全文获取类型
收费全文 | 4904篇 |
免费 | 582篇 |
国内免费 | 1篇 |
专业分类
5487篇 |
出版年
2021年 | 62篇 |
2018年 | 67篇 |
2017年 | 46篇 |
2016年 | 80篇 |
2015年 | 117篇 |
2014年 | 136篇 |
2013年 | 243篇 |
2012年 | 233篇 |
2011年 | 206篇 |
2010年 | 145篇 |
2009年 | 146篇 |
2008年 | 178篇 |
2007年 | 176篇 |
2006年 | 171篇 |
2005年 | 169篇 |
2004年 | 160篇 |
2003年 | 158篇 |
2002年 | 123篇 |
2001年 | 145篇 |
2000年 | 153篇 |
1999年 | 132篇 |
1998年 | 76篇 |
1997年 | 72篇 |
1996年 | 62篇 |
1995年 | 65篇 |
1994年 | 64篇 |
1993年 | 61篇 |
1992年 | 94篇 |
1991年 | 108篇 |
1990年 | 109篇 |
1989年 | 81篇 |
1988年 | 91篇 |
1987年 | 93篇 |
1986年 | 63篇 |
1985年 | 70篇 |
1984年 | 83篇 |
1983年 | 63篇 |
1982年 | 63篇 |
1981年 | 68篇 |
1980年 | 50篇 |
1979年 | 81篇 |
1978年 | 69篇 |
1977年 | 57篇 |
1976年 | 59篇 |
1975年 | 57篇 |
1974年 | 60篇 |
1973年 | 67篇 |
1972年 | 45篇 |
1971年 | 46篇 |
1969年 | 42篇 |
排序方式: 共有5487条查询结果,搜索用时 31 毫秒
101.
A freshwater cyanophage whose genome indicates close relationships to photosynthetic marine cyanomyophages 总被引:1,自引:0,他引:1
Dreher TW Brown N Bozarth CS Schwartz AD Riscoe E Thrash C Bennett SE Tzeng SC Maier CS 《Environmental microbiology》2011,13(7):1858-1874
Bacteriophage S-CRM01 has been isolated from a freshwater strain of Synechococcus and shown to be present in the upper Klamath River valley in northern California and Oregon. The genome of this lytic T4-like phage has a 178,563 bp circular genetic map with 297 predicted protein-coding genes and 33 tRNA genes that represent all 20-amino-acid specificities. Analyses based on gene sequence and gene content indicate a close phylogenetic relationship to the 'photosynthetic' marine cyanomyophages infecting Synechococcus and Prochlorococcus. Such relatedness suggests that freshwater and marine phages can draw on a common gene pool. The genome can be considered as being comprised of three regions. Region 1 is populated predominantly with structural genes, recognized as such by homology to other T4-like phages and by identification in a proteomic analysis of purified virions. Region 2 contains most of the genes with roles in replication, recombination, nucleotide metabolism and regulation of gene expression, as well as 5 of the 6 signature genes of the photosynthetic cyanomyophages (hli03, hsp20, mazG, phoH and psbA; cobS is present in Region 3). Much of Regions 1 and 2 are syntenic with marine cyanomyophage genomes, except that a segment encompassing Region 2 is inverted. Region 3 contains a high proportion (85%) of genes that are unique to S-CRM01, as well as most of the tRNA genes. Regions 1 and 2 contain many predicted late promoters, with a combination of CTAAATA and ATAAATA core sequences. Two predicted genes that are unusual in phage genomes are homologues of cellular spoT and nusG. 相似文献
102.
Purification of acetoacetate decarboxylase from Clostridium acetobutylicum ATCC 824 and cloning of the acetoacetate decarboxylase gene in Escherichia coli. 总被引:6,自引:1,他引:6 下载免费PDF全文
In Clostridium acetobutylicum ATCC 824, acetoacetate decarboxylase (EC 4.1.1.4) is essential for solvent production, catalyzing the decarboxylation of acetoacetate to acetone. We report here the purification of the enzyme from C. acetobutylicum ATCC 824 and the cloning and expression of the gene encoding the acetoacetate decarboxylase enzyme in Escherichia coli. A bacteriophage lambda EMBL3 library of C. acetobutylicum DNA was screened by plaque hybridization, using oligodeoxynucleotide probes derived from the N-terminal amino acid sequence obtained from the purified protein. Phage DNA from positive plaques was analyzed by Southern hybridization. Restriction mapping and subsequent subcloning of DNA fragments hybridizing to the probes localized the gene within an approximately 2.1 kb EcoRI/Bg/II fragment. A polypeptide with a molecular weight of approximately 28,000 corresponding to that of the purified acetoacetate decarboxylase was observed in both Western blots (immunoblots) and maxicell analysis of whole-cell extracts of E. coli harboring the clostridial gene. Although the expression of the gene is tightly regulated in C. acetobutylicum, it was well expressed in E. coli, although from a promoter sequence of clostridial origin. 相似文献
103.
The teratogenic effects of valproic acid and its 4-propyl-4-pentenoic acid (4-en) metabolite were investigated in three inbred mouse strains that were known to possess differing sensitivity to heat-induced neural tube defects. In the heat-resistant DBA/2J strain, administration of either valproic acid or the metabolite during the critical period of neural tube development failed to produce any abnormal offspring. Similar treatment in the moderately heat-sensitive LM/Bc strain resulted in up to 19.8% exencephalic fetuses. The highly heat-sensitive SWV strain was also very susceptible to the induction of neural tube defects by either valproic acid or its 4-en metabolite. When administered on gestational day 8 plus 12 hours, the parent compound produced 35% exencephalic fetuses, while the metabolite had a response frequency of 32.4%. Thus, the hierarchy of susceptibility for the induction of neural tube defects in these inbred mouse strains was exactly the same whether the teratogen was a physical agent such as hyperthermia or a chemical compound such as valproic acid. If such diverse agents as these should interact to produce malformations, then it is possible that a wide variety of other agents might interact in a similar manner to produce neural tube defects. 相似文献
104.
Brain Cell Biology - 440 kD ankyrin-B and 480/270 kD ankyrin-G are membrane skeletal proteins with closely related biochemical properties yet distinctive physiological roles in axons. These... 相似文献
105.
106.
Morán-Barrio J González JM Lisa MN Costello AL Peraro MD Carloni P Bennett B Tierney DL Limansky AS Viale AM Vila AJ 《The Journal of biological chemistry》2007,282(25):18286-18293
Metallo-beta-lactamases (MbetaLs) are zinc-dependent enzymes able to hydrolyze and inactivate most beta-lactam antibiotics. The large diversity of active site structures and metal content among MbetaLs from different sources has limited the design of a pan-MbetaL inhibitor. Here we report the biochemical and biophysical characterization of a novel MbetaL, GOB-18, from a clinical isolate of a Gram-negative opportunistic pathogen, Elizabethkingia meningoseptica. Different spectroscopic techniques, three-dimensional modeling, and mutagenesis experiments, reveal that the Zn(II) ion is bound to Asp120, His121, His263, and a solvent molecule, i.e. in the canonical Zn2 site of dinuclear MbetaLs. Contrasting all other related MbetaLs, GOB-18 is fully active against a broad range of beta-lactam substrates using a single Zn(II) ion in this site. These data further enlarge the structural diversity of MbetaLs. 相似文献
107.
Gardner BR Strickland MJ Correa A 《Birth defects research. Part A, Clinical and molecular teratology》2007,79(7):559-564
BACKGROUND: Although many birth defects surveillance programs incorporate georeferenced records into their databases, practical methods for routine spatial surveillance are lacking. We present a macroprogram written for the software package R designed for routine exploratory spatial analysis of birth defects data, the Automated Spatial Surveillance Program (ASSP), and present an application of this program using spina bifida prevalence data for metropolitan Atlanta. METHODS: Birth defects surveillance data were collected by the Metropolitan Atlanta Congenital Defects Program. We generated ASSP maps for two groups of years that correspond roughly to the periods before (1994-1998) and after (1999-2002) folic acid fortification of flour. ASSP maps display census tract-specific spina bifida prevalence, smoothed prevalence contours, and locations of statistically elevated prevalence. We used these maps to identify areas of elevated prevalence for spina bifida. RESULTS: We identified a large area of potential concern in the years following fortification of grains and cereals with folic acid. This area overlapped census tracts containing large numbers of Hispanic residents. CONCLUSIONS: The potential utility of ASSP for spatial disease monitoring was demonstrated by the identification of areas of high prevalence of spina bifida and may warrant further study and monitoring. We intend to further develop ASSP so that it becomes practical for routine spatial monitoring of birth defects. 相似文献
108.
Structural organization and a standardized nomenclature for plant endo-1,4-beta-glucanases (cellulases) of glycosyl hydrolase family 9 下载免费PDF全文
109.
Helen E Gruber Igor J Puzanov Michael Bennett Vinay Kumar Brian Gordon 《BMC cell biology》2001,2(1):3
Background
Although the role of the osteoclast in bone resorption is becoming better understood, much remains to be learned about osteoclastogenesis and the exact mechanism of action of anti-resorbing agents such as 17β-estradiol. This study investigated bone and morphologic osteoclast alterations following long-term estrogen administration to the B6D2F1 mouse. B6D2F1 mice aged 4-5 weeks were exposed to high levels of estrogen via implanted silastic tubing for at least 12 weeks; controls received empty tubing. Femurs of control and treated mice were assessed with radiology, quantitative histomorphometry and transmission electron microscopy. 相似文献110.
C5L2, a nonsignaling C5A binding protein 总被引:11,自引:0,他引:11
Okinaga S Slattery D Humbles A Zsengeller Z Morteau O Kinrade MB Brodbeck RM Krause JE Choe HR Gerard NP Gerard C 《Biochemistry》2003,42(31):9406-9415