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1、 第七章第七章 遗传物质的变异遗传物质的变异及修复机制及修复机制 第一节第一节 染色体畸变染色体畸变 Chromosome Aberration 一、染色体结构变异 Changes in Chromosome Structure Sometimes chromosome mutation can be detected by microscopic observation Polytene chromosome (多线染色体）A giant chromosome consisting of many identical chromatids lying in parallel register
2、Found in the nuclei of the salivary gland（唾腺）and certain other tissues of the larvae（幼虫）of Drosophila and other two-winged fliesThe polytene structures are formed by repeated replication of the DNA(10005000),but without separation of the replicated chromatin strands in a mitotic process Types of cha
3、nges in chromosome structure Deletion 缺失 Duplication 重复Inversion 倒位Translocation 易位Mechanisms of change Chromosome rearrangements can arise through physical breakage(断裂)and rejoining(重接)of the DNA moleculeSuch processes can either occur spontaneously or be induced by treatment with high-energy radia
4、tion such as X or radiation1.DeletionA deletion is a lost portion of a chromosome Terminal deletion 末端缺失 Interstitial deletion 中间缺失 The effects of deletions depend on their sizeSmall deletion within a gene inactivates the gene Large deletion affect more adjacent genes Deletion homozygote(纯合体)is ofte
5、n lethal Deletion heterozygote(杂合体)is often harmful Heterozygous for a multigenic deletion may not surviveDeletion heterozygote forms deletion loop during meiotic division Genetic properties of deletionsThe failure of the chromosome to survive as a homozygoteCan not revert to a normal conditionUncov
6、er recessive alleles(pseudodominance 假显性)Using deletion to locate genes2.Duplication A duplication is a repeated part of the genetic material串联重复(tandem duplication)非串联重复(nontandem duplication)Most duplications have no obvious phenotypic consequencesSome duplications have phenotypic consequencesThe
7、extra region of a duplication is free to undergo gene mutation Proposed generation of variant human hemoglobin subunits by unequal crossing-over in the-genetic region.3.Inversion Inversion rearrange the linear gene sequencepericentric inversion 臂间倒位 paracentric inversion 臂内倒位 Origin of InversionAn i
8、nversion can affect phenotype Inversion reduce the frequency of crossing-over and recombinationInversion homozygote is normalInversion heterozygote form an inversion loop in meiosis平衡致死系(balanced lethal system)利用倒位杂合体或紧密连锁基因的交换抑制效应，保存隐性致死基因，又叫永久杂种(permanent hybrid)展翅展翅 D+/+G/+粘胶眼粘胶眼 +/+D+/+D+/+G +/+
9、G 自交自交 D+/D+D+/+G +G/+G 致死致死 展翅、粘胶眼展翅、粘胶眼 致死致死 4.Translocation Translocation alter the location of chromosomal segmentsReciprocal translocations 相互易位 Nonreciprocal translocation 非相互易位 罗伯逊式易位罗伯逊式易位 Translocation homozygote is normalTranslocation heterozygote form a crosslike configuration in meiosisT
10、ranslocation heterozygote diminishes fertility and results in Pseudolinkage(假连锁)Some kinds of cancer are associated with translocation in somatic cells二、染色体数目的变异 Changes in Chromosome Number chromosome set(染色体组染色体组)：The group of different chromosomes that carries the basic set of genetic information
11、 of a particular species.euploid(整倍体整倍体)：Organisms with complete chromosome setsaneuploid(非整倍体非整倍体):Organisms gain or lost one or more chromosomes,but not a chromosome set 1.Euploid(整倍体)单倍体(haploid)或一倍体(monoploid):n二倍体(diploid):2n多倍体(polyploid):三倍体(triploid)3n 四倍体(tetraploid)4n 六倍体(hexaploid)6n 八倍体(
12、octploid)8n Monoploid organisms are usually infertile,but they are important in plant breeding.Polyploid autopolyploids 同源多倍体 allopolyploids 异源多倍体 Many polyploid plants are larger than their diploid counterpartsTriploids are almost always sterileTetraploidAllopolyploid 异源多倍体 Some allopolyploids have
13、 agriculturally desirable traits derived from two species2.Aneuploid(非整倍体)双体(disomic)：2n ABCD/ABCD单体(monosomic):2n-1 ABCD/ABC缺体(nullisomic):2n-2 ABC/ABC三体(trisomic):2n+1 ABCD/ABCD/A四体(tetrasomic):2n+2 ABCD/ABCD/AA双三体(double trisomic):2n+1+1 ABCD/ABCD/AB nNondisjunction（不分离）in mitosis or meiosis is t
14、he cause of most aneuploids.Aneuploid and human diseases 第二节第二节 基因突变基因突变 Gene mutation一、基因突变的类型基因突变的类型(Types of mutation)Mutation may be classified in various waysGerminal and Somatic mutationSpontaneous and induced mutationForward mutation and reverse mutationOther categories of mutation (morpholog
15、ical形态,biochemical,behavior,regulatory,lethal,conditional)二、突变的特点突变的特点(characteristics of mutation)1.Random,but exists hot spots of mutation 5-methylcytosine 5-甲基胞嘧啶2.Mutations are usually,but not always spontaneousSpontaneous mutationAdaptive mutation Cairns J.et.al 1988 Nature 335:142-145 The orig
16、in of mutants3.Mutation occurs at very low rate,the mutation rate varies among organisms and genes 4.Forward mutations occur more often than reverse mutations5.Multiple directions三、突变的检测 (detection of mutation)1.Detection in bacteria and fungi（真菌）2.Detection in Drosophila（果蝇）CLB method C:an inversio
17、n L:a recessive lethal allele B:the dominant duplicated Bar eye gene 3.Detection in humans pedigree analysis四、突变的分子基础 (Molecular basis of mutation)1.Spontaneous mutation（自发突变）Errors in DNA replication（复制错误）Transitions 转换 Transversions 颠换 Frameshift mutations 移码突变 Deletions and duplications 缺失和重复 Spo
18、ntaneous lesions (自发损伤)Depurination（脱嘌呤）Deamination（脱氨基）Oxidatively damaged bases （氧化性损伤碱基）2.Induced mutations 诱发突变 Radiation 射线Ultraviolet（UV）irradiation 紫外线Ionizing radiation 离子射线 Chemical mutagens 化学诱变剂Base analogs 碱基类似物 5-bromouracil(5-BU)5-溴尿嘧啶 5-BU pairs with G 2-amino-purine(2-AP)2-氨基嘌呤 2-AP
19、pairs with CAlkylating agents（烷化剂）ethylmethanesulfonate(EMS)乙基甲烷磺酸 nitrosoguanidine(NG)亚硝基胍Intercalating agents （DNA插入剂）proflavin 原黄素 acridine orange 吖啶橙 Cause base pairs insertions or deletions Stabilize the loop formed during frameshift The Ames test for mutagens/carcinogens detection五、动态突变与人类疾病动态
20、突变与人类疾病 Dynamic mutation and human diseasefragile X syndrome(脆性X染色体综合症)FMR-1 (fragile-X-associated-mental-retardation)The possible mechanism of trinucleotide repeat amplification（三核苷酸重复扩增）定点诱变定点诱变六、定点诱变 （site-directed mutations）Reverse Genetics第三节 DNA的修复机制 Mechanisms of DNA repair1.Photoreactivation repair(光复活)in prokaryotes2.Excision Repair（切除修复）in Prokaryotes and EukaryotesExcision Repair in E.coli3.Mismatch Repair（错配修复）4.Recombinational repair（重组修复）5.The SOS repair system -error prone(倾向差错)