全文获取类型
收费全文 | 33977篇 |
免费 | 2853篇 |
国内免费 | 1017篇 |
出版年
2023年 | 469篇 |
2022年 | 653篇 |
2021年 | 1102篇 |
2020年 | 1316篇 |
2019年 | 1669篇 |
2018年 | 1418篇 |
2017年 | 952篇 |
2016年 | 938篇 |
2015年 | 1238篇 |
2014年 | 2023篇 |
2013年 | 2160篇 |
2012年 | 1237篇 |
2011年 | 1663篇 |
2010年 | 1172篇 |
2009年 | 1547篇 |
2008年 | 1654篇 |
2007年 | 1611篇 |
2006年 | 1579篇 |
2005年 | 1361篇 |
2004年 | 1185篇 |
2003年 | 993篇 |
2002年 | 862篇 |
2001年 | 637篇 |
2000年 | 592篇 |
1999年 | 444篇 |
1998年 | 492篇 |
1997年 | 476篇 |
1996年 | 510篇 |
1995年 | 499篇 |
1994年 | 477篇 |
1993年 | 427篇 |
1992年 | 442篇 |
1991年 | 379篇 |
1990年 | 371篇 |
1989年 | 325篇 |
1988年 | 282篇 |
1987年 | 279篇 |
1986年 | 227篇 |
1985年 | 277篇 |
1984年 | 267篇 |
1983年 | 140篇 |
1982年 | 239篇 |
1981年 | 194篇 |
1980年 | 177篇 |
1979年 | 175篇 |
1978年 | 113篇 |
1977年 | 115篇 |
1976年 | 106篇 |
1973年 | 80篇 |
1972年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
Identification of a calmodulin-dependent glycogen synthase kinase in rabbit skeletal muscle, distinct from phosphorylase kinase 总被引:3,自引:0,他引:3
A glycogen synthase kinase that is completely dependent on Ca2+ and calmodulin has been identified in mammalian skeletal muscle, and purified approximately 3000-fold by chromatography on phosphocellulose and calmodulin--Sepharose. The presence of 50 mM NaCl in the homogenisation buffer was critical for extraction of the enzyme. The calmodulin-dependent glycogen synthase kinase (app. Mr 850 000) is distinct from myosin light-chain kinase and phosphorylase kinase, but phosphorylates the same serine residue on glycogen synthase as phosphorylase kinase. The physiological role of the enzyme is discussed. 相似文献
953.
Transformation of MMC-E epithelial cells by acute 3611-MSV: inhibition of collagen synthesis and induction of novel polypeptides 总被引:2,自引:0,他引:2
Mouse embryo epithelial cells MMC-E were transformed by novel fibrosarcoma-inducing murine sarcoma virus 3611-MSV. The cells were analyzed for the production and deposition of pericellular glycoproteins by immunofluorescence and by radioactive metabolic and cell surface labeling techniques followed by analysis in polyacrylamide gels and fluorography. The pericellular fibronectin matrix was lost, but unlike in virus-transformed fibroblastic cells, the production of fibronectin was not affected. The major differences detected were decrease in collagen production and initiation of synthesis of two major glycoproteins with Mr 58,000 and 60,000. Cell surface carbohydrate labeling indicated that after 3611-MSV transformation the cells expressed Mr 100,000 and 68,000 polypeptides. The present and previous results show that viral transformation of epithelial cells induces different transformed phenotypes that are associated with distinct alterations in pericellular glycoproteins. 相似文献
954.
Dicephalic — ADrosophila mutant affecting polarity in follicle organization and embryonic patterning
Margit Lohs-Schardin 《Development genes and evolution》1982,191(1):28-36
Summary The mutationdicephalic (dic) affects follicle development and thereby alters the antero-posterior polarity of embryonic patterning. It maps at a single locus (3–46.0±1.0) and can be characterized as a semi-dominant maternal effect mutation with low penetrance. Indic follicles, the 15 nurse cells form two clusters located at opposite poles of the oocyte; the numerical distribution of the nurse cells among the clusters varies from 7:8 to 1:14. Thedic egg shell carries a micropyle (anterior marker) at either pole, but the misshapen respiratory appendages are restricted to one of the two poles in most eggs. The malformed eggs rarely yield larvae and these are always abnormal anteriorly and/or posteriorly. The segment pattern expressed in their cuticle may represent two anterior parts of opposite polarities (double head type), two posterior parts of opposite polarities (double abdomen type, rare) or show uniform polarity. Lability of organization at the cystocyte stage appears as the primary developmental defect of the mutant. 相似文献
955.
Rolf Eiben 《Development genes and evolution》1982,191(4):270-276
Summary The formation of tentacles and stolons during metamorphosis is severely disturbed if inhibitors of mRNA metabolism are applied during certain phases of development. The periods of sensitivity to -amanitin are late gastrulation and the disk stage of metamorphosis. A cordycepin sensitive phase exists during the first hour of metamorphosis. In all drug sensitive phases an enhanced poly(A) synthesis is found indicating increased mRNA metabolism in these stages. Pulse-chase experiments show that planula larvae store a poly(A)-rich RNA population sedimenting between 28–18s. These long living molecules are of embryonic origin, are located in RNP particles and are degraded during metamorphosis. The particles in question appear to be stored mainly in interstitial cells. In early metamorphosis no uridine is incorporated but labelled poly(A) is added to preexisting molecules. 相似文献
956.
Stephen R. Sizemore R. David Cole 《In vitro cellular & developmental biology. Plant》1982,18(8):668-674
Summary The NMuMG cell line derived from normal mouse mammary epithelial cells was tested for responsiveness to hormones. The hormones
studied included insulin, glucocorticoids (cortisol and dexamethasone), and prolactin. In addition to membrane bound insulin
receptors and prolactin receptors, the cells had 2 × 104 cytoplasmic glucocorticoid receptors per cell. Morphological changes were observed in response to hormones. Clusters of cells
appeared with greatly increased diameter, and the number of cells per plate was reduced. The rate of DNA synthesis, corrected
by cell number, indicates that cell division, and hence cell turnover, was increased by the combination of all three hormones.
Insulin greatly enhanced protein synthesis, but glucocorticoid and prolactin did not further increase the rate. The combination
of the three hormones produced a change in the synthesis of histones, consistent with the increase in cell turnover.
There were substantial responses of enzyme activities to hormonal treatment of the cells. Insulin by itself induced a doubling
of the activity of glyceraldehyde phosphate dehydrogenase and perhaps a modest increase in NADH-cytochromec reductase. Lactose synthetase activity showed a three- to fourfold induction of both A and B subunits of the enzyme when
the cells were treated with insulin, glucocorticoid, and prolactin, and the effect of the latter two hormones was shown to
be additional to that of insulin.
This work was supported by Contract N01-CB-43866 from the National Cancer Institute, by Grants GB-38658 from the National
Science Foundation and GMS-20338 from the National Institutes of Health, and by the Agricultural Experimental Station at the
University of California. 相似文献
957.
Michael G. Gabridge Marlene J. Bright Helen R. Richards 《In vitro cellular & developmental biology. Plant》1982,18(1):55-62
Summary Cell monolayer cultures were prepared from hamster tracheal explants by a collagenase exposure and subsequent incubation in
Waymouth’s MAB 87/3 medium. The epithelial outgrowth occurred on glass cover slips. Cilia on the monolayers continued to beat
normally after the “parent” explant was removed. Monolayer cultures infected withMycoplasma pneumoniae had significant amounts of attachment. A morphological analysis of the attachment was conducted with scanning electron microscopy.
Clusters, cocci, and filaments ofM. pneumoniae all attached to the epithelial cells, but the filaments were especially common. Mycoplasmas were seen in association with
both ciliated and nonciliated cell membranes. On ciliated cells, mycoplasmas were on the ciliary strands and on the cell membrane.
When located immediately adjacent to or in between cilia, mycoplasmas were oriented vertically with the constricted attachment
tip oriented down toward the host cell membrane. When located more than a micron away from the ciliary fibers, mycoplasmas
lay horizontally along the epithelial cell membrane. The photographic data suggest that clusters or “sperules” of mycoplasmas
may liberate individual mycoplasmas that attach to the cell membrane. It appears that the receptor sites forM. pneumoniae are rather uniformly distributed along the ciliated cell membrane, and are not restricted to the interciliary areas.
Electron microscopy was done with the cooperation of Dr. R. Macleod and the staff of the Center for Electron Microscopy at
the University of Illinois. Critical editorial review was provided by C. Dayton.
This investigation was supported in part by grants to M. G. G. from the National Institute of Allergy and Infectious Diseases
(AI 12559) and the National Heart, Lung, and Blood Institute (HL 23806), Bethesda, Maryland. 相似文献
958.
Stephanie Gordon Phillips Shiu-Lan Lui David M. Phillips 《In vitro cellular & developmental biology. Plant》1982,18(8):727-738
Summary Epithelial cells may relate to their basement membrane substrates via lectin-like interactions. In a model system for study
of this type of interaction, lectin-coated bacteriological plastic petri dishes were presented as substrates for epithelial
cell adhesion. Of 21 lectins tested by mixed agglutination against two epithelial cell types, Madin-Darby canine kidney (MDCK),
and human embryonic kidney cells (HEK), nine gave less than 5% rosettes and 12 gave 5 to 50% rosettes. Wheat germ agglutinin
(WGA) andGeodia cydonium lectin gave the highest percentage of rosettes. Wheat germ agglutinin was readily adsorbed to plastic surfaces and maintained
specificity in binding interactions. Both MDCK and HEK cells attached as well to WGA coated petri dishes as to conventional
tissue culture dishes. Furthermore, both spread over the lectin-coated surfaces. The MDCK cells grew to confluence and could
be subcultured and maintained indefinitely on such surfaces, although WGA in solution was toxic to the cells in concentrations
as low as 0.1 to 1.0 μg/ml. Cell attachment to WGA coated dishes was blocked by cycloheximide only if the cells had been preincubated
with the inhibitor for several hours. Cell attachment was not inhibited by pretreatment of cells with neuraminidase. Precoating
cells with WGA blocked binding to both WGA-coated surfaces and untreated tissue culture dishes. Cells attached to WGA-coated
dishes could not be readily dislodged by trypsin-EDTA for the first 2 h after subculture. By 4 h, attachment was again trypsin
sensitive, suggesting that the cells synthesized a trypsin-sensitive material that was laid down between the cell surface
and the WGA-coated dish. Regeneration of trypsin sensitivity was not blocked by cycloheximide.
This work was supported by Research Grant AG01986 from the National Institutes of Health, Bethesda, Maryland. 相似文献
959.
James C. Fuscoe J.Patrick ONeill Richard Machanoff Abraham W. Hsie 《Mutation research》1982,96(1):15-30
We describe an assay for the quantification of reverse mutations at the hypoxanthine-guanine phosphoribosyltransferase (hgprt) locus in Chinese hamster ovary cells utilizing the selective agent L-azaserine (AS). Conditions are defined in terms of optimal AS concentration, cell density, and phenotypic expression time. After treatment, replicate cultures of 106 cells are allowed a 48-h phenotypic expression time in 100-mm plates. AS (10 μM) is then added directly to the growing culture and AS-resistant (ASr) cells form visible colonies. This assay is used to quantify ICR-191-, ICR-170-, and N-ethyl-N-nitrosourea-induced reversion of independently isolated HGPRT? clones. The ASr phenotype is characterized both physiologically and biochemically. All ASr clones isolated are stably resistant to AS and aminopterin but sensitive to 6-thioguanine. They also have re-expressed HGPRT enzyme. In addition, several revertants are shown to contain altered HGPRT. The data provide further evidence that ICR-191 and ICR-170 cause structural gene mutations in mammalian cells and also suggest that ICR-191, ICR-170, and N-ethyl-N-nitrosourea induce similar types of mutations in Chinese hamster ovary cells. 相似文献
960.
Summary Callus-derived suspension cultures of oats dramatically increase the viscosity of the culture media after one month in culture. Colorimetric assays for sugars and protein, as well as measurements of viscosity, suggest that the released material is a long-chain polysaccharide, probably a pectinaceous substance. These cells grow slowly in liquid culture, yet despite their low cell density, they are able to increase the viscosity of the media several fold within seven days after media transfer. Ultrastructural observations show that oat cells have features common to actively-secreting cells; especially evident are numerous dictyosomes with hypertrophied cisternae. Using a combination of filtering and centrifugation techniques we were able to recover large numbers of intact secretory vesicles. The interior of the vesicles stain with periodic acid-silver hexamine, and colormetric analysis of the vesicle pellet for total sugars confirms the presence of polysaccharides in this vesicle fraction. Because of the uniformity of these cells, the high rate of secretion, and the accessability of a large vesicle population, this culture system is'a useful model for studying the secretory process in plant cells.Scientific Article No. A-3128, Contribution No. 6196 of the Maryland Agricultural Experiment Station, College Park, MD. 相似文献