假基因类似于基因但不表达的DNA序列

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 4,Clusters and repeats,Gene clusters are formed by duplication and divergence,Sequence divergence is the basis for the evolutionary clock,Pseudogenes,are dead ends of evolution,Unequal crossing-over rearranges gene clusters,Genes for,rRNA,form tandem repeats,(The repeated genes for,rRNA,maintain constant sequence),Crossover fixation could maintain identical repeats,Satellite,DNAs,often lie in,heterochromatin,Arthropod satellites have very short identical repeats,Mammalian satellites consist of hierarchical repeats,Minisatellites,are useful for genetic mapping,Gene cluster,is a group of adjacent genes that are identical or related.,Gene family,consists of a set of genes whose,exons,are related;the members were derived by duplication and variation from some ancestral gene.,Satellite DNA,consists of many tandem repeats(identical or related)of a short basic repeating unit.,Unequal crossing-over,describes a recombination event in which the two recombining sites lie at,nonidentical,locations in the two parental DNA molecules.,4.1,Introduction,Figure 1.23,Chiasma,formation is responsible for generating recombinants.,4.1,Introduction,Figure 1.24 Recombination involves pairing between complementary strands of the two parental duplex,DNAs,.,4.1,Introduction,Pseudogenes,are inactive but stable components of the genome derived by mutation of an ancestral active gene.,假基因,：类似于基因但不表达的,DNA,序列。,不表现任何功能，是基因的退化形式。假基因在基因组中形成稳定的和无活性的拷贝，由活化的原始基因突变而来，这是因为存在着在某个阶段伤及基因表达的一种或多种缺陷（入启动子错误、有缺陷的剪接信号、框架中有终止信号等）之故。一旦不能产生正常的基因产物，就失去了对发生进一步突变的选择性屏障作用，因此典型的假基因都有很多缺陷。某些假基因有3-多聚,A,尾巴及准确地切掉了内含子，因而与,mRNA,类似，被认为是源自插入基因组的逆转录体（可能由某些病毒携带）。,4.2,Gene clusters are formed by duplication and divergence,Figure 2.13 All functional,globin,genes have an interrupted structure with three,exons,.The lengths indicated in the figure apply to the mammalian b-,globin,genes.,4.2,Gene clusters are formed by duplication and divergence,Figure 4.1 Each of the a-like and b-like,globin,gene families is organized into a single cluster that includes functional genes and,pseudogenes,.,4.2,Gene clusters are formed by duplication and divergence,Figure 4.2 Clusters of b-,globin,genes and,pseudogenes,are found in vertebrates.Seven mouse genes include 2 early embryonic,1 late embryonic,2 adult genes,and 2,pseudogenes,.Rabbit and chick each have four genes.,4.2,Gene clusters are formed by duplication and divergence,Figure 4.3 All,globin,genes have evolved by a series of duplications,transpositions,and mutations from a single ancestral gene.,4.2,Gene clusters are formed by duplication and divergence,Figure 4.3 All,globin,genes have evolved by a series of duplications,transpositions,and mutations from a single ancestral gene.,4.2,Gene clusters are formed by duplication and divergence,Divergence,is the percent difference in nucleotide sequence between two related DNA sequences or in amino acid sequences between two proteins.,Evolutionary clock,is defined by the rate at which mutations accumulate in a given gene.,Replacement sites,in a gene are those at which mutations alter the amino acid that is coded.,4.3,Sequence divergence is the basis for the evolutionary clock,Figure 4.4 Divergence of DNA sequences depends on evolutionary separation.Each point on the graph represents a,pairwise,comparison.,4.3,Sequence divergence is the basis for the evolutionary clock,Figure 4.5 Replacement site divergences between pairs of b-,globin,genes allow the history of the human cluster to be reconstructed.This tree accounts for the separation of classes of,globin,genes.,4.3,Sequence divergence is the basis for the evolutionary clock,Processed,pseudogene,is an inactive gene copy that lacks,introns,contrasted with the interrupted structure of the active gene.Such genes presumably originate by reverse transcription of,mRNA,and insertion of a duplex copy into the genome.,4.4,Pseudogenes,are dead ends of evolution,Figure 16.19,Pseudogenes,could arise by reverse transcription of RNA to give duplex,DNAs,that become integrated into the genome.,4.4,Pseudogenes,are dead ends of evolution,Thalassemia,is disease of red blood cells resulting from lack of either,a,or,b,globin,.,Unequal crossing-over,describes a recombination event in which the two recombining sites lie at,nonidentical,locations in the two parental DNA molecules.,4.5,Unequal crossing-over rearranges gene clusters,Figure 4.2 Clusters of b-,globin,genes and,pseudogenes,are found in vertebrates.Seven mouse genes include 2 early embryonic,1 late embryonic,2 adult genes,and 2,pseudogenes,.Rabbit and chick each have four genes.,4.5,Unequal crossing-over rearranges gene clusters,Figure 4.6 Gene number can be changed by unequal crossing-over.If gene 1 of one chromosome pairs with gene 2 of the other chromosome,the other gene copies are excluded from pairing,as indicated by the extruded loops.Recombination between the,mispaired,genes produces one chromosome with a single(recombinant)copy of the gene and on