全文获取类型
收费全文 | 67篇 |
免费 | 10篇 |
国内免费 | 2篇 |
专业分类
79篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2014年 | 2篇 |
2011年 | 3篇 |
2010年 | 4篇 |
2009年 | 3篇 |
2008年 | 2篇 |
2007年 | 1篇 |
2005年 | 1篇 |
2004年 | 3篇 |
2003年 | 3篇 |
2002年 | 1篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有79条查询结果,搜索用时 0 毫秒
51.
52.
The effective population size of Anopheles gambiae in Kenya: implications for population structure 总被引:4,自引:0,他引:4
We estimated current and long-term effective population size (Ne) of two
Anopheles gambiae (savanna cytotype) populations in Kenya. Temporal
variation at nine microsatellite loci in each population sampled 7 and 9
years apart and genetic diversity in each sample were analyzed to answer
the following questions. (1) Do bottlenecks occur in Kenyan populations of
A. gambiae? (2) How variable are different populations with respect to
their current and long-term Ne values? (3) What are the implications of
these results on population structure and history? The estimates of Ne of
Asembo and Jego were 6,359 and 4,258, respectively, and the lower 95%
limits were 2,455 and 1,669, respectively. Thus, despite the typical
observation of low density at the village level during the dry season,
large populations are maintained annually. Large current Ne is consistent
with previous studies showing low differentiation across the continent,
especially under Wright's isolation-by-distance model. Current Ne in Asembo
was 1.5-fold higher than in Jego, but this difference was not significant.
Long-term Ne in Asembo (22,667) was 2.9-fold higher than that in Jego
(7,855) based on the stepwise mutation model. The difference between
populations was significant at both time points regardless of whether
long-term Ne values were calculated based on the stepwise mutation model or
the infinite-alleles model. Heterozygosity in Jego declined significantly
between 1987 (59%) and 1996 (54%), whereas heterozygosity in Asembo was
stable (66%-65%). Despite the relatively high and significant
differentiation between Asembo and Jego (FST = 0.072-0.10, RST = 0.037-
0.038), all alleles in Jego were found in Asembo but not vice versa. All of
these findings suggest that lower Ne in Jego magnifies differentiation
between the two populations. The long-term Ne was biased downward, because
its calculation was based on an upper bound estimate of microsatellite
mutation rate. Ne values based on mtDNA and allozymes were an order of
magnitude higher. Long-term Ne therefore, is probably measured in hundreds
of thousands and hence does not support a recent expansion of this species
from a small population.
相似文献
53.
Arvid WA Kamps Dick Hendriks Jan W Smit Edo Vellenga 《Cancer immunology, immunotherapy : CII》1999,16(1):46-52
The present study focused on whether it is possible to expand monocytic cells from CD34+ progenitor cells by using macrophage colony-stimulating factor (M-CSF) in the absence and presence of mast cell growth factor
(MGF) and IL-6. It was demonstrated that CD34+ cells differentiate without expansion to functional mature monocytic cells in the presence of M-CSF or combinations of M-CSF
plus IL-6 and MGF. A different response pattern was observed for the number of clonogenic cells. The addition of IL-6 or both
IL-6 and MGF to M-CSF containing cultures resulted in significant higher numbers of colony-forming unit-macrophage (CFU-M)
as tested in clonogenic and3H-thymidine assays. Furthermore, M-CSF plus both IL-6 and MGF appeared to be the most potent combination to preserve the monocytic
precursor in cell suspension culture assays. These results indicate that IL-6 and MGF in conjunction with M-CSF affect CD34+ cells especially at precursor level without distinct effect on the more mature stages. Secondly we studied whether M-CSF
is only critical for the monocytic lineage or also affects dendritic cell (DC) development. Indeed, we were able to culture
CD83+ DC from CD34+ progenitor cells in the presence of M-CSF in conjunction with TNF-α, IL-4, and MGF although their absolute number is almost
threefold lower than the number of CD83+ cells yielded from GM-CSF plus TNF-α, IL-4, and MGF stimulated CD34+ cells. 相似文献
54.
A gas chromatographic-electron capture detection (GC-ECD) method for the analysis of deoxynivalenol (DON) in cereals was investigated.
The sample was extracted with a mixture of acetonitrile-water and purified with a MycoSep #225 column. The silylation was
performed with Tri-Sil-TBT reagent, followed by dilution with hexane and a washing step with buffer. By using Tri-Sil-TBT
reagent no double peaks were observed for DON in the gas chromatograms, in comparison with two other silylation reagents TMSI
and Tri-Sil-Z. The use of trichothecolone (TRI) as an internal standard for DON was studied in order to indicate possible
problems in the derivatisation reaction. TRI proved to be a relatively good internal standard for DON in cereal samples, as
well as 1,1-bis-(4-chlorophenyl)-2,2-dichloroethylene (DDE), which was used as a GC standard for ensuring the function of
GC-ECD. During the study, a matrix effect was clearly observed between the cereal matrix-assisted calibration curve and the
calibration curve prepared without cereal matrix. The results of spiked and reference material samples, quantified with the
calibration curve prepared without and with matrix, demonstrated that the matrix affects the results. However, after recovery
correction the results were comparable. The validation results demonstrated that the GC-ECD method for DON analysis in cereals
is sufficiently reliable. 相似文献
55.
The thermal coefficient of expansion of egg lecithin bilayer thickness, αd1, was measured as a function of its cholesterol content up to mole ratio lecithin/cholesterol of 1:1, and over the temperature range 0–40 °C. At all cholesterol contents αd1 changes abruptly at approximately 12 °C indicating a structural transition at this temperature. Above 12 °C, αd1 decreases monotonically from −2·10−3 for pure egg lecithin to −1·10–3 at mole ratio 1:1. Below 12 °C αd1 is walways higher than above 12 °C and shows a sharp, anomalously high value of −6·10−3 at the mole ratio 2:1. The results have been interpreted as the movement of cholesterol into the bilayer or the formation of lecithin-cholesterol “complexes” at temperatures below 12 °C. Similar studies with phosphatidylinositol containing cholesterol showed no structural transition and lysolecithin containing cholesterol behaved differently giving two lamellar phases in equilibrium. 相似文献
56.
The interaction of Ca2+ and Mg2+ with phosphatidylserine (PS) vesicles in 0.1 M NaCl aqueous solution was studied by equilibrium dialysis binding, X-ray diffraction, batch microcalorimetry, kinetics of cation-induced vesicle aggregation, release of vesicle contents, and fusion. Addition of either cation causes aggregation of PS vesicles and produces complexes with similar stoichiometry (1:2 cation/PS) at saturating concentrations, although the details of the interactions and the resulting complexes are quite different. Addition of Ca2+ to PS vesicles at T greater than or equal to 25 degrees C induces the formation of an "anhydrous" complex of closely apposed membranes with highly ordered crystalline acyl chains and a very high transition temperature (Tc greater than 100 degrees C). The formation of this complex is accompanied by a release of heat (5.5 kcal/mol), rapid release of vesicle contents, and fusion of the vesicles into larger membranous structures. By contrast, addition of Mg2+ produces a complex with PS which is much more hydrated, has no crystallization of the acyl chains at T greater than or equal to 20 degrees C, and has comparatively little fusion. Studies with both Ca2+ and Mg2+ added simultaneously indicate that there is a synergistic effect between the two cations, which results in an enhancement of the ability of Ca2+ to form its specific complex with PS at lower concentrations. The presence of the erythrocyte protein "spectrin" inhibits this synergism and interferes with the formation of the specific PS/Ca complex. It also inhibits the fusion of PS vesicles. It is proposed that the unique PS/Ca complex, which involves close apposition of vesicle membranes, is an intermembrane "trans" complex. We further propose that such a complex is a key step for the resultant phase transition and fusion of PS vesicles. By contrast, the PS/Mg complex is proposed to be a "cis" complex with respect to each membrane. The results are discussed in terms of the mechanism of membrane fusion. 相似文献
57.
目的:探讨胞苷酸鸟苷寡脱氧核苷酸(CpG ODN)联合铝佐剂对丙型肝炎病毒(HCV)重组免疫原的体液免疫作用。方法:采用高交叉HCV-HVR1和E1重组蛋白与CpG ODN、铝佐剂组合,免疫BALB/c小鼠后以ELISA、酶联免疫斑点测定、流式细胞术、免疫沉淀等方法检测相关体液免疫指标和免疫血清多抗的交叉反应性。结果:CpG联合铝佐剂激发了最高的特异性抗体滴度;佐剂通过提高抗体分泌细胞数量、增加脾脏中记忆B细胞数量、增加脾淋巴细胞IL-6、IL-10分泌浓度实现体液免疫增效;CpG则能提高免疫效率,联合铝佐剂时显著提高浆细胞数量;12份HCV阳性血清中有10份可与多抗HVR1 IgG发生免疫沉淀。结论:CpG和铝佐剂联合应用具有协同作用,多抗HVR1 IgG具有较好的交叉反应性。 相似文献
58.
Sterling, Clarence, and Jack Pangborn. (U. California, Davis.) Fine structure of potato starch. Amer. Jour. Bot. 47(7) : 577–582. Illus. 1960.—Electron micrographs were made from replicas of fracture surfaces of Lintnerized potato starch. These showed that much of the starch substance is organized into radially arranged microfibrils of 220–320 A diameter and considerably greater length (at least over 4000 A). The microfibrils have parallel longitudinal ridges on their surfaces. These ridges are conceived to be outer projections of micellar strands' which are 80–90 A in diameter and occasionally at least up to 4000 A long. The diametral dimension was confirmed by X-ray diffraction study of moist and dry, normal and Lintnerized potato starch. The X-ray evidence also supported the electron micrographic interpretation that amorphous regions lie between the crystalline micelles. On the basis of X-ray data, it was speculated that the molecules in a microfibril are all oriented alike. 相似文献
59.
Incubation of kidney lysosomes at 37° results in a graded release of lysosomal enzymes. The release of enzyme occurs in two stages. First the enzymes become available to the substrate but remain sedimentable. Later the amount of soluble enzyme increases and eventually is almost equal to that of the available enzyme. Morphological studies of lysosomes showed that during the process involving increasing availability of enzymes, the lysosomes remained intact. Release of the soluble enzymes was characterized ultrastructurally by a complete loss of the electron-opaque matrix contained within the lysosomal membrane. The increased release of soluble enzymes was concomitant with an increase in the number of individual lysosomes showing complete loss of contents, rather than a gradual loss or dilution of matrix density. Lysosomes which had lost their electron-opaque contents retained their outer membrane intact and were seen to contain numerous internal membranes and small vesicles. 相似文献
60.
CryIIIB2, an insecticidal protein from Bacillus thuringiensis has been crystallized from 0.6 M NaBr and HEPES buffer at pH 7.0 and X-ray diffraction data collected on a native crystal to 2.4 Å. The insecticidal protein was obtained from a Bacillus thuringiensis (Bt) strain EG7231. Crystals of the endotoxin are orthorhombic, space group C2221, with unit cell dimensions of a = 122.44, b = and c = Å. A unit cell contains one molecule of the 67,000 Da endotoxin per asymmetric unit. © 1992 Wiley-Liss, Inc. 相似文献