全文获取类型
收费全文 | 9927篇 |
免费 | 567篇 |
专业分类
10494篇 |
出版年
2022年 | 58篇 |
2021年 | 104篇 |
2020年 | 56篇 |
2019年 | 77篇 |
2018年 | 111篇 |
2017年 | 106篇 |
2016年 | 185篇 |
2015年 | 257篇 |
2014年 | 287篇 |
2013年 | 550篇 |
2012年 | 467篇 |
2011年 | 483篇 |
2010年 | 292篇 |
2009年 | 306篇 |
2008年 | 455篇 |
2007年 | 447篇 |
2006年 | 409篇 |
2005年 | 449篇 |
2004年 | 422篇 |
2003年 | 425篇 |
2002年 | 411篇 |
2001年 | 357篇 |
2000年 | 380篇 |
1999年 | 279篇 |
1998年 | 131篇 |
1997年 | 96篇 |
1996年 | 75篇 |
1995年 | 79篇 |
1994年 | 80篇 |
1993年 | 85篇 |
1992年 | 203篇 |
1991年 | 204篇 |
1990年 | 206篇 |
1989年 | 196篇 |
1988年 | 179篇 |
1987年 | 148篇 |
1986年 | 126篇 |
1985年 | 116篇 |
1984年 | 110篇 |
1983年 | 92篇 |
1982年 | 77篇 |
1981年 | 69篇 |
1979年 | 92篇 |
1978年 | 65篇 |
1977年 | 64篇 |
1976年 | 58篇 |
1975年 | 61篇 |
1974年 | 58篇 |
1973年 | 56篇 |
1972年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
We have studied in northern Japan the hitherto unidentified eggs of a species of cuckoo in the nests of the Bush Warbler Cettia diphone . The cuckoo in question appeared to be the Himalayan Cuckoo Cuculus saturatus which parasitizes mainly the Willow Warbler Phylloscopus occipitalis in southern Japan. The egg colour in this northern Cuckoo was chocolate-brown or orange-brown, similar to that of the Bush Warbler but unlike that of the southern Himalayan Cuckoo. Egg size was significantly larger than that of the southern Himalayan Cuckoo and instead similar to that of the Little Cuckoo C. poliocephalus which uses the same host species in southern Japan, to which the Little Cuckoo is confined. The shift in host species and egg colour in the northern Himalayan Cuckoo seems to be a case of character release in the absence of the Little Cuckoo. 相似文献
103.
104.
The citrate utilization (Cit+) transposon Tn3411 was shown to be flanked by directly repeated sequences (IS3411L and IS3411R) by restriction enzyme analysis and electron microscope observation. Cit- deletion mutants were frequently found to be generated in pBR322::Tn3411 by intramolecular recombination between the two copies of IS3411. The flanking IS3411 elements of Tn3411 were shown to be functional insertion sequences by Tn3411-mediated direct and inverse transposition. Tn3411-mediated inverse transposition from pBR322::Tn3411 to the F-plasmid derivative pED100 occurred more efficiently than that of direct transposition of the Cit+ determinant. This was thought to be due to the differential transposability of IS3411L and IS3411R in the transposition process. The frequency of transposition of IS3411 marked with a chloramphenicol resistance determinant was much higher than IS3411-mediated cointegrate formation, suggesting that replicon fusions are not essential intermediates in the transposition process of Tn3411 or IS3411. Spontaneous deletions occurred with high frequency in recA hosts. The spontaneous deletion promoted by homologous recombination between two IS3411 elements in Tn3411 was examined with deletion mutants. 相似文献
105.
Polypeptide involved in the Escherichia coli plasmid-mediated citrate transport system. 总被引:4,自引:4,他引:0 下载免费PDF全文
A genetic determinant conferring on Escherichia coli the ability to utilize citrate as a sole source of carbon and energy was subcloned into pBR322 from a naturally occurring, citrate utilization (Cit+) plasmid, pOH30221, and was localized to a 1.6-kilobase region by cloning and subsequent deletion analysis. Genetic expression of the Cit+ determinant in E. coli minicells revealed that the Cit+ determinant encoded a single, membrane-associated polypeptide with an apparent molecular weight of 35,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This polypeptide seemed not to be synthesized as a precursor with an amino-terminal signal sequence. 相似文献
106.
107.
The substrate specificities of the amino acid transport systems of vacuoles of the yeast, Saccharomyces cerevisiae, were investigated using purified vacuolar-membrane vesicles (Ohsumi, Y., and Anraku, Y. (1981) J. Biol. Chem. 256, 2079-2082). Ten amino acids: arginine, lysine, histidine, phenylalanine, tryptophan, tyrosine, glutamine, asparagine, isoleucine, and leucine, were taken up actively into the vesicles. Kinetic studies indicated the presence of seven independent H+/amino acid antiport systems with narrow substrate specificity, which were all driven by a proton motive force established by ATP hydrolysis. The Kt and Vmax values, and the specific inhibitors for the arginine, arginine-lysine, histidine, phenylalanine-tryptophan, tyrosine, glutamine-asparagine, and isoleucine-leucine transport systems were determined. 相似文献
108.
T Kokubu K Hiwada Y Sato T Iwata Y Imamura R Matsueda Y Yabe H Kogen M Yamazaki Y Iijima 《Biochemical and biophysical research communications》1984,118(3):929-933
We designed aldehyde derivatives of small peptides representing the C-terminal portion of angiotensin I sequence as an inhibitor of human renin. Among compounds that we synthesized, benzyloxycarbonyl (Z)-Phe-His-Leucinal (compound V), Z-Pro-Phe-His-Leucinal (Compound IV) and Z-[3-(1'-naphthyl)Ala]-His-Leucinal (compound VII) markedly inhibited human renin (IC50, 7.5 X 10(-7), 3.2 X 10(-7) and 8.0 X 10(-8) mol/l, respectively). Compound VII was shown to be noncompetitive (Ki = 2.4 X 10(-7) mol/l). It did not inhibit either cathepsin D or pepsin. Compound V had slight or no inhibitory effect at the concentration of 10(-5) mol/l on six animal renins except for monkey and rabbit renins. Results obtained show that these aldehyde compounds are highly selective and species specific inhibitors for human and monkey renins. 相似文献
109.
Makoto Daimon Ken-ichi Tsutsumi Jun-ichi Sato Reiko Tsutsumi Kiichi Ishikawa 《Biochemical and biophysical research communications》1984,124(2):337-343
The expression of aldolase A and B mRNAs during azo-dye-induced carcinogenesis in rat liver was examined. After feeding the dye for 18 weeks, the level of aldolase A mRNA increased to about 11 times that in a normal liver, with the concomitant decrease of aldolase B mRNA level to about 25% of that in a normal liver. These changes did not occur progressively during the carcinogenesis, but occurred as an additional phase after 4 week-feeding of the azo-dye. At this stage, the levels of aldolase A and B mRNAs were about 7 times and 45% of that in a normal liver, respectively. This biphasic pattern in the aldolase isozyme expression in the azo-dye-fed rat liver is discussed together with the kinetic data of the enzyme activity. 相似文献
110.
Hitoshi Sato Yuichi Sugiyama Yasufumi Sawada Tatsuji Iga Manabu Hanano 《Life sciences》1984,35(10):1051-1059
A rapid radioreceptor assay for measuring ß-endorphin (ß-EP) in unextracted serum has been developed. The method is based upon the inhibition by ß-EP of 3H-naloxone binding to the specific receptors on rat brain membranes, prepared in a stable form of pellets. Effect of serum on the assay was minimized by adding pooled serum to the equal dilution of total serum in the assay mixture. Pharmacokinetic analysis of pharmacologically active ß-EP equivalents (ß-EP eq.) in rats was performed using this method. The serum disappearance of ß-EP eq. after iv administration followed a biexponential decline and pharmacokinetic parameters were calculated by a two-compartment open model. The half-lives of α-phase and ß-phase were 2.6 ± 0.5 min and 6.2 ± 1.6 hr (mean ± SE; n=6), respectively. The volume of the central compartment (V1) and that of steady-state (Vdss) were 67 ± 16 and 480 ± 75 ml/kg (mean ± SE; n=6), respectively. The total body serum clearance (CLtot) was 2.1 ± 0.9 ml/min/kg (mean ± SE; n=6). The serum disappearance curve of ß-EP eq. obtained in the present study was similar to that previously reported by Houghten et al. (Proc. Natl. Acad. Sci. U.S.A. , 4588–4591 (1980)), in which the disapperance of total radioactivity of tritiated ß-EP in rats was examined. 相似文献