页数：46页文件大小：1.73 MB来源： 分子生物学基因Geneppt课件
本课件为《分子生物学》第4章基因簇与重复序列（Clusters and repeats），依据 Gene VII 教材编排，沿Gene clusters等小节推进，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 4 Clusters and repeats
Gene clusters are formed by duplication and divergenceSequence divergence is the basis for the evolutionary clockPseudogenes are dead ends of evolutionUnequal crossing-over rearranges gene clustersGenes for rRNA form tandem repeats( The repeated genes for rRNA maintain constant sequence)Crossover fix at ion could maintain identical repeatsSatellite DNAs often lie in heterochromatinArthropod satellites have very short identical repeatsMammalian satellites consist of hierarchical repeatsMinisatellites are useful for genetic mapping
Figure 1.23 Chiasma formation is responsible for generating recombinants.
Figure 1.24 Recombination involves pairing between complementary strands of the two parental duplex DNAs.
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）。
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.
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.
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.
Figure 4.3 All globin genes have evolved by a series of duplic at ions, trans positions, and mutations from a single ancestral gene.
Figure 4.3 All globin genes have evolved by a series of duplic at ions, trans positions, and mutations from a single ancestral gene.
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Figure 4.4 Divergence of DNA sequences depends on evolutionary separation. Each point on the graph represents a pairwise comparison.
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.
Processed pseudogeneis 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.
Figure 16.19 Pseudogenes could arise by reverse transcription of RNA to give duplex DNAs that become integrated into the genome.
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.
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 one chromosome with three copies of the gene (one from each parent and one recombinant).
Figure 4.7 Thalassemi as result from various deletions in the a-glob in gene cluster.
Figure 4.8 Deletions in the b-globin gene cluster cause several types of thalassemia.
Figure 4.9 A tandem gene cluster has an altern at ion of transcription unit and nontranscribed spacer and generates a circular restriction map.
Figure 4.10 The nucleolar core identifies rDNA under transcription, and the surrounding granular cortex consists of assembling ribosomal subunits. This thin section shows the nucleolus of the newt Notopthalmus viridescens. Photograph kindly provided by Oscar Miller.
Figure 4.11 Transcription of rDNA clusters generates a series of matrices, each corresponding to one transcription unit and separated from the next by the nontranscribed spacer. Photograph kindly provided by Oscar Miller.
Figure 4.12 The nontranscribed spacer of X. laevis rDNA has an internally repetitious structure that is responsible for its vari at ion in length.
Figure 23.4 Group I introns have a common secondary structure that is formed by 9 base paired regions. The sequences of regions P4 and P7 are conserved, and identify the individual sequence elements P, Q, R, and S. P1 is created by pairing between the end of the left exon and the IGS of the intron; a region between P7 and P9 pairs with the 3' end of the intron.
Figure 23.4 Group I introns have a common secondary structure that is formed by 9 base paired regions. The sequences of regions P4 and P7 are conserved, and identify the individual sequence elements P, Q, R, and S. P1 is created by pairing between the end of the left exon and the IGS of the intron; a region between P7 and P9 pairs with the 3' end of the intron.
说明：本页源文与上一页完全相同，疑为课件连续两页沿用同一页内容，已按原文保留。
Figure 4.13 Unequal recombination allows one particular repeating unit to occupy the entire cluster. The numbers indicate the length of the repeating unit at each stage.
Euchrom a tin comprises all of the genome in the interphase nucleus except for the heterochromatin.
Figure 4.14 Mouse DNA is separated into a main band and a satellite by centrifug at ion through a density gradient of Cs Cl.
Figure 18.16 Individual bands containing particular genes can be identified by in situ hybridization.
Figure 4.15 Cytological hybridization shows that mouse satellite DNA is located at the centromeres. Photograph kindly provided by Mary Lou Pardue and Joe Gall.
Figure 4.16 Satellite DNAs of D. virilis are related. More than 95% of each satellite consists of a tandem repetiti on of the pre dominant sequence.
Figure 4.15 Cytological hybridization shows that mouse satellite DNA is located at the centromeres. Photograph kindly provided by Mary Lou Pardue and Joe Gall.
Figure 4.17 The repeating unit of mouse satellite DNA contains two half-repeats, which are aligned to show the identities (in color).
Figure 4.18 The alignment of quarter-repeats identifies homologies between the first and second half of each half-repe at. Positions that are the same in all 4 quarter-repeats are shown in color; identities that extend only through 3 quarter-repeats are indicated by grey letters in the pink area.
Figure 4.19 The alignment of eighth-repeats shows that each quarter-repe at consists of an a and a b half. The consensus sequence gives the most common base at each position. The "ancestral" sequence shows a sequence very closely related to the consensus sequence, which could have been the predecessor to the a and b units. (The satellite sequence is continuous, so that for the purposes of deducing the consensus sequence, we can treat it as a circular per mutation, as indicated by joining the last GAA triplet to the first 6 bp.).
Figure 4.20 The existence of an overall consensus sequence is shown by writing the satellite sequence in terms of a 9 bp repeat.
Figure 4.21 The evolution of mouse satellite DNA can be explained by an altern at ion of saltatory replic at ions and accumul at ion of mutations.
Figure 4.21 Digestion of mouse satellite DNA with the restriction enzyme EcoRII identifies a series of repeating units (1, 2, 3) that are multimers of 234 bp and also a minor series (?, 1?, 2?) that includes half-repeats (see text later). The band at the far left is a fracti on resistant to digestion.
Figure 4.22 Alleles may differ in the number of repeats at a minisatellite locus, so that cleavage on either side generates restriction fragments that differ in length. By using a minisatellite with alleles that differ between parents, the pattern of inheritance can be followed.
Figure 4.22 Alleles may differ in the number of repeats at a minisatellite locus, so that cleavage on either side generates restriction fragments that differ in length. By using a minisatellite with alleles that differ between parents, the pattern of inheritance can be followed.
说明：本页源文与上一页完全相同，疑为课件连续两页沿用同一页内容，已按原文保留。
1.Almost all genes belong to families, defined by the possession of related sequences in the exons of individual members.2.An evolving set of genes may remain together in a cluster or may be dispersed to new locations by chromosomal rearrangement.3.Mutations accumulate more rapidly in silent sites than in replacement sites (which affect the amino acid sequence).4.A tandem cluster consists of many copies of a repeating unit that includes the transcribed sequence(s) and a nontranscribed spacer(s).5.Satellite DNA consists of very short sequences repeated many times in tandem.6.Unequal crossing-over appears to have been a major determinant of satellite DNA organization.
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