全文获取类型
收费全文 | 3307篇 |
免费 | 446篇 |
国内免费 | 2篇 |
学科分类
生物科学 | 3755篇 |
出版年
2017年 | 34篇 |
2016年 | 52篇 |
2015年 | 59篇 |
2014年 | 88篇 |
2013年 | 108篇 |
2012年 | 151篇 |
2011年 | 150篇 |
2010年 | 81篇 |
2009年 | 102篇 |
2008年 | 112篇 |
2007年 | 135篇 |
2006年 | 123篇 |
2005年 | 110篇 |
2004年 | 107篇 |
2003年 | 109篇 |
2002年 | 107篇 |
2001年 | 67篇 |
2000年 | 73篇 |
1999年 | 85篇 |
1998年 | 47篇 |
1997年 | 40篇 |
1996年 | 38篇 |
1995年 | 35篇 |
1994年 | 40篇 |
1993年 | 36篇 |
1992年 | 54篇 |
1991年 | 67篇 |
1990年 | 64篇 |
1989年 | 61篇 |
1988年 | 59篇 |
1987年 | 36篇 |
1986年 | 62篇 |
1985年 | 58篇 |
1984年 | 53篇 |
1983年 | 51篇 |
1982年 | 53篇 |
1981年 | 53篇 |
1980年 | 38篇 |
1979年 | 42篇 |
1978年 | 46篇 |
1977年 | 38篇 |
1976年 | 49篇 |
1975年 | 36篇 |
1974年 | 50篇 |
1973年 | 39篇 |
1972年 | 44篇 |
1971年 | 40篇 |
1969年 | 46篇 |
1968年 | 44篇 |
1967年 | 39篇 |
排序方式: 共有3755条查询结果,搜索用时 15 毫秒
91.
X-ray diffraction and electron microscope studies on the structure of bacterial F pili. 总被引:7,自引:0,他引:7
W Folkhard K R Leonard S Malsey D A Marvin J Dubochet A Engel M Achtman R Helmuth 《Journal of molecular biology》1979,130(2):145-160
F pili are hollow cylinders with 80 Å outer diameter and 20 Å inner diameter. Both X-ray fibre diffraction and optical diffraction of electron micrographs show a strong layer-line corresponding to a spacing of 32 Å, to which a J4 Bessel function is assigned on the basis of the optical diffraction. X-ray diffraction patterns show near-meridional intensity on a layer-line corresponding to a spacing of 12.8 Å, to which a J1 Bessel function is assigned. Mass per length measurements on unstained specimens in the scanning transmission electron microscope give 3000 daltons/Å, indicating that the 11,200 dalton pilin subunits are 3.7 Å apart along the axial direction of the pili. These observations show that the pilus structure can be represented as four coaxial helices of pitch 128 Å with the pilin subunits elongated and overlapping along the line of these helices. Each of these helices of subunits is translated axially with respect to its neighbour, to give a basic helix of 3.6 units per turn of 12.8 Å pitch. Radial electron density calculations indicate a 50 Å diameter girdle of hydrophobic amino acids between the inner and outer diameters of the protein shell. A molecular model of the structure at low resolution is presented. 相似文献
92.
In vivo synthesised protein with norleucine occupying one half of the normal methionine loci was prepared using a methionine auxotroph of Escherichia coli K12. The extent of charging of the analogue onto both tRNAmet species and subsequent incorporation into soluble protein was monitored with a double-labelling system comprising [G-3H]norleucine and [35S]methionine. Further experiments established that norleucine can be formylated in vivo once charged onto the initiator tRNAfmet. An N-terminal analysis of the crude soluble protein revealed that formylnorleucyl-tRNAfmet can initiate protein synthesis and that the formyl group is then removed from the nascent polypeptide. We were also led to conclude that the N-terminal methionine-amino peptidase does not recognise the analogue in this position. Slow growth rates on the methionine analogue have been partly attributed to limiting levels of charged tRNAmmet, resulting in turn from the inefficiency of norleucine charging by methionyl-tRNA synthetase. Finally no evidence has been found for the production of aberrant protein as a result of norleucine incorporation, implying that limited growth on the analogue is due to its inability to replace methionine as the precursor of S-adenosyl methionine. 相似文献
93.
Coccidial oocysts morphologically consistent with Eimeria ursini Supperer 1957, and E. tasmaniae Supperer 1957 were recovered from the feces of wild and captive hairy-nosed wombats (Lasiorhinus latifrons) in Australia. Eimeria arundeli so. n. was recovered from the feces of wild and captive common wombats (Vombatus ursinus). Eimeria arundeli oocysts are ellipsoidal to slightly ovoid 60.2--67.2 (63.7) X 40.6--47.6 (43.4); micropyle 3 in diameter usually visible; with oocyst wall granular, dark brown and occasionally opaque, 4--7 thick; inner oocyst wall clear, about 1.5 thick; small oocyst residuum present, four sporocysts ovoid 22.4--29.4 (25.8) X 12.6--15.4 (14.1) with protuberant Stieda body; opposite end of sporocyst also often slighly pointed; large granular sporocyst residuum obscuring sporozoites. Gametocytes of E. arundeli sp. n. and of an organism which is consistent with E. tasmaniae, are described developing in the lamina propria of villi in the small intestine. The stages in the hairy-nosed wombat are those described as Ileocystis wombati Gilruth and Bull 1912. It is suggested that the identification of the host of Supperer's E. ursini and E. tasmaniae as V. ursinus was in error and that the allopatric L. latifrons is the natural host. Eimeria tasmaniae Supperer 1957 is suppressed and E. wombati (Gilruth and Bull, 1912) comb. nov. is proposed and redescribed. No schizonts were identified among the endogenous stages, consistent with observations in the literature on other coccidia with similar gametocyte and oocyst structure. 相似文献
94.
95.
96.
Henry R. Black Samuel O. Thier Philip F. Felig James F. Jekel Joseph Belsky James L. Bernene Everett Cooper Vincent A. De Luca Jr. Martin H. Floch Marvin Garrell Pasquale E. Perillie Robert L. Piscatelli George F. Thornton 《The Yale journal of biology and medicine》1977,50(6):637-644
In July, 1975, the Departments of Internal Medicine at the Yale University School of Medicine and eight community hospitals in southern and western Connecticut formed the Yale Affiliated Hospital Program (YAHP) in Internal Medicine. The YAHP provides a planned and focused program of continuing education for medical staff and housestaff at the affiliated hospitals. Six formats for the over 1,000 rounds, lectures, and conferences given annually are used. The members of the YAHP also cooperate in housestaff and faculty recruiting, evaluation of quality of care and evaluation of the process of continuing medical education itself. This report summarizes the organization, goals and future plans of the YAHP. 相似文献
97.
Winona C. Barker Lynne K. Ketcham Margaret O. Dayhoff 《Journal of molecular evolution》1978,10(4):265-281
Summary We have implemented a routine procedure for screening protein sequences for evidence of intragenic duplications. We tested 163 protein sequences representing 116 superfamilies of unrelated proteins. Twenty superfamilies contain proteins with internal gene duplications. The intragenic duplications detected can be divided into two major types. (1) One or more duplications of all or part of a gene produce a protein with two or several detectable regions of sequence homology. Sequences from 18 superfamilies contained this type of duplication. (2) Repeated reduplication of a small DNA segment can produce a protein that is repetitive over most of its length. Three superfamilies contain such repetitive sequences. We also investigated the limits of detection of ancient duplications using sequences derived by random mutation of a model sequence consisting of ten 10-residue repeats. The original repetitive nature of the sequence was usually detected after 250 point mutations even though the ancestral segment could not be accurately reconstructed. 相似文献
98.
The radiological prevalence of Paget''s disease was studied in 14 towns. Routine radiographs showed that the disease was present in 5.4% of people aged 55 years and over. The disease was more prevalent in men than in women at all ages, and the prevalence increased with age. The three Lancashire towns studied (Preston, Bolton, and Blackburn) had higher rates than elsewhere. This probably reflects a real geographical variation in the prevalence of Paget''s disease in England and Wales. 相似文献
99.
Using UDP-d-galactose : 2-acetamido-2-deoxy-d-glucose 4-β-d-galactosyltransferase (EC 2.4.1.22), several methyl β-lactosides have been prepared with 2H- and/or 13C-enrichment at specific sites to facilitate study by 13C (75 MHz) and 1H (600 MHz) n.m.r. spectroscopy. 13C-Chemical shift assignments were verified and the 1H-spectrum of β-lactoside was fully assigned. Sites of enrichment were selected to permit all of the potential three-bond C-C and C-H couplings through the glycosidic bond to be obtained. Replacement of H-3 of the d-glucose residue of methyl β-lactoside with 2H allowed resolution of C-1–H-4′ coupling in the 600-MHz 1H-spectrum. Single or multiple 13C-enrichment at C-1, C-2, C-3, C-1′, C-3′, and/or C-4′ in the disaccharide allowed observation of intra- and inter-residue couplings. 13C-Spin-lattice relaxation-times (T1) are interpreted in terms of molecular motion in solution. The data suggest that methyl β-lactoside has an extended conformation with little rotation about the glycosidic bond. Inter-residue couplings are best explained by tortion angles of φ ~ 40° and ψ ~ 15°, indicating that the conformations of β-lactoside in solution and in the crystal are similar. 相似文献
100.
The purpose of this study was to learn wether a number of Ca2+ antagonists were effective in reducing contractile response of the isolated ileum of the sensitized and normal guinea pig. Contractions of the normal ileum in response to LTD4, acetylcholine, histamine, and potassium chloride were obtained before and after verapamil, diltiazen and papaverine. Ovalbumin-induced contractions of the ovalbumin-sensitized ileum were obtained in the presence of the three Ca2+ antagonists. In the normal ileum, all the Ca2+ antagonists were highly effective in diminishing the contractile responses to LTD4, acetylcholine, histamine and potassium chloride. In the sensitized ileum, ovalbumin-evoked contractions, with subsequent release of a potent contractile mediator (presumably SRS-A), were Ca2+-dependent since verapamil, diltiazem and papaverine caused a concentration-related reduction of contractions. Thus, the influx of extracellular Ca2+ plays a key role in the contractile responses of the normal and sensitized guinea pig ileum when stimulated by various potent agonists acting on specific receptors or on the cell membrane. 相似文献