全文获取类型
收费全文 | 9669篇 |
免费 | 556篇 |
国内免费 | 7篇 |
专业分类
10232篇 |
出版年
2023年 | 81篇 |
2022年 | 141篇 |
2021年 | 307篇 |
2020年 | 173篇 |
2019年 | 197篇 |
2018年 | 263篇 |
2017年 | 236篇 |
2016年 | 300篇 |
2015年 | 417篇 |
2014年 | 480篇 |
2013年 | 643篇 |
2012年 | 679篇 |
2011年 | 633篇 |
2010年 | 392篇 |
2009年 | 326篇 |
2008年 | 438篇 |
2007年 | 396篇 |
2006年 | 379篇 |
2005年 | 373篇 |
2004年 | 326篇 |
2003年 | 254篇 |
2002年 | 267篇 |
2001年 | 208篇 |
2000年 | 183篇 |
1999年 | 170篇 |
1998年 | 65篇 |
1997年 | 56篇 |
1996年 | 65篇 |
1995年 | 54篇 |
1994年 | 62篇 |
1992年 | 119篇 |
1991年 | 120篇 |
1990年 | 92篇 |
1989年 | 98篇 |
1988年 | 88篇 |
1987年 | 92篇 |
1986年 | 87篇 |
1985年 | 92篇 |
1984年 | 82篇 |
1983年 | 50篇 |
1982年 | 50篇 |
1981年 | 43篇 |
1980年 | 49篇 |
1979年 | 81篇 |
1978年 | 58篇 |
1977年 | 50篇 |
1976年 | 42篇 |
1975年 | 38篇 |
1974年 | 39篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Probing the different chaperone activities of the bacterial HSP70-HSP40 system using a thermolabile luciferase substrate 总被引:1,自引:0,他引:1
During mild heat-stress, a native thermolabile polypeptide may partially unfold and transiently expose water-avoiding hydrophobic segments that readily tend to associate into a stable misfolded species, rich in intra-molecular non-native beta-sheet structures. When the concentration of the heat-unfolded intermediates is elevated, the exposed hydrophobic segments tend to associate with other molecules into large stable insoluble complexes, also called "aggregates." In mammalian cells, stress- and mutation-induced protein misfolding and aggregation may cause degenerative diseases and aging. Young cells, however, effectively counteract toxic protein misfolding with a potent network of molecular chaperones that bind hydrophobic surfaces and actively unfold otherwise stable misfolded and aggregated polypeptides. Here, we followed the behavior of a purified, initially mostly native thermolabile luciferase mutant, in the presence or absence of the Escherichia coli DnaK-DnaJ-GrpE chaperones and/or of ATP, at 22 °C or under mild heat-stress. We concomitantly measured luciferase enzymatic activity, Thioflavin-T fluorescence, and light-scattering to assess the effects of temperature and chaperones on the formation, respectively, of native, unfolded, misfolded, and/or of aggregated species. During mild heat-denaturation, DnaK-DnaJ-GrpE+ATP best maintained, although transiently, high luciferase activity and best prevented heat-induced misfolding and aggregation. In contrast, the ATP-less DnaK and DnaJ did not maintain optimal luciferase activity and were less effective at preventing luciferase misfolding and aggregation. We present a model accounting for the experimental data, where native, unfolded, misfolded, and aggregated species spontaneously inter-convert, and in which DnaK-DnaJ-GrpE+ATP specifically convert stable misfolded species into unstable unfolded intermediates. 相似文献
992.
Kumar L Sarswat A Lal N Jain A Kumar S Kiran Kumar ST Maikhuri JP Pandey AK Shukla PK Gupta G Sharma VL 《Bioorganic & medicinal chemistry letters》2011,21(1):176-181
We designed a series of 25 3-(azol-1-yl)phenylpropanes which yielded 10 compounds (3, 4, 7, 8, 13, 14, 19, 21, 23, 26) that irreversibly immobilized 100% human sperm at 1% (w/v) concentration in 60 s; 12 compounds (8, 9, 15, 16, 19-21, 23-25, 27, 28) that showed potent microbicidal activity at 12.5-50 μg/mL against Trichomonas vaginalis; and 17 compounds (3-11, 13, 15, 19, 21, 23, 26, 28, 30) that exhibited potent anticandida activity with minimum inhibitory concentration (MIC) of 12.5-50 μg/mL. Almost all the compounds exhibited high level of safety towards normal vaginal flora (Lactobacillus) and human cervical (HeLa) cells in comparison to the marketed spermicide nonoxynol-9 (N-9). All the biological activities were evaluated in vitro. Two compounds (4, 8) with good safety profile exhibited multiple (spermicidal, antitrichomonas and anticandida) activities, warranting further lead optimization for furnishing a prophylactic vaginal contraceptive. 相似文献
993.
Melatonin secreted by the pineal gland acts as a free radical scavenger besides its role as a hormonal signaling agent. It detoxifies a variety of free radicals and reactive oxygen intermediates including hydroxyl radical, peroxynitrite anion and singlet oxygen. Ascorbic acid (Vitamin C), a water soluble vitamin, is a naturally occurring antioxidant and cofactor in various enzymes. Protein carbonyls are formed as a consequence of the oxidative modification of proteins by reactive oxygen species. Oxidative modification alters the function of protein and is thought to play an important role in the decline of cellular functions during aging. In the present study, the effect of melatonin and ascorbic acid on age-related carbonyl content of cerebral hemispheres in mice was investigated. Protein carbonyls of cerebral hemispheres have been found to be significantly higher in 18-month-old mice as compared to 1-month old mice. Administration of a single dose of melatonin (10 mg/kg body weight) and ascorbic acid (10 mg/kg body weight) intraperitoneally for three consecutive days decreases the carbonyl content in 1- and 18-month-old mice significantly. The present study thus suggests that the formation of protein carbonyls in the cerebral hemispheres of the aging mice can be prevented by the antioxidative effects of melatonin and ascorbic acid that could in turn be beneficial in having health benefits from age-related neurodegenerative diseases. 相似文献
994.
Identification of the amino acids essential for LytSR‐mediated signal transduction in Staphylococcus aureus and their roles in biofilm‐specific gene expression 下载免费PDF全文
995.
Germplasm characterization and evolutionary process in viable populations are important links between the conservation and
utilization of plant genetic resources. Here, an investigation is made, based on molecular and biochemical techniques for
assessing and exploiting the genetic variability in germplasm characterization of taro, which would be useful in plant breeding
and ex situ conservation of taro plant genetic resources. Geographical differentiation and phylogenetic relationships of Indian
taro, Colocasia esculenta (L.) Schott, were analyzed by random amplified polymorphic DNA (RAPD) and isozyme of seven enzyme systems with specific reference
to the Muktakeshi accession, which has been to be proved resistant to taro leaf blight caused by P. colocasiae. The significant differentiations in Indian taro cultivars were clearly demonstrated by RAPD and isozyme analysis. RAPD markers
showed higher values for genetic differentiation among taro cultivars and lower coefficient of variation than those obtained
from isozymes. Genetic differentiation was evident in the taro accessions collected from different regions of India. It appears
that when taro cultivation was introduced to a new area, only a small fraction of genetic variability in heterogeneous taro
populations was transferred, possibly causing random differentiation among locally adapted taro populations. The selected
primers will be useful for future genetic analysis and provide taro breeders with a genetic basis for selection of parents
for crop improvement. Polymorphic markers identified in the DNA fingerprinting study will be useful for screening a segregating
population, which is being generated in our laboratory aimed at developing a taro genetic linkage map. 相似文献
996.
Hejtmancik JF Jiao X Li A Sergeev YV Ding X Sharma AK Chan CC Medina I Edwards AO 《American journal of human genetics》2008,82(1):174-180
Snowflake vitreoretinal degeneration (SVD, MIM 193230) is a developmental and progressive hereditary eye disorder that affects multiple tissues within the eye. Diagnostic features of SVD include fibrillar degeneration of the vitreous humor, early-onset cataract, minute crystalline deposits in the neurosensory retina, and retinal detachment. A genome-wide scan previously localized the genetic locus for SVD to a 20 Mb region flanked by D2S2158 and D2S2202. This region contains 59 genes, of which 20 were sequenced, disclosing a heterozygous mutation (484C > T, R162W) in KCNJ13, member 13 of subfamily J of the potassium inwardly rectifying channel family in all affected individuals. The mutation in KCNJ13, the gene encoding Kir7.1, was not present in unaffected family members and 210 control individuals. Kir7.1 localized to human retina and retinal pigment epithelium and was especially prevalent in the internal limiting membrane adjacent to the vitreous body. Molecular modeling of this mutation predicted disruption of the structure of the potassium channel in the closed state located immediately adjacent to the cell-membrane inner boundary. Functionally, unlike wild-type Kir7.1 whose overexpression in CHO-K1 cells line produces highly selective potassium current, overexpression of R162W mutant Kir7.1 produces a nonselective cation current that depolarizes transfected cells and increases their fragility. These results indicate that the KCNJ13 R162W mutation can cause SVD and further show that vitreoretinal degeneration can arise through mutations in genes whose products are not structural components of the vitreous. 相似文献
997.
S. Gulati M. Khullar B. K. Sharma N. K. Ganguly 《Molecular and cellular biochemistry》1996,156(1):37-42
Intracellular free Ca2+ concentration has been shown to be elevated in platelets of patients with essential hypertension. This study was designed to characterize Ca2+-pump activity of the platelet membranes (surface and intracellular) in these patients. A double-blind study was carried out. Untreated and treated (on R-blockers) essential hypertensives were studied in comparison with normotensive control subjects. First degree blood relatives of essential hypertensives were also studied. The Ca2+-activation kinetics of the enzyme showed a significant decrease in the Vmax. (for the plasma- and intracellular membranes) and Km (for the intracellular membranes) in the essential hypertensive patients. Increased platelet membrane cholesterol content was observed in these patients. Lowered Ca2+-efflux by Ca2+-ATPase may lead to elevated intracellular free Ca2+-levels in platelets of essential hypertensives. A lowered Ca2+-ATPase activity may emerge as a marker for essential hypertension. 相似文献
998.
Naveen Kumar Sharma Ashwani Kumar Rai Surendra Singh Richard Malcolm Brown 《Journal of phycology》2007,43(4):615-627
Ongoing climatic changes coupled with various natural processes and the outcomes of human activities are not only loading the atmosphere with diverse kinds of biological particles but also changing their prevalence and spatial distribution. Despite having considerable ecological and economic significance, including their possible impact on human health, airborne algae are the least‐studied organisms in both aerobiological and phycological studies. The present review has been written to bring together all available information, including a brief survey of the literature, the ecology of airborne algae, mechanisms involved in their aerosolization, the role of environmental factors in shaping the structure and composition of aero‐algal flora, and other significant information associated with airborne algae. This review provides information on methodological approaches and related problems, along with suggestions for areas of future research on airborne algae. 相似文献
999.
Ihsan R Chauhan PS Mishra AK Yadav DS Kaushal M Sharma JD Zomawia E Verma Y Kapur S Saxena S 《PloS one》2011,6(12):e29431
Complex disease such as cancer results from interactions of multiple genetic and environmental factors. Studying these factors singularly cannot explain the underlying pathogenetic mechanism of the disease. Multi-analytical approach, including logistic regression (LR), classification and regression tree (CART) and multifactor dimensionality reduction (MDR), was applied in 188 lung cancer cases and 290 controls to explore high order interactions among xenobiotic metabolizing genes and environmental risk factors. Smoking was identified as the predominant risk factor by all three analytical approaches. Individually, CYP1A1*2A polymorphism was significantly associated with increased lung cancer risk (OR = 1.69;95%CI = 1.11–2.59,p = 0.01), whereas EPHX1 Tyr113His and SULT1A1 Arg213His conferred reduced risk (OR = 0.40;95%CI = 0.25–0.65,p<0.001 and OR = 0.51;95%CI = 0.33–0.78,p = 0.002 respectively). In smokers, EPHX1 Tyr113His and SULT1A1 Arg213His polymorphisms reduced the risk of lung cancer, whereas CYP1A1*2A, CYP1A1*2C and GSTP1 Ile105Val imparted increased risk in non-smokers only. While exploring non-linear interactions through CART analysis, smokers carrying the combination of EPHX1 113TC (Tyr/His), SULT1A1 213GG (Arg/Arg) or AA (His/His) and GSTM1 null genotypes showed the highest risk for lung cancer (OR = 3.73;95%CI = 1.33–10.55,p = 0.006), whereas combined effect of CYP1A1*2A 6235CC or TC, SULT1A1 213GG (Arg/Arg) and betel quid chewing showed maximum risk in non-smokers (OR = 2.93;95%CI = 1.15–7.51,p = 0.01). MDR analysis identified two distinct predictor models for the risk of lung cancer in smokers (tobacco chewing, EPHX1 Tyr113His, and SULT1A1 Arg213His) and non-smokers (CYP1A1*2A, GSTP1 Ile105Val and SULT1A1 Arg213His) with testing balance accuracy (TBA) of 0.6436 and 0.6677 respectively. Interaction entropy interpretations of MDR results showed non-additive interactions of tobacco chewing with SULT1A1 Arg213His and EPHX1 Tyr113His in smokers and SULT1A1 Arg213His with GSTP1 Ile105Val and CYP1A1*2C in nonsmokers. These results identified distinct gene-gene and gene environment interactions in smokers and non-smokers, which confirms the importance of multifactorial interaction in risk assessment of lung cancer. 相似文献
1000.
M. D. Pratheesh Nitin E. Gade Amar Nath Pawan K. Dubey T. B. Sivanarayanan D. N. Madhu T. R. Sreekumar Amarpal G. Saikumar G. Taru Sharma 《Cytotechnology》2017,69(6):841-849
The current study was designed to study the persistence and distribution of caprine bone marrow derived mesenchymal stem cells (cBM-MSCs) when administered intra-dermally in experimentally induced cutaneous wounds in rabbits. MSC’s from goat bone marrow were isolated and their differentiation potential towards adipogenic and osteogenic lineages were assayed in vitro. The isolated cells were phenotypically analysed using flow cytometry for the expression of MSC specific matrix receptors (CD73, CD105 and Stro-1) and absence of hematopoietic lineage markers. Further, these in vitro expanded MSCs were stained with PKH26 lipophilic cell membrane red fluorescent dye and prepared for transplantation into cutaneous wounds created on rabbits. Five, 2 cm linear full thickness skin incisions were created on either side of dorsal midline of New Zealand white rabbits (n = 4). Four wounds in each animal were implanted intra-dermally with PKH26 labelled cBM-MSCs suspended in 500 µl of Phosphate Buffer Saline (PBS). Fifth wound was injected with PBS alone and treated as negative control. The skin samples were collected from respective wounds on 3, 7, 10 and 14 days after the wound creation, and cryosections of 6 µM were made from it. Fluorescent microscopy of these cryosections showed that the PKH26 labelled transplanted cells and their daughter cells demonstrated a diffuse pattern of distribution initially and were later concentrated towards the wound edges and finally appeared to be engrafted with the newly developed skin tissues. The labelled cells were found retained in the wound bed throughout the period of 14 days of experimental study with a gradual decline in their intensity of red fluorescence probably due to the dye dilution as a result of multiple cell division. The retention of transplanted MSCs within the wound bed even after the complete wound healing suggests that in addition to their paracrine actions as already been reported, they may have direct involvement in various stages of intricate wound healing process which needs to be explored further. 相似文献