全文获取类型
收费全文 | 2495篇 |
免费 | 280篇 |
国内免费 | 3篇 |
出版年
2021年 | 41篇 |
2020年 | 18篇 |
2019年 | 21篇 |
2018年 | 45篇 |
2017年 | 30篇 |
2016年 | 55篇 |
2015年 | 92篇 |
2014年 | 105篇 |
2013年 | 158篇 |
2012年 | 161篇 |
2011年 | 138篇 |
2010年 | 96篇 |
2009年 | 106篇 |
2008年 | 144篇 |
2007年 | 149篇 |
2006年 | 131篇 |
2005年 | 132篇 |
2004年 | 122篇 |
2003年 | 114篇 |
2002年 | 113篇 |
2001年 | 44篇 |
2000年 | 41篇 |
1999年 | 44篇 |
1998年 | 24篇 |
1997年 | 32篇 |
1996年 | 15篇 |
1995年 | 24篇 |
1994年 | 27篇 |
1993年 | 20篇 |
1992年 | 33篇 |
1991年 | 24篇 |
1990年 | 39篇 |
1989年 | 47篇 |
1988年 | 24篇 |
1987年 | 26篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1984年 | 23篇 |
1983年 | 21篇 |
1982年 | 11篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 16篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1975年 | 17篇 |
1974年 | 10篇 |
1972年 | 14篇 |
1971年 | 19篇 |
1968年 | 13篇 |
排序方式: 共有2778条查询结果,搜索用时 268 毫秒
1.
Elena C. Guzman Alfonso Jimenez-Sanchez Elisha Orr Robert H. Pritchard 《Molecular & general genetics : MGG》1988,212(2):203-206
Summary A temperature shift-up accompanied by a reduction in RNA polymerase activity in Escherichia coli causes an increased rate of initiation leading to a 1.7- to 2.2-fold increase in chromosome copy number. A temperature shift-up without a reduction in polymerase activity induces only a transient non-scheduled initiation of chromosome replication caused by heat shock with no detectable effect on chromosome copy number. 相似文献
2.
Summary The gene specifying cytosine deaminase (cod) is shown to be located at approximately 86 minutes on the linkage map of E. coli. The corresponding gene in S. typhimurium has been reported to have a different location. 相似文献
3.
4.
5.
Low-density lipoprotein preparation by combined diafiltration and ultracentrifugation 总被引:1,自引:0,他引:1
K A Pritchard J A Holland N J Rogers C C Crean T E Britton P Onigman M B Stemerman 《Analytical biochemistry》1988,174(1):121-127
A method for isolating low-density lipoprotein by combining diafiltration and ultracentrifugation is described. Diafiltration separates plasma components by use of an ultrafiltration membrane that excludes particles of molecular weight greater than 300,000. The retentate is concentrated three- to fourfold by ultrafiltration, allowing large-scale preparation of low-density lipoprotein. Low-density lipoprotein prepared in this manner is similar in physical, chemical, and biologic properties to low-density lipoprotein isolated by sequential density ultracentrifugation alone. When low-density lipoprotein, prepared by either method, was added to human umbilical vein endothelial cell cultures, no cytotoxicity was observed. The techniques described reduce the demand on multiple rotors and ultracentrifuges for large-scale preparation of low-density lipoprotein suitable and often needed for tissue culture studies. 相似文献
6.
Robin S. Manasse Maureen L. Stanton 《Evolution; international journal of organic evolution》1991,45(4):883-890
We present evidence that extreme seed size variation in fruits of Crinum erubescens (range: 0.1 to 66.5 g per seed) occurs when mating pairs are inbred, either from selfing or biparental inbreeding. Several relatively uniform seeds of intermediate size are produced when pollen from several pollen donors is applied simultaneously to a flower. Selfed fruits and some fruits pollinated with a single pollen donor produce both large and small seeds, although selfed fruits produce fewer seeds than outcrossed fruit. These results are contrary to the hypothesis that variation in seed size is attributable to either pollen competition or differential allocation of maternal resource to seeds of different genotypes. 相似文献
7.
Physical and biological parameters that determine the fate of p-chlorophenol in laboratory test systems. 下载免费PDF全文
Shake-flask and microcosm studies were conducted to determine the fate of para-chlorophenol (p-CP) in water and sediment systems and the role of sediment and nonsediment surfaces in the biodegradation process. Biodegradation of p-CP in estuarine water samples in shake flasks was slow over incubation periods of 300 h. The addition of detrital sediment resulted in immediate and rapid degradation evidenced by the production of 14CO2 from [14C]p-CP. The addition of sterile sediment, glass beads, or sand resulted in approximately four to six times more CO2 evolution than observed in the water alone. Densities of p-CP-degrading bacteria associated with the detrital sediment were 100 times greater than those enumerated in water. Bacteria in the water and associated with the sediment after preexposure of both water and sediment of p-CP demonstrated enhanced biodegradation. In some microcosms, p-CP was degraded completely in the top 1.0 cm of intact sediment beds. Sediment reworking activities by benthic invertebrates from one site were sufficient to mix p-CP deep into the sediment bed faster than biodegradation or molecular diffusion. p-CP was persistent at lower depths of the sediment, possibly a result of reduced oxygen conditions preventing aerobic biodegradation. 相似文献
8.
Novel Pathway of Toluene Catabolism in the Trichloroethylene-Degrading Bacterium G4 总被引:41,自引:31,他引:10 下载免费PDF全文
Malcolm S. Shields Stacy O. Montgomery Peter J. Chapman Stephen M. Cuskey P. H. Pritchard 《Applied microbiology》1989,55(6):1624-1629
o-Cresol and 3-methylcatechol were identified as successive transitory intermediates of toluene catabolism by the trichloroethylene-degrading bacterium G4. The absence of a toluene dihydrodiol intermediate or toluene dioxygenase and toluene dihydrodiol dehydrogenase activities suggested that G4 catabolizes toluene by a unique pathway. Formation of a hybrid species of 18O- and 16O-labeled 3-methylcatechol from toluene in an atmosphere of 18O2 and 16O2 established that G4 catabolizes toluene by successive monooxygenations at the ortho and meta positions. Detection of trace amounts of 4-methylcatechol from toluene catabolism suggested that the initial hydroxylation of toluene was not exclusively at the ortho position. Further catabolism of 3-methylcatechol was found to proceed via catechol-2,3-dioxygenase and hydroxymuconic semialdehyde hydrolase activities. 相似文献
9.
Stable transformation of papaya via microprojectile bombardment 总被引:27,自引:0,他引:27
Maureen M. M. Fitch Richard M. Manshardt Dennis Gonsalves Jerry L. Slightom John C. Sanford 《Plant cell reports》1990,9(4):189-194
Summary Stable transformation of papaya (Carica papaya L.) has been achieved following DNA delivery via high velocity microprojectiles. Three types of embryogenic tissues, including immature zygotic embryos, freshly explanted hypocotyl sections, and somatic embryos derived from both, were bombarded with tungsten particles carrying chimeric NPTII and GUS genes. All tissue types were cultured prior to and following bombardment on half-strength MS medium supplemented with 10 mg 1–1 2,4-D, 400 mg 1–1 glutamine, and 6% sucrose. Upon transfer to 2,4-D-free medium containing 150 mg 1–1 kanamycin sulfate, ten putative transgenic isolates produced somatic embryos and five regenerated leafy shoots. Leafy shoots were produced six to nine months following bombardment. Tissues from 13 of these isolates were assayed for NPTII activity, and 10 were positive. Six out of 15 isolates assayed for GUS expression were positive. Three isolates were positive for both NPTII and GUS,Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- GUS
-glucuronidase
- X-gluc
5-Br-4-Cl-3-indolyl--D-glucuronic acid
- CaMV
cauliflower mosaic virus
- NOS
nopaline synthase
- NPTII
neomycin phosphotransferase II
Journal Series no. 3448 of the Hawaii Institute of Tropical Agriculture and Human Resources 相似文献
10.
Trichloroethylene (TCE) was metabolized by the natural microflora of three different environmental water samples when stimulated by the addition of either toluene or phenol. Two different strains of Pseudomonas putida that degrade toluene by a pathway containing a toluene dioxygenase also metabolized TCE. A mutant of one of these strains lacking an active toluene dioxygenase could not degrade TCE, but spontaneous revertants for toluene degradation also regained TCE-degradative ability. The results implicate toluene dioxygenase in TCE metabolism. 相似文献