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   165199篇
  免费   11538篇
  国内免费   31篇
  2023年   819篇
  2022年   1010篇
  2021年   2184篇
  2020年   1777篇
  2019年   2029篇
  2018年   4296篇
  2017年   3756篇
  2016年   5053篇
  2015年   7081篇
  2014年   7467篇
  2013年   9862篇
  2012年   11693篇
  2011年   10992篇
  2010年   7197篇
  2009年   5828篇
  2008年   8941篇
  2007年   8754篇
  2006年   8249篇
  2005年   7511篇
  2004年   7146篇
  2003年   6538篇
  2002年   6032篇
  2001年   3756篇
  2000年   3554篇
  1999年   3073篇
  1998年   1509篇
  1997年   1155篇
  1996年   1062篇
  1995年   1055篇
  1994年   960篇
  1993年   813篇
  1992年   1951篇
  1991年   1808篇
  1990年   1620篇
  1989年   1555篇
  1988年   1416篇
  1987年   1282篇
  1986年   1180篇
  1985年   1233篇
  1984年   1027篇
  1983年   860篇
  1982年   712篇
  1981年   643篇
  1979年   934篇
  1978年   693篇
  1977年   602篇
  1976年   595篇
  1975年   658篇
  1974年   730篇
  1973年   728篇
排序方式: 共有10000条查询结果,搜索用时 593 毫秒
931.
Summary The human genome contains a large number of interspersed simple repeat sequences that are variable in length and can therefore serve as highly informative, polymorphic markers. Typing procedures include conventional multilocus and single locus probing, and polymerase chain reaction aided analysis. We have identified simple sequences in a cosmid clone stemming from the human Y chromosome and consisting of (gata)n repeats. We have compared these with two equivalent simple repeat loci from chromosome 12. After amplifying the tandemly repeated motifs, we detected between four and eight different alleles at each of the three loci. Codominant inheritance of the alleles was established in family studies and the informativity of the simple repeat loci was determined by typing unrelated individuals. The polymorphisms are suitable for application in linkage studies, practical forensic case work, deficiency cases in paternity determination, and for studying ethnological questions. The mutational mechanisms that bring about changes in simple repeats located both on the autosomes and on the sex chromosomes, are discussed.Professor Dr. Otto Prokop (Humboldt-Universität Berlin) on the occasion of his 70th birthday  相似文献   
932.
Pig brain cerebral cortex was subfractionated by isopycnic centrifugation in sucrose gradients. In each subfraction the content of the agonist [3H]R-PIA binding, the activity of adenosine metabolizing enzymes (5-nucleotidase and adenosine deaminase) and the activity of membrane marker enzymes were determined. The fractions were also examined by electron microscope. In general, the results suggest a widespread distribution of A1 adenosine receptors in membranes from different origins. Marker enzyme profile characterization indicated an enrichment of A1 adenosine receptor in pre-synaptic membranes isolated from the crude synaptosomal fraction (P2B subfraction) as well as in membranes of glial origin such as myelin. The receptor is also present in the endoplasmic reticulum and in membranes isolated from the microsomal fraction that seem to have a post-synaptic origin (P3B). In subfractions having a high content of adenosine receptor the equilibrium binding paramters were obtained as well as the proportion of high- to low-affinity sites. From the values of the equilibrium constants it was not possible to find differences between the receptor in the different subfractions. Analysis of the affinity state distribution showed a diminished percentage of high-affinity sites in fraction P3A, which can be accounted by the existence of myelin membranes; in contrast the percentage of high-affinity states was higher in P2 and P3B, indicating that in these fractions the receptor is present in synaptosomal membranes. The close correlation shown between the enzyme 5-nucleotidase specific activity and the specific ligand binding distributions led us to postulate an important role for the enzyme in the regulation of adenosine action in pig brain cortex.  相似文献   
933.
Synopsis Leptocephali were collected in June 1981 and July 1989 over the continental shelf and slope of the Florida west coast. Tarpon larvae ranged 5.5–24.4 mm standard length (SL) and were the second most abundant leptocephalus species. Sagittae examined with compound microscopes and scanning electron microscopy had increments that were presumed to be formed daily. Increment counts made using the two microscopic techniques were not significantly different. Estimated ages ranged 2–25 days with a growth rate (± standard error) of 0.92 ± 0.04 mm d–1 The least squares linear regression equation SL = 2.78 + 0.92 (age in days) best described the relationship between estimated age and length. Adult tarpon appear to undergo a substantial spawning migration from inshore areas frequented during spring and summer to offshore spawning grounds. Spawning occurs during May, June, and July, although the spawning season may be of greater duration.  相似文献   
934.
The floral biology ofTetrastylis ovalis and its pollination by bats was studied in southeastern Brazil. The flowers ofT. ovalis seem more fitted to bat-pollination than those ofPassiflora mucronata, the other known chiropterophilous passionflower. Two species of bats were observed visiting flowers ofT. ovalis: the phyllostomidsGlossophaga soricina andRhinophylla pumilio. Due to the attributes ofTetrastylis, its presumed primitiveness and a better defined bat-pollination syndrome we suggest that chiropterophily evolved independently in the former genus and inPassiflora.  相似文献   
935.
The pollination biology and breeding systems ofEriotheca pubescens andE. gracilipes have been studied. These two species occur as trees in cerrado vegetation, the neotropical savannas of Central Brazil, with partially sympatric distributions. They have similar phenology and floral structure, although the flowers ofE. pubescens are larger. Both species have nectar flowers pollinated by largeAnthophoridae bees but the main pollinators of each species differ in size. The species have markedly different breeding systems: late-acting self-incompatibility inE. gracilipes and apomixis stimulated by pollination inE. pubescens.  相似文献   
936.
Systolic (S) and diastolic (D) blood pressure (BP) and heart rate (HR) of clinically healthy children (24 boys and 15 girls) 3 to 7 years of age were measured with a standard mercury sphygmomanometer at 3-hour intervals for 24 hours in April 1991. The children slept and/or rested from 2100 to 0700 and napped from 1230 to 1530; they had meals at 0730, 1200 and 1800. A statistically significant circadian and about 12-hour (circasemidian) component of variation is documented for SBP and DBP of boys and girls and for HR of boys. No gender difference was found for the circadian and circasemidian components. A positive correlation with age is found for the MESOR and circadian amplitude of SBP and DBP (p < 0.05); a negative correlation with age is found for the MESOR of HR (p < 0.001).  相似文献   
937.
A case of 55 years old woman with "hot" right lobe toxic thyroid nodule, presenting with paroxysmal atrial fibrillation, and therefore treated with 131I 666MBq (18 mCi) is described. After six years she became pyrexic and suffered of severe cough proxyisms. The fine needle biopsy of the above nodule showed the presence of anaplastic thyroid carcinoma. Strumectomy followed by local radiotherapy resulted in complete disappearence of all symptoms. The microscopic of the removed thyroid tissue confirmed the above diagnosis. After 22 months' observation the patient remained in good general condition. The possible reasons for the development of the thyroid carcinoma in this case are discussed.  相似文献   
938.
Normal endothelin 1/2 levels and their correlation with age were evaluated. For clinical application of the endothelin 1/2 RIA test, optimum storage conditions were investigated. Plasma endothelin 1/2 (ET) levels were measured in 36 healthy blood donors, mostly males, of mean age 36 +/- 8 years, subdivided into three age groups: 17-30, 31-40 and above 40 years old. The mean normal ET levels in the three age groups, and corresponding standard deviations, were: 0.58 +/- 0.19, 0.62 +/- 0.31, and 0.80 +/- 0.28 fmol/ml, respectively. The mean ET level for the whole normal population was 0.66 +/- 0.28 fmol/ml. Only differences between mean ET levels for the first and last groups were statistically significant (p < 0.05). Differences between mean ET levels in smokers (53% of total population) and non-smokers, women and men, irrespective of age, were found not to be statistically significant. At this stage of our work, we conclude that other factors than age alone play a role in enhancing ET levels above the age of 41 years. In our study of optimum storage conditions for endothelin 1/2, we found that after one week of storage at -24 degrees C the mean level of ET measured in frozen plasma dropped to 85% of the initial activity, while after the same period the respective decrease in ET activity in frozen extracts was 49%. Over the next two weeks, ET levels in plasma and extracts dropped to 57% and 32% of "zero time" activities, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
939.
The interaction of ovomucoid proteinase inhibitor prepared from duck egg white with a dextran of a molecular weight of 70,000 preliminary treated with potassium periodate. Irrespective of the number of the sites of the ovomucoid binding to aldehyde-dextran the anti-chymotryptic activity is equal to that of the native inhibitor, while the antitryptic activity decreases proportionally to the number of ovomucoid amino groups involved in the reaction with dextran. When a few ovomucoid molecules are immobilized on the polysaccharide macromolecule the perturbing effect of the protein-protein interactions is minimal, as the rigid polymeric chain prevents from the formation of associates of proteins immobilized on this backbone.  相似文献   
940.
A pool of nuclear proteins from adult worms of Schistosoma mansoni was analyzed for amino acid composition and found to be compatible with high mobility group (HMG) proteins. One of the schistosome HMG proteins was identified as HMG 2 by one-dimensional and two-dimensional PAGE. Stage-specific differences in the HMG-like protein composition were encountered when adult worms were compared to schistosomula, the larval form. Immobilization of the adult male and female nuclear proteins onto nitrocellulose, followed by hybridization against 32P-F-10, a schistosome sex specific gene encoding a major egg shell protein, revealed distinct banding patterns. On the other hand, a synthetic oligonucleotide, derived from the 3' untranslated end of the F-10 gene and possibly containing one regulatory element of the gene, bound mainly to male low MW proteins.  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