全文获取类型
收费全文 | 1451篇 |
免费 | 132篇 |
国内免费 | 1篇 |
专业分类
1584篇 |
出版年
2023年 | 12篇 |
2022年 | 13篇 |
2021年 | 31篇 |
2020年 | 26篇 |
2019年 | 26篇 |
2018年 | 38篇 |
2017年 | 26篇 |
2016年 | 46篇 |
2015年 | 64篇 |
2014年 | 66篇 |
2013年 | 75篇 |
2012年 | 108篇 |
2011年 | 102篇 |
2010年 | 51篇 |
2009年 | 52篇 |
2008年 | 62篇 |
2007年 | 69篇 |
2006年 | 68篇 |
2005年 | 53篇 |
2004年 | 60篇 |
2003年 | 35篇 |
2002年 | 38篇 |
2001年 | 34篇 |
2000年 | 33篇 |
1999年 | 20篇 |
1998年 | 12篇 |
1997年 | 8篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 17篇 |
1991年 | 28篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 8篇 |
1984年 | 22篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 14篇 |
1975年 | 11篇 |
1972年 | 9篇 |
1970年 | 13篇 |
1966年 | 9篇 |
1965年 | 7篇 |
排序方式: 共有1584条查询结果,搜索用时 0 毫秒
11.
The delta- and epsilon-polypeptides were removed from chloroplast coupling factor 1 (CF1). The resulting enzyme, CF1(-delta, epsilon), is a stable active ATPase containing only alpha-, beta-, and gamma-polypeptides. The dependence of the steady-state kinetics of ATP hydrolysis catalyzed by CF1(-delta, epsilon) on the concentrations of ATP and ADP was found to be essentially the same as by activated CF1. Nucleotide binding studies with CF1(-delta, epsilon) revealed three binding sites: a nondissociable ADP site (site 1), a tight MgATP binding site (site 2), and a site that binds ADP and ATP with a dissociation constant in the micromolar range (site 3). Similar results have been obtained with CF1. For both CF1 and CF1(-delta, epsilon), the binding of MgATP at site 2 is tight only in the presence of Mg2+. Fluorescence resonance energy transfer was used to map distances between the gamma-sulfhydryl ("dark" site) and gamma-disulfide and between the gamma-sulfhydryl and the three nucleotide sites. These distances are within 5% of the corresponding distances on CF1. These results indicate that removal of the delta- and epsilon-polypeptides from CF1 does not cause significant changes in the structure, kinetics, and nucleotide binding sites of the enzyme. 相似文献
12.
Haxhiu M. A.; Deal E. C. Jr; Norcia M. P.; van Lunteren E.; Mitra J.; Cherniack N. S. 《Journal of applied physiology》1986,61(2):791-796
To assess the role of structures located superficially near the ventrolateral surface of the medulla on the reflex constriction of tracheal smooth muscle that occurs when airway and pulmonary receptors are stimulated mechanically or chemically, experiments were conducted in alpha-chloralose-anesthetized, paralyzed, and artificially ventilated cats. Pressure changes within a bypassed segment of the trachea were used as an index of alterations smooth muscle tone. The effects of focal cooling of the intermediate areas or topically applied lidocaine on the ventral surface of the medulla on the response of the trachea to mechanical and chemical stimulation of airway receptors were examined. Atropine abolished tracheal constriction induced by mechanical stimulation of the carina or aerosolized histamine, showing that the responses were mediated over vagal pathways. Moderate cooling of the intermediate area (20 degrees C) or local application of lidocaine significantly decreased the tracheal constrictive response to mechanical activation of airway receptors. Furthermore, when the trachea was constricted by histamine, cooling of the intermediate area significantly diminished the increased tracheal tone, whereas rewarming restored tracheal tone to the previous level. These findings suggest that under the conditions of the experiments the ventral surface of the medulla plays an important role in constriction of the trachea by inputs from intrapulmonary receptors and in the modulation of parasympathetic outflow to airway smooth muscle. 相似文献
13.
Baseline frequency of sister-chromatid exchanges (SCE) in newborn lymphocytes and its relationship to in vivo aging in humans 总被引:1,自引:0,他引:1
Heparinised cord blood from newborns and peripheral venous blood from three other age groups of individuals (1-75 years) have been cultured in vitro to obtain baseline frequencies of SCE and to see if the frequency of baseline SCE in vitro varies as a function of aging in vivo. The results demonstrate an age-dependent variation in the frequency of SCEs. Although the SCE frequency was lowest (5.10/cell) in 1-5-year-old infants, a significantly higher (P less than 0.001) frequency (8.97/cell) was observed in the cord blood of newborns. In old age, the level of SCE also increased. The plausible reason(s) for such observations is discussed. 相似文献
14.
Purified human alpha(1) proteinase inhibitor, a plasma glycoprotein with a molecular weight of 5.3 x 10(4) daltons and a major serine protease inhibitor has been covalently coupled to dextrans with molecular weights of 1.77 x 10(4) and 1.03 x 10(4) daltons. The coupled conjugates were soluble in aqueous medium and stable up to 6 months at +5 degrees C. Increased moles of dextran/mole protein ratio during coupling resulted in progressive decreases of inhibitory capacity, immunogenicity, and the association constant (k(assoc)) between the enzyme and the inhibitor. Compared to the native protein, the soluble conjugates showed improved stability at pH 3.0 and heat stability at 60 degrees C. At 60 degrees C, no loss of inhibitory capacity has been seen up to 60 min for the conjugates during which time the native protein lost greater than 90% of its inhibitory capacity. The presence of antioxidant catalase was needed to prevent oxidative degradation by hydrogen peroxide. 相似文献
15.
Kinetics of incorporation of O6-methyldeoxyguanosine monophosphate during in vitro DNA synthesis 总被引:2,自引:0,他引:2
O6-Methyldeoxyguanosine triphosphate (m6dGTP), known to be produced in vivo by methylation of deoxyguanosine triphosphate with simple methylating mutagens, is utilized by prokaryotic DNA polymerases during in vitro replication of synthetic and natural DNA template-primers. A study of the kinetic behavior of m6dGTP during DNA replication in vitro and of its effect on DNA replication indicates that m6dGTP acts as an analogue of dATP with Kappm of about 6 microM for Escherichia coli DNA polymerase I (Klenow fragment) compared to the Kappm of about 0.8 microM for dATP. m6dGTP is not incorporated in the complete absence of dATP (a competitive inhibitor). m6dGTP also inhibits in vitro DNA synthesis. Different DNA polymerases behave differently in utilization and turnover of m6dGTP. T4 DNA polymerase shows stronger discrimination against m6dGMP incorporation than either T5 DNA polymerase or E. coli DNA polymerase I. The possibility that m6dGTP is unlikely to contribute significantly to in vivo mutation is discussed. 相似文献
16.
T. G. Krishna S. E. Pawar R. Mitra 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1986,73(1):82-87
Summary Variation in the arachin polypeptides of groundnut genotypes was observed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Three regions could be observed on the electropherogram. Region 1, corresponding to conarachin, did not show any variation; region 2, consisting of arachin acidic subunits, showed variation; region 3, containing the arachin basic subunits, did not show any variation. There are four varietal classes of arachin polypeptide patterns: class A comprised three acidic subunits of arachin of molecular weights 47.5, 45.1 and 42.6 kd and a basic subunit of 21.4 kd; class B, with three acidic subunits of molecular weights 47.5, 45.1 and 41.2 kd and a basic subunit of 21.4 kd; class C of an additive pattern of class A and class B; class D, of two acidic polypeptides of 47.5, 45.1 kd and the basic 21.4 kd subunit. Of the 90 genotypes studied, 73% belong to class A, 15% to class B and 6% each to class C and D. Analysis of F2 seeds from a cross between class A and class B genotypes showed that the two polypeptides (42.6 kd and 41.2 kd) are coded by nonallelic genes and also revealed that class C and class D patterns arose as a result of hybridisation between class A and class B. A. monticola, the progenitor of A. hypogaea, showed a pattern similar to the additive pattern of class A and class B while some diploid Arachis species had the 41.2 kd polypeptide. Based on arachin polypeptide patterns the probable origin of A. hypogaea has been suggested. 相似文献
17.
18.
19.
20.
The study of an effect of exogenous thymidine on the mitotic cycle demonstrated that a 30 minute exposure to unlabeled and to tritiated thymidine at a concentration of 2.9 × 10?6 M was sufficient to cause a significant increase in the mitotic index of root meristem cells of Haplopappus gracilis. An analysis of the data revealed that this was due to the prolongation of metaphase rather than to an increase in the actual number of cells entering division. 相似文献