首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1731篇
  免费   104篇
  1835篇
  2023年   11篇
  2022年   31篇
  2021年   42篇
  2020年   27篇
  2019年   39篇
  2018年   58篇
  2017年   48篇
  2016年   64篇
  2015年   100篇
  2014年   97篇
  2013年   154篇
  2012年   163篇
  2011年   144篇
  2010年   82篇
  2009年   82篇
  2008年   99篇
  2007年   87篇
  2006年   86篇
  2005年   51篇
  2004年   51篇
  2003年   70篇
  2002年   49篇
  2001年   13篇
  2000年   13篇
  1999年   18篇
  1998年   10篇
  1997年   16篇
  1996年   8篇
  1995年   13篇
  1994年   7篇
  1993年   8篇
  1992年   12篇
  1991年   9篇
  1990年   11篇
  1989年   9篇
  1988年   7篇
  1987年   12篇
  1986年   3篇
  1985年   5篇
  1984年   3篇
  1983年   4篇
  1982年   4篇
  1981年   3篇
  1977年   2篇
  1976年   1篇
  1975年   2篇
  1974年   1篇
  1973年   1篇
  1967年   1篇
  1965年   1篇
排序方式: 共有1835条查询结果,搜索用时 0 毫秒
81.
Past investigations of Down's syndrome (DS) have indicated that there are marked abnormalities in the craniofacial morphology. The aim of this study was to establish the craniofacial anthropometric variables which discriminate DS group from healthy population and also to observe the changes occurring with growth. Using noninvasive method of craniofacial anthropometry, craniofacial pattern profile (CFPP) analysis (from twenty-five anthropometric measurements per person) was performed in 104 DS individuals and 365 healthy controls, aged seven to fifty-seven and divided into four age ranges. Z-scores were calculated for each variable and the variations in the craniofacial region have been identified by multivariate discriminative analysis. The results showed that three variables (head length (g-op), head circumference (OFC) and outer canthal distance (ex-ex) were responsible for 85.68% variability (p < 0.001). The analysis of z-scores showed that the majority of variables were in subnormal (under -2 SD) and normal range (from -2SD to +2SD), but none of them was in the supernormal range (over the +2SD). Some craniofacial characteristics are age-related. On the basis of craniofacial anthropometric traits it was possible to separate even 91.35% of DS patients from the healthy population. It could be concluded that these findings demonstrate the usefulness of application of CFPP in defining abnormal craniofacial dimensions in DS individuals.  相似文献   
82.
The aim of our investigation was to compare the distribution of dermatomycosis species in Eastern Croatia between two different periods: first period from 1997-2001 year, and second period from 1986-88 year. The outpatients from Department of Dermatovenerology University Hospital "Osijek" with confirmed diagnosis infection. Tinea, were selected on the basis o age, gender, localization and dermatomycosis species. During the first period (1997-2001) among 75,691 outpatients Tinea infection was confirmed in 558 (0.73%), while in the second period among 47,832 outpatients there were 126 (0.26%) cases with Tinea, what showed significant increase of fungal infections among population this region. According the age and gender in both periods predominant population were under of the age 16(40.14%: 41.26%), and female population was predominant (58.60% and 57.14%) in comparison to males (41.39% and 42.85%). The most frequent localization of lesions in period I were cutis glabrae (47.31%), palms and soles (31.36%), capitis (17.38%) and unguis (9.31%) and isolated species were as followed: Trichophyton (39.06%), Microsporum (31.72%) and Candida (28.13%) species. In period II the most frequent localization were palms and soles (40.47%), cutis glabrae (36.50%), capitis (12.69%) and unguis (10.31%). The isolated species in this period were: Trichophyton (80.15%), Candida (12.69%) and Microsporum (4.76%) species. From the data collected during two different periods we can observe 1) increase of fungal infection generally in our region; 2) significant changes in causative species (increase of Microsporum and Candida species infection, but Trichophyton spp still remain the first causative agent); and 3) changes in the localization of lesions.  相似文献   
83.
We have studied topoisomerase II (topo II) in the cells of Bodo saltans, a free-living bodonid (Kinetoplastida). Phylogenetic analysis based on the sequence of the entire topo II gene, which is a single-copy gene, confirmed that B. saltans is a predecessor of parasitic trypanosomatids. Antibodies generated against either an overexpressed unique C-terminal region of topo II or a synthetic oligopeptide derived from the same region did not cross-react with cell lysates of related trypanosomatids, while they recognized a single specific band in the B. saltans lysate. Immunolocalization experiments using both antibodies showed that topo II is evenly dispersed throughout the kinetoplast. This is in striking difference from the localization of topo II in other flagellates, where it occurs in two antipodal centers flanking the kinetoplast disk. Moreover, the same topo II has a distinct localization in multiple loci at the periphery of the nucleus of B. saltans. With a minicircle probe derived from the conserved region we have shown that all relaxed non-catenated minicircles are confined to the globular kinetoplast DNA bundle. Therefore, in the mitochondrion of this primitive eukaryote topo II does not catenate relaxed DNA circles into a network in vivo, while a decatenating activity is present in partially purified cell lysates.  相似文献   
84.
Human neutrophils differ from other cells by containing high amount of IkappaBalpha in the nucleus, and this increased nuclear IkappaBalpha accumulation is associated with the inhibition of NFkappaB activity and increased apoptosis. However, the mechanisms regulating NFkappaB activation and IkappaBalpha degradation in human neutrophils are little understood. The objective of this study was to provide a further insight into the mechanisms regulating NFkappaB activity and IkappaBalpha degradation in human neutrophils. We show that okadaic acid (OA), an inhibitor of protein phosphatases PP1 and PP2A, induces sustained activation of NFkappaB and degradation of the nuclear IkappaBalpha, and increases interleukin-8 expression in the neutrophils. Furthermore, inhibitors of protein kinase C-delta (PKCdelta) and IkappaB kinase (IKK) inhibit the OA-induced activation of NFkappaB. Collectively, our results indicate that in human neutrophils, the sustained activation of NFkappaB is regulated by a continuous phosphorylation and degradation of the nuclear IkappaBalpha.  相似文献   
85.
Sso7d is a small basic protein consisting of 62 amino acids isolated from the thermoacidophilic archeobacterium Sulfolobus solfataricus. The protein is endowed with DNA binding properties, RNase activity, and the capability of rescuing aggregated proteins in the presence of ATP. In this study, the electrostatic properties of Sso7d are investigated by using the Poisson-Boltzmann calculation of the surface potential distribution and following by NMR spectroscopy the proton chemical shift pH titration of acidic residues. Although the details of the catalytic mechanism still have to be defined, the results from NMR experiments confirm the possible involvement of Glu35 as the proton acceptor in the catalytic reaction, as seen by its abnormally high pK(a) value. Poisson-Boltzmann calculations and NMR titration shifts suggest the presence of a possible hydrogen bond between Glu35 and Tyr33, with a consequent rather rigid arrangement at these positions. Comparison with RNase T1 suggests that Tyr7 may be a good candidate for acting as a proton donor in the active site of Sso7d as shown by its low phenolic pK(a) of approximately 9.3. Titration experiments performed with the UpA, a RNA dinucleotide model, showed that the protein residues affected by the interaction are mainly located in a different region with respect to the surface affected by DNA recognition, in good agreement with the surface potential distribution found with electrostatic calculations.  相似文献   
86.
We evaluated the involvement of the in vivo hormonal environment on the ability of the rat corpus luteum (CL) to undergo apoptosis. Gel electrophoretic DNA fragmentation analysis revealed no apoptosis in CL isolated either the 2 last days of pregnancy (Days 21 and 22) or throughout the 4 days following parturition, suggesting that the number of cells undergoing apoptosis at the same time is not sufficient to allow for visualization of DNA breakdown. In contrast, CL incubated in serum-free medium underwent significant apoptosis, as evaluated by chromatin condensation and DNA fragmentation, regardless of their developmental stage in pregnancy. However, CL obtained on Day 7 of pregnancy and on Day 4 postpartum demonstrated higher sensitivity to apoptosis in vitro, but lactation reduced significantly the capacity of the CL to undergo apoptosis when maintained in culture. These data suggest that the exposure of the CL to different hormonal environments throughout pregnancy and after parturition is responsible for the differential susceptibility to apoptosis observed in vitro. We have previously shown that progesterone is a direct factor for survival of the CL. Prolactin stimulates luteal progesterone production; therefore, we examined whether prolactin prevents apoptosis in luteal cells independently of its stimulatory action on progesterone production. We used a luteal cell line (GG-CL) that expresses the prolactin receptor but does not produce progesterone. These cells undergo apoptosis under conditions of serum starvation, and addition of prolactin to the culture medium significantly reduced DNA fragmentation. These results indicate that the extent of luteal cell death induced by incubation of CL under serum-free conditions depends on the hormonal environment to which this endocrine gland is exposed in vivo. These results also indicate an important role for lactation in preventing apoptosis, which is further supported by the antiapoptotic activity of prolactin observed in luteal cells.  相似文献   
87.
88.
89.
Micronucleus induction after whole-body microwave irradiation of rats   总被引:4,自引:0,他引:4  
Adult male Wistar rats were exposed for 2 h a day, 7 days a week for up to 30 days to continuous 2,450 MHz radiofrequency microwave (rf/MW) radiation at a power density of 5-10 mW/cm(2). Sham-exposed rats were used as controls. After ether anesthesia, experimental animals were euthanized on the final irradiation day for each treated group. Peripheral blood smears were examined for the extent of genotoxicity, as indicated by the presence of micronuclei in polychromatic erythrocytes (PCEs). The results for the time-course of PCEs indicated significant differences (P<0.05) for the 2nd, the 8th and the 15th day between control and treated subgroups of animals. Increased influx of immature erythrocytes into the peripheral circulation at the beginning of the experiment revealed that the proliferation and maturation of nucleated erythropoietic cells were affected by exposure to the 2,450 MHz radiofrequency radiation. Such findings are indicators of radiation effects on bone-marrow erythropoiesis and their subsequent effects in circulating red cells. The incidence of micronuclei/1,000 PCEs in peripheral blood was significantly increased (P<0.05) in the subgroup exposed to rf/MW radiation after eight irradiation treatments of 2 h each in comparison with the sham-exposed control group. It is likely that an adaptive mechanism, both in erythrocytopoiesis and genotoxicity appeared in the rat experimental model during the subchronic irradiation treatment.  相似文献   
90.
The glacial history of the alpine herb Rumex nivalis was investigated using amplified fragment length polymorphisms (AFLPs) and restriction fragment length polymorphisms with polymerase chain reaction (PCR-RFLPs) of cpDNA. Both traditional statistical methods widely applied in phylogeographic research and nested clade analysis were used. The AFLPs indicated little geographic structure probably due to the wind-pollinated reproductive system of the dioecious R. nivalis. Because cpDNA haplotypes exhibited distinct distributional patterns, correlation between AFLPs and PCR-RFLPs was low. The results of common statistical methods and of nested clade analysis were largely congruent. Both supported in situ survival of one group of common haplotypes in the Central Alps. For another group of common haplotypes, classical phylogeographic analyses gave strong evidence for survival in peripheral refugia at the northern alpine border, whereas this conclusion was not as clearly supported in the nested clade analysis. Nested clade analysis provided several detailed insights on past and ongoing populational demographic processes. Thus, it is a valuable tool in the phylogeographical analysis of haplotype data, but it should preferably be combined with other statistical analyses. In situations with low genetic variation in cpDNA, classical phylogeographic analytical tools on nuclear DNA will still be the methods of choice.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号