全文获取类型
收费全文 | 1347篇 |
免费 | 131篇 |
出版年
2021年 | 11篇 |
2020年 | 17篇 |
2019年 | 23篇 |
2018年 | 20篇 |
2017年 | 23篇 |
2016年 | 28篇 |
2015年 | 45篇 |
2014年 | 41篇 |
2013年 | 50篇 |
2012年 | 73篇 |
2011年 | 52篇 |
2010年 | 37篇 |
2009年 | 27篇 |
2008年 | 50篇 |
2007年 | 48篇 |
2006年 | 43篇 |
2005年 | 52篇 |
2004年 | 44篇 |
2003年 | 29篇 |
2002年 | 37篇 |
2001年 | 47篇 |
2000年 | 41篇 |
1999年 | 39篇 |
1998年 | 15篇 |
1997年 | 21篇 |
1996年 | 27篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1992年 | 22篇 |
1991年 | 15篇 |
1990年 | 11篇 |
1989年 | 22篇 |
1988年 | 15篇 |
1987年 | 20篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 19篇 |
1983年 | 20篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1975年 | 12篇 |
1973年 | 10篇 |
1972年 | 10篇 |
1971年 | 10篇 |
1970年 | 10篇 |
1968年 | 10篇 |
1957年 | 12篇 |
1954年 | 9篇 |
1952年 | 12篇 |
排序方式: 共有1478条查询结果,搜索用时 109 毫秒
1.
2.
3.
4.
J. D. Macdonald. 《Ibis》1952,94(1):122-127
5.
K. D. Macdonald 《Applied microbiology》1972,23(5):990-993
Conidiated slope cultures of derivative of Penicillium chrysogenum Wis 54-1255 were stored at -196 or +4 C for a period of 3.5 years. After this time, the viability fell to 68% in the former case and to 4% in the latter. At the end of the experiment, 65 single conidial isolates from each series were tested for penicillin yield. Among those from conidia stored at -196 C, the spread of penicillin yields did not differ markedly from that of 65 single conidial isolates made as controls prior to storage. However, 18% of those from conidia stored at +4 C formed a subpopulation with substantially lower penicillin titers than those of control isolates. Storage at -196 C may reduce or prevent a possible source of penicillin yield decay, namely, the selection of spontaneous mutants of low titer present in small numbers in the original culture and selected, as viability decreased, by virtue of their increased longevity relative to that of the parental culture. 相似文献
6.
7.
Ion-Exchange Chromatography Separates Activities Synthesizing and Degrading Fructose 2,6-Bisphosphate from C3 and C4 Leaves but Not from Rat Liver 总被引:1,自引:1,他引:0 下载免费PDF全文
Fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase were separated on the basis of charge from leaves of C3 (spinach, lettuce, and pea) and C4 (sorghum and amaranthus) plants but not from rat liver—a tissue known to contain a bifunctional enzyme with both activities. [2-32P]Fructose 2,6-bisphosphate binding experiments also suggest that the major forms of these activities reside on different proteins in leaves. 相似文献
8.
Early and late periodic patterns of even skipped expression are controlled by distinct regulatory elements that respond to different spatial cues 总被引:14,自引:0,他引:14
We have identified the regulatory sequences required for the periodic expression of the Drosophila pair rule gene even skipped (eve). We find that the gradually changing pattern of periodic eve expression during early embryogenesis is directed by two distinct regulatory programs. Initially, eve expression in individual stripes is established by different regulatory elements, each of which responds to nonperiodic spatial cues provided, at least in part, by the gap genes. Later, coordinate expression of eve in all seven stripes is directed by a single regulatory region that responds to periodic cues provided by primary pair rule genes, including eve itself. As a consequence of this two-step regulatory program, eve functions both in the establishment of the periodic pattern of gene expression and in the subsequent specification of parasegmental boundaries. 相似文献
9.
Structure-activity relationships for the inhibition of DNA polymerase alpha by aphidicolin derivatives. 下载免费PDF全文
Aphidicolin and 17 derivatives that have been structurally modified in the A- and D-rings were assessed for their ability to inhibit DNA polymerase alpha. No derivative surpassed the activity of aphidicolin; derivatives with structural alterations in the A-ring exhibited significantly greater loss of activity relative to derivatives with structural alterations in the D-ring. The conclusions of these studies indicate a critical role for the C-18 function in the interaction of aphidicolin with polymerase alpha. Molecular modelling studies could not identify structural features of the aphidicolin-dCTP "overlap" that is unique to dCTP, relative to the remaining dNTPs, and that is consistent with the extant structure-activity data. 相似文献
10.
Genetic variation within and the relationship betweenStellaria longipes
Goldie andS. longifolia
Muhl. were studied. Ten enzyme systems were assessed in eight natural populations ofS. longipes (25 loci) and three ofS. longifolia (20 loci) using starch and polyacrylamide gel electrophoresis. Patterns of population differentiation corresponded to geographic distance. There was no evidence that polyploidS. longipes had greater electrophoretic variability than diploidS. longipes. The isozyme data confirmed extensive population differentiation in these species and, within that context, a relatively close relationship betweenS. longipes andS. longifolia. It was postulated that diploids of these two species might be the progenitors of tetraploidS. longipes. 相似文献