全文获取类型
收费全文 | 45065篇 |
免费 | 3320篇 |
国内免费 | 11篇 |
出版年
2023年 | 280篇 |
2022年 | 355篇 |
2021年 | 650篇 |
2020年 | 619篇 |
2019年 | 661篇 |
2018年 | 1183篇 |
2017年 | 1025篇 |
2016年 | 1602篇 |
2015年 | 2297篇 |
2014年 | 2299篇 |
2013年 | 3010篇 |
2012年 | 3949篇 |
2011年 | 3608篇 |
2010年 | 2013篇 |
2009年 | 1647篇 |
2008年 | 2781篇 |
2007年 | 2715篇 |
2006年 | 2618篇 |
2005年 | 2298篇 |
2004年 | 2248篇 |
2003年 | 2039篇 |
2002年 | 1923篇 |
2001年 | 871篇 |
2000年 | 937篇 |
1999年 | 556篇 |
1998年 | 344篇 |
1997年 | 201篇 |
1996年 | 238篇 |
1995年 | 244篇 |
1994年 | 209篇 |
1993年 | 172篇 |
1992年 | 223篇 |
1991年 | 210篇 |
1990年 | 175篇 |
1989年 | 177篇 |
1988年 | 130篇 |
1987年 | 121篇 |
1986年 | 121篇 |
1985年 | 131篇 |
1984年 | 163篇 |
1983年 | 98篇 |
1982年 | 117篇 |
1981年 | 84篇 |
1980年 | 84篇 |
1979年 | 86篇 |
1978年 | 83篇 |
1976年 | 93篇 |
1975年 | 70篇 |
1974年 | 65篇 |
1973年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
The in situ localization of Ca2+ in stage I sporangiophores of the fungus Phycomyces blakesleeanus was achieved with the potassium pyroantimonate technique. Precipitates of calcium-antimonate were present in mitochondria, vacuoles, endoplasmic reticulum and adjacent cytoplasm, Golgi-like bodies, and nuclei but not cell walls. Material treated with the calcium chelator EGTA lacked these precipitates. The preferential localization of Ca2+ in mitochondria, endoplasmic reticulum and vacuoles suggests that these organelles modulate the level of this cation in sporangiophores of P. blakesleeanus.Abbreviations EGTA
ethyleneglycol-bis-(-aminoethyl ether) N,N, tetraacetic acid 相似文献
102.
On the solution of mathematical models of herd immunity in human helminth infections 总被引:1,自引:0,他引:1
Marco V. José 《Journal of mathematical biology》1989,27(6):707-715
The general solution of the mathematical model of herd immunity to human helminth infections recently proposed by Anderson and May [3] is obtained. The numerical solution of a more accurate biological model is indistinguishable from the corresponding exact solution of a more tractable mathematical model. Computer simulations of some particular cases of this model support the notion that both ecological and immunological factors determine the observed convex patterns of age-prevalence and age-intensity curves of human helminth infections.This work was made thanks to the advise and support of Dr. Robert M. May while the author was Postdoctoral Fellow at Princeton University 相似文献
103.
Carmen Di Franco Claudio Pisano Patrizio Dimitri Silvia Gigliotti Nikolaj Junakovic 《Chromosoma》1989,98(6):402-410
The genomic distribution of elements of the copia, 412, B 104, mdg 1, mdg 4 and 1731 transposon families was compared by the Southern technique in DNA preparations extracted from brains, salivary glands and adult flies of two related Drosophila lines. The copia, 412 and mdg 1 sequences were also probed in DNA from sperm, embryos, and 1st and 2nd instar larvae. The homogeneity of the patterns observed shows that somatic transposition is unlikely to occur frequently. A correlation between mobility and the euchromatic or heterochromatic location of transposable elements is discussed. In addition, an explanation of the variable band intensities of transposable elements in Southern autoradiographs is proposed. 相似文献
104.
Nora Goosen Harold P. A. Horsman René G. M. Huinen Arjan de Groot Pieter van de Putte 《Antonie van Leeuwenhoek》1989,56(1):85-91
From a gene bank of theAcinetobacter calcoaceticus genome a plasmid was isolated that complements four different classes of PQQ- mutants. Subclones of this plasmid revealed that the four corresponding PQQ genes are located on a fragment of 5 kilobases. The nucleotide sequence of this 5 kb fragment was determined and by means of Tn5 insertion mutants the reading frames of the PQQ genes could be identified. Three of the PQQ genes code for proteins of Mr 29700 (gene I), Mr 10800 (gene II) and Mr 43600 (gene III) respectively. In the DNA region where gene IV was mapped however the largest possible reading frame encodes for a polypeptide of only 24 amino acids. A possible role for this small polypeptide will be discussed. Finally we show that expression of the four PQQ genes inAcinetobacter lwoffi andEscherichia coli lead to the synthesis of the coenzyme in these organisms. 相似文献
105.
L R Chen C J Yuan G Somasekhar P Wejksnora J E Peterson A M Myers L Graves P T Cohen E F da Cruz e Silva D J Graves 《Biochemical and biophysical research communications》1989,161(2):746-753
A cDNA encoding the entire tau subunit of rabbit skeletal muscle phosphorylase kinase was reconstructed and inserted into a plasmid containing the Escherichia coli ptac promoter and a constructed plasmid containing the ptac promoter and bacterial chloramphenicol acetyl transferase (CAT) gene, respectively. A significant phosphorylase kinase activity was found, in the first case. In the second case, a fused protein containing 73 amino acids from the CAT protein was obtained. After renaturation, the CAT-tau subunit protein shows enzymatic activity similar to the HPLC-purified and renatured tau subunit. 相似文献
106.
The major type-1 protein phosphatase catalytic subunits are the same gene products in rabbit skeletal muscle and rabbit liver 总被引:3,自引:0,他引:3
P T Cohen D L Schelling O B da Cruz e Silva H M Barker P Cohen 《Biochimica et biophysica acta》1989,1008(1):125-128
The catalytic subunit of protein phosphatase-1 (PP1) isolated from rabbit liver had the same electrophoretic mobility as, and yielded peptide maps identical to those of the 33 kDa form of rabbit skeletal muscle PP1. The predicted amino-acid sequences of PP1 obtained from three rabbit liver cDNA clones were identical to that of PP1 alpha from rabbit skeletal muscle. These findings suggest that the distinctive substrate specificities and regulatory properties of hepatic and skeletal muscle type-1 protein phosphatases are not conferred by the catalytic subunits themselves, but by regulatory subunits that are complexed to the catalytic subunits in vivo. 相似文献
107.
An alkyl hydroperoxide reductase induced by oxidative stress in Salmonella typhimurium and Escherichia coli: genetic characterization and cloning of ahp 总被引:21,自引:12,他引:9 下载免费PDF全文
G Storz F S Jacobson L A Tartaglia R W Morgan L A Silveira B N Ames 《Journal of bacteriology》1989,171(4):2049-2055
The ahp genes encoding the two proteins (F52a and C22) that make up an alkyl hydroperoxide reductase were mapped and cloned from Salmonella typhimurium and Escherichia coli. Two classes of oxidant-resistant ahp mutants which overexpress the two proteins were isolated. ahp-1 was isolated in a wild-type background and is dependent on oxyR, a positive regulator of defenses against oxidative stress. ahp-2 was isolated in an oxyR deletion background and is oxyR independent. Transposons linked to ahp-1 and ahp-2 or inserted in ahp mapped the genes to 13 min on the S. typhimurium chromosome, 59% linked to ent. Deletions of ahp obtained in both S. typhimurium and E. coli resulted in hypersensitivity to killing by cumene hydroperoxide (an alkyl hydroperoxide) and elimination of the proteins F52a and C22 from two-dimensional gels and immunoblots. ahp clones isolated from both S. typhimurium and E. coli complemented the cumene hydroperoxide sensitivity of the ahp deletion strains and restored expression of the F52a and C22 proteins. A cis-acting element required for oxyR-dependent, rpoH-independent heat shock induction of the F52a protein was present at the S. typhimurium but not the E. coli ahp locus. 相似文献
108.
109.
Josée Harel Linda Duplessis Jeffrey S. Kahn Michael S. DuBow 《Archives of microbiology》1990,154(1):67-72
The 37,000 bp double-stranded DNA genome of bacteriophage Mu behaves as a plaque-forming transposable element of Escherichia coli. We have defined the cis-acting DNA sequences required in vivo for transposition and packaging of the viral genome by monitoring the transposition and maturation of Mu DNA-containing pSC101 and pBR322 plasmids with an induced helper Mu prophage to provide the trans-acting functions. We found that nucleotides 1 to 54 of the Mu left end define an essential domain for transposition, and that sequences between nucleotides 126 and 203, and between 203 and 1,699, define two auxiliary domains that stimulate transposition in vivo. At the right extremity, the essential sequences for transposition require not more than the first 62 base pairs (bp), although the presence of sequences between 63 and 117 bp from the right end increases the transposition frequency about 15-fold in our system. Finally, we have delineated the pac recognition site for DNA maturation to nucleotides 32 to 54 of the Mu left end which reside inside of the first transposase binding site (L1) located between nucleotides 1–30. Thus, the transposase binding site and packaging domains of bacteriophage Mu DNA can be separated into two well-defined regions which do not appear to overlap.Abbreviations
attL
attachment site left
-
attR
attachment site right
- bp
base pairs
- Kb
kilobase pair
- nt
nucleotide
- Pu
Purine
- Py
pyrimidine
- Tn
transposable element
State University of New York, Downstate Medical Center, Brooklyn, NY 11204 USA 相似文献
110.
Cecilia Gotor Eloísa Pajuelo Luís C. Romero Antonio J. Márquez José M. Vega 《Archives of microbiology》1990,153(3):230-234
Polyclonal antiserum specific for ferredoxin-nitrite reductase (EC 1.7.7.1) from the green alga Chlamydomonas reinhardii recognized the nitrite reductase from other green algae, but did not cross-react with the corresponding enzyme from different cyanobacteria or higher plant leaves. An analogous situation was also found for ferredoxin-glutamate synthase (EC 1.4.7.1), using its specific antiserum. Besides, the antibodies raised against C. reinhardii ferredoxin-glutamate synthase were able to inactivate the ferredoxin-dependent activity of nitrite reductase from green algae.These results suggest that there exist similar domains in ferredoxin-nitrite reductases and ferredoxin-glutamate synthases from green algae. In addition, both types of enzymes share common antigenic determinants, probably located at the ferredoxin-binding domain. In spite of their physicochemical resemblances, no apparent antigenic correlation exists between the corresponding enzymes from green algae and those from higher plant leaves or cyanobacteria.Abbreviations Fd
ferredoxin
- GOGAT
glutamate synthase
- MV+
reduced methyl viologen (radical cation)
- NiR
nitrite reductase
- PMSF
phenylmethylsulphonyl fluoride
- SDS
sodium dodecyl sulfate 相似文献