全文获取类型
收费全文 | 5747篇 |
免费 | 640篇 |
出版年
2022年 | 52篇 |
2021年 | 93篇 |
2020年 | 60篇 |
2019年 | 64篇 |
2018年 | 80篇 |
2017年 | 83篇 |
2016年 | 132篇 |
2015年 | 215篇 |
2014年 | 213篇 |
2013年 | 317篇 |
2012年 | 350篇 |
2011年 | 380篇 |
2010年 | 246篇 |
2009年 | 200篇 |
2008年 | 285篇 |
2007年 | 267篇 |
2006年 | 249篇 |
2005年 | 234篇 |
2004年 | 231篇 |
2003年 | 224篇 |
2002年 | 225篇 |
2001年 | 120篇 |
2000年 | 106篇 |
1999年 | 118篇 |
1998年 | 68篇 |
1997年 | 56篇 |
1996年 | 74篇 |
1995年 | 56篇 |
1994年 | 60篇 |
1993年 | 66篇 |
1992年 | 68篇 |
1991年 | 55篇 |
1990年 | 71篇 |
1989年 | 89篇 |
1988年 | 59篇 |
1987年 | 42篇 |
1986年 | 48篇 |
1985年 | 50篇 |
1984年 | 52篇 |
1983年 | 44篇 |
1982年 | 51篇 |
1981年 | 42篇 |
1980年 | 39篇 |
1979年 | 49篇 |
1978年 | 40篇 |
1977年 | 37篇 |
1976年 | 52篇 |
1975年 | 36篇 |
1974年 | 55篇 |
1973年 | 46篇 |
排序方式: 共有6387条查询结果,搜索用时 31 毫秒
101.
We have investigated the substrate subsite recognition requirement of the xyloglucan endo-transglycosylase/xyloglucan-specific endo-(14)--d-glucanase (NXET) from the cotyledons of nasturtium seedlings. Seed xyloglucans are composed almost entirely of the Glc4 subunits XXXG, XLXG, XXLG and XLLG, where G represents an unsubstituted glucose residue, X a xylose-substituted glucose residue and L a galactosyl-xylose-substituted glucose residue. Thus in the xyloglucan sequence shown below, the xylose (Xyl) residues at the backbone glucose (Glc) residues numbered — 3,— 2, + 2 and + 3 may be galactose-substituted, and NXET cleaves between the unsubstituted glucose at — 1 and the xylose-substituted glucose at + 1, which never carries a galactosyl substituent. We have isolated the xyloglucan oligosaccharides XXXGXXXG and XLLGXLLG from NXET digests of tamarind seed xyloglucan, have modified them enzymatically using a pure xyloglucan oligosaccharide-specific -xylosidase from nasturtium seeds to give GXXGXXXG and GLLGXLLG, and have identified and compared the products of NXET action on XXXGXXXG, GXXGXXXG, XLLGXLLG and GLLGXLLG. We have also compared the molar proportions of XXXG, XLXG, XXLG and XLLG in native tamarind and nasturtium seed xyloglucans with those in NXET digests of these polysaccharides. Using these and existing data we have demonstrated that NXET action does not require xylosesubstitution at glucose residues — 4, — 2, + 1 and + 3 and that xylose substitution at + 2, is a requirement. There may also be a requirement for xylose substitution at — 3. We have demonstrated also that galactosyl substitution of a xylose residue at + 1 prevents, and at — 2 modifies, chain-cleavage. A partial model for the minimum substrate binding requirement of NXET is proposed.Abbreviations G
unsubstituted glucose residue
- X
xylose-substituted glucose residue
- L
galactosylxylose-substituted glucose residue
- F
fucosyl-galactosylxylose-substituted glucose residue
- Gal
galactose
- Glc
glucose
- HPAE
high-performance anion-exchange chromatography
- NXET
nasturtium xyloglucan endo-transglycosylase or xyloglucan-specific endo-(14)--d-glucanase
- Xyl
xylose
This work was funded jointly by Unilever UK and the Department of Trade and Industry (UK) via the LINK initiative Agro-Food Quality. 相似文献
102.
Inactivation of Mycobacterium paratuberculosis in cows' milk at pasteurization temperatures. 总被引:7,自引:2,他引:5 下载免费PDF全文
The thermal inactivation of 11 strains of Mycobacterium paratuberculosis at pasteurization temperatures was investigated. Cows' milk inoculated with M. paratuberculosis at two levels (10(7) and 10(4) CFU/ml) was pasteurized in the laboratory by (i) a standard holder method (63.5 degrees C for 30 min) and (ii) a high-temperature, short-time (HTST) method (71.7 degrees C for 15 s). Additional heating times of 5, 10, 15, 20, and 40 min at 63.5 degrees C were included to enable the construction of a thermal death curve for the organism. Viability after pasteurization was assessed by culture on Herrold's egg yolk medium containing mycobactin J (HEYM) and in BACTEC Middlebrook 12B radiometric medium supplemented with mycobactin J and sterile egg yolk emulsion. Confirmation of acid-fast survivors of pasteurization as viable M. paratuberculosis cells was achieved by subculture on HEYM to indicate viability coupled with PCR using M. paratuberculosis-specific 1S900 primers. When milk was initially inoculated with 10(6) to 10(7) CFU of M. paratuberculosis per ml, M. paratuberculosis cells were isolated from 27 of 28 (96%) and 29 of 34 (85%) pasteurized milk samples heat treated by the holder and HTST methods, respectively. Correspondingly, when 10(3) to 10(4) CFU of M. paratuberculosis per ml of milk were present before heat treatment, M. paratuberculosis cells were isolated from 14 of 28 (50%) and 19 of 33 (58%) pasteurized milk samples heat treated by the holder and HTST methods, respectively. The thermal death curve for M. paratuberculosis was concave in shape, exhibiting a rapid initial death rate followed by significant "tailing." Results indicate that when large numbers of M. paratuberculosis cells are present in milk, the organism may not be completely inactivated by heat treatments simulating holder and HTST pasteurization under laboratory conditions. 相似文献
103.
Isolation of Y Chromosome-Specific Sequences from Silene Latifolia and Mapping of Male Sex-Determining Genes Using Representational Difference Analysis 总被引:4,自引:0,他引:4 下载免费PDF全文
The genomic subtraction method representational difference analysis (RDA) was used to identify male-specific restriction fragments in the dioecious plant Silene latifolia. Male-specific restriction fragments are linked to the male sex chromosome (the Y chromosome). Four RDA-derived male-specific restriction fragments were used to identify polymorphisms in a collection of X-ray-generated mutant plants with either hermaphroditic or asexual flowers. Some of the mutants have cytologically detectable deletions in the Y chromosome that were correlated with loss of male-specific restriction fragments. One RDA-derived probe detected a restriction fragment present in all mutants, indicating that each has retained Y chromosomal DNA. The other three probes detected genomic fragments that were linked in a region deleted in some hermaphroditic and some asexual mutants. Based on the mutant phenotypes and the correlation of cytologically visible deletions with loss of male-specific restriction fragments, these markers were assigned to positions on the Y chromosome close to the carpel suppression locus. This RDA mapping also revealed a Y-linked locus, not previously described, which is responsible for early stamen development. 相似文献
104.
Peter E. Molloy Barbara M. Graham Pauline M. Cupit Steven D. Grant Andrew J. R. Porter Charles Cunningham 《Molecular biotechnology》1995,4(3):239-245
This work describes protocols for the production of single-chain antibody and T-cell receptor fragments inE. coli. A choice of methods is given for the purification of the recombinant fragments that rely on the use of either immunoaffinity
or metal chelate affinity chromatography. The TCR fragments may have to be denatured and refolded before the fragments attain
their proper conformation. 相似文献
105.
Abstract. Eragrostis intermedia (Plains lovegrass) is a midheight perennial bunchgrass native to semi-arid grasslands of the southwestern USA, that becomes an abundant and dominant component of these grasslands in areas long protected from livestock grazing. Substantial mortality of plains lovegrass occurred on a large livestock exclosure in southeastern Arizona, after a period of declining precipitation, but only in areas that had not burned in the previous three years. Lovegrass abundance subsequently increased on both undisturbed and burned sites, but remained substantially higher on the burned area. Long-term abundance of plains lovegrass may depend on episodic fire, particularly during periods of reduced precipitation. 相似文献
106.
Uri S. Ladror Gary T. Wang William L. Klein Thomas F. Holzman Grant A. Krafft 《Journal of Protein Chemistry》1994,13(4):357-366
Fluorogenic peptide substrates designed to encompass the reported-secretory and amyloidogenic cleavage sites of the amyloid- precursor protein (PP) were used to analyze proteinase activities in brain extracts from control patients and those with Alzheimer's disease (AD). Activity against the secretory substrate atpH 7.5 in control and AD brains produced a major endopeptidase cleavage at the Lys687-Leu688 bond (PP770 numbering), consistent with thePP secretase cleavage. Activity in control brains against the amyloidogenic substrate atpH 7.5 produced one cleavage at the Ala673-Glu674 bond, two residues C-terminal to the amyloidogenic Met-Asp site. However, in three of four AD brains, the major cleavage was at the Asp-Ala bond, one residue from the amyloidogenic site. Both endopeptidase and carboxypeptidase activities in AD brains were lower than in control brains. Proteinase activities against the secretory substrate had a major optimum atpH 3.0–4.0 and another atpH 6.0–7.5. Proteinase activities against the amyloidogenic substrate had a major optimum at or belowpH 3.0 and another atpH 6.0. Using both substrates, activities at lowpH were higher in AD brains than in controls, while atpH above 6.5, activities in control brains were higher than in AD. These results indicate that the levels of proteolytic enzymes in AD brains are altered relative to controls.Abbreviations A
Amyloid-
- ACN
acetonitrile
- AD
Alzheimer's disease
- PP
amyloid- precursor protein
- DABCYL
4-(4-dimethylaminophenylazo)-benzoic acid
- EDANS
5-{(2-aminoethyl)amino}napthalene-1-sulfonic acid
- MES
morpholinoethane sulfonic acid
- MOPS
morpholino-propane sulfonic acid
- RP-HPLC
reverse-phase high-performance liquid chromatography
- SDS-PAGE
sodium do-decyl sulfate-polyacrylamide gel electrophoresis
- TFA
tri-fluoroacetic acid
- Tris
tris(hydroxyethyl)aminomethane 相似文献
107.
James S. C. Gilchrist Michael P. Czubryt Grant N. Pierce 《Molecular and cellular biochemistry》1994,135(1):79-88
Calcium has long been known to play a role as a key cytoplasmic second messenger, but until relatively recently its possible involvement in nuclear signal transduction and the regulation of nuclear events has not been extensively studied. Evidence revealing the presence of transmembrane nuclear Ca2+ gradients and a variety of intranuclear Ca2+ binding proteins has fueled renewed interest in this key ion and its involvement in cell-cycle timing and division, gene expression, and protein activation. This review will offer an overview of the current state of knowledge and theory regarding calcium orchestration of nuclear functions and events and discuss possible future directions in this field of study. 相似文献
108.
Jason R. Grant 《Nordic Journal of Botany》1994,14(3):287-288
Xerxes is proposed as a new name to replace Alcantara Glaziou ex G. M Barroso (Asteraceae) (1969), not Alcantarea (Morren ex Mez) Harms (Bromeliaceae) (1929). The genus is monotypic, represented by Xerxes ekmanianum , comb. nov. 相似文献
109.
110.
A single amino acid change determines persistence of a chimeric Theiler's virus. 总被引:12,自引:10,他引:2 下载免费PDF全文
N Jarousse R A Grant J M Hogle L Zhang A Senkowski R P Roos T Michiels M Brahic A McAllister 《Journal of virology》1994,68(5):3364-3368
The DA strain of Theiler's virus persists in the central nervous system of mice and causes chronic inflammation and demyelination. On the other hand, the GDVII strain causes an acute encephalitis and does not persist in surviving animals. Series of recombinants between infectious cDNA clones of the genomes of DA and GDVII viruses have been constructed. The analysis of the phenotypes of the recombinant viruses has shown that determinants of persistence and demyelination are present in the capsid proteins of DA virus. Chimeric viruses constructed by the different research groups gave consistent results, with one exception. Chimeras GD1B-2A/DAFL3 and GD1B-2C/DAFL3, which contain part of capsid protein VP2, capsid proteins VP3 and VP1, and different portions of P2 of GDVII in a DA background, were able to persist and cause demyelination. Chimera R4, whose genetic map is identical to that of GD1B-2A/DAFL3, was not. After exchanging the viral chimeras between laboratories and verifying each other's observations, new chimeras were generated in order to explain this difference. Here we report that the discrepancy can be attributed to a single amino acid difference in the sequence of the capsid protein VP2 of the two parental DA strains. DAFL3 (University of Chicago) and the chimeras derived from it, GD1B-2A/DAFL3 and GD1B-2C/DAFL3, contain a Lys at position 141, while TMDA (Institut Pasteur) and R4, the chimera derived from it, contain an Asn in that position. This amino acid is located at the tip of the EF loop, on the rim of the depression spanning the twofold axis of the capsid. These results show that a single amino acid change can confer the ability to persist and demyelinate to a chimeric Theiler's virus, and they pinpoint a region of the viral capsid that is important for this phenotype. 相似文献