全文获取类型
收费全文 | 405343篇 |
免费 | 46166篇 |
国内免费 | 172篇 |
出版年
2018年 | 3492篇 |
2017年 | 3481篇 |
2016年 | 4671篇 |
2015年 | 6211篇 |
2014年 | 7228篇 |
2013年 | 10808篇 |
2012年 | 12092篇 |
2011年 | 12219篇 |
2010年 | 8183篇 |
2009年 | 7636篇 |
2008年 | 11104篇 |
2007年 | 11252篇 |
2006年 | 10840篇 |
2005年 | 10237篇 |
2004年 | 10132篇 |
2003年 | 9819篇 |
2002年 | 9577篇 |
2001年 | 18886篇 |
2000年 | 19075篇 |
1999年 | 15487篇 |
1998年 | 5422篇 |
1997年 | 5684篇 |
1996年 | 5498篇 |
1995年 | 4994篇 |
1994年 | 5112篇 |
1993年 | 4977篇 |
1992年 | 12909篇 |
1991年 | 12331篇 |
1990年 | 12268篇 |
1989年 | 12230篇 |
1988年 | 11136篇 |
1987年 | 10594篇 |
1986年 | 9818篇 |
1985年 | 9857篇 |
1984年 | 7992篇 |
1983年 | 6958篇 |
1982年 | 5288篇 |
1981年 | 4666篇 |
1980年 | 4547篇 |
1979年 | 7660篇 |
1978年 | 5928篇 |
1977年 | 5290篇 |
1976年 | 5113篇 |
1975年 | 5529篇 |
1974年 | 5778篇 |
1973年 | 5700篇 |
1972年 | 5161篇 |
1971年 | 4561篇 |
1970年 | 4031篇 |
1969年 | 3732篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
101.
beta-lactamase has been purified to a homogeneous state from Mycobacterium butyricum ATCC 19979. The molecular weight (Mr = 29,000) and the isoelectric point (4,0) of the enzyme have been determined. The enzyme showed both penicillinase and cephalosporinase activity, but had relatively more of the former. With respect to substrate-profile the enzyme resembled the plasmid specified TEM-type beta-lactamases commonly encountered in Gram-negative bacteria. The enzyme was insensitive to p-chloromercuribenzoate, sodium chloride, or iodine inhibition. 相似文献
102.
Characterization of the ATP synthase of Propionigenium modestum as a primary sodium pump 总被引:10,自引:0,他引:10
The ATP synthase (F1F0) of Propionigenium modestum has been purified to a specific ATPase activity of 5.5 units/mg of protein, which is about 6 times higher than that of the bacterial membranes. Analysis by SDS gel electrophoresis indicated that in addition to the five subunits of the F1 ATPase, subunits of Mr 26,000 (a), 23,000 (b), and 7500 (c) have been purified. The ATPase activity of F1F0 was specifically activated about 10-fold by Na+ions. The enzyme was strongly inhibited by dicyclohexylcarbodiimide, venturicidin, tributyltin chloride, and azide. After incubation with [14C]dicyclohexylcarbodiimide, about 3-4 mol of the inhibitor was bound per 500,000 g of the enzyme. The radioactive label was specifically bound to submit c. These subunits form stable aggregates which resist dissociation by SDS at 100 degrees C. The monomer is formed upon heating with SDS to 121 degrees C or by extraction of the membranes with chloroform/methanol. The ATP synthase was incorporated into liposomes by a freeze-thaw-sonication procedure. The reconstituted proteoliposomes catalyzed the transport of Na+ions upon ATP hydrolysis. The transport was completely abolished by dicyclohexylcarbodiimide. Whereas monensin prevented the accumulation of Na+ions, the uptake rate was stimulated 4-5-fold in the presence of valinomycin or carbonyl cyanide m=chlorophenylhydrazone. These results indicate an electrogenic Na+ transport and also that it is a primary event and not accomplished by a H+-translocating ATP synthase in combination with a Na+/H+ antiporter. 相似文献
103.
G Rodriguez-Tarduchy P Malde A Lopez-Rivas M K Collins 《Journal of immunology (Baltimore, Md. : 1950)》1992,148(5):1416-1422
Calcium ionophores inhibit apoptosis in the IL-3-dependent cell line BAF3 and maintain the cells in a viable noncycling state. In this report, an identical effect of ionophore was also demonstrated on the multipotent IL-3-dependent progenitor cell line FDCP-MIX and on the primary IL-3-dependent cell population that could be cultured from murine bone marrow. Inhibition of apoptosis required extracellular calcium and could be blocked by cyclosporin A. Nuclei from IL-3-dependent cells were found to lack a calcium-activatable nuclease that degrades chromatin in the linker region between nucleosomes, unlike the nuclei of lymphoid cells. The mechanism of action of calcium ionophore could be divided into two distinct steps. First, ionophore induced the production of a survival factor that stimulated DNA synthesis and was identified as IL-4. Second, ionophore inhibited the cell cycle of the various IL-3-dependent cells. IL-4 production could be inhibited by cyclosporin A and required extracellular calcium, whereas cell cycle arrest did not. This implied that factor production was the step that was necessary for inhibition of apoptosis and maintenance of cell viability. This was confirmed by the use of an anti-IL-4R antibody, which blocked the inhibition of apoptosis induced by calcium ionophores. 相似文献
104.
To define catalytically essential residues of bacteriophage T7 RNA polymerase, we have generated five mutants of the polymerase, D537N, K631M, Y639F, H811Q and D812N, by site-directed mutagenesis and purified them to homogeneity. The choice of specific amino acids for mutagenesis was based upon photoaffinity-labeling studies with 8-azido-ATP and homology comparisons with the Klenow fragment and other DNA/RNA polymerases. Secondary structural analysis by circular dichroism indicates that the protein folding is intact in these mutants. The mutants D537N and D812N are totally inactive. The mutant K631M has 1% activity, confined to short oligonucleotide synthesis. The mutant H811Q has 25% activity for synthesis of both short and long oligonucleotides. The mutant Y639F retains full enzymatic activity although individual kinetic parameters are somewhat different. Kinetic parameters, (kcat)app and (Km)app for the nucleotides, reveal that the mutation of Lys to Met has a much more drastic effect on (kcat)app than on (Km)app, indicating the involvement of K631 primarily in phosphodiester bond formation. The mutation of His to Gln has effects on both (kcat)app and (Km)app; namely, three- to fivefold reduction in (kcat)app and two- to threefold increase in (Km)app, implying that His811 may be involved in both nucleotide binding and phosphodiester bond formation. The ability of the mutant T7 RNA polymerases to bind template has not been greatly impaired. We have shown that amino acids D537 and D812 are essential, that amino acids K631 and H811 play significant roles in catalysis, and that the active site of T7 RNA polymerase is composed of different regions of the polypeptide chain. Possible roles for these catalytically significant residues in the polymerase mechanism are discussed. 相似文献
105.
106.
107.
B Spyropoulos P B Moens J Davidson J A Lowden 《American journal of human genetics》1981,33(3):375-380
Chi-square analyses of new data as well as data previously reported by Myrianthopoulos have shown that grandparents of Tay-Sachs carriers die from proportionally the same causes as grandparents of noncarriers. It is unlikely that there is any advantage to being a Tay-Sachs carrier insofar as resistance to tuberculosis is concerned. Our results are further evidence to support Fraikor's claim that the high carrier frequency of the allele in Ashkenazi Jews is probably caused by a combination of founder effect, genetic drift, and differential immigration patterns. 相似文献
108.
—A resolution of the enhancement of protein synthesis in the visual cortex of rats during first exposure to light (Richardson and Rose , 1972) was achieved by polyacrylamide gel electrophoresis using a double-labelling technique. Differential incorporation of lysine was established between exposed and control animals in two fractions of the soluble proteins and seven fractions of the insoluble proteins. This suggests that exposure to a new experience of this type involves a specific effect on protein synthesis, rather than a general stimulation across all fractions. 相似文献
109.
110.