全文获取类型
收费全文 | 6788篇 |
免费 | 731篇 |
国内免费 | 210篇 |
专业分类
7729篇 |
出版年
2023年 | 49篇 |
2022年 | 133篇 |
2021年 | 216篇 |
2020年 | 146篇 |
2019年 | 161篇 |
2018年 | 184篇 |
2017年 | 113篇 |
2016年 | 218篇 |
2015年 | 335篇 |
2014年 | 367篇 |
2013年 | 447篇 |
2012年 | 507篇 |
2011年 | 483篇 |
2010年 | 334篇 |
2009年 | 274篇 |
2008年 | 323篇 |
2007年 | 285篇 |
2006年 | 304篇 |
2005年 | 254篇 |
2004年 | 260篇 |
2003年 | 203篇 |
2002年 | 178篇 |
2001年 | 157篇 |
2000年 | 160篇 |
1999年 | 133篇 |
1998年 | 63篇 |
1997年 | 60篇 |
1996年 | 50篇 |
1995年 | 51篇 |
1994年 | 53篇 |
1993年 | 55篇 |
1992年 | 89篇 |
1991年 | 95篇 |
1990年 | 72篇 |
1989年 | 72篇 |
1988年 | 80篇 |
1987年 | 69篇 |
1986年 | 64篇 |
1985年 | 73篇 |
1984年 | 59篇 |
1983年 | 48篇 |
1982年 | 41篇 |
1979年 | 38篇 |
1978年 | 41篇 |
1977年 | 27篇 |
1976年 | 28篇 |
1975年 | 27篇 |
1974年 | 34篇 |
1973年 | 31篇 |
1972年 | 26篇 |
排序方式: 共有7729条查询结果,搜索用时 0 毫秒
91.
Chang-chi Chu James S. Buckner Kamil Karut Thomas P. Freeman Dennis R. Nelson Thomas J. Hennebeny 《Insect Science》2003,10(2):121-129
Abstract Size and weight measurements were made for all the life stages of Bemisia tabaci (Gennadius) B biotype from field grown cotton ( Gossypium hirsutum L.) and cantaloupe ( Cucumis melo L., var. cantalupensis ) in Phoenix, AZ and Fargo, ND, USA in 2000 and 2001. Nymphal volumes were derived from the measurements. The average nymphal volume increase for settled 1 st to the late 4th instar was exponential. The greatest increase in body volume occurred during development from the 3rd to early 4th instar. Nymphs on cotton leaves were wider, but not longer compared with those on cantaloupe. Ventral and dorsal depth ratios of nymphal bodies from 1 st to late 4th instars from cantaloupe leaves were significantly greater compared with those from cotton leaves. During nymphal development from 1st to 4th instar, the average (from the two host species) ventral body half volume increased by nearly 51 times compared with an increase of 28 times for the dorsal body half volume. Adult female and male average lengths, from heads to wing tips, were 1 126 μm and 953 μm, respectively. Average adult female and male weights were 39 and 17 μg, respectively. Average widths, lengths, and weights of eggs from cotton and cantaloupe were, 99 μm, 197 μm, and 0.8 μg, respectively. Average widths, lengths, and weights for exuviae of non-parasitized nymphs from both cotton and cantaloupe were 492 μm, 673 μm, and 1.20 μg, respectively; and widths, lengths, and weights of parasitized nymph exuviae were 452 μm, 665 μm, and 3.62 μg, respectively. Both exuviae from non-parasitized and parasitized nymphs from cotton leaves were wider, longer, and heavier than those from cantaloupe leaves. 相似文献
92.
人β-珠蛋白基因主要在成年期的骨髓中表达,在胎儿肝,成年肝和K 562细胞中则处于关闭状态。凝胶电泳阻抑法分析发现,在人胎儿肝,成年肝及K 562细胞的核蛋白抽提物中存在着不同的与β-珠蛋白基因5'旁侧调控元件(-372到-194 bp)相结合的红系组织特异性的调控因子。竟争试验的结果表明,成年肝和K 562细胞中的调控因子与β-珠蛋白基因5'旁侧调控元件相互作用的方式具有一定的相似性,这两种细胞的调控因子既结合于负调控区(NCR 2)也结合于正调控区,说明两种细胞中β-珠蛋白基因的关闭机制可能是相似的。胎儿肝的核蛋白因子只结合于负调控区,推测在胎儿期存在独特的关闭机制。 相似文献
93.
Effects of light and chloroplast functional state on expression of nuclear genes encoding chloroplast glyceraldehyde-3-phosphate dehydrogenase in long hypocotyl (hy) mutants and wild-type Arabidopsis thaliana. 总被引:1,自引:1,他引:0 下载免费PDF全文
In a previous study of Arabidopsis thaliana (J. Dewdney, T.R. Conley, M.-C. Shih, H.M. Goodman [1993] Plant Physiol 103: 1115-1121), it was postulated that both blue light receptor- and phytochrome-mediated pathways contribute to regulation of the nuclear genes encoding A and B subunits of glyceraldehyde-3-phosphate dehydrogenase (GAPA and GAPB). Here were report on the involvement of a nuclear gene encoding a putative blue-light receptor (HY4) and of a nuclear gene encoding phytochrome A apoprotein (PHYA) in regulation of the GAPA and GAPB genes in response to blue and far-red light. Continuous light irradiation experiments with the hy4 mutant demonstrate that the HY4 gene product is required for full expression of GAPA, GAPB, and one or more of the nuclear genes encoding small subunits of of ribulose-1,5-bisphosphate carboxylase/oxygenase. Continuous light irradiation and fluence-response studies with the phyA-101 mutant show that phytochrome A functions in far-red light regulation of GAPA, GAPB, nuclear genes encoding small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase, and CAB genes. Phytochromes A and B alone either do not participate in red light-mediated gene regulation or have redundant functions, as shown by analysis of phyA-101 and phyB-1 single mutants. In addition, the hypothesis that chloroplast-nucleus interactions affect GAPA and GAPB gene regulation was tested. Herbicide-mediated photooxidative damage to chloroplasts in A thaliana seedlings strongly decreased the maximum amount of GAPA and GAPB steady-state mRNA detected in continuous-light irradiation experiments. Full expression of the GAPB genes is dependent on the presence of functional chloroplasts. 相似文献
94.
95.
96.
The protein conformation and a zinc-binding domain of an autoantigen from mouse seminal vesicle. 下载免费PDF全文
The protein conformation of a mouse seminal vesicle autoantigen was studied by circular dichroism spectroscopy. At pH 7.4, the spectrum in the UV region appears as one negative band at 217 nm and one positive band at 200 nm. This together with the predicted secondary structures indicates no helices but a mixture of beta form, beta turn, and unordered form in the protein molecule. The conformation is stable even at pH 10.5 or 3.0. The spectrum in the near-UV region consists of fine structures that are disturbed in acidic or alkaline solution. The environments around Trp2 and Trp82 of this protein were studied by intrinsic fluorescence and solute quenching. They give an emission peak at 345 nm, and about 87% of them are accessible to quenching by acrylamide. Correlating the quenching effect of CsCl and Kl on the protein fluorescence to the charged groups along the polypeptide chain suggests the difference in the "local charge" around the two tryptophan residues. The presence of ZnCl2 in the protein solution effects no change in the circular dichroism but perturbs the fluorescence due to Trp82. Analysis of the fluorescence data suggests a Zn(2+)-binding site on the protein, which cannot coordinate with both Ca2+ and Mg2+. The association constant for the complex formation is 1.35 x 10(5) +/- 0.04 x 10(5) M-1 at pH 7.4. 相似文献
97.
Long-lasting adenovirus transgene expression in mice through neonatal intrathymic tolerance induction without the use of immunosuppression. 总被引:6,自引:0,他引:6 下载免费PDF全文
The major barrier to the clinical application of adenovirus gene therapy for diseases that require stable transgene expression is the immunogenicity of recombinant adenovirus, which ordinarily limits the duration of its effects to a period of about 2 weeks. We postulated that tolerance to adenovirus could be induced and transgene expression could be prolonged if T lymphocytes underwent thymic selection in the presence of adenovirus antigens. Mice were inoculated in the thymus with a recombinant adenovirus containing the lacZ marker gene during the neonatal period at a time before T-cell maturation had occurred. When the virus was administered intravenously to these mice in adulthood, they were found to have an impaired adenovirus-specific cytotoxic T-lymphocyte response which allowed prolonged hepatic lacZ expression, for up to 260 days. The ability to achieve unresponsiveness to a recombinant adenovirus after inoculation of the thymus in neonates extends the paradigm of intrathymic tolerance induction. Furthermore, this model will enable the study of stable adenovirus transgene expression in vivo without the use of immunosuppression and ultimately may have clinical utility. 相似文献
98.
Lun He Kurt J. Isselbacher Jack R. Wands Howard M. Goodman Chiaho Shih Andrea Quaroni 《In vitro cellular & developmental biology. Plant》1984,20(6):493-504
Summary A human hepatocellular carcinoma cell line (FOCUS—Friendship of China and United States) was derived from a patient with primary
hepatocellular carcinoma. This cell line has been in continuous culture over an 18-mo period. The morphological and ultrastructural
features of FOCUS are consistent with its neoplastic hepatocellular orgin. FOCUS cells contain aspartate aminotransferase
and glucose-6-phosphatase activity. In addition, α1-antitrypsin, fibrinogen, alpha fetoprotein, and carcinoembryonic antigens were detectble in the cytoplasm of the cultured
cells by immunochemical staining techniques. The karyotype of the FOCUS cell is human in origin and it contains human DNA
sequences as detected by molecular hybridization analysis. The FOCUS cells do not show evidence of density-dependent inhibition
of growth under confluent conditions. Repeated growth curves over an 18-mo period were identical, revealing a doubling time
of 42 to 48 h. The malignant potential of FOCUS cells was further demonstrated by their ability to lead to gross tumor formation
after subcutaneous infection into nude mice. From one of the solid tumors grown in nude mice, recultured cell lines have been
established and found to have properties identical to the original FOCUS cell line. This FOCUS cell line represents an additional
model for further investigation of tumor specific antigens and the relationship between hepatitis B virus (HBV) and hepatocellular
carcinoma. Preliminary molecular characterization has indicated the existence of integrated HBV sequences within the FOCUS
genome. 相似文献
99.
Antibodies against aflatoxin Q1 (AFQ1) were obtained from rabbits after immunization of either AFQ1-hemisuccinate or AFQ2a conjugated to bovine serum albumin. Both radioimmunoassay and enzyme-linked immunosorbent assaY (ELISA) were used for the determination of antibody titers and specificities. Antibodies obtained from rabbits after immunization with AFQ1-hemisuccinate-bovine serum albumin had the highest affinity to aflatoxin B1, whereas antibodies obtained from rabbits after immunization with AFQ2a-bovine serum albumin bound most effectively with AFQ2a. AFQ2a antibody was selected for the subsequent direct and indirect ELISA for the detection of AFQ1 in biological fluids. When AFQ2a-peroxidase and AFQ2a antibody were used, direct ELISA was able to detect as low as 2 ppb (ng/ml) of AFQ1 spiked in the urine samples that had been subjected to a Sep-Pak cleanup treatment. In indirect ELISA in which the antigen (AFQ2a-bovine serum albumin) was coated to the solid phase followed by reaction with rabbit antibody and goat anti-rabbit immunoglobulin G-peroxidase conjugate, 50-fold less antibody was used without sacrificing sensitivity. Recoveries of AFQ1 added to urine samples (2 to 40 ppb) were 46.3 to 73% and 65.8 to 85.8% for direct and indirect ELISA, respectively. 相似文献
100.
Production and characterization of monoclonal antibodies against aflatoxin M1. 总被引:4,自引:1,他引:3 下载免费PDF全文
By using an indirect enzyme-linked immunosorbent assay, four monoclonal antibodies were selected after fusion of mouse P3-NS1-Ag4-1 myeloma cells with spleen cells isolated from BALB/c mice that had been immunized with aflatoxin M1 (AFM1) conjugated to bovine serum albumin. Two of these antibodies were found to be specific for AFM1 and were designated AMW-1 and AMW-4. The specificities of AMW-1, which had higher affinity to AFM1, were determined by a competitive direct enzyme-linked immunosorbent assay with peroxidase-AFM1 as the marker. The relative cross-reactivity of each toxin (relative to AFM1) with AMW-1, as determined by the amount of aflatoxin necessary to cause 50% inhibition of enzyme activity, was 12, greater than 40, 12, and greater than 40 for B1, B2, G1, and G2, respectively. 相似文献