全文获取类型
收费全文 | 4792篇 |
免费 | 354篇 |
国内免费 | 4篇 |
专业分类
5150篇 |
出版年
2022年 | 44篇 |
2021年 | 68篇 |
2020年 | 56篇 |
2019年 | 68篇 |
2018年 | 100篇 |
2017年 | 86篇 |
2016年 | 93篇 |
2015年 | 140篇 |
2014年 | 198篇 |
2013年 | 297篇 |
2012年 | 311篇 |
2011年 | 333篇 |
2010年 | 204篇 |
2009年 | 158篇 |
2008年 | 199篇 |
2007年 | 231篇 |
2006年 | 215篇 |
2005年 | 179篇 |
2004年 | 164篇 |
2003年 | 140篇 |
2002年 | 119篇 |
2001年 | 129篇 |
2000年 | 134篇 |
1999年 | 101篇 |
1998年 | 53篇 |
1997年 | 44篇 |
1996年 | 48篇 |
1995年 | 42篇 |
1994年 | 35篇 |
1993年 | 34篇 |
1992年 | 75篇 |
1991年 | 93篇 |
1990年 | 90篇 |
1989年 | 86篇 |
1988年 | 47篇 |
1987年 | 63篇 |
1986年 | 75篇 |
1985年 | 76篇 |
1984年 | 68篇 |
1983年 | 43篇 |
1982年 | 47篇 |
1981年 | 34篇 |
1980年 | 27篇 |
1979年 | 33篇 |
1978年 | 31篇 |
1977年 | 33篇 |
1976年 | 24篇 |
1975年 | 22篇 |
1974年 | 24篇 |
1972年 | 27篇 |
排序方式: 共有5150条查询结果,搜索用时 15 毫秒
31.
Anthony P. Fordham-Skelton F. Safadi M. Golovkin A. S. N. Reddy 《Plant Molecular Biology Reporter》1994,12(4):358-366
Calmodulin labeled with125I or34S has been used to screen expression libraries to isolate cDNAs encoding calmodulin-binding proteins (CBPs) from several eukaryotic
systems. The use of radiolabeled calmodulin has, however, several disadvantages. We have developed a nonradiactive method
to isolate cDNAs for CBPs using biotinylated calmodulin. Screening of a cDNA library in an expression vector with biotinylated
calmodulin resulted in the isolation of cDNAs encoding CBPs. Avidin and biotin blocking steps, prior to incubation of the
filters with biotinylated calmodulin, are found to be essential to eliminate the cDNAs that code for biotin-containing polypeptides.
The cDNA clones isolated using this nonradioactive method bound calmodulin in a calcium-dependent manner. The binding of biotinylated
calmodulin to these clones was completely abolished by ethylene glycolbis(\-aminoethylether)-N,N′-tetraacetic acid (EGTA),
a calcium chelator. Furthermore, the isolated cDNAs were confirmed by probing the clones with35S-labeled calmodulin. All the isolated clones bound to radiolabeled calmodulin in the presence of calcium but not in the presence
of EGTA. The method described here is simple, fast, and does not involve preparation and handing of radiolabeled calmodulin.
All the materials used in this method are commercially available; hence, this procedure should be widely applicable to isolate
cDNAs encoding CBPs from any eukaryotic organism. 相似文献
32.
Sarvesh Adda T. P. Reddy P. B. Kavi Kishor 《In vitro cellular & developmental biology. Plant》1994,30(2):104-107
Summary Induction of somatic embryogenesis, shoot organogenesis, and subsequent plant regeneration in niger seem to be dependent on
genotype, choice of explant, and composition of media growth regulators. Two distinct regeneration protocols have been developed
for somatic embryogenesis and shoot organogenesis. Somatic embryogenesis was induced from epicotyls and cotyledonary explants
(9 to 35%) (but not from hypocotyls and roots) in presence of 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic
acid, and 2,4,5-trichlorophenoxypropionic acid. These embryos matured in MS medium containing Kinetin plus naphthalene acetic
acid (NAA), Kinetin plus Zeatin, and Kinetin plus abscisic acid (ABA). Matured embryos could be germinated on LS and MS basal
media without hormones. Non-embryogenic callus initiated on Linsmaier and Skoog’s (LS) medium from cotyledons of six
different genotypes produced shoots (9 to 32%) on Murashige and Skoog’s (MS) medium fortified with 6-benzylaminopurine
(BAP, 0.5 mg · liter−1), and BAP (1 mg · liter−1) plus NAA (0.1 mg · liter−1). These shoots were rooted with 100% frequency by using indole-3-acetic acid or NAA and transferred successfully to the soil. 相似文献
33.
v-myb blocks granulocyte colony-stimulating factor-induced myeloid cell differentiation but not proliferation. 总被引:7,自引:2,他引:5 下载免费PDF全文
To understand the effects of v-myb expression on mammalian hematopoietic cell differentiation, we have constructed a retroviral vector which can efficiently express v-myb gene product in mammalian cells. Infection of interleukin-3-dependent murine progenitor cell line 32D Cl3, which undergoes terminal differentiation to mature granulocytes in the presence of granulocyte colony-stimulating factor (GCSF), with this recombinant retrovirus does not abrogate its requirement of interleukin-3 for growth. However, expression of v-myb in these cells blocks their ability to differentiate in response to GCSF. Instead, the v-myb-infected cells proliferate indefinitely in the presence of GCSF. 32D Cl3 cells infected with empty vector carrying only the neomycin resistance gene responded to the addition of GCSF in a manner identical to that of the uninfected cells and underwent terminal differentiation into granulocytes. These results suggest that oncogenic forms of myb gene bring about transformation by blocking the differentiation signal derived by cytokines while promoting the proliferative signal transduction pathways. 相似文献
34.
Syntheses of the title glycosides are described. The critical O-glycosylations were carried out in the presence of boron trifluoride etherate with a high degree of alpha-selectivity. 相似文献
35.
Mei-Ling Siu-Caldera Jeffrey W. Clark Anabela Santos-Moore Sara Peleg Yan Yun Liu Milan R. Uskokovi Surendra Sharma G. Satyanarayana Reddy 《The Journal of steroid biochemistry and molecular biology》1996,59(5-6)
1α,25(OH)2-16-ene-D3, a synthetic analog of the steroid hormone, 1α,25(OH)2D3, has great potential to become a drug in the treatment of leukemia and other proliferative disorders, because of its minimal in vivo calcemic activity associated with a potent inhibitory effect on cell growth. However, at present, the mechanisms through which 1α,25(OH)2-16-ene-D3 expresses its biological activities are still not completely understood. Our previous in vitro study in a perfused rat kidney indicated for the first time that 1α,25(OH)2-16-ene-D3 and 1α,25(OH)2D3 are metabolized differently. 1α,25(OH)2-24-oxo-16-ene-D3, an intermediary metabolite of 1α,25(OH)2-16-ene-D3 formed through the C-24 oxidation pathway, accumulated significantly in the perfusate when compared to 1α,25(OH)2-24-oxo-D3, the corresponding intermediary metabolite of 1α,25(OH)2D3. In a subsequent in vivo study, we also reported that 1α,25(OH)2-24-oxo-16-ene-D3 exerted immunosuppressive activity equal to its parent, without causing significant hypercalcemia. In order to establish further the critical role of 1α,25(OH)2-24-oxo-16-ene-D3, in generating some of the key biological activities ascribed to its parent, we performed the present in vitro study using a human myeloid leukemic cell line (RWLeu-4) as a model. Comparative target tissue metabolism studies indicated that 1α,25(OH)2-16-ene-D3 and 1α,25(OH)2D3 are metabolized differently in RWLeu-4 cells, and the differences were similar to the ones we previously observed in the rat kidney. The significant finding was the accumulation of 1α,25(OH)2-24-oxo-16-ene-D3 in RWLeu-4 cells because of its resistance to further metabolism. Biological activity studies indicated that both 1α,25(OH)2-16-ene-D3 and its 24-oxo metabolite produced growth inhibition and promoted differentiation of RWLeu-4 cells to the same extent, and these activities were several fold higher than those exerted by 1α,25(OH)2D3. In addition, the genomic action of each vitamin D compound was assessed in a rat osteosarcoma cell line (ROS 17/2.8) by measuring its ability to transactivate a gene construct containing the vitamin D response element of the osteocalcin gene linked to the growth hormone reporter gene. In these studies, both 1α,25(OH)2-16-ene-D3 and its 24-oxo metabolite exerted similar but potent transactivation activity which was several fold greater than that exerted by 1α,25(OH)2D3 itself. In summary, our results indicate that the production and slow clearance of the bioactive intermediary metabolite, 1α,25(OH)2-24-oxo-16-ene-D3, in RWLeu-4 cells contributes significantly to the final expression of the enhanced biological activities ascribed to its parent analog, 1α,25(OH)2-16-ene-D3. 相似文献
36.
Comparative analyses of human immunodeficiency virus type 1 (HIV-1) and HIV-2 Vif mutants. 总被引:6,自引:6,他引:0 下载免费PDF全文
T R Reddy G Kraus O Yamada D J Looney M Suhasini F Wong-Staal 《Journal of virology》1995,69(6):3549-3553
Virion infectivity factor (vif), a gene found in all lentiviruses, plays an essential role in virus replication in certain target cells. We examined the replication competence of the human immunodeficiency virus type 2 (HIV-2) vif mutant in different T-cell lines and primary cells in comparison with that of the HIV-1 vif mutant. Both mutant viruses were unable to replicate in peripheral blood-derived mononuclear cells but replicated with wild-type efficiency in certain T-cell lines, such as SupT1 and MOLT-4/8. These results confirm the importance of vif in the infection of relevant target cells and imply that some cellular factor(s) could compensate for vif function. However, HIV-1 and HIV-2 vif mutant viruses also show differential replications in other cell lines, suggesting either different threshold requirements for the same cellular factor(s) or the involvement of different factors to compensate for vif-1 and vif-2 functions. By cross complementation experiments, we showed that vif-1 and vif-2 have similar functions. Our studies further indicate the existence of two kinds of nonpermissive cells: H9 is unable to complement HIV-1 delta vif but is susceptible to a one-round infection with HIV-1 delta vif produced from permissive cells. In contrast, U937 is nonpermissive for HIV-2 delta vif produced from permissive cells but, once infected, is able to complement the delta vif function. In both types of nonpermissive cells, a step prior to proviral DNA synthesis is affected. 相似文献
37.
Lectin recognition of host-like saccharide motifs in streptococcal cell wall polysaccharides 总被引:10,自引:0,他引:10
Cisar John O.; Sandberg Ann L.; Abeygunawardana Chitrananda; Reddy G.Prabhakar; Bush C.Allen 《Glycobiology》1995,5(7):655-662
Viridans streptococci that participate in the microbial colonizationof teeth have cell wall polysaccharides composed of linear phosphodiester-linkedhexa- or heptasaccharide repeating units, each containing ahost-like disaccharide motif, either Galß1 相似文献
38.
In vitro flowering and pod formation from cotyledons of groundnut (Arachis hypogaea L.) 总被引:2,自引:0,他引:2
S. B. Narasimhulu G. M. Reddy 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1984,69(1):87-91
Summary The response of groundnut cotyledons to the presence of various growth regulators in concentrations from 0.1 to 5 mg/l has been studied in detail using several genotypes of groundnut on two different media. Cotyledons with embryo axis, cultured on Blaydes' medium with cytokinins, produced shoots, in the axils of which 2–7 flower buds could be seen. The frequency of flower bud induction in general increased with increasing concentrations of cytokinins, the optimal levels being 3 mg/l of KN or 4 mg/l of BAP. Cotyledons without embryo axis, cultured on Blaydes' medium with BAP (0.5 mg/l), produced a cluster of flower buds directly, ranging in number from 8–28, without any vegetative growth. Excised embryo axes cultured on the same medium gave plantlets without flower buds. The growth regulators IAA, NAA, GA3 and ABA failed to induce flower buds in independent treatments. However, lower concentrations of IAA and NAA in combination with cytokinins exerted a positive influence on flowering. The blooming of the flower buds was facilitated on media supplemented with low concentrations of cytokinins. Six percent of the induced flowers resulted in gynophore development and ultimately formed pods when cultured under complete dark conditions in modified MS medium supplemented with kinetin. 相似文献
39.
Previous investigation of the development of humoral immunologic responsiveness following cryostimulation of the monkey prostate up to 30 days postoperatively disclosed low to modest titres of antibodies to prostatic tissue antigens. In the present study, the possible occurrence and predominance of a cellular response and its ensuing immunopathologic effects on the prostate and other accessory sexual glands of these animals, not initially examined, together with further serologic evaluation and correlation of this cellular response with the presence of antiprostatic antibodies, has been made 41 to 90 days following freezing. A reduction in the size of the prostate observed following freezing was accompanied by what were suggestive of immunopathologic alterations. Alterations occurred principally in the caudal lobe, and were characterized by what appeared to be specific periacinar foci of lymphocytic infiltrates. These lymphocytes were observed to infiltrate onto the acinar epithelial cells with subsequent separation of epithelial cells from the basal lamina and epithelial cell destruction. Other observed alterations in the prostate consisting of stromal fibrosis, periodic presence of inflammatory cells, proliferation of regenerating glands, and squamous metaplasia were in consonance with previous histologic studies of the prostate following cryosurgery by others. Antiprostatic antibodies were present in only one of the seven animals evaluated at the time which these observations were made. The present preliminary observations provide evidence suggestive of the development of a specific cellular immunologic response following cryosurgery of the prostate. Fending confirmation of the study of a larger series of animals, these observations may be of potential significance in providing an explanation of reported cases of eradication of human prostatic carcinomas following cryotherapy. 相似文献
40.
Coupled ribonucleoside diphosphate reduction, channeling, and incorporation into DNA of mammalian cells 总被引:8,自引:0,他引:8
There is rapid and specific channeling of ribonucleoside diphosphates into DNA through reactions beginning with ribonucleotide reductase and terminating with DNA polymerase. Lysolecithin-permeabilized Chinese hamster embryo fibroblasts in culture rapidly reduced ribonucleoside diphosphates by ribonucleotide reductase action when dithiothreitol was provided as a reducing agent and incorporated these deoxynucleotides into DNA. The radioactive label provided in ribo-CDP was not diluted by added deoxyribo-CTP during its incorporation into DNA, showing that the ribo-CDP does not pass through a deoxy-CTP pool. Under the conditions that permitted rapid incorporation of ribonucleoside diphosphates, deoxynucleoside triphosphates were very poorly incorporated. Ribonucleotide reductase with the rate-limiting enzyme for the overall process. The Km values for the reductase reaction and the overall process were similar and low enough for saturation by in vivo pools. Natural feedback inhibitors dATP or dTTP inhibited incorporation of labeled ribo-CDP into deoxyribonucleotides and into DNA to the same extent. Ribonucleotide reductase behaved like other enzymes that are associated in a rapidly sedimenting form. It was concentrated in the nucleus during S phase, and most of the enzyme activity in these nuclear extracts was co-sedimented with DNA polymerase on sucrose density gradients. These data support the hypotheses that a physically associated complex of enzymes (replitase) catalyzes the production of deoxynucleotides and their incorporation into DNA in S phase cells. 相似文献