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   1503篇
  免费   60篇
  国内免费   6篇
  1569篇
  2023年   20篇
  2022年   27篇
  2021年   47篇
  2020年   34篇
  2019年   31篇
  2018年   33篇
  2017年   25篇
  2016年   63篇
  2015年   68篇
  2014年   70篇
  2013年   86篇
  2012年   108篇
  2011年   108篇
  2010年   54篇
  2009年   60篇
  2008年   78篇
  2007年   76篇
  2006年   53篇
  2005年   52篇
  2004年   79篇
  2003年   61篇
  2002年   51篇
  2001年   17篇
  2000年   23篇
  1999年   21篇
  1998年   14篇
  1996年   7篇
  1995年   6篇
  1994年   9篇
  1992年   9篇
  1991年   17篇
  1990年   19篇
  1989年   7篇
  1988年   13篇
  1987年   6篇
  1985年   10篇
  1984年   13篇
  1983年   7篇
  1982年   4篇
  1981年   6篇
  1980年   4篇
  1979年   10篇
  1978年   7篇
  1977年   5篇
  1975年   4篇
  1974年   5篇
  1973年   5篇
  1972年   5篇
  1971年   4篇
  1966年   4篇
排序方式: 共有1569条查询结果,搜索用时 0 毫秒
121.
122.
The sequencing of the complete genome of Anaplasma marginale has enabled the identification of several genes that encode membrane proteins, thereby increasing the chances of identifying candidate immunogens. Little is known regarding the genetic variability of genes that encode membrane proteins in A. marginale isolates. The aim of the present study was to determine the degree of conservation of the predicted amino acid sequences of OMP1, OMP4, OMP5, OMP7, OMP8, OMP10, OMP14, OMP15, SODb, OPAG1, OPAG3, VirB3, VirB9-1, PepA, EF-Tu and AM854 proteins in a Brazilian isolate of A. marginale compared to other isolates. Hence, primers were used to amplify these genes: omp1, omp4, omp5, omp7, omp8, omp10, omp14, omp15, sodb, opag1, opag3, virb3, VirB9-1, pepA, ef-tu and am854. After polimerase chain reaction amplification, the products were cloned and sequenced using the Sanger method and the predicted amino acid sequence were multi-aligned using the CLUSTALW and MEGA 4 programs, comparing the predicted sequences between the Brazilian, Saint Maries, Florida and A. marginale centrale isolates. With the exception of outer membrane protein (OMP) 7, all proteins exhibited 92-100% homology to the other A. marginale isolates. However, only OMP1, OMP5, EF-Tu, VirB3, SODb and VirB9-1 were selected as potential immunogens capable of promoting cross-protection between isolates due to the high degree of homology (over 72%) also found with A. (centrale) marginale.  相似文献   
123.
124.
Thirty-four spermidine (SPD) and spermine (SPM) derivatives with aromatic substituents were synthesized and tested as inhibitors of specific binding of the NMDA channel blocker [3H]MK-801 to membranes prepared from rat hippocampus and cerebral cortex. SPD and SPM derivatives with aromatic substituents at the primary amino groups were the most potent inhibitors (IC50 3.9-4.7 microM). These compounds most likely act directly at the NMDA ion channel, since 30 microM SPM had no pronounced influence on their inhibiting activities. SPD derivatives with aromatic substituents at the secondary amino group were either inactive or highly SPM-sensitive inhibitors (IC50 10-82 microM), depending on the size of the substituent. Our results support the hypothesis that an aromatic interaction site near the center of polyamine derivatives contributes to polyamine inverse agonism.  相似文献   
125.
BackgroundCOVID-19 is a new pandemic that has infected millions of people worldwide and caused a high morbidity and mortality rate. COVID-19 may have a harmful effect on organs, especially the kidneys. Aims: The main aim of our research is to study the association between the severity of COVID-19 disease and biochemical parameters related to kidney function and to investigate certain risk factors of COVID-19-associated kidney disease.MethodsA total of 174 individuals, 121 COVID-19 positive and 53 COVID-19 negative, were enrolled in this study. The relation between COVID-19 infection, severity, kidney function test, and hematological indicators were examined.ResultsThe most prominent symptoms among COVID-19 were fever (95% ) and fatigue (92%). Regarding biochemical parameters, median creatinine, MPV, and CRP were significantly higher in COVID-19 patients, whereas median eGFR, Na+, WBC, MCH, MCHC, and eosinophil percentages were significantly lower in this group. Severely infected patients were observed to have higher urea, creatinine, neutrophils, and NLR. However, median sodium, eGFR, hemoglobin, hematocrit, RBC, lymphocytes, and platelet count were significantly lower in the severe group. Urine examination of the severe group showed a significantly lower specific gravity, while urine pH, protein, and glucose were significantly higher.ConclusionsOur analysis indicates that COVID-19 infection affects kidney function, mainly creatinine level, urea, eGFR, Na+ and urine protein. Additionally, comorbidities such as older age (>65), hypertension, taking medications, and CRP (>33.55 mg/L) are considered risk factors that are more likely to contribute to kidney impairment in COVID-19 positive patients.  相似文献   
126.
127.
S I Said  J C Porter 《Life sciences》1979,24(3):227-230
Immunoreactive vasoactive intestinal polypeptide (VIP) was present in portal hypophyseal blood of 24 male rats in concentrations (mean, 995 pg per ml) that were approximately 19 times as high as those in systemic arterial blood (mean, 52 pg per ml). The results demonstrate release of VIP from the hypothalamic-neurohypophyseal complex into the portal circulation, and establish a mechanism for direct influence of the peptide on pituitary function.  相似文献   
128.
129.
In this paper we discuss about the utility of the species concept as real definition, particularly the Mayr concept. We propose a method for the logical separation of taxa based in the statements of the logical mathematics and the application of the sets theory to the concepts in systematic. We attempt to provide an objective methodology for the interpretation of natural groups in biology including the species as a basic group in evolution. We introduce the concept of the hypothetical ancestor as a mathematical possibility derived from the use of matrix calculations for non square matrix.  相似文献   
130.
Increasing pollution of water and soils by xenobiotic compounds has led in the last few decades to an acute need for understanding the impact of toxic compounds on microbial populations, the catabolic degradation pathways of xenobiotics and the set-up and improvement of bioremediation processes. Recent advances in molecular techniques, including high-throughput approaches such as microarrays and metagenomics, have opened up new perspectives and pointed towards new opportunities in pollution abatement and environmental management. Compared with traditional molecular techniques dependent on the isolation of pure cultures in the laboratory, microarrays and metagenomics allow specific environmental questions to be answered by exploring and using the phenomenal resources of uncultivable and uncharacterized micro-organisms. This paper reviews the current potential of microarrays and metagenomics to investigate the genetic diversity of environmentally relevant micro-organisms and identify new functional genes involved in the catabolism of xenobiotics.  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