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   6611篇
  免费   561篇
  国内免费   5篇
  2023年   25篇
  2022年   40篇
  2021年   102篇
  2020年   74篇
  2019年   83篇
  2018年   103篇
  2017年   127篇
  2016年   165篇
  2015年   293篇
  2014年   398篇
  2013年   445篇
  2012年   560篇
  2011年   497篇
  2010年   327篇
  2009年   284篇
  2008年   345篇
  2007年   378篇
  2006年   353篇
  2005年   339篇
  2004年   311篇
  2003年   305篇
  2002年   286篇
  2001年   72篇
  2000年   62篇
  1999年   72篇
  1998年   83篇
  1997年   60篇
  1996年   61篇
  1995年   54篇
  1994年   63篇
  1993年   53篇
  1992年   74篇
  1991年   48篇
  1990年   56篇
  1989年   51篇
  1988年   34篇
  1987年   35篇
  1986年   30篇
  1985年   30篇
  1984年   47篇
  1983年   34篇
  1982年   32篇
  1981年   38篇
  1980年   20篇
  1979年   20篇
  1978年   20篇
  1976年   19篇
  1974年   23篇
  1973年   17篇
  1972年   23篇
排序方式: 共有7177条查询结果,搜索用时 203 毫秒
31.
32.
Summary A new gene for trimethoprim resistance, dhfrV, found in several plasmid isolates with different characteristics, was sequenced and found to correspond to a peptide of 157 amino acids showing 75% similarity with the previously characterized, drug resistant dihydrofolate reductase of type I. The sequenced surroundings of dhfrV in plasmid pLMO20, were found to be almost identical with genetic areas surrounding resistance genes in transposon Tn21 and in R plasmid R388. The trimethoprim resistance genes of pLMO20 and R388 and the spectinomycin resistance gene of Tn21 could be regarded as having been inserted, by recombination, into an evolutionary older structure containg the sulfonamide resistance gene, sulI. The latter gene was sequenced and found to correspond to a peptide of 279 amino acids and with a molecular weight of 30126 daltons. The inserted genes were found to be governed by a promoter situated in the highly conserved structure and also controlling expression of sulI. The insertion points of the different resistance genes were precisely defined, and at the 3 ends of the inserted genes inverted repeats allowing the formation of stem and loop structures were found. Similar structures were found at the 3 ends of the antibiotic resistance genes in Tn7, which could indicate similar recombination mechanisms to be effective in the evolutionary construction of all these different resistance elements.  相似文献   
33.
A soil nitrogen model was used for a 4-year simulation of nitrogen dynamics and nitrate leaching, both during grass ley growth and after ploughing a grass ley. Model results were compared with field measurements of soil mineral-N status and leaching. A soil water and heat model provided daily values for abiotic conditions, which were used as driving variables in the nitrogen simulation. Simulated values for mineral-N levels in the soil agreed well with field data for the first 3 years of the simulation. During the final year the model predicted considerably higher levels of soil mineral-N content compared with measurements. To reach the mineral-N level measured at the time of ploughing the ley, the simulated N-uptake by plants had to be increased by 8 g N m−2. Simulations of nitrate leaching suggested that estimates of leaching based on measurements in tile-drained plots can be considerably underestimated. Accurate quantification of leaching in tile-drained plots often requires additional information on water-flow paths. A substantial increase in simulated and measured values for the mineral-N content of the soil occurred after ploughing the ley. In the simulation, most of the increase was due to a high crop residue input and the absence of a growing crop after ploughing. Litter accumulations in the soil during the 4-year period contributed little to the increase in soil mineral-N.  相似文献   
34.
We here report an enzyme linked immunosorbent assay (ELISA) and a scintillation proximity assay (SPA) for detection of the ganglioside FucGM1 in sera from small cell lung cancer (SCLC) patients. The SPA was more sensitive and reproducible than the ELISA. In this assay, monoclonal antibodies specific for FucGM1 were bound to SPA particles and incubated with labelled FucGM1 and 100 µl test-serum overnight, and counted in a -counter. The sensitivity was 0.2 ng. Seven out of twenty sera from SCLC patients were positive, whereas none of twenty sera from healthy individuals were positive for FucGM1. The SPA was more sensitive than the previously reported HPTLC as well as a direct ELISA.Abbreviations MAb monoclonal antibody - SPA scintillation proximity assay - HPTLC high performance thin layer chromatography - SCLC small cell lung cancer - FucGM1 Fuc1-2Gal1-3GalNAc1-4(NeuAc2-3)-Gal1-4Glc1-1Cer - ELISA enzyme linked immunosorbent assay - FCS foetal calf serum - PBS phosphate buffered saline  相似文献   
35.
Abstract A number of sediment incubations were set up to reproduce some of the conditions used by Kristensen and Blackburn [1] and to make a comparison with their results. There were three types of microcosm: aerobic (OX), anaerobic (AN) and aerobic with Nephtys (NOX). In addition to other measurements, dissolved organic nitrogen (DON) pools and fluxes, were measured. The sediment in this experiment contained more particulate organic matter (POM). Nephtys (NOX) had the same effect as Nereis in increasing the rate of mineralization of POC and PON, compared with the OX-cores (2.1 and 2.6 times, respectively). Again, the AN-cores had a higher mineralization rate (loss of POM) than that of the OX-cores, but in addition, mineralization in NOX-cores was not significantly different from AN-cores. It was thus confirmed that anoxic mineralization could be as high, or higher, than the oxic process. Both the temporal patterns of O2-and and CO2-fluxes and their magnitudes were very similar to those reported earlier. This contrasts with the higher loss of POM in the present experiment. However, the loss of C in DOC (associated with the measured DON) can account for the extra POM loss. The pore-water profiles of σCO2 and NH4+ were similar to those in the earlier report, and the fluxes of σCO2, O2, NH4+ and NO3 followed the same temporal pattern.  相似文献   
36.
Floral scents of male and female inflorescences of three dioeciousSalix species were collected by head-space adsorption, and analysed by GC-MS. InSalix caprea andS. cinerea 1,4-dimethoxy benzene was the main compound, and male and female scents showed a high degree of resemblance. No dominant compound was found inS. repens and malefemale scent similarity was low. Floral scent inSalix is likely a strong orientation cue, guiding pollinators between male and female plants ensuring pollen transfer and pollination. We suggest that a high degree of male-female floral scent resemblance is coupled to a high degree of insect pollination. Floral scent does not promote reproductive isolation betweenS. caprea andS. cinerea.  相似文献   
37.

Book Review

Plant protoplasts and genetic engineering IIY.P.S. Bajaj (Ed.), (Biotechnology in Agriculture and Forestry, Vol. 9). Berlin: Springer-Verlag, 1989. 510 pages. DM398.00. ISBN 3-540-50789-2  相似文献   
38.
39.
Ochratoxin A contamination of cereal feed grain was monitored during October 1989–September 1990 by analysis of blood samples from slaughter swine in Sweden. The detection of ochratoxin A in swine blood was used as a method to identify swine herds fed ochratoxin A contaminated feed. The contamination level of ochratoxin A in the blood of the positive herds was in the range 2–45 ng/ml with the mean concentration 5.2 ng/ml. Feed samples for mycological analysis were collected from both ochratoxin A positive herds (2 ng/ml blood) and ochratoxin A negative herds (<2 ng/ml blood). From the ochratoxin A positive herds and the ochratoxin A negative herds 22 and 21 feed samples were collected, respectively. No quantitative differences in mould content, as determined by colony forming units, were observed between the two groups. However, there were differences in the mycoflora. The incidence of storage fungi (Penicillium and Aspergillus spp.) was significantly higher (p < 0.05) in feed from ochratoxin A positive herds. Particularly, Penicillium verrucosum was found to be significantly more common (p < 0.001). Altogether 274 isolates were screened for their ability to produce ochratoxin A. Ochratoxin A producers were found only within P. verrucosum; 38% of the 63 isolates produced detectable amounts of ochratoxin A. Ochratoxin A producing isolates of P. verrucosum were found in 60% of the feed samples collected from ochratoxin A positive swine herds and in one sample (5% ) of the feed samples collected from the ochratoxin A negative herds.  相似文献   
40.
Summary A series of experiments has established the molecular defect in the medium-chain acyl-coenzyme A (CoA) dehydrogenase (MCAD) gene in a family with MCAD deficiency. Demonstration of intra-mitochondrial mature MCAD indistinguishable in size (42.5-kDa) from control MCAD, and of mRNA with the correct size of 2.4 kb, indicated a point-mutation in the coding region of the MCAD gene to be disease-causing. Consequently, cloning and DNA sequencing of polymerase chain reaction (PCR) amplified complementary DNA (cDNA) from messenger RNA of fibroblasts from the patient and family members were performed. All clones sequenced from the patient exhibited a single base substitution from adenine (A) to guanine (G) at position 985 in the MCAD cDNA as the only consistent base-variation compared with control cDNA. In contrast, the parents contained cDNA with the normal and the mutated sequence, revealing their obligate carrier status. Allelic homozygosity in the patient and heterozygosity for the mutation in the parents were established by a modified PCR reaction, introducing a cleavage site for the restriction endonuclease NcoI into amplified genomic DNA containing G985. The same assay consistently revealed A985 in genomic DNA from 26 control individuals. The A to G mutation was introduced into an E. coli expression vector producing mutant MCAD, which was demonstrated to be inactive, probably because of the inability to form active tetrameric MCAD. All the experiments are consistent with the contention that the G985 mutation, resulting in a lysine to glutamate shift at position 329 in the MCAD polypeptide chain, is the genetic cause of MCAD deficiency in this family. We found the same mutation in homozygous form in 11 out of 12 other patients with verified MCAD deficiency.  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