全文获取类型
收费全文 | 249篇 |
免费 | 26篇 |
国内免费 | 1篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 3篇 |
2019年 | 2篇 |
2018年 | 6篇 |
2017年 | 4篇 |
2016年 | 10篇 |
2015年 | 11篇 |
2014年 | 9篇 |
2013年 | 20篇 |
2012年 | 19篇 |
2011年 | 11篇 |
2010年 | 17篇 |
2009年 | 20篇 |
2008年 | 9篇 |
2007年 | 7篇 |
2006年 | 10篇 |
2005年 | 6篇 |
2004年 | 12篇 |
2003年 | 9篇 |
2002年 | 5篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有276条查询结果,搜索用时 662 毫秒
1.
Experiments using plasmid-borne gene fusions and direct RNA measurements have revealed that expression from the nifA gene is induced in Rhizobium meliloti when the external oxygen concentration is reduced to microaerobic levels. Induction occurs in the absence of alfalfa and in the presence of fixed nitrogen and does not require ntrC. The production of functional nifA gene product (NifA) can be demonstrated by its ability to activate the nitrogenase promoter P1. Aerobic induction of nifA can also occur during nitrogen starvation at low pH, but in this case induction is dependent on ntrC and does not lead to P1 activation. The data indicate that reduced oxygen tension is potentially a major trigger for symbiotic activation of nitrogen fixation in Rhizobium species. 相似文献
2.
We have isolated the delta-globin gene of the New-World spider monkey,
Ateles geoffroyi, and compared its nucleotide sequence with those of other
primate delta- and beta-globin genes. Among primate delta-globin genes, the
rate of nonsynonymous substitutions is much less than the rate of
synonymous substitutions. This suggests that primate delta- globin genes
may remain under evolutionary conservation, perhaps because hemoglobin A2
has an as yet unknown physiological importance.
相似文献
3.
A fluorometric procedure for the detection of DNA-DNA hybrids is described. The procedure involved the detection of probe-bound alkaline phosphatase with the fluorescent substrate ATTOPHOS. This substrate is converted to ATTOFLUOR by alkaline phosphatase and fluoresces strongly at 550 nm when excited with a wavelength of 440 nm. DNA hybridization assays were performed both with dilutions of purified target plasmid DNA (pSE9 or PBR322) and whole bacterial cells. Streptavidin-alkaline phosphatase conjugates were added to react with bound probe. Fluorometric assays, as well as colorimetric assays, using 5-bromo-4-chloro-3-indolylphosphate + nitroblue tetrazolium for alkaline phosphatase activity were performed. The fluorescence of the substrate was measured at time intervals, and the slope of the regression line calculated. A slope four times greater than that of background was considered positive. One hundred femtograms or 2.2 x 10(4) molecules of homologous DNA were detected with the fluorescent assay as compared with 10,000 femtograms or 2.2 x 10(6) molecules of homologous DNA with the colorimetric assay. Similar results were obtained with whole cells. Approximately 1 x 10(3) homologous cells were detected fluorometrically and 1 x 10(5) cells were detected colorimetrically. Based on these results, we conclude that, in our hands, the DNA hybridization assay described here using ATTOPHOS as the substrate for alkaline phosphatase is a very sensitive assay for the detection of DNA-DNA hybrids. 相似文献
4.
Biochemical pathways in prokaryotes can be traced backward through evolutionary time 总被引:10,自引:0,他引:10
For the first time, a credible prokaryotic phylogenetic tree is being
assembled by Woese and others using quantitative sequence analysis of
oligonucleotides in the highly conservative rRNA. This provides an
evolutionary scale against which the evolutionary steps that led to the
arrangement and regulation of contemporary biochemical pathways can be
measured. This paper presents an emerging evolutionary picture of aromatic
amino acid biosynthesis within a large superfamily assemblage of
prokaryotes that is sufficiently developed to illustrate a new perspective
that will be applicable to many other biochemical pathways.
相似文献
5.
Dr. Manuel Megias Miguel A. Caviedes Francisco Rodriguez-Quiñones Antonio J. Palomares Francisco Ruiz-Berraquero 《Current microbiology》1985,12(6):325-328
Rhizobium trifolii was highly resistant to the lethal effect ofN-methyl-N-nitro-N-nitrosoguanidine (MNNG), but it was sensitive to the mutagenic action of this chemical. A concentration of 500g/ml yields a survival of between 1% and 10%, which allows us to obtain a higher number of mutants than lower concentrations that yield higher survival rates. Lethal damage produced by nitrosoguanidine was repaired, and repair is inhibited by acriflavine. 相似文献
6.
In Drosophila pseudoobscura, the amylase (Amy) multigene family is
contained within a series of inversions, or gene arrangements, on the third
chromosome. The Standard (ST), Santa Cruz (SC), and Tree Line (TL)
inversions are central to the phylogeny of arrangements, and have clusters
of other arrangements derived from them. The gene arrangements belonging to
each of these three clusters have a characteristic number of Amy genes,
ranging from three in ST to two in SC to one in TL. This distribution
pattern can reflect a history of either duplications or deletions, although
the data available in the past did not permit a decision between these
alternatives. We provide unambiguous evidence that three Amy genes were
present before the divergence of the ST, SC, and TL arrangements. Thus, the
current status of the Amy multigene family is the result of deletions in
the TL and SC arrangements, which created three new pseudogenes: TL
Amy2-psi, TL Amy3-psi, and SC Amy3- psi. Analysis of pseudogene sequences
revealed that, in the SC and ST arrangements, pseudogene evolution has been
retarded, most likely due to the homogenization effect of gene conversion.
Finally, by determining the original copy number, we have reconstructed the
evolutionary history of the Amy multigene family and linked it with the
evolution of the central gene arrangements.
相似文献
7.
Dissolved oxygen tension and oxygen uptake rate are critical parameters in animal cell culture. However, only scarce information of such variables is available for insect cell culture. In this work, the effect of dissolved oxygen tension (DOT) and the utility of on-line oxygen uptake rate (OUR) measurements in monitoring Spodoptera frugiperda (Sf9) cultures were determined. Sf9 cells were grown at constant dissolved oxygen tensions in the range of 0 to 30%. Sf9 metabolism was affected only at DOT below 10%, as no significant differences on specific growth rate, cell concentration, amino acid consumption/production nor carbohydrates consumption rates were found at DOT between 10 and 30%. The specific growth rate and specific oxygen uptake rate followed typical Monod kinetics with respect to DOT. The calculated max and
max were 0.033 h-1 and 3.82×10-10 mole cell-1h-1, respectively, and the corresponding saturation constants were 1.91 and 1.57%, respectively. In all aerated cultures, lactate was consumed only after glucose and fructose had been exhausted. The yield of lactate increased with decreasing DOT. It is proposed, that an apparent DOT in non-instrumented cultures can be inferred from the lactate yield of bioreactors as a function of DOT. Such a concept, can be a useful and important tool for determining the average dissolved oxygen tension in non-instrumented cultures. It was shown that the dynamic behavior of OUR can be correlated with monosaccharide (fructose and glucose) depletion and viable cell concentration. Accordingly, OUR can have two important applications in insect cell culture: for on-line estimation of viable cells, and as a possible feed-back control variable in automatic strategies of nutrient addition.Abbreviations DOT
Dissolved oxygen tension
- OUR
Oxygen uptake rate
-
specific oxygen uptake rate
-
specific growth rate
- Xv
viable cell concentration
- CL, C*, and
oxygen concentrations in liquid phase, in equilibrium with gas phase, and medium molar concentration, respectively
- H
Henry's constant
- KLa
volumetric oxygen transfer coefficient
- PT
total pressure
-
oxygen partial pressure
-
oxygen molar fraction
- i
discrete element 相似文献
8.
9.
Fluorescent detection-polymerase chain reaction (FD-PCR) assay on microwell plates as a screening test for salmonellas in foods 总被引:7,自引:1,他引:6
R.J. Cano S.R. Rasmussen Gloria Sánchez Fraga J.C. Palomares 《Journal of applied microbiology》1993,75(3):247-253
This study evaluates a polymerase chain reaction assay coupled with a fluorescent detection in microwell plates for salmonellas in food samples. Chelex 100-extracted cultures and bulk and processed food samples were used as templates for a PCR assay in microwell plates, with a primer pair that amplifies a 206 bp segment of IS 200 . The PCR products were then denatured by heat and transferred to CovaLink NH plates (Nunc) to which capture oligonucleotides were covalently bound. Hybridization was performed for 1 h at 55°C, the microwells were washed and an alkaline phosphatase-labelled probe, complementary of an internal sequence of the PCR product, was added. After stringent washes, 100 μl of 1 mmol 1-1 AttoPhosTM (JBL Scientific) was then added to the wells and the fluorescence measurement system (Millipore). The level of detection of the assay was as low as 1–10 cfu. A total of 172 food samples were tested, both by culture and FD-PCR. Of these 53 were culture positive and 119 culture negative. The sensitivity of the FD-PCR assay was 100% and the specificity was 90.1%. Positive and negative predictive values were 82.8 and 100%, respectively. Based on the results obtained in this study it appears that the FD-PCR. assay described here can be useful to screen a large number of food samples for contamination by salmonellas. 相似文献
10.
The application of sensory methodology for measuring deodorizing effect of an air conditioner equipped with electric plasma was introduced. Deodorizing effect was measured using chemical and sensory methods at different time (0, 30 and 60 min) and mode (control, blowing and cooling) of an air conditioner. Smoke from a roll of cigarette in a closed room was used as a source of odor and the concentrations of acetic acid and ammonia were measured as odorous chemical components. As one of the sensory methods triangle test was used and as a first step to obtain deodorizing effects by triangle test, the threshold of each panelist was obtained as the log dilution ratio of odor concentration at which the difference from odorless air was detected. The odor concentration at each time and mode was calculated using the threshold of the panel and the deodorizing effect was obtained on the basis of the odor concentration. In addition to a triangle test, scaling methods such as category scaling or magnitude estimation were used to measure deodorizing effect of an air conditioner. Deodorizing effects by scaling methods were calculated based on odor intensity with time at each mode. The regression analysis was done between the efficacy of deodorizing effect by sensory test and those by acetic acid and ammonia, the R2 values of the regression equations for triangle test, category scale, and magnitude estimation were 0.84, 0.72 and 0.69, respectively. Deodorizing effect by triangle test explained the decrease of acetic acid and ammonia better than those by category scaling or magnitude estimation while high cost and time consuming labor involved in triangle tests reduced the merit. The results of this study demonstrated that various sensory methods could be used to measure deodorizing effect of air conditioners and further researches on fast and reliable methods are needed to establish the official procedures. 相似文献