全文获取类型
收费全文 | 1602篇 |
免费 | 271篇 |
专业分类
1873篇 |
出版年
2021年 | 19篇 |
2017年 | 23篇 |
2016年 | 29篇 |
2015年 | 26篇 |
2014年 | 42篇 |
2013年 | 72篇 |
2012年 | 68篇 |
2011年 | 50篇 |
2010年 | 40篇 |
2009年 | 40篇 |
2008年 | 48篇 |
2007年 | 65篇 |
2006年 | 47篇 |
2005年 | 34篇 |
2004年 | 58篇 |
2003年 | 46篇 |
2002年 | 36篇 |
2001年 | 61篇 |
2000年 | 52篇 |
1999年 | 55篇 |
1998年 | 36篇 |
1997年 | 21篇 |
1996年 | 18篇 |
1995年 | 18篇 |
1994年 | 17篇 |
1993年 | 25篇 |
1992年 | 35篇 |
1991年 | 41篇 |
1990年 | 39篇 |
1989年 | 36篇 |
1988年 | 30篇 |
1987年 | 41篇 |
1986年 | 26篇 |
1985年 | 32篇 |
1984年 | 30篇 |
1983年 | 32篇 |
1982年 | 15篇 |
1981年 | 24篇 |
1979年 | 28篇 |
1978年 | 20篇 |
1977年 | 31篇 |
1976年 | 21篇 |
1975年 | 18篇 |
1974年 | 28篇 |
1973年 | 31篇 |
1972年 | 15篇 |
1971年 | 18篇 |
1970年 | 15篇 |
1968年 | 20篇 |
1966年 | 16篇 |
排序方式: 共有1873条查询结果,搜索用时 15 毫秒
1.
Keratinocyte growth-promoting activity from human placenta 总被引:4,自引:0,他引:4
Extracts of term human placenta were tested for enhancement of proliferative growth of primary cultures of human keratinocytes. Saline extracts or supernatants from homogenates were dialyzed extensively, lyophilized, and tested in subcultures of keratinocytes in MCDB 153 medium with 0.1 mM Ca++ containing only defined supplements (insulin, hydrocortisone, transferrin, ethanolamine, phosphoethanolamine). Cells plated in the absence of EGF at moderately high densities (1000-3000 cells per cm2) formed colonies and grew in the presence of placental extract at 25-500 micrograms/ml. Extracts of cord serum or maternal serum were inactive, suggesting that the activity is derived from placental tissue. The activity is not EGF, since the activity in the placental extract, unlike EGF, did not promote growth at low cell density, was synergistic with EGF under some conditions, and did not produce changes in colonial morphology which occurred in the presence of EGF. Unlike keratinocyte growth-promoting activity in bovine hypothalamic extract, the activity is non-dialyzable and is destroyed at 100 degrees C. Placental extract could not replace any of the defined components of the medium and is therefore distinct from them. The presence of activity in the placenta with distinctive properties suggests that this is a previously undescribed material with growth-promoting properties for epithelium. 相似文献
2.
The dynamics of fish populations in two African rivers are being investigated with a view to developing fishery management models for tropical non-floodplain rivers. Preliminary work was carried out in the Rufiji River, East Africa, to establish growth and mortality rates along with L ∞ and K for six species in a river where fishing mortality is zero. These parameters, derived from scale-reading and length frequency analysis, also allowed the development of phased management plans for the fishery resulting from impoundment.
A subsequent investigation has begun on a West African river, the Taia in Sierra Leone, to pursue observations over four seasons, two of which are now complete. A systematic daily sampling programme is being carried out of the whole community, to accumulate similar but more extensive data to those obtained previously. In the Taia, however, fishing mortality must also be estimated from catch surveys, and analysis is carried out using the recently developed ELEFAN programs which can be compared with results obtained by other methods such as scale-reading. 相似文献
A subsequent investigation has begun on a West African river, the Taia in Sierra Leone, to pursue observations over four seasons, two of which are now complete. A systematic daily sampling programme is being carried out of the whole community, to accumulate similar but more extensive data to those obtained previously. In the Taia, however, fishing mortality must also be estimated from catch surveys, and analysis is carried out using the recently developed ELEFAN programs which can be compared with results obtained by other methods such as scale-reading. 相似文献
3.
John R Payne 《Journal of applied physiology》2002,92(4):1775-6; author reply 1776-7
4.
5.
DNAase I, an endonuclease which interacts with G-actin, also affects tropomyosin polymerization. With chicken pectoralis or bovine cardiac ventricle tropomyosin, DNAase I both prevents tropomyosin from polymerizing and disrupts already formed tropomysin filaments. DNAase I and filament tropomyosin can also form a precipitable complex. In the electron microscope, the complex is observed as irregularly margined stellate-shaped structures with a maximum size of 9 micron. Isolated DNAase I-tropomyosin stellate complex consists of a 2:1 molar ratio of DNAase I and tropomyosin, suggesting that each tropomyosin subunit can bind DNAase I. 相似文献
6.
Expression of tetanus toxin fragment C in yeast: gene synthesis is required to eliminate fortuitous polyadenylation sites in AT-rich DNA. 总被引:6,自引:1,他引:5 下载免费PDF全文
M A Romanos A J Makoff N F Fairweather K M Beesley D E Slater F B Rayment M M Payne J J Clare 《Nucleic acids research》1991,19(7):1461-1467
7.
8.
Course and extent of variation of equine infectious anemia virus during parallel persistent infections. 总被引:16,自引:14,他引:2 下载免费PDF全文
Comparisons of peptide and oligonucleotide maps of glycoproteins and RNA from nine isolates of equine infectious anemia virus (EIAV) that were generated during parallel infections of two Shetland ponies revealed that each isolate was structurally unique. Each EIAV isolate contained a unique subset of variant peptides, oligonucleotides, or both, indicating that structural variation in EIAV is a random and noncumulative process and that a large spectrum of possible EIAV variants can be generated in infected animals. 相似文献
9.
Aflatoxin contamination of agricultural commodities continues to be a serious problem in the United States. Breeding for resistant genotypes has been unsuccessful and detoxification of food sources is not economically feasible. New strategies for control may become apparent once more is known about the biosynthesis and regulation of aflatoxin. Although the biosynthetic pathway of aflatoxin has been extensively studied, little is known about the regulation of the individual steps in the pathway. We have developed a genetic transformation system forAspergillus flavus that provides a new and expedient approach to studying the biosynthesis of aflatoxin and its regulation. Through the use of this genetic transformation system, genes for aflatoxin biosynthesis can be identified and isolated by the complementation of aflatoxin negative mutants. In this paper we discuss molecular strategies for studying the regulation and biosynthesis of aflatoxin. 相似文献
10.
Ex planta, bacteroids of the sulla-symbiont Rhizobium hedysari strain HCNT 1 terminated reduction of nitrite at nitrous oxide irrespective of the presence or absence of acetylene. Nitrate was not reduced during the experimental period, but slight nitrate reductase activity occurred if incubation with nitrate was prolonged (up to 15 h). As was observed in free-living cells, exposure of the bacteroids to the metal chelator, diethyldithiocarbamate, prevented reduction of nitrite, indicating the presence of a copper-containing nitrite reductase. Pulses of 10–75 M nitrite transiently impeded O2 uptake in bacteroids, which resumed consumption of O2 when the nitrite had been reduced. Exposure to >1.0 mM nitrite for 24h greatly inhibited nitrogenase activity (assayed as acetylene reduction activity) of bacteroids in planta. Exposure to the same concentrations of nitrite after 1h of incubation in the presence of acetylene almost completely stopped ongoing ethylene production in bacteroids of strain HCNT 1 extracted from nodules. Free cells of the non-nitrite-reducing R. hedysari strain CC 1335 were lacking in nitrogenase (acetylene-reduction) activity, whereas identically cultured (low-oxygen) strain HCNT 1 cells reduced both nitrite and acetylene.Abbreviations PMS
phenazine methosulfate
- DDC
diethyldithiocarbamate 相似文献