全文获取类型
收费全文 | 3137篇 |
免费 | 315篇 |
国内免费 | 8篇 |
专业分类
3460篇 |
出版年
2023年 | 14篇 |
2022年 | 39篇 |
2021年 | 52篇 |
2020年 | 29篇 |
2019年 | 36篇 |
2018年 | 51篇 |
2017年 | 44篇 |
2016年 | 87篇 |
2015年 | 154篇 |
2014年 | 155篇 |
2013年 | 173篇 |
2012年 | 237篇 |
2011年 | 232篇 |
2010年 | 151篇 |
2009年 | 126篇 |
2008年 | 168篇 |
2007年 | 172篇 |
2006年 | 165篇 |
2005年 | 140篇 |
2004年 | 161篇 |
2003年 | 111篇 |
2002年 | 108篇 |
2001年 | 72篇 |
2000年 | 60篇 |
1999年 | 90篇 |
1998年 | 39篇 |
1997年 | 36篇 |
1996年 | 27篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 20篇 |
1992年 | 52篇 |
1991年 | 40篇 |
1990年 | 41篇 |
1989年 | 30篇 |
1988年 | 38篇 |
1987年 | 29篇 |
1986年 | 29篇 |
1985年 | 19篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1973年 | 11篇 |
1972年 | 10篇 |
排序方式: 共有3460条查询结果,搜索用时 15 毫秒
1.
A patient with chronic anemia is presented who radiologically showed prominent rugae of the stomach. Angiography demonstrated an arteriovenous malformation with a large feeding artery and prominent draining veins. 相似文献
2.
Ultrastructure of the epithelial cells of the ventral prostate from the hopping mouse Notomys alexis
The large ventral prostate of the hopping mouse has abundant secretory units whose epithelial cells vary in height and which often have nuclei in the apical region of the cell. TEM observations indicated two epithelial cell types in which some unusual features occurred. Type A cells had granular endoplasmetic reticulum (GER) whose membranes often formed intracytoplasmic confronting cisternae. Type B cells had more fragmented and vesiculated GER with sparse ribosomes and less frequently also intracytoplasmic confronting confronting cisternae. In the latter cells, two types of granules were found, one of which was derived from the Golgi and the other possibly directly from the GER. Type A cells only had one type of granule present. A highly convoluted membrane was also found at the basal region in many of the cells. The significance of these unusual ultrastructural features has yet to be ascertained. 相似文献
3.
Electron microscope study of the structures of the Bacillus subtilis prophages, SPO2 and phi105 总被引:14,自引:0,他引:14
Circular duplex structures of the correct length are observed in the electron microscope in hybridization mixtures of lysogen DNA and mature phage DNA for the case of the temperate Bacillus subtilis bacteriophage SPO2. This result shows that the sequence order of the prophage is a circular permutation of that of the mature phage. By making heteroduplexes of prophage DNA with that of the SPO2 deletion mutants, R90 and S25, the att site of the phage has been mapped at 61.2 ± 0.6% from one end of the mature phage DNA, which has a length of 38,600 base pairs. In the same co-ordinate system, the R90 deletion extends from 58.9 ± 0.7 to 66.8 ± 0.8% on the SPO2 chromosome, whereas the S25 deletion extends from 63.2 ± 0.6 to 66.9 ± 0.7%. In similar experiments with lysogen and mature phage DNA's of the temperate B. subtilis phage, φ105, no circular structures were seen. This result shows that the sequence order in the prophage and the phage are colinear, without circular permutation. 相似文献
4.
Helen M P Chui May Meroueh Stephen A Scaringe Christine S Chow 《Bioorganic & medicinal chemistry》2002,10(2):325-332
The synthesis of a 5'-O-BzH-2'-O-ACE-protected-3-methyluridine phosphoramidite is reported [BzH, benzhydryloxy-bis(trimethylsilyloxy)silyl; ACE, bis(2-acetoxyethoxy)methyl]. The phosphoramidite was employed in solid-phase RNA synthesis to generate a series of RNA hairpins containing single or multiple modifications, including the common nucleoside pseudouridine. Three 19-nucleotide hairpin RNAs that represent the 1920-loop region (G(1906)-C(1924)) of Escherichia coli 23S ribosomal RNA were generated. Modifications were present at positions 1911, 1915, and 1917. The stabilities and structures of the three RNAs were examined by using thermal melting, circular dichroism, and NMR spectroscopy 相似文献
5.
6.
D A Hartman S R Kuo T R Broker L T Chow R D Wells 《The Journal of biological chemistry》1992,267(8):5488-5494
A conformational distortion in the DNA duplex at the regulatory region of human papillomavirus type-11 next to an intermolecular triplex, formed with a synthetic oligonucleotide, was investigated with several chemical probes. The sequence targeted for triplex formation borders on the binding sites for the regulatory proteins encoded by the viral E2 open reading frame. Dimethyl sulfate, diethyl pyrocarbonate, and OsO4 all react to a greater extent with nucleotides in the duplex that are immediately adjacent to the triplex as compared to other bases throughout the duplex. This hypermodification was observed on both the polypurine and polypyrimidine strands of the duplex DNA. Similar hyperreactivity of bases flanking a triplex also was seen when the contiguous target polypurine tract was effectively extended by mutating interrupting pyrimidines in the human papillomavirus type-11 sequence to purines. We propose that this hyperreactivity is due to a structural distortion caused by the junction between the triplex and the duplex tracts. 相似文献
7.
8.
9.
Effects of tissue position (viz. outer vs inner mesocarp) and heat treatment (48°C, 20 min) on variations in polygalacturonase (EC 3.2.1.15 and EC 3.2.1.67) activity and ripening of fruits of Carica papaya L. cv. Backcross Solo were investigated. Polygalacturonase activity increased during ripening concomitantly with an increase in tissue softness and soluble polyuronide level. Throughout ripening, inner mesocarp tissue was softer and contained higher polygalacturonase activity than outer mesocarp tissue. Titratable acidity as well as ß-galactosidase (EC 3.2.1.23) activity also increased during ripening; however, unlike polygalacturonase, their level or activity was lower in inner than in outer mesocarp. Ascorbic acid could partially account for the increase in titratable acidity during ripening but contributed very little to the differences in titratable acid levels between outer and inner mesocarp. Heat treatment had no effect on either fruit softness or titratable acidity, but it markedly reduced the increase in ascorbic acid and polygalacturonase activity during ripening. Ripening, as reflected by changes in tissue softness and polygalacturonase activity, progressed outwardly from the interior towards the exterior of the fruit. The effect of heat treatment in suppressing polygalacturonase activity was relatively greater in inner than in outer mesocarp, suggesting that sensitivity of the enzyme to heat treatment may vary with stage of ripeness of the tissue. 相似文献
10.