全文获取类型
收费全文 | 941篇 |
免费 | 50篇 |
专业分类
991篇 |
出版年
2015年 | 9篇 |
2013年 | 23篇 |
2012年 | 12篇 |
2011年 | 11篇 |
2010年 | 27篇 |
2009年 | 25篇 |
2008年 | 28篇 |
2007年 | 47篇 |
2006年 | 20篇 |
2005年 | 20篇 |
2004年 | 18篇 |
2003年 | 9篇 |
2002年 | 15篇 |
2001年 | 9篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 18篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 13篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 17篇 |
1982年 | 16篇 |
1980年 | 13篇 |
1979年 | 14篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 15篇 |
1973年 | 18篇 |
1972年 | 16篇 |
1971年 | 10篇 |
1969年 | 9篇 |
1968年 | 10篇 |
1965年 | 36篇 |
1964年 | 19篇 |
1963年 | 32篇 |
1962年 | 23篇 |
1961年 | 33篇 |
1960年 | 21篇 |
1959年 | 27篇 |
1958年 | 27篇 |
1957年 | 12篇 |
排序方式: 共有991条查询结果,搜索用时 0 毫秒
901.
902.
WILLIAM P. KUVLESKY JR. LEONARD A. BRENNAN MICHAEL L. MORRISON KATHY K. BOYDSTON BART M. BALLARD FRED C. BRYANT 《The Journal of wildlife management》2007,71(8):2487-2498
ABSTRACT Wind energy development represents significant challenges and opportunities in contemporary wildlife management. Such challenges include the large size and extensive placement of turbines that may represent potential hazards to birds and bats. However, the associated infrastructure required to support an array of turbines—such as roads and transmission lines—represents an even larger potential threat to wildlife than the turbines themselves because such infrastructure can result in extensive habitat fragmentation and can provide avenues for invasion by exotic species. There are numerous conceptual research opportunities that pertain to issues such as identifying the best and worst placement of sites for turbines that will minimize impacts on birds and bats. Unfortunately, to date very little research of this type has appeared in the peer-reviewed scientific literature; much of it exists in the form of unpublished reports and other forms of gray literature. In this paper, we summarize what is known about the potential impacts of wind farms on wildlife and identify a 3-part hierarchical approach to use the scientific method to assess these impacts. The Lower Gulf Coast (LGC) of Texas, USA, is a region currently identified as having a potentially negative impact on migratory birds and bats, with respect to wind farm development. This area is also a region of vast importance to wildlife from the standpoint of native diversity, nature tourism, and opportunities for recreational hunting. We thus use some of the emergent issues related to wind farm development in the LGC—such as siting turbines on cropland sites as opposed to on native rangelands—to illustrate the kinds of challenges and opportunities that wildlife managers must face as we balance our demand for sustainable energy with the need to conserve and sustain bird migration routes and corridors, native vertebrates, and the habitats that support them. 相似文献
903.
904.
Stable expression of mammalian beta 1,4-galactosyltransferase extends the N-glycosylation pathway in insect cells 总被引:3,自引:2,他引:3
An established lepidopteran insect cell line (Sf9) was cotransfected with
expression plasmids encoding neomycin phosphotransferase and bovine beta
1,4-galactosyltransferase. Neomycin-resistant transformants were selected,
assayed for beta 1,4-galactosyltransferase activity, and the transformant
with the highest level of enzymatic activity was characterized. Southern
blots indicated that this transformed Sf9 cell derivative contained
multiple copies of the galactosyltransferase- encoding expression plasmid
integrated at a single site in its genome. One-step growth curves showed
that these cells supported normal levels of baculovirus replication.
Baculovirus infection of the transformed cells stimulated beta
1,4-galactosyltransferase activity almost 5-fold by 12 h postinfection.
This was followed by a gradual decline in activity, but the infected cells
still had about as much activity as uninfected controls as late as 48 h
after infection and they were able to produce a beta 1,4-galactosylated
virion glycoprotein during infection. Infection of the transformed cells
with a conventional recombinant baculovirus expression vector encoding
human tissue plasminogen activator also resulted in the production of a
galactosylated end-product. These results demonstrate that stable
transformation can be used to add a functional mammalian
glycosyltransferase to lepidopteran insect cells and extend their N-
glycosylation pathway. Furthermore, stably-transformed insect cells can be
used as modified hosts for conventional baculovirus expression vectors to
produce foreign glycoproteins with "mammalianized" glycans which more
closely resemble those produced by higher eucaryotes.
相似文献
905.
906.
907.
908.
909.
910.
The phosphoprotein phosphatase 2A (PP2A) catalytic subunit contains a methyl ester on its C-terminus, which in mammalian cells is added by a specific carboxyl methyltransferase and removed by a specific carboxyl methylesterase. We have identified genes in yeast that show significant homology to human carboxyl methyltransferase and methylesterase. Extracts of wild-type yeast cells contain carboxyl methyltransferase activity, while extracts of strains deleted for one of the methyltransferase genes, PPM1, lack all activity. Mutation of PPM1 partially disrupts the PP2A holoenzyme in vivo and ppm1 mutations exhibit synthetic lethality with mutations in genes encoding the B or B' regulatory subunit. Inactivation of PPM1 or overexpression of PPE1, the yeast gene homologous to bovine methylesterase, yields phenotypes similar to those observed after inactivation of either regulatory subunit. These phenotypes can be reversed by overexpression of the B regulatory subunit. These results demonstrate that Ppm1 is the sole PP2A methyltransferase in yeast and that its activity is required for the integrity of the PP2A holoenzyme. 相似文献