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   10347篇
  免费   1098篇
  国内免费   4篇
  2021年   123篇
  2020年   86篇
  2019年   101篇
  2018年   127篇
  2017年   114篇
  2016年   209篇
  2015年   371篇
  2014年   389篇
  2013年   466篇
  2012年   580篇
  2011年   630篇
  2010年   424篇
  2009年   347篇
  2008年   514篇
  2007年   472篇
  2006年   440篇
  2005年   459篇
  2004年   432篇
  2003年   423篇
  2002年   429篇
  2001年   208篇
  2000年   164篇
  1999年   202篇
  1998年   139篇
  1997年   111篇
  1996年   104篇
  1995年   103篇
  1994年   117篇
  1993年   108篇
  1992年   148篇
  1991年   145篇
  1990年   150篇
  1989年   153篇
  1988年   145篇
  1987年   114篇
  1986年   104篇
  1985年   107篇
  1984年   110篇
  1983年   88篇
  1982年   101篇
  1981年   101篇
  1980年   78篇
  1979年   96篇
  1978年   74篇
  1977年   66篇
  1976年   70篇
  1975年   72篇
  1974年   78篇
  1973年   84篇
  1972年   67篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Proteolytic control of Caulobacter cell cycle proteins is primarily executed by ClpXP, a dynamically localized protease implicated in turnover of several factors critical for faithful cell cycle progression. Here, we show that the transient midcell localization of ClpXP that precedes cytokinesis requires the FtsZ component of the divisome. Although ClpAP does not exhibit subcellular localization, FtsZ is a substrate of both ClpXP and ClpAP in vivo and in vitro. A peptide containing the C‐terminal portion of the FtsA divisome protein is a substrate of both ClpXP and ClpAP in vitro but is primarily degraded by ClpAP in vivo. Caulobacter carries out an asymmetric division in which FtsZ and FtsA are stable in stalked cells but degraded in the non‐replicative swarmer cell where ClpAP alone degrades FtsA and both ClpAP and ClpXP degrade FtsZ. While asymmetric division in Caulobacter normally yields larger stalked and smaller swarmer daughters, we observe a loss of asymmetric size distribution among daughter cells when clpA is depleted from a strain in which FtsZ is constitutively produced. Taken together, these results suggest that the activity of both ClpXP and ClpAP on divisome substrates is differentially regulated in daughter cells.  相似文献   
2.
J Stewart 《CMAJ》1995,153(5):667-668
Manitoba''s Norway House offers a ruggedly beautiful location and a challenging medical practice, but the physical and professional appeal has not been strong enough to entice physicians to make a long-term commitment to the community, which has needed medical service since September 1994. A physician who left there this spring said concerns about practice restrictions convinced him to move south.  相似文献   
3.
The cold-shock response — a hot topic   总被引:4,自引:2,他引:2  
  相似文献   
4.
Gender differences in anaerobic power tests   总被引:1,自引:0,他引:1  
The purpose of this study was to determine if the differences in anaerobic power between males and females could be accounted for by differences in body composition, strength, and neuromuscular function. A total of 82 untrained men and 99 women took part in the study. Body composition, somatotype, isometric strength, neuromuscular function were measured, and four anaerobic power tests performed. The men were significantly different from the women on all strength, power, and neuromuscular measurements except reaction time and on all anthropometric and somatotype dimensions except ectomorphy. Strength and anthropometric dimensions were similarly related to anaerobic power values within each sex. Relative fat (%fat) exerted different degrees of influence on sprint and jump performances in each sex. Removing the influence of anthropometric, strength, and neuromuscular differences by analysis of covariance reduced, but did not remove, the significant differences between the sexes. Therefore, factors other than lean body mass, leg strength, and neuromuscular function may be operating in short-term, explosive power performances to account for the differences between the sexes. The task-specific nature of anaerobic power tests and the relatively large influence of anthropometric factors on power production were confirmed.  相似文献   
5.
6.
Tomaymycin is a member of the pyrrolo[1,4]benzodiazepine [P(1,4)B] antitumor antibiotic group. This antibiotic is proposed to react with the exocyclic 2-amino group (N2) of guanine to form a covalent adduct that lies snugly within the minor groove of DNA. While DNA-footprinting experiments using methidiumpropyl-EDTA have revealed the favored bonding sequences for tomaymycin and related drugs on DNA, the stereochemistry at the covalent bonding site (C-11) and orientation in the minor groove were not established by these experiments. In previous studies using a combined fluorescence, high-field NMR, and molecular modeling approach, we have shown that for tomaymycin there are two diastereomeric species (11R and 11S) on both calf thymus DNA and d(ATGCAT)2. Although we were able to infer the identity (stereochemistry at C-11 and orientation in the minor groove) of the two species on d(ATGCAT)2 by high-field NMR and fluorescence studies, in combination with molecular mechanics calculations, definitive experimental evidence was lacking. We have designed and synthesized a self-complementary 12-mer [d(CICGAATTCICG)2] based on the Dickerson dodecamer [d(CGCGAATTCGCG)2] that bonds identically two tomaymycin molecules, each having a defined orientation and stereochemistry. Thus the bis(tomaymycin)-12-mer adduct maintains the self-complementarity of the original duplex molecule. Two-dimensional proton J-correlated spectroscopy (COSY) of the bis(tomaymycin)-d(CICGAATTCICG)2 adduct (I = inosine) unequivocally shows that C-11 of tomaymycin covalently bonds through N2 of guanine with an 11S stereochemistry in the sequence 5'-CGA-3'.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
7.
Species of the marine mussel family Mytilidae have two types of mitochondrial DNA: one that is transmitted from the mother to both female and male offspring (the F type) and one that is transmitted from the father to sons only (the M type). By using pair matings that produce only female offspring or a mixture of female and male offspring and a pair of oligonucleotide primers that amplify part of the COIII gene of the M but not the F mitochondrial genome, we demonstrate that both male and female embryos receive M mtDNA through the sperm and that within 24 hr after fertilization the M mtDNA is eliminated or is drastically reduced in female embryos but maintained in male embryos. These observations are important for understanding the relationship between mtDNA transmission and sex determination in species with doubly uniparental inheritance of mitochondrial DNA.  相似文献   
8.
Development of the yolk sac of squamate reptiles (lizards and snakes) differs from other amniote lineages in the pattern of growth of extraembryonic mesoderm, which produces a cavity, the yolk cleft, within the yolk. The structure of the yolk cleft and the accompanying isolated yolk mass influence development of the allantois and chorioallantoic membrane. The yolk cleft of viviparous species of the Eugongylus group of scincid lizards is the foundation for an elaborate yolk sac placenta; development of the yolk cleft of oviparous species has not been studied. We used light microscopy to describe the yolk sac and chorioallantoic membrane in a developmental series of an oviparous member of this species group, Oligosoma lichenigerum. Topology of the extraembryonic membranes of late stage embryos differs from viviparous species as a result of differences in development of the yolk sac. The chorioallantoic membrane encircles the egg of O. lichenigerum but is confined to the embryonic hemisphere of the egg in viviparous species. Early development of the yolk cleft is similar for both modes of parity, but in contrast to viviparous species, the yolk cleft of O. lichenigerum is transformed into a tube‐like structure, which fills with cells. The yolk cleft originates as extraembryonic mesoderm is diverted from the periphery of the egg into the yolk sac cavity. As a result, a bilaminar omphalopleure persists over the abembryonic surface of the yolk. The bilaminar omphalopleure is ultimately displaced by intrusion of allantoic mesoderm between ectodermal and endodermal layers. The resulting chorioallantoic membrane has a similar structure but different developmental history to the chorioallantoic membrane of the embryonic hemisphere of the egg. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.  相似文献   
9.
Type IB topoisomerases cleave and rejoin DNA through a DNA-(3'-phosphotyrosyl)-enzyme intermediate. A constellation of conserved amino acids (Arg-130, Lys-167, Arg-223, and His-265 in vaccinia topoisomerase) catalyzes the attack of the tyrosine nucleophile (Tyr-274) at the scissile phosphodiester. Previous studies implicated Arg-223 and His-265 in transition state stabilization and Lys-167 in proton donation to the 5'-O of the leaving DNA strand. Here we find that Arg-130 also plays a major role in leaving group expulsion. The rate of DNA cleavage by vaccinia topoisomerase mutant R130K, which was slower than wild-type topoisomerase by a factor of 10(-4.3), was stimulated 2600-fold by a 5'-bridging phosphorothiolate at the cleavage site. The catalytic defect of the R130A mutant was also rescued by the 5'-S modification (190-fold stimulation), albeit to a lesser degree than R130K. We surmise that Arg-130 plays dual roles in transition state stabilization and general acid catalysis. Whereas the R130A mutation abolishes both functions, R130K permits the transition state stabilization function (via contact of lysine with the scissile phosphate) but not the proton transfer function. Our results show that the process of general acid catalysis is complex and suggest that Lys-167 and Arg-130 comprise a proton relay from the topoisomerase to the 5'-O of the leaving DNA strand.  相似文献   
10.
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