全文获取类型
收费全文 | 337篇 |
免费 | 33篇 |
出版年
2021年 | 3篇 |
2020年 | 1篇 |
2018年 | 3篇 |
2017年 | 6篇 |
2016年 | 5篇 |
2015年 | 7篇 |
2014年 | 11篇 |
2013年 | 18篇 |
2012年 | 14篇 |
2011年 | 9篇 |
2010年 | 11篇 |
2009年 | 14篇 |
2008年 | 8篇 |
2007年 | 11篇 |
2006年 | 12篇 |
2005年 | 16篇 |
2004年 | 8篇 |
2003年 | 9篇 |
2002年 | 17篇 |
2001年 | 12篇 |
2000年 | 10篇 |
1999年 | 10篇 |
1998年 | 13篇 |
1997年 | 10篇 |
1996年 | 13篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 9篇 |
1985年 | 2篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 5篇 |
1972年 | 1篇 |
1971年 | 4篇 |
1970年 | 3篇 |
1969年 | 1篇 |
排序方式: 共有370条查询结果,搜索用时 46 毫秒
1.
J M Trzaskos M F Favata R T Fischer S H Stam 《The Journal of biological chemistry》1987,262(25):12261-12268
Biphasic modulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) has been demonstrated in primary hepatocyte cultures treated with the lanosterol 14 alpha-methyl demethylase inhibitor miconazole. At concentrations of the drug which lead to suppressed levels of reductase activity, the appearance of a polar, mevalonate-derived sterol is noted. Cochromatography of the identified sterol with 3 beta-hydroxylanost-8-en-32-aldehyde tentatively identified the metabolite as a lanosterol 14 alpha-methyl demethylation intermediate. Subsequent isolation and characterization of the metabolite by gas chromatography/mass spectroscopy confirmed this structural assignment. When the lanosterol 14 alpha-methyl demethylase-deficient mutant, AR45, was treated with authentic metabolite, a suppression of HMG-CoA reductase was observed. These results demonstrate that metabolism of the oxygenated biosynthetic intermediate is not required to suppress reductase activity. The results also strongly support the hypothesis that oxygenated 14 alpha-methyl demethylase intermediates are endogenously generated modulators of HMG-CoA reductase activity. 相似文献
2.
In traditional chlorophytan systems the organizational level was the primary character for the distinction of main groups (classes and orders). For instance, in Fott (1971), the flagellate level corresponds with the Volvocales, the coccoid level with the Chlorococcales, the filamentous level with the Ulotrichales, the siphonocladous level with the Siphonocladales, and the siphonous level with the Bryopsidales. The new system presented here is an elaboration and emendation of recently proposed taxonomies and their underlying phylogenetic hypotheses, and it is mainly based on ultrastructural features which have become available over the last 15 years. The following criteria are used for the distinction of classes and orders: (1) architecture of the flagellate cell (flagellate cells are considered as the depositories of primitive characters); (2) type of mitosis-cytokinesis; (3) place of meiosis in the life history and, consequently, the sexual life history type; (4) organizational level and thallus architecture; (5) habitat type (marine versus feshwater and terrestrial); (6) chloroplast type. The following classes are presented: Prasinophyceae, Chlamydophyceae, Ulvophyceae (orders Codiolales, Ulvales, Cladophorales, Bryopsidales, Dasycladales), Pleurastrophyceae (?), Chlorophyceae s.s. (orders Cylindrocapsales, Oedogoniales, Chaetophorales), Zygnematophyceae, Trentepohliophyceae, Charophyceae (orders Klebsormidiales, Coleochaetales, Charales). The new system no longer reflects the traditional hypothesis of a stepwise evolutionary progression of organizational levels in which the flagellate level represents the most primitive lineage, the coccoid and sarcinoid levels lineages of intermediate derivation, and the filamentous, siphonocladous and siphonous levels the most derived lineages. Instead, it is now hypothesized that these levels have arisen over and over again in different chlorophytan lineages which are primarily characterized by their type of flagellate cell. The flagellate green algal classes Prasinophyceae (with organic body scales) and Chlamydophyceae probably represent bundles of highly conservative lineages that diverged very long ago. Consequently, extant genera and species in these classes can be expected to have emerged long ago. Fossil evidence points to a minimum age of 600 Ma of certain extant Prasinophycean genera, and molecular evidence to a minimum age of 400–500 Ma of a fewChlamydomonas species. On the contrary, the most derived “green algal” lineage, the Angiosperms, can be expected to consist of, on average, much younger genera and species. Fossil evidence points to a minimum age of genera of 5–60 Ma. Lineages of intermediate evolutionary derivation (Ulvophyceae, Chlorophyceae, Charophyceae) can be expected to encompass genera and species of intermediate age. Fossil and (limited) molecular evidence point to a minimum age of 230–70 Ma of extant genera in Bryopsidales, Dasycladales and Cladophorales (Ulvophyceae) and of 250–80 Ma of extant genera in Charales (Charophyceae). 相似文献
3.
4.
Sulfur dynamics of two Spodosols were ascertained using soil columns constructed from homogenized mineral soil from nothern hardwood ecosystems at the Huntington Forest (HF) in the Adirondack Mountains of New York and Bear Brook Watershed in Maine (BBWM). Columns were leached for 20 weeks with a simulated throughfall solution with35SO4
2-. Sulfur constituents were similar to those of other Spodosols, with the organic S fractions (C-bonded S and ester sulfate) constituting over 90% of total S. HF soil columns had higher total S (14.9 mol S g-1) than that for the BBWM soil columns (7.4 mol g-1) primarily due to higher C-bonded S in the former.Initially, adsorbed SO4
- accounted for 5 and 4% of total S for the BBWM and HF soil columns, respectively. After 20 weeks, adsorbed SO4
2- decreased (81%) in BBWM and increased (33%) in HF soil columns. For both HF and BBWM soil columns, C-bonded S increased and ester sulfate decreased, but only for HF columns was there a net mineralization of organic S (5.6% of total S). The greatest decrease in ester sulfate occurred at the top of the columns.Leaching of35S was less than 0.5% of the35S added due to its retention in various S constituents. There was an exponential decrease in35S with column depth and most of the radioisotope was found in C-bonded S (70–88 and 70–91% for BBWM and HF, respectively). The rapid turnover of adsorbed SO2-
4 was reflected in its high specific activity (834 and 26 kBq mol-1 S for BBWM and HF, respectively). The lower specific activity of adsorbed SO4
2- in HF was attributable to greater isotopic dilution by non-radioactive SO2-
4 derived from greater organic S mineralization in the HF versus the BBWM columns.Both soil columns initially had high levels of NO-
3 which resulted in the generation of H+ and net retention of SO4
2- in the early phase of the experiment due to pH dependent sulfate adsorption; later NO3
- decreased and SO4
2- was desorbed. Leaching of NIO3
- and SO4
2- was correlated with losses of Mg2+ and Ca2+ of which the latter was the dominant cation.Analyses using both S mass balances and radioisotopes corroborate that for BBWM soil columns, SO2-
4 adsorption-desorption dominated the S biogeochemistry while in HF soil columns, organic S mineralization-immobilization processes were more important. It is suggested that similar techniques can be applied to soils in the field to ascertain the relative importances of SO4
2- adsorption processes and organic S dynamics. 相似文献
5.
Studies on the relationship between insect pests and their predators and parasites were conducted in the Syrian cotton agro-ecosystem
from 1980 up to 1983.Earias insulana (Boisduval) was found to be the most damaging pest, whileHeliothis armigera (Hübner) was not an annual returning problem. Among the entomophagous insects found, coccinellids were most numerous during
the months June and July and the hemipterous predators were more abundant during August and September.
When predator numbers were reduced with insecticide applications, a significant increase in the bollworm populations occurred,
resulting in significant reductions in seed cotton yields. Parasites were relatively not important for the control of lepidopterous
pests on cotton. Egg and larva populations ofE. insulana andH. armigera were parasitized byTrichogramma spp. andHabrobracon brevicornis Wesmael respectively. Populations ofBemisia tabaci (Gennadius) were heavily parasitized byEretmocerus mundus Mercet.
相似文献
6.
7.
WhileEscherichia coli is common as a commensal organism in the distal ileum and colon, the presence of colonization factors (CF) on pathogenic strains ofE. coli facilitates attachment of the organism to intestinal receptor molecules in a species- and tissue-specific fashion. After the initial adherence, colonization occurs, and the involvement of additional virulence determinants leads to illness. EnterotoxigenicE. coli (ETEC) is the most extensively studied of the five categories ofE. coli that cause diarrheal disease, and has the greatest impact on health worldwide. ETEC can be isolated from domestic animals and humans. The biochemistry, genetics, epidemiology, antigenic characteristics, and cell and receptor binding properties of ETEC have been extensively described. Another major category, enteropathogenicE. coli (EPEC), has virulence mechanisms, primarily effacement and cytoskeletal rearrangement of intestinal brush borders, that are distinct from ETEC. An EPEC CF receptor has been purified and characterized as a sialidated transmembrane glycoprotein complex directly attached to actin, thereby associating CF-binding with host-cell response. Three, additional categories ofE. coli diarrheal disease, their colonization factors and their host cell receptors are discussed. It appears that biofilms exist in the intestine in a manner similar to oral bacterial biofilms, and thatE. coli is part of these biofilms as both commensals and pathogens.Abbreviations CF
colonization factor
- CFA
Colonization Factor Antigen
- CS
coli-surface-associated antigen
- EAggEC
enteroaggregativeE. coli
- ECDD
E. coli diarrheal disease
- EHEC
enterohemorrhagicE. coli
- EIEC
enteroinvasiveE. coli
- EPEC
enteropathogenicE. coli
- ETEC
enterotoxigenicE. coli
- Gal
galactose
- GalNAc
N-acetyl galactosamine
- LT
heat-labile toxin
- NeuAc
N-acetyl neuraminic acid
- PCF
Putative colonization factor
- RBC
red blood cells
- SLT
Shiga-like toxin
- ST
heat-stable toxin 相似文献
8.
R. Weide M. F. van-Wordragen R. K. Lankhorst R. Verkerk C. Hanhart T. Liharska E. Pap P. Stam P. Zabel M. Koornneef 《Genetics》1993,135(4):1175-1186
In the past, a classical map of the tomato genome has been established that is based on linkage data from intraspecific Lycopersicon esculentum crosses. In addition, a high density molecular linkage map has recently been constructed using a L. esculentum X L. pennellii cross. As the respective maps only partially match, they provide limited information about the relative positions of classical and molecular markers. In this paper we describe the construction of an integrated linkage map of tomato chromosome 6 that shows the position of cDNA-, genomic DNA- and RAPD markers relative to 10 classical markers. Integration was achieved by using a L. esculentum line containing an introgressed chromosome 6 from L. pennellii in crosses to a variety of L. esculentum marker lines. In addition, an improved version of the classical linkage map is presented that is based on a combined analysis of new linkage data for 16 morphological markers and literature data. Unlike the classical map currently in use, the revised map reveals clustering of markers into three major groups around the yv, m-2 and c loci, respectively. Although crossing-over rates are clearly different when comparing intraspecific L. esculentum crosses with L. esculentum X L. pennellii crosses, the clusters of morphological markers on the classical map coincide with clusters of genomic- and cDNA-markers on the molecular map constructed by Tanksley and coworkers. 相似文献
9.
Tamara Schmey Corinn Small Severin Einspanier Lina Muñoz Hoyoz Tahir Ali Soledad Gamboa Betty Mamani German C. Sepulveda Marco Thines Remco Stam 《Environmental microbiology》2023,25(10):1830-1846
The wild relatives of modern tomato crops are native to South America. These plants occur in habitats as different as the Andes and the Atacama Desert and are, to some degree, all susceptible to fungal pathogens of the genus Alternaria. Alternaria is a large genus. On tomatoes, several species cause early blight, leaf spots and other diseases. We collected Alternaria-like infection lesions from the leaves of eight wild tomato species from Chile and Peru. Using molecular barcoding markers, we characterized the pathogens. The infection lesions were caused predominantly by small-spored species of Alternaria of the section Alternaria, like A. alternata, but also by Stemphylium spp., Alternaria spp. from the section Ulocladioides and other related species. Morphological observations and an infection assay confirmed this. Comparative genetic diversity analyses show a larger diversity in this wild system than in studies of cultivated Solanum species. As A. alternata has been reported to be an increasing problem in cultivated tomatoes, investigating the evolutionary potential of this pathogen is not only interesting to scientists studying wild plant pathosystems. It could also inform crop protection and breeding programs to be aware of potential epidemics caused by species still confined to South America. 相似文献
10.
DNA sequences were determined for three to five alleles of the bride-of-
sevenless (boss) gene in each of four species of Drosophila. The product of
boss is a transmembrane receptor for a ligand coded by the sevenless gene
that triggers differentiation of the R7 photoreceptor cell in the compound
eye. Population parameters affecting the rate and pattern of molecular
evolution of boss were estimated from the multinomial configurations of
nucleotide polymorphisms of synonymous codons. The time of divergence
between D. melanogaster and D. simulans was estimated as approximately 1
Myr, that between D. teissieri and D. yakuba as approximately 0.75 Myr, and
that between the two pairs of sibling species as approximately 2 Myr. (The
boss genes themselves have estimated divergence times approximately 50%
greater than the species divergence times.) The effective size of the
species was estimated as approximately 5 x 10(6), and the average mutation
rate was estimated as 1-2 x 10(-9)/nucleotide/generation. The ratio of
amino acid polymorphisms within species to fixed differences between
species suggests that approximately 25% of all possible single-step amino
acid replacements in the boss gene product may be selectively neutral or
nearly neutral. The data also imply that random genetic drift has been
responsible for virtually all of the observed differences in the portion of
the boss gene analyzed among the four species.
相似文献