  2012年   33篇
  2011年   27篇
  2010年   28篇
  2009年   21篇
  2008年   31篇
  2007年   25篇
  2006年   36篇
  2005年   29篇
  2004年   40篇
  2003年   37篇
  2002年   32篇
  2001年   30篇
  2000年   24篇
  1999年   26篇
  1998年   16篇
  1997年   15篇
  1996年   18篇
  1995年   20篇
  1994年   13篇
  1993年   11篇
  1992年   32篇
  1991年   13篇
  1990年   6篇
  1989年   17篇
  1988年   17篇
  1987年   14篇
  1986年   17篇
  1985年   16篇
  1984年   11篇
  1983年   14篇
  1982年   7篇
  1981年   10篇
  1979年   13篇
  1978年   13篇
  1977年   7篇
  1975年   9篇
  1973年   14篇
  1972年   14篇
  1971年   4篇
  1970年   4篇
  1969年   5篇
  1957年   5篇
排序方式: 共有928条查询结果,搜索用时 15 毫秒
51.
52.
The genus Phrynosoma includes 13 species of North American lizards characterized by unique and highly derived morphologies and ecologies. Understanding interspecific relationships within this genus is essential for testing hypotheses about character evolution in this group. We analyzed mitochondrial ND4 and cytochrome b gene sequence data from all species of Phrynosoma in conjunction with a previously published dataset including 12S and 16S rRNA gene sequences and morphological characters. We used multiple phylogenetic methods and diagnostic tests for data combinability and taxonomic congruence to investigate the data in separate and combined analyses. Separate data partitions resulted in several well-supported lineages, but taxonomic congruence was lacking between topologies from separate and combined analyses. Partitioned Bremer support analyses also reveals conflict between data partitions in certain tree regions. When taxa associated with well-supported clades were removed from analyses, phylogenetic signal was lost. Combined, our results initially suggest conflict between data partitions, but further tests show the data are only appropriate for phylogenetic reconstruction of those parts of the topology that were well resolved. Nonetheless, our data analyses reveal five well-supported clades: (1) Phrynosoma ditmarsi and Phrynosoma hernandesi, (2) P. ditmarsi, P. hernandesi, and Phrynosoma douglasii, (3) P. ditmarsi, P. hernandesi, P. douglasii, and Phrynosoma orbiculare, (4) Phrynosoma mcallii and Phrynosoma platyrhinos, and (5) Phrynosoma braconnieri and Phrynosoma taurus.  相似文献   
53.
A cDNA from Arabidopsis thaliana and four related cDNAs from Nicotiana tabacum that we have isolated encode hitherto unidentified members of the mitochondrial carrier family. These proteins have been overexpressed in bacteria and reconstituted into phospholipid vesicles. Their transport properties demonstrate that they are orthologs/isoforms of a novel mitochondrial carrier capable of transporting both dicarboxylates (such as malate, oxaloacetate, oxoglutarate, and maleate) and tricarboxylates (such as citrate, isocitrate, cis-aconitate, and trans-aconitate). The newly identified dicarboxylate-tricarboxylate carrier accepts only the single protonated form of citrate (H-citrate2-) and the unprotonated form of malate (malate2-) and catalyzes obligatory, electroneutral exchanges. Oxoglutarate, citrate, and malate are mutually competitive inhibitors, showing K(i) close to the respective K(m). The carrier is expressed in all plant tissues examined and is largely spread in the plant kingdom. Furthermore, nitrate supply to nitrogen-starved tobacco plants leads to an increase in its mRNA in roots and leaves. The dicarboxylate-tricarboxylate carrier may play a role in important plant metabolic functions requiring organic acid flux to or from the mitochondria, such as nitrogen assimilation, export of reducing equivalents from the mitochondria, and fatty acid elongation.  相似文献   
54.
Aqueous solutions of 5-500 μg/ml aldicarb inhibited hatching of Heterodera schachtii. Addition of hatching agents, zinc chloride, or sugarbeet root diffusate, to the aldicarb solutions did not decrease the inhibition of hatching. When cysts were removed from the aldicarb solufions and then treated for 4 wk in sugarbeet root diffusate, larvae hatched and emerged. Treatments of newly hatched larvae of H. schachtii with 5-100 μg/ml aldicarb depressed later development of larvae on sugarbeet (Beta vulgaris). Similar treatments with aldicarb sulfoxide had less effect on larval development, and aldicarb sulfone had no effect. Numbers of treated larvae that survived and developed were inversely proportional to concentration (0.1-5.0 μg/ml) and duration (0-14 days) of aldicarb treatments. Development of H. schachtii on sugarbeet grown in aldicarb-treated soil was inversely proportional to the concentration of aldicarb in the tested range of 0.75 - 3.0 μg aldicarb/g of soil. Transfer of nematode-infected plants to soil with aldicarb retarded nematode development, whereas transfer of plants first grownin treated soil to nematode-infested soil only slightly suppressed nematode development. Development of H. schachtii was inhibited in slices of storage roots of table beet (B. vulgaris), sugarbeet and turnip, (Brassica rapa), that had grown in soil treated with aldicarb.  相似文献   
55.
Adrenocortical cells were prepared from rats maintained on essential fatty acid-deficient diets and control litter mates. Cells from control rats had high concentrations of essential fatty acids in the cholesteryl ester fraction of which approximately 22% was arachidonate. In contrast, cells from EFA-deficient rats had only 2.5% arachidonate in the cholesteryl esters, even though the total esterified cholesterol level was comparable to that of controls. In place of the essential fatty acids, the cholesteryl esters of these cells were rich in 20:3(n--9) and 22:3(n--9). When cells from EFA-deficient rats were incubated with ACTH or dibutyryl cyclic AMP, the output of corticosterone was the same as in controls. Also sterol esters were hydrolyzed to the same extent as in controls despite the unusual composition of the fatty acid esters. The phospholipids in both control and EFA-deficient cells contained high levels of arachidonate but were not hydrolyzed in either type of cell during incubation with ACTH or dibutyryl cyclic AMP. The results indicate that high levels of the prostaglandin precursors, namely linoleate and arachidonate, are not a sine qua non for the steroidogenic action of ACTH or cyclic AMP.  相似文献   
56.
Eight Danish Holstein cows were milked with a 1-mm thick specially designed soft liner on their right rear teat and a standard liner mounted under extra high tension on their left rear teat. Four of the animals were overmilked for 5 min. Rear teats were subjected to ultrasound examination on the first day and to infrared thermography on the second day. Teats were submersed in ethanol 20 min post-milking on the second day. Ultrasonography measurements showed that teat canal length increased by 30–41% during milking. Twenty minutes after milking, teats milked with modified standard liners still had elongated teat canals while teats milked with the soft liner were normalized. Overmilking tended to increase teat wall thickness. Approximately 80% of variability in teat canal length, from before teat preparation to after milking, could be explained by changes during teat preparation. Thermography indicated a general drop in teat temperature during teat preparation. Teat temperature increased during milking and continued to increase until the ethanol challenge induced a significant drop. Temperatures approached pre-challenge rather than pre-milking temperatures within 10 minutes after challenge. Teat temperatures were dependent on type of liner. Mid-teat temperatures post-challenge relative to pre-teat preparation were dependent on overmilking. Thermography and ultrasound were considered useful methods to indirectly and non invasively evaluate teat tissue integrity.  相似文献   
57.
The biosynthesis of mammalian mitochondrial cytochromes was explored in primary hepatocyte cultures. When these were pulsed with [35S]methionine in the presence of cycloheximide, eight discrete mitochondrial polypeptides were detected by fluorography after their resolution under denaturing conditions by polyacrylamide gel electrophoresis. Since the pulse labeling of the polypeptides was sensitive to chloramphenicol, an inhibitor of mitochondrial translation, they must be translated on mitochondrial ribosomes. Three were identified as the largest subunits of cytochrome oxidase by their immunoprecipitation with antibody directed against purified rat liver cytochrome oxidase. Another (Mr = 28,000) was identified as one of eight subunits of purified rat liver cytochrome b-c1 complex by its immunoprecipitation with antibody directed against bovine heart b-c1 complex. Since cytochrome b apoprotein is the only product of the mitochondrial genome in the yeast cytochrome b-c1 complex (Krieke, J., Bechmann, H., van Hemert, F. J., Schweyan, R. J., Boer, P. H., Kaudewitz, F., and Groot, G. S. P. (1979) Eur. J. Bio-chem. 101, 607-617), the results strongly suggest that the Mr = 28,000 subunit of liver b-c1 complex is cytochrome b apoprotein. Thus the contribution of the mitochondrial translation system to the cytochrome complexes in liver is identical to that of yeast and Neurospora, and there appears to be no deletion or transfer to the nuclear genome of structural genes for mitochondrially synthesized cytochromes during eukaryotic evolution.  相似文献   
58.
Proteus mirabilis, a leading cause of catheter-associated urinary tract infection (CaUTI), differentiates into swarm cells that migrate across catheter surfaces and medium solidified with 1.5% agar. While many genes and nutrient requirements involved in the swarming process have been identified, few studies have addressed the signals that promote initiation of swarming following initial contact with a surface. In this study, we show that P. mirabilis CaUTI isolates initiate swarming in response to specific nutrients and environmental cues. Thirty-three compounds, including amino acids, polyamines, fatty acids, and tricarboxylic acid (TCA) cycle intermediates, were tested for the ability to promote swarming when added to normally nonpermissive media. l-Arginine, l-glutamine, dl-histidine, malate, and dl-ornithine promoted swarming on several types of media without enhancing swimming motility or growth rate. Testing of isogenic mutants revealed that swarming in response to the cues required putrescine biosynthesis and pathways involved in amino acid metabolism. Furthermore, excess glutamine was found to be a strict requirement for swarming on normal swarm agar in addition to being a swarming cue under normally nonpermissive conditions. We thus conclude that initiation of swarming occurs in response to specific cues and that manipulating concentrations of key nutrient cues can signal whether or not a particular environment is permissive for swarming.  相似文献   
59.
Cycle and gestation lengths, menstruation patterns, female genital swelling characteristics, and male-female consortship durations are reported in a semifree-ranging group of Tonkean macaques (Macaca tonkeana) studied over a 12 year period. In addition, profiles of urinary estrone conjugates (E1C) and immunoreactive pregnanediol glucuronide (PdG) throughout four complete menstrual cycles in two females and three full-term pregnancies are presented. Based on intermenstrual intervals, a mean cycle length of 37–41 days (n = 55 cycles in 10 females) was found. Gestation length averaged 173 days (n = 27 pregnancies in eight females). Measurement of PdG immunoreactivity in urine revealed a cyclic pattern with a 5–15-fold increase between follicular and luteal phase concentrations, suggesting that PdG is a reliable indicator of ovarian cyclicity and luteal function. In contrast to PdG, E1C excretion showed no clear pattern throughout the cycle; however, highest values of E1C were usually found shortly before the onset of the luteal phase PdG rise at the presumed time of ovulation. Levels of both hormones were elevated during the first half of gestation and showed a marked increase throughout the second half, with maximum E1C concentrations being up to 100-fold higher than nonpregnant levels. Consortships by the male and occurrence of female genital swelling were long lasting (on average 5–10 days and 13 days, respectively) and were restricted to the follicular phase of the cycle. The day of maximal swelling and day of detumescence as well as the end of male consortship were closely associated with the periovulatory period. Swellings and consortships were longer following lactational ammenorhea than for subsequent cycles. The evolutionary significance of the cyclical changes undergone by females upon their relations with males is discussed. © 1996 Wiley-Liss, Inc.  相似文献   
60.
To find out the infection efficiency of recombinant adeno-as sociated virus 2-mediated exogenou s genes in human peripheral blood monocyte-derived dendritic cells(D Cs),the process of transfection wa s investigated by FITC-labeled rAA V2,and observed under confocal mic roscope.Newly separated dendritic cells were tranfected by rAAV2-luc and rAAV2-GFP at different MOI,and transfection efficiency were detec ted by luminometer for rAAV2-luc a nd flow cytometry for rAAV2-GFP.Th e results were elucidated in four different assay systems:(1)60min w as needed for rAAV2 to bind on den dritic cells,and got into cells in the following 10min;(2)the express ion of luc could be detected at th e MOI as low as 1×10~5v.g/cell,and the expression plateau was reached by the MOI of 10~6~10~7v.g/cell,furt her increase of MOI had no functio n on expression level;(3)transgene expression was detected after 48h, and maintained a higher expression level from 96h to 240h after infec tion;(4)7days postinfection of rAA V2-GFP,5%~18% dendritic cells were GFP positive. These data suggest th at rAAV2 vector can efficiently in fect monocyte-derived dendritic ce lls and mediate exogenous gene exp ression,and that the application o f rAAV2 as vector may be useful fo r gene transfer to dendritic cells in ex vivo immunotherapy.  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