共查询到20条相似文献，搜索用时 31 毫秒
1.
B Singer H Fraenkel-Conrat J Greenberg A M Michelson 《Science (New York, N.Y.)》1968,160(833):1235-1237
"Nitrosoguanidine" (N-methyl-N'-nitro-N-nitrosoguanidine) acts on polynucleotides in neutral aqueous solution, methylating guanine in the 7-position, and, to a much lesser extent, adenine. In 67 percent dimethylformamide the effect on the purines decreased, and what seems to be 3-methyl-cytosine appeared. In formamide solution no base changes were detected. Nitrosoguanidine had little mutagenic effect on tobacco mosaic virus RNA in water; it had greater effect in dimethylformamide, and even greater in formamide; it was a strong mutagen only when it acted on intact tobacco mosaic virus particles. Thus, neither the methylation of guanine nor the cytosine modification represents the main mutagenic event. 相似文献
2.
RNA composition and base pairing 总被引：4，自引：0，他引：4
If RNA may contain a small proportion of adenine-guanine base pairs, these could interrupt the continuity of helical structure in a polynucleotide, in keeping with current theories of RNA structure, and could also account for the experimentally observed tendency for 6-amino bases to equal 6-keto bases and for purines to exceed pyrimidines. 相似文献
3.
Electrophoresis in polyacrylamide gel of homogenates of various organs from the mouse yields five major lactic dehydrogenase bands. If the gels are treated with beta-mercaptoethanol, subsequent electrophoresis produces 15 bands which show lactic dehydrogenase activity. This could be explained if one molecule of nicotinamide adenine dinucleotide (coenzyme) is attached to each of the monomeric subunits of lactic dehydrogenase and if mercaptoethanol can remove the coenzyme only from the muscle type. This is consistent with the hypothesis that intact lactic dehydrogenase is a tetramer. 相似文献
4.
Takeda S Gapper C Kaya H Bell E Kuchitsu K Dolan L 《Science (New York, N.Y.)》2008,319(5867):1241-1244
The specification and maintenance of growth sites are tightly regulated during cell morphogenesis in all organisms. ROOT HAIR DEFECTIVE 2 reduced nicotinamide adenine dinucleotide phosphate (RHD2 NADPH) oxidase-derived reactive oxygen species (ROS) stimulate a Ca2+ influx into the cytoplasm that is required for root hair growth in Arabidopsis thaliana. We found that Ca2+, in turn, activated the RHD2 NADPH oxidase to produce ROS at the growing point in the root hair. Together, these components could establish a means of positive feedback regulation that maintains an active growth site in expanding root hair cells. Because the location and stability of growth sites predict the ultimate form of a plant cell, our findings demonstrate how a positive feedback mechanism involving RHD2, ROS, and Ca2+ can determine cell shape. 相似文献
5.
以永磁电动机模型为例,研究如何运用延迟反馈来控制混沌。首先在理论上分析永磁电动机模型延迟反馈控制混沌的可行性,然后通过仿真实验验证其正确性。通过研究可知运用延迟反馈可以很好地消除混沌使系统达到一个稳定的状态。 相似文献
6.
以促进NC297产量和质量协调和最优化为目的，研究施氮量和留叶数互作对NC297的物理特性、化学成分、经济性状的影响。结果表明：在施氮量7.5～60 kg/hm2、留叶数17～26片时，NC297叶长、叶宽、叶面积、叶厚、单叶重均随施氮量的增加而增加；含梗率、叶质重随施氮量的增加先降低后升高；烟碱和总氮含量随着施氮量的增加而上升显著，随着留叶数的增加而显著下降；产量持续上升，产值、均价、中上等烟比例随着施氮量和留叶数的增加先增大后减小；在物理性状、化学成分、经济性状上，除还原糖和糖碱比外，施氮量对其余各指标的影响均最大，留叶数的影响次之，二者交互作用产生的影响最小。NC297的适宜施氮量为30 kg/hm2，适宜留叶数为20～23片。 相似文献
7.
Infrared spectroscopy is used extensively in the study of isolated biomolecules, but it becomes less useful as it is applied to systems of increasing complexity. Even if the individual vibrational bands can be resolved spectroscopically, their assignment becomes problematic when they are more closely spaced than can be determined using ab initio methods. We describe a method that helps to alleviate this difficulty by measuring the direction of the vibrational transition moment for each vibrational band. The molecules of interest (adenine and cytosine) are cooled to 0.37 kelvin in liquid helium nanodroplets and oriented in a large dc electric field. A polarized infrared laser is then used to determine the directions of the infrared transition moments relative to the permanent dipole moment. Comparisons with ab initio calculations provide detailed structural information, including experimental evidence for nonplanarity of adenine and three tautomers of cytosine. 相似文献
8.
以永磁同步电动机为例,研究如何利用线性反馈控制混沌.首先在理论上分析该方法并构造李亚普诺夫函数,然后通过实验仿真来验证它的正确性.结果表明:利用线性反馈可以很好地消除混沌,使方程达到一个稳定的状态. 相似文献
9.
The effect of variations in cloud cover, optical properties, and fractional distribution with altitude on the mean surface temperature of a model of the early earth has been investigated. In all cases examined, cloud-climate feedbacks result in temperatures greater than those in models with no cloud feedbacks. If the model of hydrospheric feedback effects is correct, then cloud feedbacks are as important to the climate as changes in solar luminosity and atmospheric composition during the earth's atmospheric evolution. In particular, the early earth need not become completely ice-covered if strong negative cloud feedbacks occur. However, until a proper understanding of cloud feedbacks is available, conclusions regarding conditions in the early atmosphere must remain in doubt. 相似文献
10.
The photochemical rearrangement of a tetramer of hydrogen cyanide to 4-aminoimidazole-5-carbonitrile, a critical step in the proposed prebiotic synthesis of purines, proceeds in high yield in the absence of oxygen without photodestruction of the reaction product. The mechanism of the rearrangement involves the conversion of the excited singlet of the hydrogen cyanide tetramer to 2-imino-3-cyano-4-aminoazetine, which then rearranges to the imidazole product. The photolysis of the vinylogous enaminonitrile 1,3-dicyano4-amino-1,3-butadiene yields 6-aminonicotinonitrile. The latter reaction affords an efficient route to a nicotinamide derivative from cyanoacetylene. 相似文献
11.
Enzyme regulation in a trypanosomatid: effect of purine starvation on levels of 3'-nucleotidase activity 总被引：3，自引：0，他引：3
M Gottlieb 《Science (New York, N.Y.)》1985,227(4682):72-74
Crithidia fasciculata, a nonpathogenic relative of the leishmanial and trypanosomal pathogens of humans and animals, showed a 3'-ribonucleotidase activity similar to that in Leishmania donovani. The level of 3'-nucleotidase activity in Crithidia was regulated by the availability of purines in the culture medium. Specifically, organisms obtained from culture medium depleted of purines contained elevated levels of enzyme activity compared to those grown in complete medium. The 3'-nucleotidase, located at the cell surface, may serve as a first step in purine salvage for these protozoa, which are unable to synthesize the purine ring de novo. 相似文献
12.
对引入的IS-LM-BP模型在选取适当的状态向量、控制输入向量、输出向量及干扰输入向量后将其转化为状态空间模型,在此基础上,判断出该模型可任意极点配置,进而设计了一种纯增益反馈控制器.计算得到适当的状态反馈增益矩阵K_1,K_2,通过反馈控制,使该经济系统渐近稳定,同时可对输出进行调节,即实际总供给和实际汇率趋于计划的总供给和汇率.借助Matlab软件对模型进行数值模拟,验证了系统在该控制器下可达到输出调节.通过对IS-LM-BP模型纯增益反馈控制的设计,为该模型宏观经济政策设计提供了一定的依据. 相似文献
13.
壳聚糖对啤酒中嘌呤的吸附性能 总被引：2，自引：0，他引：2
以不同浓度的壳聚糖加入啤酒中,吸附以高氯酸水解出的嘌呤碱基,用高效液相色谱法(HPLC)测定吸附前后4种嘌呤碱基量,结果显示:壳聚糖可以有效吸附啤酒中的嘌呤碱基,降低啤酒中的嘌呤含量。 相似文献
14.
Receptors for the age of anxiety: pharmacology of the benzodiazepines 总被引：40，自引：0，他引：40
Investigation of the actions of the benzodiazepines has provided insights into the neurochemical mechanisms underlying anxiety, seizures, muscle relaxation, and sedation. Behavioral, electrophysical, pharmacological, and biochemical evidence indicates that the benzodiazepines exert their therapeutic effects by interacting with a high-affinity binding site (receptor) in the brain. The benzodiazepine receptor interacts with a receptor for gamma-aminobutyric acid, a major inhibitory neurotransmitter, and enhances its inhibitory effects. The benzodiazepine receptor may also interact with endogenous substances and several naturally occurring compounds, including the purines and nicotinamide, are candidates for this role. Both the purines and nicotinamide possess some benzodiazepine-like properties in vivo, although further work will be required to confirm their possible roles as endogenous benzodiazepines. 相似文献
15.
RopGAP4-dependent Rop GTPase rheostat control of Arabidopsis oxygen deprivation tolerance 总被引：1，自引：0，他引：1
Baxter-Burrell A Yang Z Springer PS Bailey-Serres J 《Science (New York, N.Y.)》2002,296(5575):2026-2028
Transient soil flooding limits cellular oxygen to roots and reduces crop yield. Plant response to oxygen deprivation involves increased expression of the alcohol dehydrogenase gene (ADH) and ethanolic fermentation. Disruption of the Arabidopsis gene that encodes Rop (RHO-like small G protein of plants) guanosine triphosphatase (GTPase) activating protein 4 (ROPGAP4), a Rop deactivator, elevates ADH expression in response to oxygen deprivation but decreases tolerance to stress. Rop-dependent production of hydrogen peroxide via a diphenylene iodonium chloride-sensitive calcium-dependent reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is necessary for induction of both ADH and RopGAP4 expression. Tolerance to oxygen deprivation requires Rop activation and RopGAP4-dependent negative feedback regulation. This Rop signal transduction rheostat balances the ability to increase ethanolic fermentation with survival. 相似文献
16.
Interaction between purine and benzodiazepine: Inosine reverses diazepam-induced stimulation of mouse exploratory behavior 总被引：1，自引：0，他引：1
J N Crawley P J Marangos S M Paul P Skolnick F K Goodwin 《Science (New York, N.Y.)》1981,211(4483):725-727
Inosine, 2-deoxyinosine, and 2-deoxyguanosine completely reversed the increase in exploratory activity elicited in mice by diazepam. The inhibition of exploratory behavior by purines occurred at doses that when given alone have no effect on exploratory behavior. 7-Methylinosine, which does not bind to the brain benzodiazepine binding site in vitro, had no effect on the diazepam-induced increase in exploratory behavior. Behavioral effects produced by various combinations of inosine and diazepam indicate that the interaction between purine and benzodiazepine is antagonistic and support the hypothesis that the naturally occurring purines function in anxiety-related behaviors that respond to benzodiazepine treatment. 相似文献
17.
Holt NE Zigmantas D Valkunas L Li XP Niyogi KK Fleming GR 《Science (New York, N.Y.)》2005,307(5708):433-436
Photosynthetic light harvesting in excess light is regulated by a process known as feedback deexcitation. Femtosecond transient absorption measurements on thylakoid membranes show selective formation of a carotenoid radical cation upon excitation of chlorophyll under conditions of maximum, steady-state feedback deexcitation. Studies on transgenic Arabidopsis thaliana plants confirmed that this carotenoid radical cation formation is correlated with feedback deexcitation and requires the presence of zeaxanthin, the specific carotenoid synthesized during high light exposure. These results indicate that energy transfer from chlorophyll molecules to a chlorophyllzeaxanthin heterodimer, which then undergoes charge separation, is the mechanism for excess energy dissipation during feedback deexcitation. 相似文献
18.
Decay of visual information from a single letter 总被引：2，自引：0，他引：2
If the trace of a letter can be matched more rapidly with a physically identical letter (as in the pair AA) than it can be with a letter having only the same name (as in the pair Aa), then the trace must preserve the visual aspect of the letter. The visual information from a single letter decays in about 1.5 seconds if the task provides little incentive for preservation. 相似文献
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20.
[目的]筛选耐盐性较强的小桐子新品种。[方法]以小桐子种胚为材料,采用含不同浓度6-苄基腺嘌呤(6-BA)和吲哚丁酸(IBA)组合的培养基对其进行愈伤组织诱导,筛选出最佳诱导培养基;在最佳诱导培养基中分别加入0、20、40、60、80、100 mmol/L NaCl对小桐子胚愈伤组织进行诱导,研究胚愈伤组织的耐盐性。[结果]小桐子胚愈伤组织的最佳诱导培养基为MS+1.0 mg/L6-BA+0.1mg/LIBA;胚愈伤组织在含40、60 mmol/LNaCl的培养基上生长状况最佳,当培养基中NaCl浓度达到100 mmol/L时,小桐子胚愈伤组织无法存活。[结论]在MS+1.0 mg/L6-BA+0.1 mg/LIBA培养基中,小桐子胚愈伤组织具有较好的耐NaCl性。 相似文献