全文获取类型
收费全文 | 630486篇 |
免费 | 72575篇 |
国内免费 | 472篇 |
专业分类
703533篇 |
出版年
2016年 | 6215篇 |
2015年 | 8704篇 |
2014年 | 10411篇 |
2013年 | 14684篇 |
2012年 | 16462篇 |
2011年 | 16839篇 |
2010年 | 11152篇 |
2009年 | 10595篇 |
2008年 | 15026篇 |
2007年 | 15659篇 |
2006年 | 14977篇 |
2005年 | 14492篇 |
2004年 | 14293篇 |
2003年 | 13814篇 |
2002年 | 13696篇 |
2001年 | 30550篇 |
2000年 | 31150篇 |
1999年 | 24459篇 |
1998年 | 7926篇 |
1997年 | 8400篇 |
1996年 | 7824篇 |
1995年 | 7365篇 |
1994年 | 7280篇 |
1993年 | 7372篇 |
1992年 | 19830篇 |
1991年 | 19507篇 |
1990年 | 18788篇 |
1989年 | 18370篇 |
1988年 | 17298篇 |
1987年 | 16357篇 |
1986年 | 15257篇 |
1985年 | 15293篇 |
1984年 | 12432篇 |
1983年 | 10847篇 |
1982年 | 8307篇 |
1981年 | 7610篇 |
1980年 | 7027篇 |
1979年 | 12023篇 |
1978年 | 9554篇 |
1977年 | 8760篇 |
1976年 | 8077篇 |
1975年 | 9146篇 |
1974年 | 9923篇 |
1973年 | 9839篇 |
1972年 | 8924篇 |
1971年 | 8162篇 |
1970年 | 7129篇 |
1969年 | 6978篇 |
1968年 | 6292篇 |
1967年 | 5401篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
The role of DNA sequence in determining nucleosome positions in vivo was investigated by comparing the positions adopted by nucleosomes reconstituted on a yeast plasmid in vitro using purified core histones with those in native chromatin containing the same DNA, described previously. Nucleosomes were reconstituted on a 2.5 kilobase pair DNA sequence containing the yeast TRP1ARS1 plasmid with CUP1 as an insert (TAC-DNA). Multiple, alternative, overlapping nucleosome positions were mapped on TAC-DNA. For the 58 positioned nucleosomes identified, the relative positioning strengths and the stabilities to salt and temperature were determined. These positions were, with a few exceptions, identical to those observed in native, remodeled TAC chromatin containing an activated CUP1 gene. Only some of these positions are utilized in native, unremodeled chromatin. These observations suggest that DNA sequence is likely to play a very important role in positioning nucleosomes in vivo. We suggest that events occurring in yeast CUP1 chromatin determine which positions are occupied in vivo and when they are occupied. 相似文献
42.
43.
44.
45.
46.
T. B. Lapirova V. R. Mikryakova A. S. Mavrin G. A. Yinogradova 《Journal of Evolutionary Biochemistry and Physiology》2000,36(1):47-51
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon
exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic
character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and
structural-functional organization of the studied organs. 相似文献
47.
Staphylococcal gamma-hemolysin and leukocidin are bi-component cytolysins, consisting of LukF (or Hlg1)/Hlg2 and LukF/LukS, respectively. Here, we purified serum inhibitors of gamma-hemolysin and leukocidin from human plasma. Protein sequencing showed that the purified inhibitors of 62, 57, 50 and 38 kDa were the vitronectin fragments with truncation(s) of the C-terminal or both N- and C-terminal regions. The purified vitronectin fragments specifically bound to the Hlg2 component of gamma-hemolysin and the LukS component of leukocidin to form high-molecular-weight complexes with them, leading to inhibition of the toxin-induced lysis of human erythrocytes and human polymorphonuclear leukocytes, respectively. Intact vitronectin also showed inhibitory activity to the toxins. The ability of gamma-hemolysin and leukocidin to bind vitronectin and its fragments is a novel function of the pore-forming cytolysins. 相似文献
48.
(Z)- and (E)-4-amino-2-(trifluoromethyl)-2-butenoic acid (4 and 5, respectively) were synthesized and investigated as potential mechanism-based inactivators of gamma-aminobutyric acid aminotransferase (GABA-AT) in a continuing effort to map the active site of this enzyme. The core alpha-trifluoromethyl-alpha,beta-unsaturated ester moiety was prepared via a Reformatsky/reductive elimination coupling of the key intermediates tert-butyl 2,2-dichloro-3,3,3-trifluoropropionate and N,N-bis(tert-butoxy-carbonyl)glycinal. Both 4 and 5 inhibited GABA-AT in a time-dependent manner, but displayed non-pseudo-first-order inactivation kinetics; initially, the inactivation rate increased with time. Further investigation demonstrated that the actual inactivator is generated enzymatically from 4 or 5. This inactivating species is released from the active site prior to inactivation, and as a result, 4 and 5 cannot be defined as mechanism-based inactivators. Furthermore, 4 and 5 are alternate substrates for GABA-AT, transaminated by the enzyme with Km values of 0.74 and 20.5 mM, respectively. Transamination occurs approximately 276 and 305 times per inactivation event for 4 and 5, respectively. The enzyme also catalyzes the elimination of the fluoride ion from 4 and 5. A mechanism to account for these observations is proposed. 相似文献
49.
Christopher T. Clarke 《Restoration Ecology》1997,5(3):256-264
No guidelines are currently available that advise which soil properties of arable land can be used to suggest suitable locations for heathland reconstruction. This paper reviews studies comparing soil properties of heathland or semi-natural grassland with those of adjacent arable fields, investigations in the autecology of the dominant heathland plant, Calluna vulgaris (common heather), and long-term experiments of fertilizer inputs on arable soils. Three properties must be assessed before the suitability of a field can be determined: extractable phosphorus, exchangeable calcium, and pH. A number of other nutrients may also be important, but evidence is currently insufficient to substantiate this. Natural changes in levels of extractable phosphorus, exchangeable calcium, and pH appear to be very slow, so nutrient stripping and acidification will be necessary where recommended levels are exceeded to successfully restore heathland vegetation. 相似文献
50.
OBJECTIVE--To examine the relation between bone density and indices of calcium metabolism including parathyroid hormone and 25-hydroxyvitamin D concentrations in middle aged women. DESIGN--A cross sectional study. SETTING AND SUBJECTS--138 women volunteers aged 45-65 with no known osteoporosis and unselected for disease status recruited for a dietary assessment study from the community using general practice registers. Volunteer rate was 20%. MAIN OUTCOME MEASURE--Bone mineral density measured with dual energy x ray absorptiometry. RESULTS--Bone density at the lumbar spine and neck and trochanteric regions of the femur was inversely related to serum intact parathyroid hormone concentrations and positively related to serum 25-hydroxyvitamin D concentrations. These associations were independent of possible confounding factors, including age, body mass index, cigarette smoking habit, menopausal status, and use of diuretics and postmenopausal hormone replacement therapy. These associations were apparent throughout the whole distribution of bone density and 25-hydroxyvitamin D and parathyroid hormone concentrations within the normal range, suggesting a physiological relation. CONCLUSIONS--The findings are consistent with the hypothesis that parathyroid hormone and 25-hydroxyvitamin D concentrations influence bone density in middle aged women. Findings from this study together with other work suggest that the role of vitamin D in osteoporosis should not be neglected. The associations with parathyroid hormone also indicate plausible biological mechanisms. The roughly 5-10% difference in bone density between top and bottom tertiles of serum 25-hydroxyvitamin D concentrations, though not large in magnitude, may have considerable public health implications in terms of prevention of osteoporosis and its sequelae, fractures. 相似文献