全文获取类型
收费全文 | 1310篇 |
免费 | 185篇 |
国内免费 | 1篇 |
出版年
2022年 | 11篇 |
2021年 | 12篇 |
2020年 | 14篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 47篇 |
2014年 | 39篇 |
2013年 | 49篇 |
2012年 | 69篇 |
2011年 | 45篇 |
2010年 | 41篇 |
2009年 | 37篇 |
2008年 | 46篇 |
2007年 | 67篇 |
2006年 | 69篇 |
2005年 | 67篇 |
2004年 | 61篇 |
2003年 | 76篇 |
2002年 | 53篇 |
2001年 | 29篇 |
2000年 | 27篇 |
1999年 | 26篇 |
1998年 | 17篇 |
1997年 | 15篇 |
1996年 | 12篇 |
1995年 | 14篇 |
1994年 | 9篇 |
1993年 | 17篇 |
1992年 | 17篇 |
1991年 | 22篇 |
1990年 | 25篇 |
1989年 | 22篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 26篇 |
1985年 | 16篇 |
1984年 | 18篇 |
1983年 | 20篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 17篇 |
1975年 | 17篇 |
1974年 | 23篇 |
1972年 | 18篇 |
1969年 | 8篇 |
1967年 | 7篇 |
排序方式: 共有1496条查询结果,搜索用时 15 毫秒
1.
2.
Prostaglandin E2-9- oxoreductase (PGE2-9-OR), the enzyme which converts prostaglandin E2 (PGE2) to prostaglandin F2 alpha (PGF2 alpha), has been detected in human decidua vera. A 105-fold purification was achieved when the centrifuged homogenate was fractionated sequentially by DEAE-Trisacryl, hydroxyapatite-agarose gel, ultrogel AcA 44 and Matrex gel blue A gel chromatographies. The following kinetic constants for PGE2-9-OR have been obtained. The equilibrium constant with respect to PGE2 is 83 microM, the Michaelis constant, Km, for PGE2 is 80 microM, for NADPH 1.6 microM. The maximal velocity for the forward reaction is V1 = .203 pmol/min. The enzyme was inhibited by progesterone, oestradiol-17 beta, cortisol and pharmaceutical drugs. An activating effect could be demonstrated with Ca2+ and oxytocin. The occurrence of PGE2-9-OR in the decidua vera suggests that this enzyme may be responsible for the transformation of PGE2 to PGF2 alpha in these tissues. This may be an important mechanism for the initiation and maintenance of uterine contractions. 相似文献
3.
4.
phi6 bacteriophage from Pseudomonas phaseolicola has a pleomorphic shape due to an outer layer of lipid. This layer was removed with organic solvents or sodium dodecyl sulfate revealing a 50-nm cubic particle. The virus was found only in the central portion of the cell and not near the cell wall. 相似文献
5.
Irene Jao Vicki Marsh Primus Che Chi Melissa Kapulu Mainga Hamaluba Sassy Molyneux Philip Bejon Dorcas Kamuya 《Bioethics》2020,34(8):819-832
Controlled human malaria infection (CHMI) studies involve the deliberate infection of healthy volunteers with malaria parasites under controlled conditions to study immune responses and/or test drug or vaccine efficacy. An empirical ethics study was embedded in a CHMI study at a Kenyan research programme to explore stakeholders’ perceptions and experiences of deliberate infection and moral implications of these. Data for this qualitative study were collected through focus group discussions, in-depth interviews and non-participant observation. Sixty-nine participants were involved, including CHMI study volunteers, community representatives and research staff. Data were managed using QSR Nvivo 10 and analysed using an inductive-deductive approach, guided by ethics literature. CHMI volunteers had reasonable understanding of the study procedures. Decisions to join were influenced by study incentives, trust in the research institution, their assessment of associated burdens and motivation to support malaria vaccine development. However, deliberate malaria infection was a highly unusual research strategy for volunteers, community representatives and some study staff. Volunteers’ experiences of physical, emotional and social burdens or harms were often greater than anticipated initially, and fluctuated over time, related to specific procedures and events. Although unlikely to deter volunteers' participation in similar studies in furture, we argue that the dissonance between level of understanding of the burdens involved and actual experiences are morally relevant in relation to community engagement, informed consent processes, and ongoing support for volunteers and research staff. We further argue that ethics oversight of CHMI studies should take account of these issues in deciding whether consent, engagement and the balance of benefits and harms are reasonable in a given context. 相似文献
6.
Matthew L. Knope Clifford W. Morden Vicki A. Funk Tadashi Fukami 《Journal of Biogeography》2012,39(7):1206-1216
Aim To estimate the rate of adaptive radiation of endemic Hawaiian Bidens and to compare their diversification rates with those of other plants in Hawaii and elsewhere with rapid rates of radiation. Location Hawaii. Methods Fifty‐nine samples representing all 19 Hawaiian species, six Hawaiian subspecies, two Hawaiian hybrids and an additional two Central American and two African Bidens species had their DNA extracted, amplified by polymerase chain reaction and sequenced for four chloroplast and two nuclear loci, resulting in a total of approximately 5400 base pairs per individual. Internal transcribed spacer sequences for additional outgroup taxa, including 13 non‐Hawaiian Bidens, were obtained from GenBank. Phylogenetic relationships were assessed by maximum likelihood and Bayesian inference. The age of the most recent common ancestor and diversification rates of Hawaiian Bidens were estimated using the methods of previously published studies to allow for direct comparison with other studies. Calculations were made on a per‐unit‐area basis. Results We estimate the age of the Hawaiian clade to be 1.3–3.1 million years old, with an estimated diversification rate of 0.3–2.3 species/million years and 4.8 × 10?5 to 1.3 × 10?4 species Myr?1 km?2. Bidens species are found in Europe, Africa, Asia and North and South America, but the Hawaiian species have greater diversity of growth form, floral morphology, dispersal mode and habitat type than observed in the rest of the genus world‐wide. Despite this diversity, we found little genetic differentiation among the Hawaiian species. This is similar to the results from other molecular studies on Hawaiian plant taxa, including others with great morphological variability (e.g. silverswords, lobeliads and mints). Main conclusions On a per‐unit‐area basis, Hawaiian Bidens have among the highest rates of speciation for plant radiations documented to date. The rapid diversification within such a small area was probably facilitated by the habitat diversity of the Hawaiian Islands and the adaptive loss of dispersal potential. Our findings point to the need to consider the spatial context of diversification – specifically, the relative scale of habitable area, environmental heterogeneity and dispersal ability – to understand the rate and extent of adaptive radiation. 相似文献
7.
T J Biden B Peter-Riesch W Schlegel C B Wollheim 《The Journal of biological chemistry》1987,262(8):3567-3571
The role of Ca2+ in the generation of inositol phosphates was investigated using rat pancreatic islets after steady state labeling with myo-[2-3H]inositol. Depolarizing K+ concentrations (24 mM) evoked early (2 s) increases in inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) and inositol 1,3,4,5-tetrakisphosphate (Ins-1,3,4,5-P4) as measured by high performance anion-exchange chromatography. The increase in Ins-1,4,5-P3 was transient and was followed by a more pronounced rise in Ins-1,3,4-P3. These effects were dependent on the presence of extracellular Ca2+ but were not secondary to release of either neurotransmitters or metabolites of arachidonic acid. K+ also promoted the breakdown of phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) and of the other phosphoinositides. Glucose (16.7 mM) was less marked in its effects but still promoted rapid increases in Ins-1,3,4,5-P4 (2 s) and Ins-1,4,5-P3 (10 s) and a slower rise in Ins-1,3,4-P3 (30 s). The levels of all three metabolites rose steadily over 10 min stimulation. These responses to glucose could be largely, although not entirely, inhibited by depletion of extracellular Ca2+ or by Ca2+ channel blockade with verapamil (20 microM). Carbamylcholine (0.5 mM) was the most potent stimulus used evoking early rises in Ins-1,4,5-P3 and Ins-1,3,4,5-P4 (2 s) followed by Ins-1,3,4-P3 (10 s), effects which were only partially dependent on extracellular Ca2+. The results suggest that a Ca2+-mediated PtdIns-4,5-P2 hydrolysis accounts for most of the Ins-1,4,5-P3 generated in response to glucose but not carbamylcholine. In addition, glucose may exert effects on inositol phosphate metabolism which are Ca2+ independent. 相似文献
8.
Exposure to caffeine and suppression of DNA replication combine to stabilize the proteins and RNA required for premature mitotic events 总被引:4,自引:0,他引:4
Caffeine had been shown to induce mitotic events in Syrian hamster fibroblast (BHK) cells that were arrested during DNA replication (Schlegel and Pardee, Science 232:1264-1266, 1986). Inhibition of protein synthesis blocked these caffeine-induced events, while inhibition of RNA synthesis showed little effect. We now report that the protein(s) that are required for inducing mitosis in these cells were synthesized shortly after caffeine addition, the activity was very labile in the absence of caffeine, and the activity was lost through an ATP-dependent mechanism. Caffeine dramatically increased the stability of these putative proteins while having no effect on overall protein degradation. Experiments with an inhibitor of RNA synthesis indicated that mitosis-related RNA had accumulated during the suppression of DNA replication, and this RNA was unstable when replication was allowed to resume. These results suggest that the stability of RNA needed for mitosis is regulated by the DNA replicative state of the cell and that caffeine selectively stabilizes the protein product(s) of this RNA. Conditions can therefore be selected that permit mitotic factors to accumulate in cells at inappropriate times in the cell cycle. Two-dimensional gel electrophoresis has demonstrated several protein changes resulting from caffeine treatment; their relevance to mitosis-inducing activity remains to be determined. 相似文献
9.
Three different proteins exhibiting NAD-dependent acetaldehyde dehydrogenase activity from Alcaligenes eutrophus 总被引:9,自引:0,他引:9
The existence of three different proteins exhibiting NAD-dependent acetaldehyde dehydrogenase activity was confirmed in Alicaligenes eutrophus. The fermentative alcohol dehydrogenase, which also exhibits acetaldehyde dehydrogenase activity, is one of these proteins. The other two proteins were purified from A. eutrophus N9A mutant AS4 grown on ethanol applying chromatography on DEAE-Sephacel and triazine-dye affinity media. Acetaldehyde dehydrogenase II, which amounts to about 14% of the total soluble protein in cells grown on ethanol, was purified to homogeneity. The relative molecular masses of the native enzyme and of the subunits were 195,000 or 56,000, respectively. This enzyme exhibits a high affinity for acetaldehyde (Km = 4 microM). Acetaldehyde dehydrogenase I amounts only to less than 1% of the total soluble protein. The relative molecular masses of the native enzyme and of the subunits were 185,000 and 52,000, respectively. This enzyme exhibits a low affinity for acetaldehyde (Km = 2.6 mM). Antibodies raised against acetaldehyde dehydrogenase II did not react with acetaldehyde dehydrogenase I. Two different strains, A. eutrophus N9A mutant AS1, which represents a different mutant type and can utilize both ethanol or 2,3-butanediol, and the type strain of A. eutrophus (TF93), which can utilize ethanol, form two acetaldehyde dehydrogenases during growth on ethanol, too. As in AS4, one of these enzymes from each strain amounted to a substantial portion of the total soluble protein in the cells. These major acetaldehyde dehydrogenases were purified from both strains; they resemble acetaldehyde dehydrogenase II isolated from AS4 in all relevant properties. Antibodies against the enzyme isolated from AS4 gave identical cross-reactions with the enzymes isolated from AS1 and TF93. 相似文献
10.
C Zaborosch K Schneider H G Schlegel H Kratzin 《European journal of biochemistry》1989,181(1):175-180
The cytoplasmic, NAD-linked hydrogenase of the Gram-positive hydrogen-oxidizing bacterium Nocardia opaca 1b was compared with the analogous enzyme isolated from the Gram-negative bacterium Alcaligenes eutrophus H16. The hydrogenase of N. opaca 1b was purified by a new procedure applying chromatography on phenyl-Sepharose and DEAE-Sephacel with two columns in series. A homogeneous enzyme preparation with a specific activity of 74 mumol H2 oxidized.min-1.mg protein-1 and a yield of 32% was isolated. The A. eutrophus enzyme was purified as previously published. Both enzymes are tetrameric proteins composed of four non-identical subunits (alpha, beta, gamma, delta). The four subunits of both of these enzymes were separated and isolated as single polypeptides by preparative polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Immunological comparison of the four subunits of the Nocardia hydrogenase with those of the Alcaligenes enzyme showed that the alpha, beta, gamma, and delta subunits of one organism were serologically related to the analogous subunits of the other organism. Among themselves, the four subunits do not have any serological relationship. The eight individual polypeptides were also compared with respect to the NH2-terminal amino acid sequences determined by automated Edman degradation and to the amino acid compositions. Strong sequence similarities exist between the analogous subunits isolated from the two bacteria. Within the established N-terminal sequences the similarities between both alpha, beta, gamma and delta subunits amount to 63%, 79%, 80% and 65%, respectively. No similarities exist between the different, non-analogous subunits alpha, beta, gamma and delta. 相似文献