全文获取类型
收费全文 | 3052篇 |
免费 | 299篇 |
国内免费 | 1篇 |
专业分类
3352篇 |
出版年
2023年 | 20篇 |
2022年 | 29篇 |
2021年 | 43篇 |
2020年 | 40篇 |
2019年 | 47篇 |
2018年 | 48篇 |
2017年 | 53篇 |
2016年 | 72篇 |
2015年 | 148篇 |
2014年 | 152篇 |
2013年 | 169篇 |
2012年 | 264篇 |
2011年 | 246篇 |
2010年 | 146篇 |
2009年 | 136篇 |
2008年 | 208篇 |
2007年 | 189篇 |
2006年 | 165篇 |
2005年 | 175篇 |
2004年 | 171篇 |
2003年 | 159篇 |
2002年 | 155篇 |
2001年 | 35篇 |
2000年 | 25篇 |
1999年 | 28篇 |
1998年 | 36篇 |
1997年 | 24篇 |
1996年 | 20篇 |
1995年 | 33篇 |
1994年 | 15篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 17篇 |
1990年 | 20篇 |
1989年 | 12篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1981年 | 7篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 7篇 |
1976年 | 12篇 |
1974年 | 13篇 |
1973年 | 8篇 |
1969年 | 6篇 |
1967年 | 7篇 |
排序方式: 共有3352条查询结果,搜索用时 15 毫秒
31.
Holger Hackstein Gerhard Jahn Holger Kirchner Gregor Bein 《Histochemistry and cell biology》1996,106(2):229-234
Radioactive in situ hybridization techniques or enzymatic detection procedures of hapten-modified human cytomegalovirus (HCMV)
probes have been widely used for studying the infection of peripheral blood leukocytes with HCMV. This report describes significant
improvements in terms of signal resolution which can be obtained by applying a highly sensitive fluorescence in situ hybridization
(FISH) technique in conjunction with a large subgenomic HCMV DNA probe. Three cosmid clones spanning 119.1 kb of the HCMV
genome (230 kb) were used to construct the digoxigenin-11-dUTP-labeled probe which was found to be superior to a total HCMV
probe representing the entire genome. Crucial hybridization parameters were analyzed systematically in order to ensure optimal
resolution power and sensitivity. The protocol was successfully applied to HCMV-infected fibroblasts and peripheral blood
leukocytes of 12 transplant patients and unambiguously facilitated the precise intracellular localization of HCMV genomes
in infected cells. Because of its excellent resolution properties, accompanied by the virtual absence by any background staining,
we recommend the use of this protocol as a sensitive approach for further virological analyses of the interactions between
HCMV and peripheral blood leukocytes at the single-cell level.
Accepted: 16 February 1996 相似文献
32.
33.
William r. Anderson Willi Frick G.Doyle Daves Douglas F. Barofsky Isomaro Yamaguchi Ding Chang Karl Folkers Sune Rosell 《Biochemical and biophysical research communications》1977,78(1):372-376
Mass spectra of underivatized hexa- and heptapeptide amides related to Substance P have been obtained with a conventional electron ionization mass spectrometer using sample vaporization from a tungsten wire by the technique of rapid heating, proton transfer ionization using ammonia, and photoplate recording of spectra. These spectra exhibit little evidence of sample pyrolysis and are readily interpreted to yield amino acid sequences. 相似文献
34.
Analysis of the distribution of 35S-sulfate and 14C-glutamate in major biochemical components of the two marine bacteria, Pseudomonas halodurans and Alteromonas luteo-violaceus, was compared with cell density and total cellular protein during exponential growth in batch culture. For both organisms, the sulfur distribution was restricted principally to the low molecular weight organic and protein fractions, which together accounted for over 90% of the total sulfur. Carbon was more widely distributed, with these two fractions containing only 70% of the total label.Growth rate constants calculated from increases in cell numbers, protein, and 35S and 14C in the various fractions indicated nearly balanced growth in A. luteo-violaceus, with constants derived from all biosynthetic parameters agreeing within 5% during the exponential phase. In contrast, protein synthesis and 35S incorporation into residue protein constants were 30% higher than constants derived from cell counts and incorporation of 14C in P. halodurans. Therefore the cellular protein content P. halodurans varied over a two-fold range, with maximum protein per cell in the late exponential phase. A distinct reduction in the rate constants for total protein and 35S incorporation into residue protein foreshadowed entry into the stationary phase more than one generation before other parameters.Incorporation of 35S-sulfate into residue protein paralleled protein synthesis in both bacteria. The weight percent S in protein agreed well with the composition of an average protein derived from the literature. Sulfur incorporation into protein may be a useful measurement of marine bacterial protein synthesis.Abbreviations L.M.W.
low molecular weight
- TCA
trichloroacetic acid
- CFU
colony forming unit 相似文献
35.
Assimilatory Sulfur Metabolism in Marine Microorganisms: Considerations for the Application of Sulfate Incorporation into Protein as a Measurement of Natural Population Protein Synthesis 总被引:8,自引:6,他引:2
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The sulfur content of residue protein was determined for pure cultures of Nitrosococcus oceanus, Desulfovibrio salexigens, 4 mixed populations of fermentative bacteria, 22 samples from mixed natural population enrichments, and 11 nutritionally and morphologically distinct isolates from enrichments of Sargasso Sea water. The average 1.09 ± 0.14% (by weight) S in protein for 13 pure cultures agrees with the 1.1% calculated from average protein composition. An operational value encompassing all mixed population and pure culture measurements has a coefficient of variation of only 15.1% (n = 41). Short-term [35S]sulfate incorporation kinetics by Pseudomonas halodurans and Alteromonas luteoviolaceus demonstrated a rapid appearance of 35S in the residue protein fraction which was well modelled by a simple exponential uptake equation. This indicates that little error in protein synthesis determination results from isotope dilution by endogenous pools of sulfur-containing compounds. Methionine effectively competed with sulfate for protein synthesis in P. halodurans at high concentrations (10 μM), but had much less influence at 1 μM. Cystine competed less effectively with sulfate, and glutathione did not detectably reduce sulfate-S incorporation into protein. [35S]sulfate incorporation was compared with [14C]glucose assimilation in a eutrophic brackish-water environment. Both tracers yielded similar results for the first 8 h of incubation, but a secondary growth phase was observed only with 35S. Redistribution of 14C from low-molecular-weight materials into residue protein indicated additional protein synthesis. [35S]sulfate incorporation into residue protein by marine bacteria can be used to quantitatively measure bacterial protein synthesis in unenriched mixed populations of marine bacteria. 相似文献
36.
Microbial Activities in Undecompressed and Decompressed Deep-Seawater Samples 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Microbial transformations of 14C-labeled substrates (sodium glutamate, Casamino Acids, glucose, and sodium acetate) were measured in undecompressed seawater samples collected from depths of 1,800 to 6,000 m, during 14- to 21-day incubation periods at in situ temperature (3°C). Each substrate was tested at two concentrations (ca. 0.5 and 5.0 μg/ml) and two in situ pressures. The data were compared to 1-atmosphere (ca. 1.013 × 102 kPa) controls. The rates of 14C incorporation and 14CO2 production as well as the amounts of total substrate utilization were generally lower at pressure than in the decompressed controls but were significantly different for each of the four substrates used. The utilization of acetate was the least affected by pressure; rates were similar to those measured at 1 atmosphere in two out of four experiments. In contrast, transformation rates of the amino acids at pressure averaged to only 38% of those in the controls. A single but reproducible “barophilic” response was observed with glucose as a substrate in samples collected from a depth of 4,500 m at a specific area in the northwestern Atlantic Ocean. Except for this latter set of experiments, the transformation of all substrates showed an increased lag period at pressure as compared to the 1-atmosphere controls. 相似文献
37.
Summary Cell recovery by means of continuous flotation of the Hansenula polymorpha cultivation medium without additives was investigated as a function of the cultivation conditions as well as of the flotation equipment construction and flotation operational parameters. The cell enrichment and separation is improved at high liquid residence times, high aeration rates, small bubble sizes, increasing height of the aerated column, and diameter of the foam column. Increasing cell age and cultivation with nitrogen limitation reduce the cell separation.Symbols CP
cell mass concentration in medium g·l–1
- CR
cell mass concentration in residue g·l–1
- CS
cell mass concentration in foam liquid g·l–1
-
V
equilibrium foam volume cm3
- V
gas flow rate through the aerated liquid column cm3·s–1
- VF
feed rate to the flotation column ml/min
-
1
V
S/V foaminess s
-
mean liquid residence time in the column s 相似文献
38.
The seasonal change in gut contents of nymphs of Isoperla grammatica and I. difformis from six streams in southern Sweden was analysed. Both species had ingested a variety of benthic prey and vegetable matter, predominantly diatoms. Some seasonality was evident with high percentages of diatoms in spring in I. grammatica, and in autumn in I. difformis. The scope of food was larger in the latter species which contained about equal proportions of vegetable matter, chironomids, mayfly, stonefly, and black fly larvae. In I. grammatica plant matter and chironomids dominated strongly, comprising > 50% of the gut contents on an annual basis, > 90 % in spring. While small nymphs of I. difformis contained a low proportion of animal matter, only gradually increasing with size, the nymphs of I. grammatica were carnivorous from very early instars. Both species switched to a temporarily strong utilization of algae in spring. This differed among sites, and appeared to reflect differences in insolation and thus the availability of algae. There was a significant negative relationship between the mean densities of Isoperla nymphs and the proportion of animal material found in the guts of I. grammatica (R
2 = 0.86). Considering the density of I. grammatica alone, the relationship was weaker (R
2 = 0.56). A positive correlation between predator and prey size was observed. With chironomid prey the size range increased with predator size. With simuliid prey, however, prey size increased with predator size in such a way that it suggests selection rather than just an expanding prey size range. Correlations were stronger and regression coefficients significantly higher for I. grammatica than for I. difformis. We suggest that I. grammatica, which ingests a much wider size range of prey might choose prey of optimal sizes more readily than the more synchronously developing I. difformis. Although the life cycles of the two species are staggered, overlap in size distribution indicates that competition for food could be important in spring. However, observed differences in diet should facilitate coexistence. Gut content differences might in turn be accomplished through microhabitat segregation. 相似文献
39.
The reaction of methyl 4,6-O-benzylidene-3(2)-deoxy--
-erythro-hexopyranosid-2(3)-ulose with carbon disulfide, alkyl iodide, and sodium hydride gave methyl 4,6-O-benzylidene-3(2)-[bis(alkylthio)methylene]-3(2)-deoxy--
-erythro-hexopyranosid-2(3)-uloses. Methyl 4,6-O-benzylidene-2-[bis(methylthio)methylene]-2-deoxy--
-erythro-hexopyranosid-3-ulose (5) reacted with aromatic amines to give, in a rearrangement process, N-aryl-2-aryliminomethyl-4,6-O-benzylidene-2-deoxy--
-erythro-hex-1-enopyranosylamin-3-uloses. The reaction of 5 which hydrazine hydrate afforded 5-methylthio-(methyl-4,6-O-benzylidene-2,3-dideoxy--
-erythro-hexopyranosido)[3,2-c]pyrazole. 相似文献
40.
The gene for the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT) is expressed at a low level in many cells. As is the case with several other “housekeeping genes,” thorough studies of hprt gene regulation have been hampered by the low levels of its mRNA. We have used RNA/RNA hybridization in solution to determine the concentration of hprt-RNA in human cells. The sensitivity and specificity of the method have been validated, and it is shown that hprt-RNA can be accurately determined at a level of a few mRNA molecules per cell. As expected for a housekeeping gene, low and relatively constant hprt-RNA levels (0.3–0.8 pg/μg DNA) were found in primary cultures of normal amnion cells and fibroblasts, EBV-transformed lymphoblastoid cell lines, neuroblastoma, glioblastoma, and melanoma cell cultures. While resting lymphocytes were found to contain very low amounts of hprt-RNA, lymphocytes stimulated with phytohemagglutinin (PHA) showed a 10-fold increase to about 0.8–1.2 pg/μg DNA, which corresponds to 6–10 hprt-RNA molecules per cell. The level started to increase about 20 h after PHA stimulation, 5–10 h before the onset of DNA synthesis, and a steady-state level was reached after 2–3 days in culture. In PHA-stimulated lymphocytes from two brothers with inherited HPRT deficiency (LeschNyhans syndrome), the hprt-RNA level in PHA-stimulated lymphocytes was only about 25% of that in normal subjects. In T-cells selected for HPRT deficiency by growth in 6-thioguanine medium, the levels of hprt-RNA were either normal or very low, which probably reflects the different nature of the mutations involved. These results demonstrate the sensitivity of this method for determinations of low levels of RNA and clearly show induction of hprt-RNA after mitogenic stimulation of human lymphocytes. 相似文献