全文获取类型
收费全文 | 36570篇 |
免费 | 4188篇 |
国内免费 | 30篇 |
专业分类
40788篇 |
出版年
2022年 | 307篇 |
2021年 | 633篇 |
2020年 | 320篇 |
2019年 | 449篇 |
2018年 | 525篇 |
2017年 | 454篇 |
2016年 | 804篇 |
2015年 | 1354篇 |
2014年 | 1545篇 |
2013年 | 1908篇 |
2012年 | 2414篇 |
2011年 | 2345篇 |
2010年 | 1532篇 |
2009年 | 1290篇 |
2008年 | 1952篇 |
2007年 | 1940篇 |
2006年 | 1750篇 |
2005年 | 1693篇 |
2004年 | 1686篇 |
2003年 | 1614篇 |
2002年 | 1515篇 |
2001年 | 712篇 |
2000年 | 625篇 |
1999年 | 655篇 |
1998年 | 499篇 |
1997年 | 393篇 |
1996年 | 352篇 |
1995年 | 350篇 |
1994年 | 329篇 |
1993年 | 297篇 |
1992年 | 465篇 |
1991年 | 436篇 |
1990年 | 452篇 |
1989年 | 382篇 |
1988年 | 359篇 |
1987年 | 374篇 |
1986年 | 348篇 |
1985年 | 383篇 |
1984年 | 320篇 |
1983年 | 293篇 |
1982年 | 272篇 |
1981年 | 273篇 |
1980年 | 238篇 |
1979年 | 310篇 |
1978年 | 293篇 |
1977年 | 225篇 |
1976年 | 211篇 |
1975年 | 216篇 |
1974年 | 221篇 |
1973年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Cultured Friend murine erythroleukemia cells (Friend cells) are induced to undergo erythroid differentiation when grown in the presence of dimethylsulfoxide (DMSO) and other compounds. The effects of unifilar substitution of bromouracil (BU) for thymidine in the DNA (BU-DNA) of Friend cells were examined. Cells were grown in the presence of 5-bromodeoxy-uridine (BrdU) for one generation, then centrifuged and resuspended in medium containing DMSO without BrdU. These cells exhibited a delay in the appearance of heme-producing, benzidine-reative (B+) cells and a decreased rate of cell proliferation in comparison to the control not containing BU-DNA. A transient inhibition of entry into S phase was observed when control cells or cells containing BU-DNA were grown in the presence of DMSO) for 10 to 20 hours. This transient inhibition was increased in the BrdU culture. Thus BU-substitution in Friend cells alters other cellular functions in addition to erythroid differentiation. The rate of increase in the percent of cells committed to differentiate (those forming B+ colonies in plasma clots) was similar in the BrdU and control cultures until 40 to 50 hours. After this time, a delay in the appearance of committed cells was observed in the BrdU culture. The effect of BrdU on the appearance of B+ cells was more pronounced and occurred earlier than its effect on the rate of commitment. Therefore, the delay in the appearance of B+ cells in the BrdU culture was due primarily to perturbation of post-commitment events such as the accumulation of hemoglobin. We also examined the effect on growth and differentiation after BrdU was incorporated during different intervals of S phase in cells synchronized by centrifugal elutriation or by double thymidine block and hydroxyurea treatment. The delay in the appearance of B+ cells and inhibition of cell proliferation were only observed when BrdU was incorporated in the first half of S phase. BrdU (10 muM) had no effect on growth or differentiation when present during late S or G1 and G2. These results, using two very different methods to achieve cell synchrony, indicate that the effects of BrdU on growth and differentiation described above are due to its incorporation into DNA sequences replicating during early S. 相似文献
3.
4.
5.
6.
7.
M C Brown G C Lunt A Stapleton 《Comp. Biochem. Physiol. C, Comp. Pharmacol. Toxicol.》1989,92(1):9-13
1. Using homogenates of supraoesophageal ganglia from locust we observed specific binding of [35S]-TBPS which was linear with protein concentration up to 7 mg/ml, showed a pH optimum at pH 9.0 and was linear with NaCl concentration up to 350 mM. 2. Kinetic analysis of the binding showed positive cooperativity as a result of changes in on and off-rates with occupation of the binding site by the ligand. The apparent KD = 417 nM and Bmax = 1083 fmol/mg of membrane protein were calculated using a computer program for dose-response curve fitting. 3. The binding was enhanced by GABA, pentobarbital and benzodiazepines. Picrotoxinin had no effect on the binding at 0.1 mM. Only the cage convulsants TBPS and IBP were able to displace the binding. 4. Whilst the characteristics of the binding are similar to those reported for house fly thorax and abdomen preparations they are significantly different from those reported for house fly head, cockroach nerve cord and rat brain. 相似文献
8.
David M. Anderson Richard H. Scheller James W. Posakony Linda B. McAllister Steven G. Trabert Clifford Beall Roy J. Britten Eric H. Davidson 《Journal of molecular biology》1981,145(1):5-28
Three repetitive sequence families from the sea urchin genome were studied, each defined by homology with a specific cloned probe one to a few hundred nucleotides long. Recombinant λ-sea urchin DNA libraries were screened with these probes, and individual recombinants were selected that include genomic members of these families. Restriction mapping, gel blot, and kinetic analyses were carried out to determine the organization of each repeat family. Sequence elements belonging to the first of the three repeat families were found to be embedded in longer repeat sequences. These repeat sequences frequently occur in small clusters. Members of the second repeat family are also found in a long repetitive sequence environment, but these repeats usually occur singly in any given region of the DNA. The sequences of the third repeat are only 200 to 300 nucleotides long, and are generally terminated by single copy DNA, though a few examples were found associated with other repeats. These three repeat sequence families constitute sets of homologous sequence elements that relate distant regions of the DNA. 相似文献
9.
Tanner Miest Dyana Saenz Anne Meehan Manuel Llano Eric M. Poeschla 《Methods (San Diego, Calif.)》2009,47(4):298-303
RNAi is a powerful technology for analyzing gene function in human cells. However, its utility can be compromised by inadequate knockdown of the target mRNA or by interpretation of effects without rigorous controls. We review lentiviral vector-based methods that enable transient or stable knockdowns to trace mRNA levels in human CD4+ T cell lines and other targets. Critical controls are reviewed, including rescue of the pre-knockdown phenotype by re-expression of the targeted gene. The time from thinking about a potential knockdown target to analysis of phenotypes can be as short as a few weeks. 相似文献
10.