全文获取类型
收费全文 | 1261篇 |
免费 | 97篇 |
国内免费 | 9篇 |
出版年
2021年 | 24篇 |
2020年 | 9篇 |
2019年 | 21篇 |
2018年 | 10篇 |
2017年 | 20篇 |
2016年 | 23篇 |
2015年 | 32篇 |
2014年 | 26篇 |
2013年 | 42篇 |
2012年 | 57篇 |
2011年 | 46篇 |
2010年 | 35篇 |
2009年 | 49篇 |
2008年 | 59篇 |
2007年 | 56篇 |
2006年 | 59篇 |
2005年 | 50篇 |
2004年 | 30篇 |
2003年 | 36篇 |
2002年 | 29篇 |
2001年 | 26篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 13篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1994年 | 12篇 |
1992年 | 20篇 |
1991年 | 20篇 |
1990年 | 9篇 |
1989年 | 14篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1984年 | 9篇 |
1981年 | 14篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 13篇 |
1974年 | 8篇 |
1961年 | 9篇 |
1959年 | 9篇 |
1958年 | 26篇 |
1957年 | 26篇 |
1956年 | 29篇 |
1955年 | 23篇 |
1954年 | 22篇 |
1953年 | 14篇 |
1952年 | 14篇 |
1951年 | 10篇 |
1950年 | 9篇 |
排序方式: 共有1367条查询结果,搜索用时 15 毫秒
21.
22.
23.
Deep-level diagnostic value of the rDNA-ITS region 总被引:14,自引:0,他引:14
The similarity of certain reported angiosperm rDNA internal transcribed
spacer (ITS) region sequences to those of green algae prompted our analysis
of the deep-level phylogenetic signal in the highly conserved but short
5.8S and hypervariable ITS2 sequences. We found that 5.8S sequences yield
phylogenetic trees similar to but less well supported than those generated
by a ca. 10-fold longer alignment from rDNA-18S sequences, as well as
independent evidence. We attribute this result to our finding that,
compared to 18S, the 5.8S has a higher proportion of sites subject to vary
and greater among-site substitution rate homogeneity. We also determined
that our phylogenetic results are not likely affected by intramolecular
compensatory mutation to maintain RNA secondary structure nor by evident
systematic biases in base composition. Despite historical homology, there
appears to be no ITS2 primary sequence similarity shared sufficient
similarity to cluster correctly on the basis of alignability. Our results
indicate that groups, however, share sufficient similarity to cluster
correctly on the basis of alignability. Our results indicate that ITS
region sequences can diagnose organismal origins and phylogenetic
relationships at many phylogenetic levels and provide a useful paradigm for
molecular evolutionary study.
相似文献
24.
Production of Serine Proteases by the Oyster Pathogen Perkinsus marinus (Apicomplexa) In Vitro 总被引:1,自引:1,他引:0
JEROME F. LA PEYRE DORIS Y. SCHAFHAUSER ESAM H. RIZKALLA MOHAMED FAISAL 《The Journal of eukaryotic microbiology》1995,42(5):544-551
ABSTRACT. Analysis of the cell-free supernatants of Perkinsus marinus cultures by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining revealed the presence of as many as 17 bands ranging in molecular weight from 239 to 32 kDa. These bands were not present in un-inoculated medium. Moreover, P. marinus produces extracellular proteins that possess proteolytic activities; the cell-free supernatants of P. marinus cultures could digest a variety of proteins including gelatin, casein, fibronectin and laminin. Oyster plasma was also digested by cell-free culture supernatants. The proteolytic activity in cell-free culture supernatants was detected 24 h post-inoculation, while no proteolytic activity could be detected in cell lysates. The proteolytic activities were characterized using substrate-impregnated sodium dodecylsulfate-polyacrylamide gels and had approximate molecular weights ranging from 55 to 35 kDa. The proteolytic activity of cell-free culture supernatants was inhibited by the serine protease inhibitors phenylmethylsulphonyl fluoride, 3,4-dichloroisocoumarin and soybean trypsin inhibitor. In contrast, inhibitors (i.e. trans-epoxysuccinyll-leucylamido(4-guanidino)-butane, 1, 10-phenanthroline, captopril, ethylenediaminetetracetic acid, pepstatin A or diazoacetyl-DL-norleucine methyl ester) from the other three classes of proteases had no effect. It was concluded that the P. marinus proteases in cell-free culture supernatants are serine proteases. 相似文献
25.
Analysis of nonmethylated GATC sites in the Escherichia coli chromosome and identification of sites that are differentially methylated in response to environmental stimuli. 总被引:3,自引:1,他引:2 下载免费PDF全文
Seven GATC sites that are nonmethylated in logarithmic growth phase cells using glycerol as a carbon source were isolated from the Escherichia coli chromosome. Three of these GATC sites are located upstream of the operons gut, mtl, and ppiA, whereas DNA sequences adjacent to three other nonmethylated GATC sites are not homologous to previously identified genes. The seventh nonmethylated GATC site is located downstream of uspA. The protection of this site from DNA methylation requires leucine-responsive regulatory protein and is leucine responsive. The carbon source and the growth phase influenced the protection of the GATC site 5' of the ppiA gene. The other five sites were protected under all the environmental conditions examined. 相似文献
26.
27.
28.
29.
30.