全文获取类型
收费全文 | 65716篇 |
免费 | 4446篇 |
国内免费 | 2808篇 |
出版年
2023年 | 1062篇 |
2022年 | 1072篇 |
2021年 | 2088篇 |
2020年 | 2050篇 |
2019年 | 2778篇 |
2018年 | 2470篇 |
2017年 | 1638篇 |
2016年 | 1767篇 |
2015年 | 2208篇 |
2014年 | 3731篇 |
2013年 | 4847篇 |
2012年 | 2719篇 |
2011年 | 3554篇 |
2010年 | 2585篇 |
2009年 | 2941篇 |
2008年 | 3008篇 |
2007年 | 3131篇 |
2006年 | 2769篇 |
2005年 | 2561篇 |
2004年 | 2275篇 |
2003年 | 1961篇 |
2002年 | 1838篇 |
2001年 | 1432篇 |
2000年 | 1240篇 |
1999年 | 1199篇 |
1998年 | 1079篇 |
1997年 | 971篇 |
1996年 | 936篇 |
1995年 | 873篇 |
1994年 | 821篇 |
1993年 | 767篇 |
1992年 | 722篇 |
1991年 | 673篇 |
1990年 | 526篇 |
1989年 | 511篇 |
1988年 | 462篇 |
1987年 | 392篇 |
1986年 | 345篇 |
1985年 | 501篇 |
1984年 | 745篇 |
1983年 | 505篇 |
1982年 | 578篇 |
1981年 | 501篇 |
1980年 | 406篇 |
1979年 | 370篇 |
1978年 | 282篇 |
1977年 | 239篇 |
1976年 | 231篇 |
1974年 | 135篇 |
1973年 | 152篇 |
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
991.
992.
ts7, a temperature-sensitive mutant defective in neuraminidase (NA) of influenza B/Kanagawa/73, lacks NA enzymatic activity at the nonpermissive temperature (37.5 C). When MDCK cells were infected with the mutant at the permissive temperature (32 C) and exposed to pH 5.2 medium, extensive cell fusion occurred. In contrast, at the nonpermissive temperature cells did not show cell fusion at all unless they were pretreated with trypsin, suggesting that at 37.5 C the hemagglutinin (HA) of ts7 is expressed at the cell surface in an uncleaved form. It was also found that the replacement of RNA segment 6 of ts7 with that of wild-type B/Lee resulted in the emergence of low pH-induced fusion activity as well as NA enzymatic activity at the incubation temperature of 37.5 C and that the addition of bacterial NA to the cultures infected with ts7 at 37.5 C early in infection brought about low pH-induced cell fusion. We suggest that the removal of neuraminic acid from the carbohydrate moiety of HA by NA is essential for the cleavage of HA by cellular protease. 相似文献
993.
刺果番荔枝中的番荔枝内酯 总被引:4,自引:0,他引:4
从刺果番荔枝(Annona m uricata L.)的种子中分离到3 个单四氢呋喃型番荔枝内酯类化合物,用波谱方法鉴定为海南哥纳香甲素(how iicin A, S13)、乙素(how iicin B, S5)和新化合物4-去氧海南哥纳香乙素(4-desoxyhow iicin B, S2)。 相似文献
994.
995.
Protoplasts isolated from both 7-day-old light-grown and 4-day-old dark/dim light-grown cotyledons of four Brassica campestris varieties (Arlo, Sonja, Bunyip and Wonk Bok) were cultured in three liquid media: modified K8P, modified MS and modified Pelletier's B to compare the capacities for cell division and plant regeneration. Following cell wall regeneration the cultured protoplasts from dark/dim light-grown cotyledons of four varieties showed rapid division and high frequency of cell division compared with those isolated from light-grown cotyledons. The frequencies of cell division were significantly influenced by varieties and culture media but only in cultured protoplasts isolated from dark/dim light-grown cotyledons. The interaction between varieties and media was also significant. Cell colonies formed within 7–14 days in protoplast cultures from dark/dim light-grown cotyledons, and calli subsequently grown on a solid medium developed shoots when transferred onto a regeneration medium. Three of four tested varieties (Arlo, Sonja and Bunyip) showed shoot regeneration within 2–3 months after protoplast isolation, with a high degree of reproducibility in Arlo and Bunyip. Regenerated shoots, which were induced to root on half-strength MS medium with 0.1 mg.l–1 IBA, survived in soil and grew to produce siliques and set viable seeds in the greenhouse. The present report is the first to document the production of regenerated plants that set seeds in Brassica campestris from cotyledonary protoplasts.Abbreviations BAP
benzylaminopurine
- CPW
Composition of Protoplast Washing-solution
- 2,4-D
2,4-dichlorophenoxyacetic acid
- EDTA
ethylenediamine-tetraacetic acid
- GA3
gibberellic acid
- IBA
indole-3-butyric acid
- IAA
indole-3-acetic acid
- MS
Murashige and Skoog medium
- NAA
-naphthaleneacetic acid
- KT
kinetin
- FDA
fluorescein diacetate
- SDS
sodium dodecyl sulfate 相似文献
996.
Millie Hughes-Fulford 《Journal of cellular biochemistry》1994,54(3):265-272
Our previous studies have implied that prostaglandins inhibit cell growth independent of cAMP. Recent reports, however, have suggested that prostaglandin arrest of the cell cycle may be mediated through protein kinase A. In this report, in order to eliminate the role of c-AMP in prostaglandin mediated cell cycle arrest, we use the-49 lymphoma variant (cyc?) cells that lack adenylate cyclase activity. We demonstrate that dimethyl prostaglandin A1 (dmPGA1) inhibits DNA synthesis and cell growth in cyc? cells. DNA synthesis is inhibited 42% by dmPGA1 (50 μM) despite the fact that this cell line lacks cellular components needed for cAMP generation. The ability to decrease DNA synthesis depends upon the specific prostaglandin structure with the most effective form possessing the α,β unsaturated ketone ring. Dimethyl PGA1 is most effective in inhibiting DNA synthesis in cyc? cells, with prostaglandins PGE1 and PGB1 being less potent inhibitors of DNA synthesis. DmPGE2 caused a significant stimulation of DNA synthesis. S-49 cyc- variant cells exposed to (30–50 μm) dmPGA1, arrested in the G1 phase of the cell cycle within 24 h. This growth arrest was reversed when the prostaglandin was removed from the cultured cells; growth resumed within hours showing that this treatment is not toxic. The S-49 cyc? cells were chosen not only for their lack of adenylate cyclase activity, but also because their cell cycle has been extensively studied and time requirements for G1, S, G2, and M phases are known. Within hours after prostaglandin removal the cells resume active DNA synthesis, and cell number doubles within 15 h suggesting rapid entry into S-phase DNA synthesis from the G1 cell cycle block. The S-49 cyc? cells are known to have a G1/S boundary through M phase transition time of 14.8 h, making the location of the prostaglandin cell cycle arrest at or very near the G1/S interface. The oncogenes, c-fos and c-myc which are normally expressed during G1 in proliferating cells have a 2–3 fold enhanced expression in prostaglandin G1 arrested cells. These data using the S-49 variants demonstrate that dmPGA1 inhibits DNA synthesis and arrests the cell cycle independent of cAMP-mediated effects. The prostaglandin arrested cells maintain the gene expression of a G1 synchronous cell which suggests a unique molecular mechanism for prostaglandin action in arresting cell growth. These properties indicate that this compound may be an effective tool to study molecular mechanisms of regulation of the cell cycle. 相似文献
997.
998.
999.
Kumar B. Reddy Barbara A. Hocevar Philip H. Howe 《Journal of cellular biochemistry》1994,56(3):418-425
Transforming growth factor β1 (TGFβ1) inhibits epithelial cell proliferation late in the G1 phase of the cell cycle. We examined the effect of TGFβ1 on known late G1 cell cycle regulators in an attempt to determine the molecular mechanism of growth inhibition by this physiological inhibitor. The results demonstrate the TGFβ1 inhibits the late G1 and S phase specific histone H1 kinase activity of p33cdk2. This inhibitiion is not dur to TGFβ1's effect on p33cdk2 synthesis, but rather due to its negative effect on the late G1 phosphorylation of p33cdk2. It is also shown that TGFβ1 inhibits both late G1 cyclin A and cyclin E associated histon H1 kinase activities. The inhibitor has no effects on the synthesis of cyclin E but to inhibit the synthesis of cyclin A protein in a cell cycle dependent manner. If TGFβ1 is added to cells which have progressed futher than 8 hours into G1, then it is without inhibitory effect on cyclin A synthesis. These effect on TGFβ1 on late G1 cell cycle regulators correlate well with its inhibitory effects on cellular growth and suggest that these G1 cyclin dependent kinases might serve as targets for TGFβ1-mediated growth arrest. 相似文献
1000.
J. Berreur-Bonnenfant M. Ammar A. Dubreuil H. Kiefer G Diogene P. Metezeau S. Puiseux-Dao 《Cell biology and toxicology》1994,10(5-6):423-427
Maitotoxin (MTX) induces an increase of [Ca2+]i and of phosphoinositide breakdown in various cell types. The [Ca2+]i increase followed with fluorescent probes on cell suspensions has been described as slow and lasting, in contrast to the signal induced by calcium ionophores such as ionomycin. MTX effects have been studied on two fibroblastic cell lines, BHK21 C13 and FR 3T3, synchronized by serum deprivation treatment performed in an isoleucine-free medium for BHK21 C13 cells. In BHK21 C13 cells, flow cytometry analysis showed that two stages, G1/S and G2/M, were particularly susceptible to MTX treatment. Scanning laser cytometry demonstrated that calcium response of FR 3T3 fibroblasts followed with Indo-1 varied during the cell division cycle. The [Ca2+]i increase was almost always vertical, but its delay after MTX addition lasted from zero (S and G2/M transition) to 10–20 min (G1) or more (G2). No [Ca2+]i change could be detected during mitosis. The [Ca2+]i response at the S phase was biphasic. These observations suggest that (1) the lasting response described in the literature represents a global cell population effect, and (2) cells are more sensitive to MTX at specific stages of the cell division cycle, which could correspond to periods when calcium signals have been detected in different cell types.Abbreviations MTX
maitotoxin
- [Ca2+]i
intracellular calcium concentration
- IP3
inositol triphosphate 相似文献