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   1156008篇
  免费   131972篇
  国内免费   695篇
  1288675篇
  2018年   9850篇
  2016年   13752篇
  2015年   19124篇
  2014年   22437篇
  2013年   32097篇
  2012年   36018篇
  2011年   36365篇
  2010年   24674篇
  2009年   23111篇
  2008年   32713篇
  2007年   33690篇
  2006年   31545篇
  2005年   30434篇
  2004年   30082篇
  2003年   28916篇
  2002年   28196篇
  2001年   51197篇
  2000年   51590篇
  1999年   41123篇
  1998年   15108篇
  1997年   15957篇
  1996年   15111篇
  1995年   14080篇
  1994年   13891篇
  1993年   13724篇
  1992年   34552篇
  1991年   33370篇
  1990年   32628篇
  1989年   31700篇
  1988年   29283篇
  1987年   28253篇
  1986年   26150篇
  1985年   26179篇
  1984年   21790篇
  1983年   18774篇
  1982年   14665篇
  1981年   13199篇
  1980年   12550篇
  1979年   20908篇
  1978年   16602篇
  1977年   15099篇
  1976年   14062篇
  1975年   15345篇
  1974年   16654篇
  1973年   16334篇
  1972年   14744篇
  1971年   13657篇
  1970年   11748篇
  1969年   11086篇
  1968年   10099篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
Studies of association between candidate genes and disease can be designed to use cases with disease, and in place of nonrelated controls, their parents. The advantage of this design is the elimination of spurious differences due to ethnic differences between cases and nonrelated controls. However, several statistical methods of analysis have been proposed in the literature, and the choice of analysis is not always clear. We review some of the statistical methods currently developed and present two new statistical methods aimed at specific genetic hypotheses of dominance and recessivity of the candidate gene. These new methods can be more powerful than other current methods, as demonstrated by simulations. The basis of these new statistical methods is a likelihood approach. The advantage of the likelihood framework is that regression models can be developed to assess genotype-environment interactions, as well as the relative contribution that alleles at the candidate-gene locus make to the relative risk (RR) of disease. This latter development allows testing of (1) whether interactions between alleles exist, on the scale of log RR, and (2) whether alleles originating from the mother or father of a case impart different risks, i.e., genomic imprinting.  相似文献   
44.
The Fis protein: it''s not just for DNA inversion anymore   总被引:36,自引:0,他引:36  
  相似文献   
45.
Catalytic properties of a human cytomegalovirus-induced protein kinase   总被引:4,自引:0,他引:4  
Human cytomegalovirus, a DNA virus whose genome contains a fragment of transforming DNA, induces a threonine-serine protein kinase having a molecular mass of 68 kDa (p68). p68 was extracted from cells 96-144 h after infection, and immunoprecipitated with a monoclonal antibody (F6b). Antibody-enzyme complexes were immobilized on heat/formaldehyde-inactivated Staphylococcus aureus. The best substrates for p68 were acidic proteins, phosvitin and casein. Glycogen synthase, phosphorylase alpha and histones were phosphorylated at rates not higher than 1-4% that obtained with phosvitin as substrate. ATP and GTP were equally good substrates of p68. p68 is able to autophosphorylate at the same residues (i.e. threonine and serine) as the protein substrates. Autophosphorylation does not seem to represent an intermediate in substrate phosphorylation. The protein kinase activity of p68 was not enhanced by cAMP, calcium ions, or polyamines like spermine or spermidine. Only at low Mg2+ concentration spermine enhanced by 68% the rate of casein phosphorylation. Heparin, a potent inhibitor of casein kinase II, inhibits p68 activity too, but ten-times higher concentrations were required for the same degree of inhibition. Quercetin, a bioflavonoid, acts as a strong inhibitor of p68 protein kinase activity. The inhibitory effect of quercetin was competitive towards the nucleotide substrate (Ki = 2.8 microM), and non-competitive towards the protein substrate (Ki = 15 microM).  相似文献   
46.
Five open reading frames designated nirB, nirD, nirE, nirC and cysG have been identified from the DNA sequence of the Escherichia coli nir operon. Complementation experiments established that the NirB, NirD and CysG polypeptides are essential and sufficient for NADH-dependent nitrite reductase activity (EC 1.6.6.4). A series of plasmids has been constructed in which each of the open reading frames has been fused in-phase with the beta-galactosidase gene, lacZ. Rates of beta-galactosidase synthesis during growth in different media revealed that nirB, -D, -E and -C are transcribed from the FNR-dependent promoter, p-nirB, located just upstream of the nirB gene: expression is co-ordinately repressed by oxygen and induced during anaerobic growth. Although the nirB, -D and -C open reading frames are translated into protein, no translation of nirE mRNA was detected. The cysG gene product is expressed from both p-nirB and a second, FNR-independent promoter, p-cysG, located within the nirC gene. No NADH-dependent nitrite reductase activity was detected in extracts from bacteria lacking either NirB or NirD, but a mixture of the two was as active as an extract from wild-type bacteria. Reconstitution of enzyme activity in vitro required stoichiometric quantities of NirB and NirD and was rapid and independent of the temperature during mixing. NirD remained associated with NirB during the initial stages of purification of the active enzyme, suggesting that NirD is a second structural subunit of the enzyme.  相似文献   
47.
Changes in the properties of adenylate cyclase from the lungs of tuberculotic guinea pigs were revealed. The number of beta-adrenergic receptors in the lungs was found to be reduced by 30% at the second and by 70% at the third stage of the disease. The degree and the value of Ka for adenylate cyclase activation by isoproterenol remained thereby unchanged. The basal activity of adenylate cyclase was increased by 20% against the control level at the second stage and decreased by 20% at the third stage of the disease. At these periods, the stimulating effects of guanylyl imidodiphosphate, NaF and forskolin on lung adenylate cyclase were diminished. The experimental results point to the significant role of the enzymes of cAMP metabolism and reflect the course of the tuberculosis process in experimental animals.  相似文献   
48.
49.
Intra-individual length heterogeneity of Rana esculenta mitochondrial DNA   总被引:4,自引:0,他引:4  
Mitochondrial DNA extracted from Rana esculenta oocytes appears heterogeneous in size. The length of these molecules varies continuously from 18,700 bp to 19,700 bp. Each animal is heteroplasmic and can be characterized by the range of the variation (400-700 bp) and the extreme sizes of the various molecules it carries. The variable region of the genome has been localized between the coding region and the replication origin area.  相似文献   
50.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982).  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