全文获取类型
收费全文 | 18255篇 |
免费 | 1635篇 |
国内免费 | 10篇 |
专业分类
19900篇 |
出版年
2022年 | 131篇 |
2021年 | 205篇 |
2020年 | 144篇 |
2019年 | 192篇 |
2018年 | 236篇 |
2017年 | 234篇 |
2016年 | 332篇 |
2015年 | 692篇 |
2014年 | 758篇 |
2013年 | 954篇 |
2012年 | 1164篇 |
2011年 | 1211篇 |
2010年 | 799篇 |
2009年 | 751篇 |
2008年 | 1024篇 |
2007年 | 1109篇 |
2006年 | 1037篇 |
2005年 | 1044篇 |
2004年 | 1026篇 |
2003年 | 847篇 |
2002年 | 849篇 |
2001年 | 264篇 |
2000年 | 216篇 |
1999年 | 243篇 |
1998年 | 274篇 |
1997年 | 161篇 |
1996年 | 133篇 |
1995年 | 124篇 |
1994年 | 141篇 |
1993年 | 128篇 |
1992年 | 157篇 |
1991年 | 142篇 |
1990年 | 124篇 |
1989年 | 150篇 |
1988年 | 112篇 |
1987年 | 135篇 |
1986年 | 109篇 |
1985年 | 136篇 |
1984年 | 125篇 |
1983年 | 119篇 |
1982年 | 136篇 |
1981年 | 121篇 |
1980年 | 97篇 |
1979年 | 93篇 |
1978年 | 83篇 |
1977年 | 95篇 |
1976年 | 73篇 |
1974年 | 93篇 |
1973年 | 79篇 |
1972年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
51.
The isolation and immunolocalization of iron-binding compounds produced by Gloeophyllum trabeum 总被引:2,自引:0,他引:2
Jody Jellison Vikas Chandhoke Barry Goodell Frank A. Fekete 《Applied microbiology and biotechnology》1991,35(6):805-809
Summary Low molecular weight iron-binding compounds are produced by the brown-rot fungus Gloeophyllum trabeum. These chelators may function in scavenging transition metals for fungal metabolism and extracellular enzyme production. Because of the low molecular mass of the chelate-metal complex (below 1000 Da), and the oxidizing potential of the bound transition metals, certain chelating compounds could also play a role in the early stages of cellulose depolymerization by brown-rot fungi. High-affinity iron-binding compounds were isolated and partially purified from both liquid cultures of the brown-rot Gloeophyllum trabeum and from infected wood. Chelating compounds purified by thin-layer chromatography were used to prepare specific antibodies. These antibodies were shown to detect the chelator in infected wood and liquid fungal cultures by enzyme-linked immunosorbent assay and could be used in immunotransmission electron microscopy to visualize the high-affinity iron-binding compounds in situ. Elucidating the physiological roles of fungal chelate-metal complexes and determining their function in lignocellulose depolymerization will help us to better understand the mechanism of wood biodegradation.Publication no. 1549 Maine Agricultural Experiment Station
Offprint requests to: J. Jellison 相似文献
52.
Determination of the origin of nondisjunction in a 49,XXXXY male using hypervariable dinucleotide repeat sequences 总被引:1,自引:1,他引:0
Summary We present a patient with a 49,XXXXY chromosome constitution in whom the origin of the extra X chromosomes was determined by analysis of five polymorphic CA (or GT) dinucleotide repeat sequences. This class of DNA marker has recently been demonstrated to be hypervariable with heterozygosity values up to 80%. By polymerase chain reaction (PCR) analysis of the dinucleotide repeat length polymorphisms, we have shown that all four X chromosomes were of maternal origin. 相似文献
53.
Lymphocyte activation and serine-esterase induction following recombinant interleukin-2 infusion for lymphomas and acute leukaemias 总被引:3,自引:0,他引:3
Seah H. Lim Colin Worman Andrew Jewell Chris Tsakona Frank J. Giles Anthony Goldstone 《Cancer immunology, immunotherapy : CII》1991,33(2):133-137
Summary C57BL mice inoculated with radiation leukemia virus (RadLV) develop preleukemic cells long before the onset of leukemia. These cells are potentially immunogenic but fail to elicit an immune response in the host because of the appearance of virus-specific suppressor T cells. We have studied the effect of polysaccharide K (PSK) on the generation of RadLV-specific cell-mediated immune responses in vitro. Long-term exposure to PSK in culture potentiated the ability of immunized T cells to respond to a RadLV-induced lymphoma. It also abrogated the suppressive activity of suppressor T cells and simultaneously boosted the ability of reactive T cells to respond. The dual immunostimulating activity of PSK resulted in the generation of T cytotoxic lymphocytes that could lyse lymphoma cells in vitro. The results suggest that PSK could be used as a prophylactic immune response modifier in preleukemia. 相似文献
54.
Z L Hegedus H A Frank T I Steinman M D Altschule U Nayak 《Archives internationales de physiologie et de biochimie》1988,96(5):211-221
The fluorescence excitation and emission spectra observed in plasma from patients with chronic renal failure were reproduced by the generation of soluble lipofuscins in normal plasma samples by incubation with mixtures of L-dopa, dopamine, L-norepinephrine, L-epinephrine, 3-hydroxy-DL-kynurenine and 3-hydroxy-anthranilic acid for 24 h at 37 degrees C. Relative fluorescence intensity measurements consistently showed elevated plasma levels of the soluble lipofuscins in chronic renal failure: the means (n = 27) were 73.9 +/- 33.4 (SD) and 71.1 +/- 14.8 at emissions 413 nm and 445 nm respectively, in contrast to those of normal plasma samples (n = 11), 18.2 +/- 5.3 and 23.1 +/- 5.6. The maximum or shoulder at approximately 413 nm represents soluble lipofuscin that can be generated from 3-hydroxyanthranilic acid and the maximum or shoulder at approximately 445 nm represents soluble lipofuscins derived from the precursors listed above and probably from other related precursors. Gravimetric measurements also showed elevated levels of melanins in the plasma samples of patients with chronic renal failure: 2.72 +/- 0.38 mg/ml (n = 16), as compared to normal values: 1.70 +/- 0.10 mg/ml (n = 6). In individual patients haemodialysis reduced the fluorescence intensities to a range of 65-99% and the melanin levels to a range of 86-99% of the pre-dialysis values. 相似文献
55.
Kenneth D. Vernick Frank H. Collins Douglas C. Seeley Robert W. Gwadz Louis H. Miller 《Biochemical genetics》1988,26(5-6):367-379
The main polymorphic system of esterase isoenzymes in adults of the G3 laboratory strain ofAnopheles gambiae consists of two to five major bands of activity per individual. The bands are designated 5S, 5F, 13, 14, and 15. In genetic
crosses, the genes which coded for the bands assorted as three codominant alleles, Est A, Est B, and Est C, at a single autosomal
locus. Homozygotes for the Est C allele were significantly underrepresented among backcross progeny. The developmental pattern
of esterase expression was examined. Esterase gene expression in embryos was first detectable between 2 and 12 hr after oviposition.
The initiation or termination of expression of some of the bands corresponded to boundaries between developmental stages.
Most of the esterase fractions were not specifically localized within the tissues tested, with the exception of a series of
bands which were restricted largely to adult male testes. 相似文献
56.
Of 24 ethyl methanesulphonate-induced, recessive-lethal mutations in the region 9E1-9F13 of the X chromosome of Drosophila melanogaster, eight fall into a typically homogeneous lethal complementation group associated with the raspberry (ras) locus. Mutations in this group have previously been shown to be pleiotropic, affecting not only ras but also two other genetic entities, gua 1 and pur 1, which yield auxotrophic mutations.--The eight new mutations have been characterized phenotypically in double heterozygotes with gua 1, pur 1 and ras mutations. Despite their homogeneity in lethal complementation tests, the mutations prove quite diverse. For example, two mutations have little or no effect on eye color in double heterozygotes with ras2. The differences between the lethals are allele-specific and cannot be explained as a trivial outcome of a hypomorphic series.--Taken alone, the lethal complementation studies mask the complexity of the locus and the diversity of its recessive lethal alleles. By extension, we argue that the general use of lethal saturation studies provides an unduly simplified image of genetic organization. We suggest that the reason why recessive lethal mutations rarely present complex complementation patterns is that complex loci tend to produce mutations that affect several subfunctions. 相似文献
57.
58.
Frank E. Poirier 《American journal of physical anthropology》1985,66(2):113-113
Empirical analyses and models of the lineal effects of fission indicate that considerable genetic differentiation may occur at the time of group formation, thus confusing the usual positive relationship between historical affiliation and genetic differentiation. We analyze the effects of fission pattern on variation in highly heritable morphological traits among eight social groups on Cayo Santiago. The analysis is performed using general network autocorrelation methods that quantitatively and directly measure the amount of variation in social group mean morphology that can be explained by fission. All of the fission autocorrelation coefficients are strongly negative, indicating that groups most recently formed by fission are most dissimilar. Also, most of the variation between groups can be explained by the fission pattern, indicating that lineal fission is the most important process generating between-group variation on Cayo Santiago. 相似文献
59.
Summary A model has been developed for 5-nitroxide stearate, I(12,3), distribution in human erythrocyte ghosts which accurately predicts ESR spectral alterations observed with increased probe/total lipid (P/L) at 37°C. This spin probe occupies a class of high-affinity, noninteracting sites at low loading. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membranebound clusters of variable size. No low probe remains at high P/L where all I(12,3) clusters in a concentrated phase. This model allows determination of the dilute/clustered probe ratio, and shows that I(12,3) segregates in erythrocytes at what might otherwise be considered low P/L (e.g., 1/359). These findings validate the earlier use of empirical parameters to estimate probe sequestration in biological membranes. 相似文献
60.
Computer-Assisted Image Analysis of Plant Growth, Thigmomorphogenesis, and Gravitropism 总被引:2,自引:0,他引:2 下载免费PDF全文
Mordecai J. Jaffe Andrew H. Wakefield Frank Telewski Edward Gulley Ronald Biro 《Plant physiology》1985,77(3):722-730
A nonintrusive auxonometric system, based on the DARWIN image processor (Telewski et al. 1983 Plant Physiol 72: 177-181), is described and demonstrated in the analysis of gravitropism and thigmomorphogenesis in corn seedlings (Zea mays). Using this system, growth and bending of regularly shaped plants or organs can be quickly and accurately measured without, in any way, interfering with the plant. Furthermore, the growth and bending curves are automatically plotted. Thigmomorphogenesis in the aerial part of corn seedlings involves growth promotion at a low force load and growth retardation at higher force loads. The time courses of the two kinds of response are somewhat different, with retardation occurring immeditely after mechanical perturbation and growth promotion taking somewhat longer to begin. Gravitropic experiments show that when dark-grown corn seedlings are placed on their side in the light, the resulting curvature is due to two consecutive morphological mechanisms. In the first instance, lasting for about 15 minutes, the elongation of the bottom edge of the plant accelerates, while the elongation of the top edge remains constant. After that, for the next 1.75 hours, the elongation of the top edge decelerates and stops while that of the bottom edge remains constant at the increased rate for most of the period. The measurements taken from both experiments at relatively high resolution (0.08-0.1 millimeter) show that the growth curves are not smooth but show many small irregularities which may or may not involve micronutations. 相似文献