全文获取类型
收费全文 | 5397篇 |
免费 | 523篇 |
出版年
2021年 | 52篇 |
2019年 | 48篇 |
2018年 | 56篇 |
2017年 | 51篇 |
2016年 | 82篇 |
2015年 | 133篇 |
2014年 | 172篇 |
2013年 | 208篇 |
2012年 | 270篇 |
2011年 | 277篇 |
2010年 | 199篇 |
2009年 | 168篇 |
2008年 | 243篇 |
2007年 | 261篇 |
2006年 | 252篇 |
2005年 | 225篇 |
2004年 | 239篇 |
2003年 | 236篇 |
2002年 | 215篇 |
2001年 | 123篇 |
2000年 | 109篇 |
1999年 | 119篇 |
1998年 | 98篇 |
1997年 | 54篇 |
1996年 | 73篇 |
1995年 | 53篇 |
1994年 | 56篇 |
1993年 | 62篇 |
1992年 | 75篇 |
1991年 | 85篇 |
1990年 | 83篇 |
1989年 | 51篇 |
1988年 | 79篇 |
1987年 | 49篇 |
1986年 | 50篇 |
1985年 | 75篇 |
1984年 | 60篇 |
1983年 | 49篇 |
1982年 | 55篇 |
1981年 | 47篇 |
1980年 | 46篇 |
1979年 | 40篇 |
1977年 | 47篇 |
1976年 | 45篇 |
1975年 | 38篇 |
1974年 | 48篇 |
1972年 | 41篇 |
1971年 | 35篇 |
1969年 | 32篇 |
1968年 | 37篇 |
排序方式: 共有5920条查询结果,搜索用时 15 毫秒
11.
Ryan K. Cheu Andrew T. Gustin Christina Lee Luca Schifanella Charlene J. Miller Avie Ha Casey Kim Violeta J. Rodriguez Margaret Fischl Adam D. Burgener Kelly B. Arnold Maria L. Alcaide Nichole R. Klatt 《PLoS pathogens》2020,16(12)
Despite the efficacy of antiretroviral-based pre-exposure prophylactics (PrEP) in men who have sex with men, studies in women have produced widely varying outcomes. Recent evidence demonstrates that vaginal microbial communities are associated with increased HIV acquisition risk and may impact PrEP efficacy. Here, we investigate the mechanisms underlying how vaginal bacteria alter PrEP drug levels and impact HIV infection rates ex vivo. Using cervicovaginal lavages (CVLs) from women with or without bacterial vaginosis (BV), we identified microbial metabolism of PrEP drugs in BV samples through LC-MS/MS analysis of soluble drug levels and metabolite formation in dual T-cell cultures. CVL samples were assessed for microbiome analysis using sequencing of bacterial 16S rRNA genes. We also observed non-Lactobacillus bacteria that are associated with BV may potentially impact PrEP efficacy through increased HIV infection rates in co-cultures containing Lactobacillus or BV bacteria, PrEP drugs, CEM-GFP cells, and HIV-1LAI virus. Finally, we used these data to develop a novel predictive mathematical simulation modeling system to predict these drug interactions for future trials. These studies demonstrate how dysbiotic vaginal microbiota may impact PrEP drugs and provides evidence linking vaginal bacteria to PrEP efficacy in women. 相似文献
12.
Arnold Simanowitz 《BMJ (Clinical research ed.)》1988,297(6641):133-134
13.
14.
15.
Arnold ten Thoren und Hans-Heiner Bergmann 《Journal of Ornithology》1987,128(2):181-207
Zusammenfassung Das Lautrepertoire von sechs handaufgezogenen Graugänsen wurde in individuellen Längsschnitten vom Schlüpfen bis zum Alter von 100 Tagen mit sonagraphischer Methode analysiert. Die Einteilung des Repertoires erfolgte nach drei verschiedenen Kriterien: Klang, Sonagramm und Kontext. Im jugendlichen Repertoire ließen sich 4 Rufklassen voneinander trennen. Im Laufe der Jugendentwicklung veränderten sich vor allem die phonetischen Parameter der Rufe, während die syntaktischen weitgehend konstant blieben. Nach einem Anstieg in der ersten Woche fiel die Tonhöhe der Rufe bei allen untersuchten Individuen und Rufklassen gleichmäßig ab. Der Stimmbruch, meßbar an der Zunahme geräuschhafter Komponenten, war ein kontinuierlicher, langfristiger Prozeß. Die Individualität war nicht in einem einzelnen, wohl aber in der Kombination mehrerer Merkmale faßbar. Die Geschlechter waren schon früh an Unterschieden in der Tonhöhe erkennbar. An sonstigen ontogenetischen Veränderungen innerhalb des Untersuchungszeitraumes konnten die Erweiterung des Repertories durch Aufspaltung und Reifung sowie ein möglicher Verlust von Rufklassen beobachtet werden.
Herrn Dr. Hans Löhrl zum 75. Geburtstag gewidmet 相似文献
Juvenile development of vocalizations in Greylag Geese (Anser anser)
Summary The juvenile vocal development of six Greylag Geese from hatching up to the age of 100 days was analysed by using sonagraphic methods. The calls were classified by means of the acoustic phenomenon, sonagrams, and context criteria. The juvenile repertoire included four main call types. During ontogeny, changes of call parameters mainly referred to phonetic characteristics, whereas syntactic parameters remained more or less constant. The frequency pitch of calls changed to higher levels during the first week of development while in older goslings this parameter decreased constantly. This held true for each individual and call type. Breaking of the voice, defined as an increment of noisy parts in the vocalizations, is described as a continuous long term process. Structural individuality of calls apparently was not based on one single parameter, but on the combination of several ones. Sex differences were found in the pitch of vocalizations, correlating with different body weights of male and female geese. Additional developmental processes of the vocal repertoire included differentiation, maturation of new call types, and apparent suppression of juvenile ones. The functional organization of the juvenile call system in Greylags is discussed.
Herrn Dr. Hans Löhrl zum 75. Geburtstag gewidmet 相似文献
16.
17.
Comparison of the three-dimensional structure of two human rhinoviruses (HRV2 and HRV14) 总被引:1,自引:0,他引:1
An attempt has been made to build a model of human rhinovirus 2 (HRV2) based on the known human rhinovirus 14 (HRV14) structure. HRV2 was selected because its amino acid sequence is known and because it belongs to the minor rhinovirus receptor class as compared to HRV14, which belongs to the major class. Initial alignment of HRV2 with HRV14 based on the primary sequence and the knowledge of the three-dimensional structure of HRV14 showed that the most probable position of the majority of insertions and deletions occurred in the vicinity of the neutralizing immunogenic sites (NIm). Out of a total of 855 amino acids present in one copy of each of the capsid proteins VP1 through VP4 of HRV14, 411 are different between the two viruses. There are also 6 amino acid residues inserted and 14 residues deleted in HRV2 relative to HRV14. Examination of amino acid interactions showed several cases of conservation of function, e.g., salt bridges or the filling of restricted space. The largest variation amongst the residues lining the canyon, the putative receptor binding site, was in the carboxy-terminal residues of VP1. 相似文献
18.
Mono- through hexanucleotide composition of the Escherichia coli genome: a Markov chain analysis. 总被引:15,自引:9,他引:6
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Several statistical methods were tested for accuracy in predicting observed frequencies of di- through hexanucleotides in 74,444 bp of E. coli DNA. A Markov chain was most accurate overall, whereas other methods, including a random model based on mononucleotide frequencies, were very inaccurate. When ranked highest to lowest abundance, the observed frequencies of oligonucleotides up to six bases in length in E. coli DNA were highly asymmetric. All ordered abundance plots had a wide linear range containing the majority of the oligomers which deviated sharply at the high and low ends of the curves. In general, values predicted by a Markov chain closely followed the overall shape of the ordered abundance curves. A simple equation was derived by which the frequency of any nucleotide longer than four bases in the E. coli genome (or any genome) can be relatively accurately estimated from the nested set of component tri- and tetranucleotides by serial application of a 3rd order Markov chain. The equation yielded a mean ratio of 1.03 +/- 0.94 for the observed-to-expected frequencies of the 4,096 hexanucleotides. Hence, the method is a relatively accurate but not perfect predictor of the length in nucleotides between hexanucleotide sites. Higher accuracy can be achieved using a 4th order Markov chain and larger data sets. The high asymmetry in oligonucleotide abundance means that in the E. coli genome of 4.2 X 10(6) bp many relatively short sequences of 7-9 bp are very rare or absent. 相似文献
19.
Arnold Tiehm 《Brittonia》1989,41(2):152-155
Types are indicated for 28 new names inadvertently published in Rydberg’sFlora of Colorado. Included are a discussion as to the validity of these new names and an explanation of the causes of this oversight. A lectotype ofLupinus decumbens var.argentatus is herein designated by Rupert C. Barneby. 相似文献
20.
Bactericidal effect of lactoferrin on Legionella pneumophila: effect of the physiological state of the organism 总被引:6,自引:0,他引:6
Lactoferrin has been previously shown to be bactericidal for Legionella pneumophila. The current study showed that CaCl2, Mg(NO3)2, and MgCl2, but not NaCl, blocked killing. Activity was pH dependent with the greatest activity at 5.0. Sensitivity of the organism was dramatically affected by the growth conditions. Log phase 12 h, broth-grown cells were most sensitive, with older cultures becoming more resistant. Plate-grown cells were completely resistant. Lactoferrin binding, as detected by immunofluorescence microscopy, was temperature dependent (no binding at 4 degrees C), but was independent of killing. 相似文献