全文获取类型
收费全文 | 1250篇 |
免费 | 18篇 |
国内免费 | 102篇 |
出版年
2022年 | 3篇 |
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 24篇 |
2018年 | 29篇 |
2017年 | 20篇 |
2016年 | 24篇 |
2015年 | 11篇 |
2014年 | 26篇 |
2013年 | 46篇 |
2012年 | 28篇 |
2011年 | 42篇 |
2010年 | 20篇 |
2009年 | 41篇 |
2008年 | 49篇 |
2007年 | 60篇 |
2006年 | 56篇 |
2005年 | 37篇 |
2004年 | 56篇 |
2003年 | 51篇 |
2002年 | 86篇 |
2001年 | 55篇 |
2000年 | 60篇 |
1999年 | 62篇 |
1998年 | 43篇 |
1997年 | 53篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 22篇 |
1993年 | 25篇 |
1992年 | 26篇 |
1991年 | 47篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 21篇 |
1984年 | 21篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有1370条查询结果,搜索用时 78 毫秒
31.
本文对鸭跖草科的穿鞘花Amischotolypehispida(LessetA.Rich.)Hong进行了核形态研究。其静止核和有丝分裂前期染色体分别属于复杂中央染色微粒型和中间型;染色体数目为2n=36,核型公式为2n=4m+28sm+4st。本种的核型为首次报道。 相似文献
32.
33.
34.
Assessment of a model for intron RNA secondary structure relevant to RNA self-splicing--a review 总被引:36,自引:0,他引:36
A widespread class of introns is characterized by a particular RNA secondary structure, based upon four conserved nucleotide sequences. Among such "class I" introns are found the majority of introns in fungal mitochondrial genes and the self-splicing intron of the large ribosomal RNA of several species of Tetrahymena. A model of the RNA secondary structure, which must underlie the self-splicing activity, is here evaluated in the light of data on 16 further introns. The main body or "core structure" of the intron always consists of the base-paired regions P3 to P9 with the associated single-stranded loops, with P2 present also in most cases. Two minority sub-classes of core structure occur, one of which is typical of introns in fungal ribosomal RNA. Introns in which the core structure is close to the 5' splice site all have an internal guide sequence (IGS) which can pair with exon sequences adjacent to the 5' and 3' splice sites to align them precisely, as proposed by Davies et al. [Nature 300 (1982) 719-724]. In these cases, the internal guide model allows us to predict correctly the exact location of splice sites. All other introns probably use other mechanisms of alignment. This analysis provides strong support for the RNA splicing model which we have developed. 相似文献
35.
Alwyn H. Gentry 《Plant Systematics and Evolution》1981,137(1-2):95-105
Field studies in Amazonian Peru show that thePassiflora vitifolia complex, a microcosm of many larger taxa, is concentrated in Amazonia with 4 ecologically separated species. One species is restricted to seasonally inundated forests, a second to non-inundated white sand soils, a third to non-inundated lateritic soils, and the fourth mostly to non-inundated alluvial soils. A new combination is formally proposed to recognize the white sand taxon, previously regarded as a variety, as a distinct species:P. involucrata. The importance of -diversity in maintaining high Amazonian species diversity is emphasized. 相似文献
36.
37.
Shigenobu Kanba Nobuyuki Sasakawa Toshio Nakaki Kiyoko-Shimizu Kanba Gohei Yagi Ryuichi Kato Elliott Richelson 《Journal of neurochemistry》1991,57(6):2011-2015
Prostaglandin E1 (PGE1)-mediated transmembrane signal control systems were investigated in intact murine neuroblastoma cells (clone N1E-115). PGE1 increased intracellular levels of total inositol phosphates (IP), cyclic GMP, cyclic AMP, and calcium ([Ca2+]i). PGE1 transiently increased inositol 1,4,5-trisphosphate formation, peaking at 20 s. There was more than a 10-fold difference between the ED50 for PGE1 at cyclic AMP formation (70 nM) and its ED50 values at IP accumulation (1 microM), cyclic GMP formation (2 microM), and [Ca2+]i increase (5 microM). PGE1-mediated IP accumulation, cyclic GMP formation, and [Ca2+]i increase depended on both the concentration of PGE1 and extracellular calcium ions. PGE1 had more potent intrinsic activity in cyclic AMP formation, IP accumulation, and cyclic GMP formation than did PGE2, PGF2 alpha, or PGD2. A protein kinase C activator, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate, had opposite effects on PGE1-mediated IP release and cyclic GMP formation (inhibitory) and cyclic AMP formation (stimulatory). These data suggest that there may be subtypes of the PGE1 receptor in this clone: a high-affinity receptor mediating cyclic AMP formation, and a low-affinity receptor mediating IP accumulation, cyclic GMP formation, and intracellular calcium mobilization. 相似文献
38.
Classical Conditioning-Induced Changes in Low-Molecular-Weight GTP-Binding Proteins in Rabbit Hippocampus 总被引:1,自引:0,他引:1
T. J. Nelson J.-V. Sanchez-Andres B. G. Schreurs D. L. Alkon 《Journal of neurochemistry》1991,57(6):2065-2069
Classical conditioning of Hermissenda, involving paired light-rotation events, results in a 30-35% decrease in the levels of a 20-kDa G protein (cp20). To test whether a similar protein exists in vertebrates, rabbits were trained to associate a tone with periorbital electrical stimulation and G proteins were analyzed by photoaffinity labeling with [alpha-32P]GTP-azidoanilide. A 20-kDa G protein similar to cp20 decreased by 36% in the hippocampus of rabbits subjected to paired tone and electrical stimulation, but not in unpaired controls. Learning-specific decreases were also found in the amount of ras protein. 相似文献
39.
Jean C. Delumeau François Petitet Jocelyne Cordier Jacques Glowinski Joël Prémont 《Journal of neurochemistry》1991,57(6):2026-2035
The effects on cytosolic Ca2+ concentration of 2-chloroadenosine and [L-Pro9]-substance P, a selective agonist of NK1 receptors, were investigated on astrocytes from embryonic mice in primary culture. Cells responded to [L-Pro9]-substance P with a transitory increase in cytosolic Ca2+ which was of shorter duration when external Ca2+ was removed. A transient response to 2-chloroadenosine alone occurred. When simultaneously applied, [L-Pro9]-substance P and 2-chloroadenosine evoked a prolonged elevation of cytosolic Ca2+ (up to 30 min). This phenomenon was dependent on the presence of extracellular Ca2+, but insensitive to dihydropyridines, La3+, and Co2+, excluding the implication of voltage-operated Ca2+ channels. Arachidonic acid also induced a sustained elevation of cytosolic Ca2+, but did not increase further the response evoked by [L-Pro9]-substance P and 2-chloroadenosine. The activation of protein kinase C by a diacylglycerol analogue mimicked the effect of [L-Pro9]-substance P in potentiating the 2-chloroadenosine-evoked response. Like 2-chloroadenosine, pinacidil, which hyperpolarizes the cells by opening K+ channels, prolonged the elevation of cytosolic Ca2+ concentration induced by [L-Pro9]-substance P. Conversely, depolarization with 50 mM KCl canceled the effects of either pinacidil or 2-chloroadenosine applied with [L-Pro9]-substance P. Pertussis toxin pretreatment suppressed all the effects induced by 2-chloroadenosine. 相似文献
40.
温度和食物对稻虱缨小蜂发育、存活和繁殖的影响 总被引:5,自引:0,他引:5
在白背飞虱(Sogatella furcifera(Hovàrth))卵内,稻虱缨小蜂(Anagrus nilparvatae Pang et Wang)的发育起点温度为10.6℃,有效积温为162.3日度,随着温度上升,发育加快;寿命(y_1,D_(10.4)~0C)、产卵量(y_2,粒/♀)、产出卵率则在适宜温度范围(23—26℃)内最大,高低温区内均呈下降趋势,其方程分别为:y_1=exp(-0.04187+0.3612x-7.4654×10~(-3)x~2),y_2=exp(-1.9539+0.4563x-0.01001x~2),式中x为温度。温度对未产出卵量影响不显著;对子代雌性比的影响也不显著,其平均值为0.7631。温度主要通过对产卵量和产卵速率来影响繁殖力,理论上27.3℃时周限增长率λ最大,达1.2374倍/天。成虫期供蜜加水时的产卵量、产出卵率和寿命显著比仅供水时的高或长。此研究为预测害虫种群发展,充分利用天敌资源,达到综合管理害虫提供了科学依据。 相似文献