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   3305篇
  免费   196篇
  国内免费   5篇
  3506篇
  2023年   9篇
  2022年   20篇
  2021年   46篇
  2020年   23篇
  2019年   26篇
  2018年   46篇
  2017年   27篇
  2016年   53篇
  2015年   119篇
  2014年   149篇
  2013年   200篇
  2012年   237篇
  2011年   229篇
  2010年   153篇
  2009年   154篇
  2008年   215篇
  2007年   235篇
  2006年   220篇
  2005年   255篇
  2004年   216篇
  2003年   184篇
  2002年   188篇
  2001年   24篇
  2000年   20篇
  1999年   43篇
  1998年   53篇
  1997年   32篇
  1996年   35篇
  1995年   33篇
  1994年   39篇
  1993年   23篇
  1992年   16篇
  1991年   21篇
  1990年   18篇
  1989年   13篇
  1988年   17篇
  1987年   9篇
  1986年   4篇
  1985年   15篇
  1984年   15篇
  1983年   8篇
  1982年   7篇
  1981年   22篇
  1980年   8篇
  1979年   4篇
  1978年   6篇
  1977年   3篇
  1976年   3篇
  1975年   4篇
  1961年   2篇
排序方式: 共有3506条查询结果,搜索用时 15 毫秒
21.
Seven different restriction fragment length polymorphisms (RFLPs) at the N-acetylgalactosamine-6-sulfate sulfatase (GALNS) locus were analyzed using Southern blotting and polymerase chain reaction based techniques to search for the frequency of each RFLP produced by StyI, SphI, HaeIII, StuI, HapII, XhoI, and BamHI restriction endonucleases, respectively, in 36 mutant alleles, including two sibling cases and 100 normal alleles. Calculation of heterozygosity indexes showed that these RFLPs were polymorphic, ranging from 0.31 to 0.69 in mucopolysaccharidosis IVA (MPS IVA) patients compared with 0.21 to 0.65 in normal individuals. There was some significant difference in several RFLPs and in the combination with four kinds of RFLPs (SphI, StuI, HapII, XhoI polymorphisms). The normal alleles were composed of 13 different RFLPs haplotypes; the most common among the Japanese population carrying normal alleles was haplotype 8 (bDEF1) (31.3%), the others being dispersed. The same haplotype 8 was the most frequent in the mutant alleles (44.4%), with seven further haplotypes. These findings revealed the striking variety of polymorphic haplotypes in the MPS IVA gene. By using these five kinds of RFLPs, we examined the theoretical informativity of haplotype analysis in heterozygote detection in nine unrelated MPS IVA families and ten unrelated normal families. All the members of the MPS IVA families studied were diagnosed as a patient, carrier, or noncarrier. We propose that prenatal diagnosis or family analysis in cases in which mutations have not been characterized is now feasible.  相似文献   
22.
23.
When mt+ and mt gametes of Chlamydomonas reinhardtiiwere mixed, shedding of cell walls took place in both matingtypes during massive agglutination and/or pairing. This wascaused by a cell wall lytic factor that had been induced byflagellar agglutination and excreted into the medium by cellsconcurrently with their cell wall release. When glutaraldehyde-fixed gametes and isolated flagella of onemating type caused isoagglutination of live gametes of the othermating type, the live mt+ gametes induced the lytic factor andshed their walls, whereas none of the live mt did this.The cell walls of mt gametes were lost only when thelytic factor, which had been excreted by mt+ gametes into themedium, acted from the outside. These data imply that mt+ gametesare responsible for the induction of the lytic factor by agglutination,which acts on cell walls of both mating types either endogenouslyor exogenously. (Received February 28, 1978; )  相似文献   
24.
A new type polyamide containing a glucose unit in the main chain has been synthesized by the polymerization of C1, C3, C4 blocked C6-carboxymethylglucosamine, prepared from chitin. The deblocking procedure gave the water-soluble polyamide, of MW 1.5 × 104, which can be regarded as a model for the recognition site of lectin.  相似文献   
25.
The ability to form functionally active chloroplasts is determined at a certain early stage of leaf development in three non-allelic temperature-sensitive virescent mutants of rice. Temperature-shift analysis, together with anatomical observations, indicates that the intrinsic developmental signals of the virescent genes are expressed at the stage immediately following the formation of basic leaf structure, but just before the onset of leaf elongation. These signals control the expression of chloroplast-encoded genes but do not affect the subsequent morphological development of the leaf or the photo-regulation of the expression of nuclear genes encoding chloroplast proteins.  相似文献   
26.
Summary Lipase was modified with several hydrophilic and hydrophobic synthetic polymers. The modified lipase was solubilized into chloroform by. The catalytic esterification activity of modified lipase increased linearly with the increase of its solubility in chloroform.  相似文献   
27.
Summary The effect of ion-supplemented medium on peroxidase excretion from horseradish (Armoracia rusticana) hairy roots was studied. Supplementation of mannitol instead of ions revealed that the excretion was stimulated not by osmotic pressure in the medium but by ionic properties. Extracellular peroxidase activity per dry cell was proportionally correlated with the ionic strength of the cations. CaCl2 or MgCl2 was found to be the most effective agent for excretion among other combinations. CaCl2 supplementation at the beginning of the culture caused higher peroxidase production in the medium without a significant loss of final cell mass compared with CaCl2 addition during the culture. Repeated batch culture with 50 mM CaCl2 supplementation allowed a continuous retention of cell viability over 149 days and produced a great amount of extracellular peroxidase, 12-fold higher than that achieved in a 40-day-old batch culture with 50 mM CaCl2 supplementation. Correspondence to: T. Kobayashi  相似文献   
28.
The trypsin-catalyzed coupling of bovine (Boc)2-desoctapeptide (B23-B30)-insulin with synthetic octapeptides, H-Gly-X2-X3-X4-Thr-Pro-Lys(Boc)-Thr-OH (X2 = Phe or Ala, X3 = Phe or Ala, X4 = Tyr or Ala), followed by deprotection and purification produced the [AlaB24, ThrB30]-, [AlaB25, ThrB30]-, and [AlaB26, ThrB30]-analogs of bovine insulin in yields of 32, 35, and 32%, respectively. The biological activity of these analogs decreased in the order, normal insulin ([ThrB30]-bovine insulin) = AlaB26-insulin > AlaB25-insulin > AlaB24-insulin, as assayed for receptor binding and some other biological effects, in contrast with the corresponding Leu-analogs of human insulin, in which the activity decreased in the order, normal insulin > LeuB24-insulin > LeuB25-insulin. The affinity to insulin antibodies greatly diminished in both AlaB24-insulin and LeuB24-insulin but not in the B25-substituted analogs. The CD spectra of the Leu- and the Ala-analogs were compared with those of normal insulins to show that no apparent correlation seems to exist between the decrease in biological activity and the conformational changes observed in solution. The effects of organic solvents on the peptide-bond equilibrium and on the stability of trypsin are also discussed.  相似文献   
29.
A thiosulfate-oxidizing enzyme was partially purified fromChromatium vinosum, and some of its properties were studied. The enzyme rapidly reducede HiPIP (high-potential nonheme iron protein) in the presence of thiosulfate. Cytochromesc of yeast and tuna and ferricyanide also acted well as electron acceptors for the enzyme; horse cytochromec was a poor electron acceptor. Cytochromec-552, cytochromec′, and cytochromec-553 did not act as electron acceptors. The enzyme was inhibited by cyanide and sulfite. On the basis of the stoichiometry in reduction of ferricyanide catalyzed by the enzyme in the presence of thiosulfate, the oxidized product of thiosulfate was inferred to be tetrathionate.  相似文献   
30.
BackgroundWe evaluated the setup error and dose absorption of an immobilization system with a shell and wooden baseplate (SW) for lung stereotactic body radiotherapy (SBRT).Materials and methodsSetup errors in 109 patients immobilized with an SW or BodyFix system (BF) were compared. Dose attenuation rates of materials for baseplates were measured with an ion-chamber. Ionization measurements were performed from 90° to 180° gantry angle in 10° increments, with the ball water equivalent phantom placed at the center of the wood and carbon baseplates whose effects on dose distribution were compared using an electron portal imaging device.ResultsThe ratio for the anterior-posterior, cranial-caudal, and right-left of the cases within 3-mm registered shifts in interfractional setup error were 90.9%, 89.2%, and 97.4% for the SW, and 93.2%, 91.6%, and 98.0% for the BF, respectively. For intrafractional setup error, 98.3%, 97.4%, and 99.1% for the SW and 96.6%, 95.8%, and 98.7% for the BF were within 3-mm registered shifts, respectively. In the center position, the average (minimum/maximum) dose attenuation rates from 90° to 180° for the wooden and carbon baseplates were 0.5 (0.1/2.8)% and 1.0 (–0.1/10.1)% with 6 MV, respectively. The gamma passing rates of 2%/2 mm for the wooden and carbon baseplates were 99.7% and 98.3% (p < 0.01).ConclusionsThe immobilization system with an SW is effective for lung SBRT since it is comparable to the BF in setup accuracy. Moreover, the wooden baseplate had lower radiation attenuation rates and affected the dose distribution less than the carbon baseplate.  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