全文获取类型
收费全文 | 2418篇 |
免费 | 226篇 |
出版年
2021年 | 30篇 |
2020年 | 17篇 |
2019年 | 20篇 |
2018年 | 25篇 |
2017年 | 32篇 |
2016年 | 56篇 |
2015年 | 68篇 |
2014年 | 91篇 |
2013年 | 128篇 |
2012年 | 106篇 |
2011年 | 103篇 |
2010年 | 85篇 |
2009年 | 72篇 |
2008年 | 108篇 |
2007年 | 119篇 |
2006年 | 108篇 |
2005年 | 92篇 |
2004年 | 92篇 |
2003年 | 68篇 |
2002年 | 82篇 |
2001年 | 87篇 |
2000年 | 93篇 |
1999年 | 87篇 |
1998年 | 34篇 |
1997年 | 36篇 |
1996年 | 23篇 |
1995年 | 31篇 |
1994年 | 37篇 |
1993年 | 21篇 |
1992年 | 58篇 |
1991年 | 51篇 |
1990年 | 64篇 |
1989年 | 57篇 |
1988年 | 51篇 |
1987年 | 47篇 |
1986年 | 42篇 |
1985年 | 34篇 |
1984年 | 22篇 |
1983年 | 20篇 |
1982年 | 25篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 29篇 |
1978年 | 25篇 |
1977年 | 15篇 |
1976年 | 13篇 |
1975年 | 19篇 |
1974年 | 19篇 |
1973年 | 12篇 |
1968年 | 9篇 |
排序方式: 共有2644条查询结果,搜索用时 31 毫秒
101.
Primary and secondary structure of 5.8S rRNA from the silkgland of Bombyx mori. 总被引:5,自引:5,他引:0
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Nucleotide sequence of 5.8S rRNA of the silkworm, Bombyx mori has been determined by gel sequencing methods. The 5.8S rRNA was the longest so far reported, with the 5'-terminal sequence several nucleotides longer than those of the other organisms. Upon constructing the secondary structure in accordance with the "burp gun" model (12), the Bombyx 5.8S rRNA formed a wide-open "muzzle" due to several unpaired bases at the ends. The overall structure also appeared less stable with less G . C pairs and more unpaired bases than that of the HeLa 5.8S rRNA. These structural features may be essential for those 5.8S rRNAs which interact with 28S rRNAs containing the hidden break to form a stable complex. 相似文献
102.
Primary and secondary structures of Tetrahymena and aphid 5.8S rRNAs: structural features of 5.8S rRNA which interacts with the 28S rRNA containing the hidden break 总被引:6,自引:6,他引:0
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The Tetrahymena 5.8S rRNA is 154 nucleotides long, the shortest so far reported except for the split 5.8S rRNAs of Diptera (m5.8S plus 2S rRNA). In this molecule several nucleotides are deleted in the helix e (GC-rich stem) region. Upon constructing the secondary structure in accordance with "burp-gun" model, the Tetrahymena 5.8S rRNA forms a wide-open "muzzle" of the terminal regions due to both extra nucleotides and several unpaired bases. The aphid 5.8S rRNA consists of 161 nucleotides and can form stable helices in both terminal and helix e regions. As a whole, the secondary structure of Tetrahymena 5.8S rRNA resembles that of Bombyx 5.8S molecule while the aphid 5.8S rRNA shares several structural features with the HeLa 5.8S molecule. Likely, the 5.8S rRNA attached to the 28S rRNA with the hidden break differs in structure from those interacting with the 28S partners without the break. Nucleotide sequences of 5.8S rRNA in insects as well as in protozoans are not so conservative evolutionarily as in vertebrates. 相似文献
103.
H Fujiwara T Tsuchida R B Levy G M Shearer 《Journal of immunology (Baltimore, Md. : 1950)》1982,129(3):1189-1193
The present study investigates the effect of trinitrophenyl- (TNP) modified H-2Kk (TNP-Kk) antigens on the generation of anti-TNP-Dk restricted cytotoxic T lymphocyte (CTL) responses. C3H.OH mice were primed to TNP-self by skin-painting with trinitrochlorobenzene, and spleen cells from these primed mice were subsequently stimulated in vitro with TNP-self. The effector cells generated exhibited appreciable lysis of TNP-modified C3H.OH blast target cells. Cold target inhibition studies demonstrated the generation of two effector cell populations: one that recognizes TNP in association with unique Dk self determinants, and one that recognizes TNP in association with self determinants shared between TNP-Kk and TNP-Dk. This was in contrast to primed C3H/He spleen cells, which did not generate CTL that recognized TNP in association with unique Dk self determinants. When spleen cells from (C3H/He x C3H.OH)F1 mice primed to TNP were stimulated in vitro with TNP-C3H.OH cells, unique Dk self determinants were recognized in association with TNP. However, in vitro stimulation of the same F1 responding cells with TNP-C3H/He or TNP-F1 cells failed to elicit CTL that utilized these Dk-unique self determinants. The findings of this study demonstrate that unique or shared H-2Dk determinants can be differentially utilized by CTL populations, depending on the H-2 alleles expressed by the stimulator cells. 相似文献
104.
Antipain (AP) and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) were tested in V79 Chinese hamster cells for cytotoxicity and effects on survival and 6-thioguanine-resistant (6TGr) mutation after UV-irradiation. AP and/or TPA were relatively non-cytotoxic and had no significant effects on UV survival. Despite their non-mutagenicity, the recovery of UV-induced 6TGr colonies was significantly enhanced by the pretreatment with either AP (0.5–2 mM) or TPA (0.1–1 μg/ml) only during the expression period before the 6TG selection at a low density of cells in the absence of AP or TPA. Such enhancing effects were maximal when AP or TPA was present during the late expression period after the mutation fixation and extensive dilution of DNA lesions. Reconstruction experiments revealed the antagonistic actions that TPA and AP tended to eliminate and increase, respectively, the metabolic co-operation. In the TPA-plus-AP treatment, AP abolished the TPA-enhanced recovery of induced mutants. Thus, it seems that TPA increases the mutant recovery largely through decreased metabolic co-operation and AP could modulate the mutation expression. Further, an error-prone inducible repair may not exist or, if it exists, AP may not inhibit it in V79 Chinese hamster cells. 相似文献
105.
Hydrocephalus in suckling rats infected intracerebrally with mouse hepatitis virus, MHV-A59 总被引:2,自引:0,他引:2
After intracerebral inoculation of mouse hepatitis virus, MHV-A59 strain, into 3- to 5-day-old Wistar rats, some survivors at 14 days postinoculation (p.i.) were found to lack the cerebral cortex and to have an accumulation of considerable amount of cerebrospinal fluid. The virus titer in the brain increased exponentially after inoculation, reaching a maximum 4 to 6 days p.i. when immunofluorescence revealed virus-specific antigen within neurons in the cerebral cortex. A small amount of infectious virus was also detectable 14 days p.i. when the cerebral anomaly was evident. This brain malformation causing hydrocephalus was due to cerebral damage by viral infection. 相似文献
106.
Entamoeba histolytica: localization and characterization of ca2+-dependent nucleotidases 总被引:2,自引:0,他引:2
T Takeuchi S Kobayashi M Masuda M Tanabe S Miura T Fujiwara 《International journal for parasitology》1981,11(3):209-215
An activity of Ca2+-dependent nucleotidase was detected in axenically-cultivated trophozoites of Entamoeba histolytica. The enzyme was concentrated by differential and sucrose density gradient centrifugation and catalyzed hydrolysis of nucleoside tri- and diphosphates and also thiamine pyrophosphate. Hydrolysis of nucleoside mono-phosphates was not affected by Ca2+. Among substrates tested, ATP was most active. Addition of Zn2+ or heat treatment almost abolished the enzyme activity. The enzyme exhibited almost the identical activity at acid and neutral pH. Among 6 bands isolated by polyacrylamide gel electrophoresis, 4 were stained with ATP, UTP, CTP and ADP, whereas the other 2 were stained only with ATP, UTP and CTP. The concentrated enzyme preparation, primarily composed of membrane fragments, also had activities of acid phosphatase, acid inorganic pyrophosphatase, 5'-nucleotidase and Mg2+-dependent ATPase. These observations suggest that E. histolytica has 2 Ca2+-dependent nucleotidases, i.e. one Ca2+-dependent ATPase and the other Ca2+-dependent nucleoside diphosphatase or an apyrase-like enzyme, and that these nucleotidases are at least partially associated with the plasma membrane or an organelle of lysosomal nature in this parasite. 相似文献
107.
Kenji Fujiwara Shigeki Hayashi Shunji Mishiro Hiroshi Oka Toshitsugu Oda 《Biochemical and biophysical research communications》1980,96(3):1135-1142
In primary culture of normal adult rat hepatocytes, human serum heated at 56°C for 30 min stimulated dose-dependently [3H]thymidine incorporation into trichloroacetic acid insoluble fraction of the cells, most of which was solubilized into hot trichloroacetic acid solution. The solubilized fraction was reduced when hydroxyurea was added to the culture. The heated serum also increased dose-dependently protein synthesis and cell viability determined from morphological findings. These results suggest that human serum has heat-stable factors stimulating DNA synthesis and maintaining cell viability of cultured rat hepatocytes. 相似文献
108.
The effect of methylation of histidine-57 of alpha-chymotrypsin with Streptomyces subtilisin inhibitor was examined. Methylchymotrypsin was isolated by affinity chromatography on inhibitor-Sepharose, and the interaction of this inactive enzyme with inhibitor was quantitatively analyzed by two different methods: the spectrophotometric titration of difference spectrum resulted in the complex formation and the application of competitive enzyme assay by using substrates of large Km values. The former method gave values of 8.6 . 10(-6) M as dissociation constant (Kd) of methylchymotrypsin . inhibitor complex and 0.91 as the number of binding sites (n) per inhibitor monomer, both of which were almost equivalent to those for native enzyme . inhibitor complex. By the latter novel method, values of 7.9 . 10(-6) M and 1.08 were obtained for Kd and n, respectively, for interaction of inhibitor with alpha-chymotrypsin, and 8 . 10(-6) M as Kd for methylchymotrypsin . inhibitor complex. These results indicate that methylation of histidine-57 of active site in alpha-chymotrypsin molecule does not affect essentially the binding ability to inhibitor and the modified enzyme binds stoichiometrically to inhibitor, as the native enzyme does, with a molar ratio of 1:1 per inhibitor monomer. 相似文献
109.
Hapten-reactive helper T cells were generated in the spleen of C57BL/6 mice primed with sulfanilated syngeneic IgG (S-MGG). Specific immunological tolerance was induced in vitro in these helper T cells, when spleen cell suspension was passed through Sephadex G-10 column to remove adherent cells and cultured in the presence of soluble S-MGG for 21 to 24 hours. On the other hand, tolerance was not inducible in unfractionated, primed spleen cells. When G-10-passed spleen cells were added to the culture dishes containing phagocytic, adherent cells of the spleen, tolerance was no more inducible in these reconstituted cell population. From these experimental results, it was concluded that macrophages played an interfering role in tolerance induction. The experimental data were also discussed in terms of macrophage function in the recognition of antigen by T lymphocytes. 相似文献
110.
Replicative bypass repair of UV damage to DNA was studied in wide variety of human, mouse and hamster cells in culture. Survival curve analysis revealed that in established cell lines (mouse L, Chinese hamster V79, HeLa S3 and SV40-transformed xeroderma pigmentosum (XP)), post-UV caffeine treatment potentiated cell killing by reducing the extrapolation number and mean lethal UV fluence (Do). In the Do reduction as the result of random inactivation by caffeine of sensitive repair there were marked clonal differences among such cell lines, V79 being most sensitive to caffeine potentiation. However, other diploid cell lines (normal human, excision-defective XP and Syrian hamster) exhibited no obvious reduction in Do by caffeine. In parallel, alkaline sucrose sedimentation results showed that the conversion of initially smaller segments of DNA synthetized after irradiation with 10 J/m2 to high-molecular-weight DNA was inhibited by caffeine in transformed XP cells, but not in the diploid human cell lines. Exceptionall, diploid XP variants had a retarded ability of bypass repair which was drastically prevented by caffeine, so that caffeine enhanced the lethal effect of UV. Neutral CsCl study on the bypass repair mechanism by use of bromodeoxyuridine for DNA synthesis on damaged template suggests that the pyrimidine dimer acts as a block to replication and subsequently it is circumvented presumably by a new process involving replicative bypassing following strand displacement, rather than by gap-filling de novo. This mechanism worked similarly in normal and XP cells, whether or not caffeine was present, indicating that excision of dimer is not always necessary. However, replicative became defective in XP variant and transformed XP cells when caffeine was present. It appears, therefore, that the replicative bypass repair process is either caffeine resistant or sensitive, depending on the cell type used, but not necessarily on the excision repair capability. 相似文献