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21.
Slipped-strand mispairing: a major mechanism for DNA sequence evolution 总被引:141,自引:13,他引:128
Simple repetitive DNA sequences are a widespread and abundant feature of
genomic DNA. The following several features characterize such sequences:
(1) they typically consist of a variety of repeated motifs of 1-10
bases--but may include much larger repeats as well; (2) larger repeat units
often include shorter ones within them; (3) long polypyrimidine and poly-CA
tracts are often found; and (4) tandem arrangements of closely related
motifs are often found. We propose that slipped-strand mispairing events,
in concert with unequal crossing- over, can readily account for all of
these features. The frequent occurrence of long tandem repeats of
particular motifs (polypyrimidine and poly-CA tracts) appears to result
from nonrandom patterns of nucleotide substitution. We argue that the
intrahelical process of slipped-strand mispairing is much more likely to be
the major factor in the initial expansion of short repeated motifs and
that, after initial expansion, simple tandem repeats may be predisposed to
further expansion by unequal crossing-over or other interhelical events
because of their propensity to mispair. Evidence is presented that
single-base repeats (the shortest possible motifs) are represented by
longer runs in mammalian introns than would be expected on a random basis,
supporting the idea that SSM may be a ubiquitous force in the evolution of
the eukaryotic genome. Simple repetitive sequences may therefore represent
a natural ground state of DNA unselected for coding functions.
相似文献
22.
Complete sequences of the rRNA genes of Drosophila melanogaster 总被引:19,自引:0,他引:19
In this, the first of three papers, we present the sequence of the
ribosomal RNA (rRNA) genes of Drosophila melanogaster. The gene regions of
D. melanogaster rDNA encode four individual rRNAs: 18S (1,995 nt), 5.8S
(123 nt), 2S (30 nt), and 28S (3,945 nt). The ribosomal DNA (rDNA) repeat
of D. melanogaster is AT rich (65.9% overall), with the spacers being
particularly AT rich. Analysis of DNA simplicity reveals that, in contrast
to the intergenic spacer (IGS) and the external transcribed spacer (ETS),
most of the rRNA gene regions have been refractory to the action of
slippage-like events, with the exception of the 28S rRNA gene expansion
segments. It would seem that the 28S rRNA can accommodate the products of
slippage-like events without loss of activity. In the following two papers
we analyze the effects of sequence divergence on the evolution of (1) the
28S gene "expansion segments" and (2) the 28S and 18S rRNA secondary
structures among eukaryotic species, respectively. Our detailed analyses
reveal, in addition to unequal crossing-over, (1) the involvement of
slippage and biased mutation in the evolution of the rDNA multigene family
and (2) the molecular coevolution of both expansion segments and the
nucleotides involved with compensatory changes required to maintain
secondary structures of RNA.
相似文献
23.
24.
25.
An overview of models of stomatal conductance at the leaf level 总被引:4,自引:0,他引:4
GAËLLE DAMOUR THIERRY SIMONNEAU HERVÉ COCHARD LAURENT URBAN 《Plant, cell & environment》2010,33(9):1419-1438
Stomata play a key role in plant adaptation to changing environmental conditions as they control both water losses and CO2 uptake. Particularly, in the context of global change, simulations of the consequences of drought on crop plants are needed to design more efficient and water‐saving cropping systems. However, most of the models of stomatal conductance (gs) developed at the leaf level link gs to environmental factors or net photosynthesis (Anet), but do not include satisfactorily the effects of drought, impairing our capacity to simulate plant functioning in conditions of limited water supply. The objective of this review was to draw an up‐to‐date picture of the gs models, from the empirical to the process‐based ones, along with their mechanistic or deterministic bases. It focuses on models capable to account for multiple environmental influences with emphasis on drought conditions. We examine how models that have been proposed for well‐watered conditions can be combined with those specifically designed to deal with drought conditions. Ideas for future improvements of gs models are discussed: the issue of co‐regulation of gs and Anet; the roles of CO2, absissic acid and H2O2; and finally, how to better address the new challenges arising from the issue of global change. 相似文献
26.
Background
The cell shape and morphology of plant tissues are intimately related to structural modifications in the primary cell wall that are associated with key processes in the regulation of cell growth and differentiation. The primary cell wall is composed mainly of cellulose immersed in a matrix of hemicellulose, pectin, lignin and some structural proteins. Xyloglucan is a hemicellulose polysaccharide present in the cell walls of all land plants (Embryophyta) and is the main hemicellulose in non-graminaceous angiosperms. 相似文献27.
Galbeta1-3GalNAc (T-disaccharide) and related molecules were assayed to
describe the structural requirements of carbohydrates to bind Agaricus
bisporus lectin (ABL). Results provide insight into the most relevant
regions of T-disaccharide involved in the binding of ABL. It was found that
monosaccharides bind ABL weakly indicating a more extended
carbohydrate-binding site as compared to those involvedin the T-
disaccharide specific lectins such as jacalin and peanut agglutinin.
Lacto-N-biose (Galbeta1-3GlcNAc) unlike T-disaccharide, is unable to
inhibit the ABL interaction, thus showing the great importance of the
position of the axial C-4 hydroxyl group of GalNAc in T-disaccharide. This
finding could explain the inhibitory ability of Galbeta1-6GlcNAc and
lactose because C-4 and C-3 hydroxyl groups of reducing Glc, respectively,
occupy a similar position as reported by conformational analysis. From the
comparison of different glycolipids bearing terminal T-disaccharide bound
to different linkages, it can be seen than ABL binding is even more
impaired by an adjacent C-6 residual position than by the anomeric
influence of T-disaccharide. Furthermore, the addition of beta-GlcNAc to
the terminal T-disaccharide in C-3 position of Gal does not affect the ABL
binding whereas if an anionic group such as glucuronic acid is added to
C-3, the binding is partially affected. These findings demonstrate that ABL
holds a particular binding nature different from that of other
T-disaccharide specific lectins.
相似文献
28.
目的:研究脂多糖(LPS)对人血清中补体系统的激活及在小鼠模型中诱导产生白三烯B4(LTB4)。方法:LPS包被ELISA板,利用血清中补体C4、C3沉积实验检测补体成分被LPS活化的情况,通过尾静脉注射小鼠LPS后不同时间点ELISA定量检测LTB4,评价补体系统的活化和炎症因子的产生。结果与结论:血清系统ELISA检测发现LPS可以激活补体系统,且以凝集素途径为主;动物实验中LTB4被LPS诱导后1~3 h达到峰值,之后回落。C1INH对血清补体活化和动物模型中LTB4的产生均有显著抑制。 相似文献
29.
摘要目的:探讨氨氯地平联合依那普利治疗原发性高血压的临床效果,观察联合用药对左心室肥厚的影响。方法:选择本院收治的原发性高血压患者92例,随机分为观察组和对照组,各46例,对照组给予苯磺酸左旋氨氯地平5mg,1次/d,口服;观察组在对照组基础上加用马来酸依那普利10mg,2次/d,口服,疗程均为24周。观察两组治疗前后血压变化,应用超声心动图测量两组左心室厚度变化。结果:治疗后,观察组总有效率为91_3%;对照组总有效率为73.9%,观察组总有效率高于对照组(P〈0.05)。治疗前两组心率、血压比较无统计学差异(P〉0.05),治疗后两组血压均明显降低,观察组收缩压、舒张压明显低于对照组(P〈O.05);观察组心率明显低于对照组(P〈0.01)。治疗前两组左心室舒张末期室间隔厚度(Leaventricularend—diastolicventricularseptalthickness,IVST)、左心室后壁厚度(1eftventricularposteriorwallthickness,U,PwT)和左室射血分数(Leftventricularejectionfxaction,LVEF)比较无统计学差异(P〉0.05);治疗后观察组IVST、L、,PwT明显低于对照组,LVEF明显高于对照组(P〈0.05)。结论:氨氯地平联合依那普利治疗原发性高血压能有效扭转左心室肥厚,降压效果较单独应用氨氯地平更佳。 相似文献
30.
目的:获得酶原形式的重组人甘露聚糖结合凝集素相关丝氨酸蛋白酶2(MASP2)。方法:在大肠杆菌中诱导表达重组人MASP2全长蛋白,包涵体裂解后,经复性、透析、浓缩、考马斯亮蓝染色、SDS-PAGE及Western印迹,鉴定纯化结果及酶活性。结果:复性后的MASP2蛋白经考马斯亮蓝染色未见杂带。自激活实验表明,当MASP2浓度在1μmool/L以下时,无论在4℃还是37℃,都能较稳定地保持酶原形式;蛋白浓度为3.5μmool/L时只能在4℃保持稳定,37℃发生自激活;蛋白浓度达到12μmool/L后,在4℃时已不能稳定存在。结论:获得了较纯的重组人MASP2蛋白,且具有自激活活性。 相似文献