全文获取类型
收费全文 | 6017篇 |
免费 | 293篇 |
国内免费 | 305篇 |
出版年
2023年 | 97篇 |
2022年 | 158篇 |
2021年 | 202篇 |
2020年 | 140篇 |
2019年 | 183篇 |
2018年 | 149篇 |
2017年 | 93篇 |
2016年 | 133篇 |
2015年 | 150篇 |
2014年 | 231篇 |
2013年 | 378篇 |
2012年 | 206篇 |
2011年 | 245篇 |
2010年 | 165篇 |
2009年 | 231篇 |
2008年 | 238篇 |
2007年 | 251篇 |
2006年 | 208篇 |
2005年 | 271篇 |
2004年 | 201篇 |
2003年 | 169篇 |
2002年 | 193篇 |
2001年 | 125篇 |
2000年 | 100篇 |
1999年 | 124篇 |
1998年 | 131篇 |
1997年 | 139篇 |
1996年 | 96篇 |
1995年 | 121篇 |
1994年 | 88篇 |
1993年 | 100篇 |
1992年 | 78篇 |
1991年 | 77篇 |
1990年 | 64篇 |
1989年 | 70篇 |
1988年 | 72篇 |
1987年 | 74篇 |
1986年 | 53篇 |
1985年 | 122篇 |
1984年 | 116篇 |
1983年 | 100篇 |
1982年 | 110篇 |
1981年 | 85篇 |
1980年 | 58篇 |
1979年 | 66篇 |
1978年 | 34篇 |
1977年 | 31篇 |
1976年 | 22篇 |
1974年 | 16篇 |
1973年 | 22篇 |
排序方式: 共有6615条查询结果,搜索用时 952 毫秒
121.
Abstract Isolation and identification of allatostatin 4 (AST 4) from crude brain extracts of female Diploptera punctata are described. Synthetic analogues of allatostatin 4 truncated from the N-terminal were evaluated to gain some knowledge of structure-activity correlation. AST 4 reversibly inhibits the biosynthesis of JH in vitro . AST 4 has the following sequence: Asp-Arg-Leu-Tyr-Ser-Phe-Gly-Leu-NH2 . This neurohormone (AST 4) has no sequence similarity with any known neuropeptide from other organisms. Synthetic AST 4 as well as truncation fragments, including two with the five and six amino acids terminal residues deleted showed in vitro activity distinguishable from that of the native allatostatin. 相似文献
122.
三种鱼类生长激素cDNA基因的结构比较研究 总被引:3,自引:0,他引:3
从厦门海区选取3种生长速度不同的鱼类,真鲈、高体Shi、褐菖You,由它们的脑下垂体中分别提取出总RNA,用逆转录PCR方法(RT-PCR)扩增出生长激素cDNA,克隆到pBluescript载体上的EcoRI位点,并分别测定了这3种鱼的成熟生长激素cRNA序列。将由这3种序列推导出的氨基酸序列同已知的8种不同科鱼类的生长激素氨基酸序列进行比较,分析它们序列的同源性,结果表明它们间的同源性与这些鱼 相似文献
123.
七星瓢虫(Coccmella septimpunctata)为了适应环境的变化,通过咽侧体产生保幼激素的活动调节其生殖作用。为了探索内外因素对咽侧体活动的影响,应用放射化学法及免疫电泳测定了食物、卵巢发育、脑神经肽、保幼激素类似物对卵黄发生期成虫保幼激素生物合成及血淋巴中卵黄原蛋白含量的影响。结果表明咽侧体活性受上述各种因子的影响。咽侧体活性与卵黄原蛋白含量及卵母细胞生长密切相关,说明有反馈作用。食物的质与量影响着咽侧体活性的变化。低剂量外源保幼激素类似物处理成虫则可促使咽侧体活性的变化。脑分泌的神经肽(allatotropin)可活化咽侧体。这些结果表明雌瓢虫保幼激素的生产主要是受起源于脑的促咽侧体信号的调节作用。 相似文献
124.
构建了含有pGHcDNA的重组痘苗病毒,用ELISA证明该重组病毒在被感染的h143细胞中,可表达出猪生长激素并将之分泌到培养基中,表达量约为1.05μg/10 ̄6细胞(24h)。用定位免疫化学法进一步证明该病毒可感染小鼠并在小鼠体内表达pGHcDNA。同时还构建了含双拷贝pGHcDNA的重组痘苗病毒,并证明其pGH表达量比单拷贝重组病毒有明显提高,约为1.50μg/10 ̄6细胞(24h)。 相似文献
125.
Corticotropin-releasing hormone (CRH) has been shown to be a central mediator for most, if not all, stress-induced responses. Since stressful stimuli may decrease hypothalamic tuberoinfundibular and tuberohypophysial dopaminergic neuronal activities, we aimed to determine whether CRH is involved. Using central administration of various doses of ovine CRH (oCRH; 1, 3 and 10 µg/rat) into the lateral cerebroventricle of either male or female rats, the neurochemical changes in various parts of the central nervous system, including the hypothalamus, were determined by high-performance liquid chromatography at various times after the injection (30, 60, 120 and 240 min). The concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG), two major metabolites of dopamine and norepinephrine, respectively, in discrete brain regions were used as indices for catecholaminergic neuron activity. Plasma corticosterone levels increased significantly after all doses of oCRH and at all time points studied. oCRH also exerted significant stimulatory effects on noradrenergic neuron terminals in the frontal cortex, and on dopaminergic neuron terminals in the nucleus accumbens, hypothalamic paraventricular and periventricular nuclei, and intermediate pituitary lobe. Dopaminergic neuron terminals in the median eminence and the neural lobe of the pituitary, however, were not affected. There was no major difference in the responses between male and female rats. We conclude that CRH has a differential effect on central catecholaminergic neurons. 相似文献
126.
Roberto E. Izquierdo Kimberly Breese Shalini Jain Daniel Carestio Lawrence Jung James Figge 《In vitro cellular & developmental biology. Animal》1995,31(1):71-76
Summary Gene transfer techniques can be used to encode the production of a polypeptide product, such as human growth hormone (hGH),
that is missing in an acquired or inherited disease state such as growth hormone deficiency. In one model system, engineered
C2C12 myoblasts are injected intramuscularly into a mouse and subsequently secrete hGH into the circulation. In this regard,
a gene-expression regulatory system that functions in myoblasts would be of interest. We demonstrate that theEscherichia coli lac operon system can be used to stringently regulate the expression of hGH in engineered C2C12 myoblasts in tissue culture.
A DNA segment encoding hGH was linked to a DNA segment containing an SV40 enhancer and promoter. The latter components were
positioned between two syntheticlac operators.Lac repressor expression was driven by a simian cytomegalovirus promoter. In transient co-transfection assays, hGH expression
from cultured C2C12 myoblasts could be modulated up to 60-fold (P = 0.002) with the inducing agent, isopropyl-β-d-thiogalactoside (IPTG). In the absence of IPTG, hGH expression was almost fully repressed. These results show that the components
of theE. coli lac operon provide a stringent regulatory system for use in myoblasts. The system might prove to be useful for the regulation
of transferred genes in animals. 相似文献
127.
An analogue of human melanin-concentrating hormone (MCH) suitable for radioiodination was designed in which Tyr13 and Val19 of the natural peptide were replaced by phenylalanyl and tyrosyl residues: [Phe13, Tyr19] -MCH. The peptide was synthesized by the continuous-flow solid-phase methodology using Fmocstrategy and Polyhipe PA 500 and PEG-PS resins. The linear MCH peptides with either acetamidomethyl-protected or free cysteinyl residues were purified to homogeneity and cyclized by iodine oxidation, yielding the final product with the correct molecular weight of 2434.61. Radioiodination of the C-terminal tyrosine was carried out enzymatically using solid-phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed-phase mini-column and by high-pressure liquid chromatography. The resulting [125I]-[Phe13, Tyr19]-MCH tracer was the first radiolabelled MCH peptide suitable for radioreceptor assay: saturation binding analysis using mouse G4F-7 melanoma cells demonstrated the presence of 1090 MCH receptors per cell. The dissociation constant (KD ) was 1.18 × 10?10 M, indicating high-affinity MCH receptors on these cells. MCH receptors were also found in other cell lines such as mouse B16-F1 and G4F and human RE melanoma cells as well as in PC12 and COS-7 cells. Competition binding analyses with a number of other peptides such as α-MSH, neuropeptide Y, substance P and pituitary adenylate cyclase activating peptide, demonstrated that the binding to the MCH receptor is specific. Atrial natriuretic factor was found to be a weak competitor of MCH, indicating topological similarities between MCH and ANF when interacting with MCH receptors. 相似文献
128.
Josiane Girardie Adrien Girardie Alain Van Dorsselaer Odile Sorokine Serge Geoffre Michel Hospital Gilles Precigoux 《Journal of peptide science》1995,1(5):311-318
An original insect neurohormone of 65 residues was synthesized by the solid-phase methodology using t-Boc strategy and Boc-Val-PAM–resin. The purification, conducted by several steps of liquid chromatography having mass, polarity or charge as separative criteria, yielded the product with the correct molecular weight of 6922 Da determined by mass spectrometry. The synthetic peptide had both the same affinity for the antinative neurohormone serum and the same biological activity as the native neurohormone. 相似文献
129.
Direct shoot formation in spontaneously occurring root pseudonodules of alfalfa (Medicago sativa L.)
P. Sarul M. Vlahova A. Ivanova A. Atanassov 《In vitro cellular & developmental biology. Plant》1995,31(1):21-25
Summary A simple and rapid procedure for direct organogenesis from root nodulelike structures of alfalfa (Medicago sativa L.) line SGg, spontaneously induced on growth regulator-free Gamborg (B5) medium, was developed. Prolific adventitious shoot initiation was obtained using a combination of 1.0 mg/liter TIBA and
0.5 mg/liter 2iP. Transfer of shoots to a medium containing 0.5 mg/liter ABA and reduced concentration of TIBA (0.5 mg/liter)
before rooting markedly stimulated shoot development. Regenerated shoots rooted easily and revealed the early appearance of
nodulelike structures on basal medium (B5) lacking growth regulators. Analysis of endogenous growth regulator levels of SGg roots maintained on growth regulators free
media, showed that spontaneous shoot appearances was correlated with high cytokinin-to-auxin ratios. 相似文献
130.
Effect of season and photoperiod on the follicle-stimulating hormone receptors in a subtropical bird
Annual changes in and photoperiodic influence oh the weight of gonads, a parameter of gonadal activity, are much smaller in
female birds than in males. Effect of season and photoperiod on the follicle-stimulating hormone receptors in the testis or
ovary was studied using a subtropical weaver finch. The number of follicle-stimulating hormone binding sites per unit testicular
weight showed a peak in the non-breeding phase; while the total number of binding sites per two testes was maximal in the
breeding phase and minimal in the regressive phase. In contrast, seasonal changes in follicle-stimulating hormone binding
sites in the ovary were less marked. Exposure to short-day during the breeding phase induced marked decreases in the numbers
of binding sites per unit testicular weight and per two testes. These numbers markedly increased after transfer to long-day
during the non-breeding phase. However, there was no significant effect of short-day or long-day exposure on follicle-stimulating
hormone binding sites in the ovary. These results suggest that photoperiod is an effective environmental factor in the regulation
of follicle-stimulating hormone receptors in the testis and the effect is manifested by pronounced changes in the testicular
weight during annual breeding cycle. 相似文献