全文获取类型
收费全文 | 756785篇 |
免费 | 86440篇 |
国内免费 | 363篇 |
出版年
2016年 | 8385篇 |
2015年 | 11543篇 |
2014年 | 13511篇 |
2013年 | 20006篇 |
2012年 | 21849篇 |
2011年 | 22480篇 |
2010年 | 15270篇 |
2009年 | 14081篇 |
2008年 | 19970篇 |
2007年 | 20878篇 |
2006年 | 19551篇 |
2005年 | 18795篇 |
2004年 | 18537篇 |
2003年 | 18189篇 |
2002年 | 17702篇 |
2001年 | 32554篇 |
2000年 | 32790篇 |
1999年 | 26047篇 |
1998年 | 9163篇 |
1997年 | 9903篇 |
1996年 | 9284篇 |
1995年 | 9043篇 |
1994年 | 8803篇 |
1993年 | 8997篇 |
1992年 | 22689篇 |
1991年 | 22446篇 |
1990年 | 21897篇 |
1989年 | 21610篇 |
1988年 | 20041篇 |
1987年 | 19131篇 |
1986年 | 17841篇 |
1985年 | 18292篇 |
1984年 | 15254篇 |
1983年 | 13167篇 |
1982年 | 10188篇 |
1981年 | 9094篇 |
1980年 | 8657篇 |
1979年 | 14597篇 |
1978年 | 11458篇 |
1977年 | 10665篇 |
1976年 | 10163篇 |
1975年 | 11064篇 |
1974年 | 11803篇 |
1973年 | 11655篇 |
1972年 | 10722篇 |
1971年 | 9755篇 |
1970年 | 8328篇 |
1969年 | 8140篇 |
1968年 | 7461篇 |
1967年 | 6549篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
A highly abundant satellite DNA comprising 17% of the Tribolium castaneum
(Insecta, Coleoptera) genome was cloned and sequenced. The satellite
monomer is 360 bp long, has a high A+T content of 73%, and lacks
significant internal substructures. The sequence variability is 3.6%,
essentially due to random distribution of single-point mutations. The
satellite is evenly distributed in the regions of centromeric
heterochromatin of all 20 chromosomes, as shown by fluorescent in situ
hybridization. Comparison of T. castaneum satellite with those from three
different but congeneric species reveals the highest sequence similarity of
47.1% with the satellite from the sibling species Tribolium freemani. The
phylogenetic relationships among Tribolium species deduced from satellite
sequence agree with those based on karyological, chemotaxonomic, and
hybridization data. This indicates a parallel in the divergence of
satellites and some genetic and cytogenetic characters. Despite low mutual
sequence similarity, which makes them species-specific, Tribolium
satellites have a common structural characteristic: a block of about 95%
A+T content, 20 to 42 bp long, flanked at one side by an inverted repeat
which can potentially form a thermodynamically stable dyad structure. Since
similar structural features are found in centromeric DNA of Saccharomyces
cerevisiae and Chironomus pallidivittatus, their possible importance in
centromere function may be inferred.
相似文献
22.
23.
24.
25.
26.
27.
H.U. Schweikert U.W. Tunn U.-F. Habenicht J. Arnold Th. Senge H. Schulze F.H. Schrder J.H.M. Blom O. Ennemoser W. Horniger G. Bartsch 《The Journal of steroid biochemistry and molecular biology》1993,44(4-6):573-576
Sex steroids are thought to play an essential role in the pathogenesis of human benign prostatic hyperplasia (BPH). Since recent studies in animal models and in men have shown that estrogens might be causally linked to the onset and maintenance of BPH, we examined the effect of 1-methyl-androsta-1,4-diene-3,17-dione (Atamestane), a newly developed aromatase inhibitor, in men with BPH. In an open multicenter study 49 men (mean age 70.1 years, range 55 to 84) with obstructive BPH were treated with atamestane (3 × 200 mg/day) for 3 months. Of the 49 patients 44 completed the treatment period; the other patients discontinued the study for reasons unrelated to treatment. With treatment BPH-related symptoms such as daytime voiding frequency, nycturia, peak flow and residual urine improved considerably; however, these parameters did not reach statistical significance. The mean prostatic volume decreased significantly from 74.2 ± 31.7 to 64.0 ± 31 ml (mean ± SD). Serum estrogen levels decreased markedly during treatment. In addition intraprostatic estrogen concentration decreased with treatment as compared to estrogen levels in hyperplastic prostates from untreated patients. The following conclusions can be drawn from this study: first, estrogens appear to have an important supportive role in established BPH, and second, estrogen deprivation improved BPH-related symptoms and reduced significantly prostatic volume. 相似文献
28.
29.
30.
E. Smyk-Randall O. R. Brown A. Wilke A. Eisenstark D. H. Flint 《Free radical biology & medicine》1993,14(6):609-613
The effects of near ultraviolet (NUV) light on a NUV chromophore-containing oxidant-sensitive enzyme, dihydroxyacid dehydratase (DHAD), were measured in seven strains of Escherichia coli. The strains differed in production of the oxidant-defense enzymes, superoxide dismutases (Fe-SOD and Mn-SOD), and catalases HPI and HPII. With the stress of aerobic growth but without NUV exposure, the strains lacking either Fe or Mn SOD or both SODs had 57%, 25%, and 12%, respectively, of the DHAD-specific activity of the parent (K12) strain. Under the same conditions, the catalase strains that were wild type, overproducing, and deficient had comparable DHAD-specific activities. When aerobic cultures were exposed for 30 min to NUV with a fluence of 216 J/m2/s at 310–400 nm, the percentage decreases in DHAD-specific activities were similar (ranging from 75% to 89%) in strains with none, either, or both SODs missing, and in the catalase-overproducing strain. However, the decreases were only 58% and 52% in the strain with catalase missing and in its parent, respectively. The NUV-induced loss of DHAD enzyme activity was not accompanied by any detectable loss of the DHAD protein as measured by polyclonal antibody to DHAD. 相似文献