全文获取类型
收费全文 | 640篇 |
免费 | 54篇 |
国内免费 | 9篇 |
专业分类
703篇 |
出版年
2022年 | 8篇 |
2021年 | 8篇 |
2020年 | 6篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 7篇 |
2015年 | 19篇 |
2014年 | 18篇 |
2013年 | 16篇 |
2012年 | 22篇 |
2011年 | 25篇 |
2010年 | 17篇 |
2009年 | 22篇 |
2008年 | 29篇 |
2007年 | 19篇 |
2006年 | 29篇 |
2005年 | 13篇 |
2004年 | 15篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2001年 | 19篇 |
2000年 | 11篇 |
1999年 | 14篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 16篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1992年 | 11篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1983年 | 7篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1974年 | 7篇 |
1973年 | 4篇 |
1959年 | 7篇 |
1958年 | 23篇 |
1957年 | 26篇 |
1956年 | 26篇 |
1955年 | 22篇 |
1954年 | 22篇 |
1953年 | 13篇 |
1952年 | 13篇 |
1951年 | 10篇 |
1950年 | 9篇 |
排序方式: 共有703条查询结果,搜索用时 15 毫秒
1.
A method for the detection of the specific binding of 3-methylcholanthrene to rat liver cytosolic proteins is described. The separation of the protein-bound 3-methylcholanthrene from the free 3-methylcholanthrene was achieved using a batch DEAE-cellulose technique. Extraction of the DEAE-cellulose with 0.3 M KCl allowed the selective release and measurement of the amount of protein-bound 3-methylcholanthrene. The assay was optimized for the following parameters: time of incubation with DEAE-cellulose, time required for salt extraction, protein concentration, the concentration of KCl required to elute the specific binding proteins, the amount of DEAE-cellulose required to bind the specific binding proteins, and ligand specificity. The sedimentation properties of those 3-methylcholanthrene-binding proteins which were extracted with salt from DEAE-cellulose were examined on 5 to 20% sucrose gradients; the major binding species sedimented as a broad peak at 4.5 S. 相似文献
2.
3.
Antigenic and functional organization of human parainfluenza virus type 3 fusion glycoprotein. 总被引:10,自引:8,他引:2 下载免费PDF全文
Twenty-six monoclonal antibodies (MAbs) (14 neutralizing and 12 nonneutralizing) were used to examine the antigenic structure, biological properties, and natural variation of the fusion (F) glycoprotein of human type 3 parainfluenza virus (PIV3). Analysis of laboratory-selected antigenic variants and of PIV3 clinical isolates indicated that the panel of MAbs recognizes at least 20 epitopes, 14 of which participate in neutralization. Competitive binding assays indicated that the 14 neutralization epitopes are organized into three nonoverlapping antigenic sites (A, B, and C) and one bridge site (AB) and that the 6 nonneutralization epitopes form four sites (D, E, F, and G). Most of the neutralizing MAbs were involved in nonreciprocal competitive binding reactions, suggesting that they induce conformational changes in other neutralization epitopes. Fusion-inhibition and complemented-enhanced neutralization assays indicated that antigenic sites AB, B, and C may correspond to functional domains of the F molecule. Our results indicated that antibody binding alone is not sufficient for virus neutralization and that many anti-F MAbs neutralize by mechanisms not involving fusion-inhibition. The degree of antigenic variation in the F epitopes of clinical strains was examined by binding and neutralization tests. It appears that PIV3 frequently develops mutations that produce F epitopes which efficiently bind antibodies, but are completely resistant to neutralization by these antibodies. 相似文献
4.
5.
6.
7.
8.
H Bartsch C Malaveille B Tierney P L Grover P Sims 《Chemico-biological interactions》1979,26(2):185-196
The mutagenic activities of benz[alpha]anthracene, 7-methylbenz[alpha]anthracene, 7,12-dimethylbenz[alpha]anthracene, 3-methylcholanthrene and benzo[alpha]pyrene, together with those of the trans-dihydrodiols derived from these hydrocarbons that would be expected to yield 'bay-region' vicinal diolepoxides on further metabolism have been examined in assays with S. typhimurium TA100 using post-mitochondrial supernatant fractions prepared from the livers of 3-methylcholanthrene-treated rats. Mutagenic activities obtained have been compared with: (a) the extents of reaction with DNA that occur in mouse skin following treatment with these hydrocarbons; (b) the carcinogenicities of the hydrocarbons expressed as Iball indices; (c) their activities as tumour-initiating agents on mouse skin. Close positive associations were found between the microsome-mediated mutagenicities of the dihydrodiols that could yield "bay-region" diol-epoxides and: (a) the extents of reaction with DNA in hydrocarbon-treated mouse skin; (b) the carcinogenic potencies of the parent hydrocarbons; although these correlations are not perfect, the mutagenic activities of the hydrocarbons themselves in microsome-mediated assays with S. typhimurium show no correlation with their extents of DNA binding on mouse skin and a poor correlation with their activities as initiating agents. These comparisons also indicated a statistically-significant positive correlation between carcinogenicity and the in vivo DNA binding on mouse skin treated with the hydrocarbons. Differences in the metabolic pathways by which polycyclic hydrocarbons are activated in vivo and in vitro are discussed in relation to the improved correlations found with the dihydrodiols. 相似文献
9.
Snow depth manipulation and its influence on soil frost and water dynamics in a northern hardwood forest 总被引:8,自引:0,他引:8
Janet P. Hardy Peter M. Groffman Ross D. Fitzhugh Karen S. Henry Adam T. Welman Jason D. Demers Timothy J. Fahey Charles T. Driscoll Geraldine L. Tierney Scott Nolan 《Biogeochemistry》2001,56(2):151-174
Climate change will likelyresult in warmer winter temperatures leading toless snowfall in temperate forests. Thesechanges may lead to increases in soil freezingbecause of lack of an insulating snow cover andchanges in soil water dynamics during theimportant snowmelt period. In this study, wemanipulated snow depth by removing snow for twowinters, simulating the late development of thesnowpack as may occur with global warming, toexplore the relationships between snow depth,soil freezing, soil moisture, and infiltration.We established four sites, each with two pairedplots, at the Hubbard Brook Experimental Forest(HBEF) in New Hampshire, U.S.A. and instrumentedall eight plots with soil and snow thermistors,frost tubes, soil moisture probes, and soillysimeters. For two winters, we removed snowfrom the designated treatment plots untilFebruary. Snow in the reference plots wasundisturbed. The treatment winters (1997/1998 and1998/1999) were relatively mild, withtemperatures above the seasonal norm and snowdepths below average. Results show the treatedplots accumulated significantly less snow andhad more extensive soil frost than referenceplots. Snow depth was a strong regulator ofsoil temperature and frost depth at all sites.Soil moisture measured by time domainreflectometry probes and leaching volumescollected in lysimeters were lower in thetreatment plots in March and April compared tothe rest of the year. The ratio of leachatevolumes collected in the treatment plots tothat in the reference plots decreased as thesnow ablation seasons progressed. Our data showthat even mild winters with low snowfall,simulated by snow removal, will result inincreased soil freezing in the forests at theHBEF. Our results suggest that a climate shifttoward less snowfall or a shorter duration ofsnow on the ground will produce increases insoil freezing in northern hardwood forests.Increases in soil freezing will haveimplications for changes in soil biogeochemicalprocesses. 相似文献
10.
ELISA LOBATO SANTIAGO MERINO JUDITH MORALES GUSTAVO TOMÁS JOSUÉ MARTÍNEZ‐DE LA PUENTE ESTRELLA SÁNCHEZ SONIA GARCÍA‐FRAILE JUAN MORENO 《Ibis》2008,150(4):799-806
Sex differences in immune function are relatively well studied in vertebrate animals, although the patterns are not always clear in birds. The study of immune responses in nestlings of wild bird populations may constitute an appropriate way to investigate inherent intersexual differences while controlling for environmental conditions such as parasitism that affect male and female individuals growing in the same nest. We studied whether the cell‐mediated immune response, as measured by phytohaemagglutinin (PHA) injection, and the levels of circulating antibodies differ between sexes of Pied Flycatcher nestlings Ficedula hypoleuca. No sex differences in nestling cell‐mediated immune response were found, but females showed significantly higher levels of plasma immunoglobulins than males did. Although nestling birds may not have a fully functional humoral immune defence, our study indicates that sex differences in the humoral component exist at this early stage of life. Given the importance of antibodies in the fight against parasite, bacterial and viral infections, the intrinsic sex disparity in circulating antibodies may have important implications for the life history of each sex. 相似文献