全文获取类型
收费全文 | 147790篇 |
免费 | 4849篇 |
国内免费 | 5267篇 |
专业分类
157906篇 |
出版年
2024年 | 144篇 |
2023年 | 790篇 |
2022年 | 1865篇 |
2021年 | 3050篇 |
2020年 | 2088篇 |
2019年 | 2512篇 |
2018年 | 13589篇 |
2017年 | 11928篇 |
2016年 | 9615篇 |
2015年 | 4193篇 |
2014年 | 4657篇 |
2013年 | 4719篇 |
2012年 | 9325篇 |
2011年 | 17154篇 |
2010年 | 14509篇 |
2009年 | 10550篇 |
2008年 | 12487篇 |
2007年 | 13750篇 |
2006年 | 2510篇 |
2005年 | 2382篇 |
2004年 | 2450篇 |
2003年 | 2410篇 |
2002年 | 1838篇 |
2001年 | 1161篇 |
2000年 | 1047篇 |
1999年 | 833篇 |
1998年 | 508篇 |
1997年 | 476篇 |
1996年 | 489篇 |
1995年 | 424篇 |
1994年 | 419篇 |
1993年 | 354篇 |
1992年 | 466篇 |
1991年 | 361篇 |
1990年 | 291篇 |
1989年 | 268篇 |
1988年 | 227篇 |
1987年 | 207篇 |
1986年 | 176篇 |
1985年 | 154篇 |
1984年 | 122篇 |
1983年 | 139篇 |
1982年 | 83篇 |
1980年 | 52篇 |
1979年 | 63篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1973年 | 45篇 |
1972年 | 299篇 |
1971年 | 300篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
Tianhu Wang Zhenhua Yang Fang Yang Mingye Zhang Jinlong Zhao Jinwu Chen Yongming Li 《PloS one》2014,9(4)
Objective
The study was performed to compare the 3D pharyngeal airway dimensions in adult skeletal Class II patients with different vertical growth patterns (low, normal, and high angle) and to investigate whether the upper airway dimensions of untreated skeletal Class II adults were affected by vertical skeletal variables.Methods
Cone-beam computed tomography (CBCT) records of 64 untreated adult skeletal Class II patients (34 male and 30 female) were collected to evaluate the pharyngeal airway dimensions. Subjects were divided into three subgroups according to the GoGn-SN angle (low angle, normal angle or high angle). All subgroups were matched for sex. ANOVA and SNK - q tests were used to identify differences within and among groups (p<0.05). Coefficient of product-moment correlation (Pearson correlation coefficient) was used to analyze the association between pharyngeal airway dimensions and vertical growth patterns.Results
The results showed that pharyngeal airway measurements were statistically significantly less (p<0.05) in high angle group as compared to normal angle or low angle group.Conclusions
Adult skeletal Class II subjects with vertical growth patterns have significantly narrower pharyngeal airways than those with normal or horizontal growth patterns, confirming an association between pharyngeal airway measurements and a vertical skeletal pattern. 相似文献962.
963.
964.
A study was conducted to analyze the regional distribution of Ca, Cu, Fe, Mg, and Zn contents in brain tissues after animals
were given liquor of brick tea that contained a high Al content. In 25 normal adult male mice given either water or 0.9% NaCl
for 1 mo or 2 mo, the metal concentrations in the serum, liver, frontal cortex, hippocampus, cerebellum, and brain stem were
comparable (p>0.05). When the drinking water was replaced by a 1% brick tea liquor, which contained a high Al content, serum Al concentration
was increased significantly 1 mo after the onset of the experiment and remained high at the end of the second month. The level
of Al was also elevated in both the cortex and hippocampus at 1 mo after replacing tea for drinking water. In addition to
Al, there were a significant increase in hippocampal Zn and a decrease in Cu contents. There was no change in tissue Mg or
Fe contents, but there was a significant increase in Ca content in every brain region studied. It was suggested that the increase
in Ca might be the result of the effect of other components in tea. Unlike the brain, there was no change in the concentration
of any of the metals, including Al, in the liver, which further demonstrated that the changes observed in the brain was specific.
The results of the present study confirmed that Al, when given orally in the form of tea, could be absorbed into the bloodstream.
The absorbed Al could accumulate in selected brain regions. The presence of Al might also change the tissue content of endogenous
trace metals. 相似文献
965.
Statistical methods for detecting molecular adaptation 总被引:2,自引:0,他引:2
The past few years have seen the development of powerful statistical methods for detecting adaptive molecular evolution. These methods compare synonymous and nonsynonymous substitution rates in protein-coding genes, and regard a nonsynonymous rate elevated above the synonymous rate as evidence for darwinian selection. Numerous cases of molecular adaptation are being identified in various systems from viruses to humans. Although previous analyses averaging rates over sites and time have little power, recent methods designed to detect positive selection at individual sites and lineages have been successful. Here, we summarize recent statistical methods for detecting molecular adaptation, and discuss their limitations and possible improvements. 相似文献
966.
Human monoclonal antibodies that inhibit binding of hepatitis C virus E2 protein to CD81 and recognize conserved conformational epitopes 总被引:9,自引:0,他引:9 下载免费PDF全文
Hadlock KG Lanford RE Perkins S Rowe J Yang Q Levy S Pileri P Abrignani S Foung SK 《Journal of virology》2000,74(22):10407-10416
The intrinsic variability of hepatitis C virus (HCV) envelope proteins E1 and E2 complicates the identification of protective antibodies. In an attempt to identify antibodies to E2 proteins from divergent HCV isolates, we produced HCV E2 recombinant proteins from individuals infected with HCV genotypes 1a, 1b, 2a, and 2b. These proteins were then used to characterize 10 human monoclonal antibodies (HMAbs) produced from peripheral B cells isolated from an individual infected with HCV genotype 1b. Nine of the antibodies recognize conformational epitopes within HCV E2. Six HMAbs identify epitopes shared among HCV genotypes 1a, 1b, 2a, and 2b. Six, including five broadly reactive HMAbs, could inhibit binding of HCV E2 of genotypes 1a, 1b, 2a, and 2b to human CD81 when E2 and the antibody were simultaneously exposed to CD81. Surprisingly, all of the antibodies that inhibited the binding of E2 to CD81 retained the ability to recognize preformed CD81-E2 complexes generated with some of the same recombinant E2 proteins. Two antibodies that did not recognize preformed complexes of HCV 1a E2 and CD81 also inhibited binding of HCV 1a virions to CD81. Thus, HCV-infected individuals can produce antibodies that recognize conserved conformational epitopes and inhibit the binding of HCV to CD81. The inhibition is mediated via antibody binding to epitopes outside of the CD81 binding site in E2, possibly by preventing conformational changes in E2 that are required for CD81 binding. 相似文献
967.
968.
B→O血型转变工具酶α-半乳糖苷酶cDNA克隆及表达 总被引:10,自引:0,他引:10
α-半乳糖苷酶是实现 B→O血型转变、制备通用型血的关键工具酶 .利用反转录 PCR方法从中国海南 Catimor咖啡豆中克隆α-半乳糖苷酶 c DNA,插入嗜甲基酵母 P.pastoris分泌表达载体 p PIC9K中 ,转化 P.pastoris GS1 1 5,筛选高表达重组菌株 .经甲醇诱导表达 7d后 ,发酵液总蛋白分泌量约 1 .2 mg/ml,SDS- PAGE呈现约 41 k D特异表达带 ,与专一性底物对 -硝基 -苯基 -α- D-吡喃半乳糖苷反应证明 ,表达产物具有 α-半乳糖苷酶活性 ,最高达到 1 8U/ml.初步实验表明 ,表达的 α-半乳糖苷酶可酶解 B型红细胞 ,成功实现 B→O血型转变 . 相似文献
969.
Late-acting self-incompatibility in tea plant (<Emphasis Type="Italic">Camellia sinensis</Emphasis>)
The self-incompatibility of tea plant (Camellia sinensis (L.) O. Kuntze) was studied with the methods of aniline blue fluorescence assay and paraffin sections. The characteristics
of pollen tube elongation after hand pollination was analyzed in 4 tea cultivars, including ‘Keemenzhong’, ‘Longjing-changye’,
‘Fuding-dabaicha’ and ‘Yabukita’, under self-pollination and cross-pollination, respectively. Although there were some difference
among cultivars, pollen tubes elongated through the style and reach the ovary successfully at 48 h after pollination for both
cross- and self-pollen tubes in all the four cultivars of tea. Pollen tubes entered into the ovule micropyles, however, only
for cross-pollination, but not for self-pollination. Pollen tubes of selfing plants, failed in fertilizing, seemed have some
difficulties to enter the ovule. All of which indicated that the self-incompatibility of tea plant is a late-acting self-incompatibility
system (LSI) or an ovarian sterility (OS), in which the self incompatibility was due to none self pollen tube penetrating
into the ovule and no fertilization. 相似文献
970.
Ke-Jing Wang Xiang-Hua Li Tetsuro Yamashita Yoshihito Takahata 《Plant Systematics and Evolution》2012,298(1):1-7
The wild soybean (Glycine soja), which is the progenitor of cultivated soybean (Glycine max), is expected to offer more information about genetic variability and more useful mutants for evolutionary research and breeding
applications. Here, a total of 1,600 wild soybean samples from China were investigated for genetic variation with regard to
the soybean Kunitz trypsin inhibitor (SKTI). A new mutant SKTI, Tik, was identified. It was found to be a Tia-derived codominant allele caused by a transversion point mutation from C to G at nucleotide +171, leading to an alteration
of one codon (AAC → AAG) and a corresponding amino acid substitution (Asn → Lys) at the ninth residue. Upon examination of
this variant and others previously found in wild soybeans, it became clear that SKTI has undergone high-level evolutionary
differentiation. There were more abundant polymorphisms in the wild than in the cultivated soybean. 相似文献