Download Gene Mutation

第六章 遗传物质的变异
及修复机制
Molecular mechanism of
mutation and DNA repair
The capacity to blunder slightly is the real
marvel of DNA. Without this special
attribute, we would still be anaerobic
bacteria and there would be no music.
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Chromosome Aberration（染色体畸变）
Changes in Chromosome Structure
Changes in Chromosome Number
Gene Mutation
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第一节 染色体结构变异
Changes in Chromosome Structure
Sometimes chromosome mutation can be
detected by microscopic observation
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Types of changes in
chromosome structure

Deletion 缺失

Duplication 重复
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Inversion 倒位
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Translocation 易位
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Mechanisms of change

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Chromosome rearrangements can arise
through physical breakage(断裂)and
rejoining(重接) of the DNA molecule
Such processes can either occur
spontaneously or be induced by
treatment with high-energy radiation
such as X or γ radiation
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Approaches for detecting
chromosome rearrangements
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Chromosome banding
Polytene chromosome
Fluorescence in situ Hybridization (FISH)
Genome sequencing and single-cell
sequencing
……
Chromosome banding is revealed by special staining techniques
(a) G banding. (b) Q banding. (c) C banding. (d) R banding
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Polytene chromosome
(多线染色体)
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A giant chromosome consisting of many
identical chromatids lying in parallel register
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 flies
The interphase chromosomes go through 10
rounds of replication without ever entering
mitosis. In addition, homologous chromosomes
remain tightly paired throughout interphase.

210 (1024) ×2=2048 double helixes of DNA
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Deletion loops form in the paired polytene
chromosomes of Drosophila deletion heterozygotes.
Fluorescence in situ Hybridization
(FISH) 荧光原位杂交
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Spectral karyotype of human chromosomes
utilizing differentially labeled “painting” probes
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SKY of a Thyroid (甲状腺) Cancer Cell
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Multicolor Banding FISH
Region-specific partial chromosomal paints
were generated by microdissection
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1. Deletion
A deletion is a lost portion of a chromosome
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Terminal deletion
末端缺失
Interstitial deletion
中间缺失
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Deletion heterozygote forms deletion
loop during meiotic division
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The effects of deletions depend on their size

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Small deletion within a gene inactivates the gene
Large deletion affect more adjacent genes
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Deletion homozygote(纯合体) is often lethal
Deletion heterozygote (杂合体) is often harmful
Heterozygous for a multigenic deletion may not
survive
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Genetic properties of deletions

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The failure of the chromosome to
survive as a homozygote
Can not revert to a normal condition
Uncover recessive alleles
(pseudodominance 假显性 )
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(a) Scarlet eye color in fly
(b)
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人5P－引起的猫叫综合症（cri-du-chat syndrome）
（46, 5p－）
contains TERT (telomerase reverse transcriptase)gene
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2. Duplication
A duplication is a repeated part of the
genetic material

串联重复(tandem duplication)
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非串联重复(nontandem duplication)
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Duplication arise in several ways
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Duplication loop
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Most duplications have no obvious
phenotypic consequences
Some duplications have phenotypic
consequences
In human, heterozygosity for
duplications covering more than 5% of
the haploid genomes is most often lethal
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Unequal crossing-over
between duplications
increases or decreases
gene copy number
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Proposed generation of variant human hemoglobin subunits
by unequal crossing-over in the γ-δ-β genetic region.
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The extra region of a duplication is
free to undergo gene mutation
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3. Inversion
Inversion rearrange the linear gene
sequence
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Pericentric inversion
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Paracentric inversion 臂内倒位
臂间倒位
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Chromosome 4 differs in humans and
chimpanzees in a pericentric inversion
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Inversion homozygote is normal
Inversion heterozygote form an inversion
loop in meiosis
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Genetic properties of inversions
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Most inversions do not result in an abnormal
phenotype
If one end of an inversion lies within the DNA
of a gene, inversion can affect phenotype
Inversions reduce the frequency of crossingover and recombination
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movie
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Crossing-over within an inversion loop
produces aberrant recombinant
chromatids
The genetically unbalanced gametes are
lethal to the zygote’s development
Inversions act as crossover suppressors
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Balancer chromosome
Geneticists use crossover suppression to create
balancer chromosomes, which contain multiple,
overlapping inversions (both pericentric and
paracentric), as well as with a dominant
morphological phenotype to mark the balancer
chromosome.
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Cy: Curly wings, Dominant,
Recessive lethal
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Balancer chromosomes are useful in stock keeping
(平衡致死系)
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4. Translocation
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Translocation alter the location of
chromosomal segments
Reciprocal translocations 相互易位
Nonreciprocal translocation 非相互易位
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罗伯逊式易位
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Human chromosome 2 contains a Robertsonian translocation that is
not present in chimps(黑猩猩), gorillas(大猩猩), or orangutans(猩猩)
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Translocation homozygote is normal
Translocation heterozygote form a
crosslike configuration in meiosis
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Translocation heterozygote diminishes
fertility（semisterility半不育） and
results in pseudolinkage(假连锁)
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See
movie
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Some kinds of cancer are associated
with translocation in somatic cells
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Reciprocal translocation causes one
kind of leukemia
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Somatic genome variations
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Single-cell Sequencing
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Tumour evolution inferred by single-cell sequencing.
Nature 472, 90–94 (2011)
Mosaic Copy Number Variation in Human Neurons.
Science 342, 632 (2013)
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Identified aneuploid neurons, as well as numerous DNA
copy number variation (CNV) in euploid neurons.
Somatic CNVs occurred in almost half of human frontal
cortex neurons. These deletions and duplications ranged
from about 3 Mb to an entire chromosome in size.
第二节 染色体数目变异
Changes in Chromosome Number
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Chromosome set (染色体组)：The group of
different chromosomes that carries the basic
set of genetic information of a particular
species.
Euploid (整倍体) ：Organisms with complete
chromosome sets
Aneuploid (非整倍体) : Organisms gain or
lost one or more chromosomes, but not a
chromosome set
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1. Euploid (整倍体 )
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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(octaploid) 8n
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Monoploid organisms are usually infertile,
but they are important in plant breeding
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Using monoploid plants
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Polyploid
autopolyploids 同源多倍体
allopolyploids 异源多倍体
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Many polyploid plants are larger than
their diploid counterparts
Octaploid
Diploid
Diploid
Tetraploid
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Triploids
Formation of a triploid organism
Triploids are almost sterile
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Fish tale : The Triploid Grass Carp
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Taking triploid carp as an example, what
controversies may emerge as similar modified
species become available for widespread use?
If you were a state employee in charge of a
specific waterway, what questions would you ask
before you approved the introduction of a
laboratory-produced, polyploid species into your
waterway?
Why would the creation and use of a tetraploid
grass carp species be less desirable in the above
situation?
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Tetraploid
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Tissue-specific polyploidy
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Endopolyploidy is the condition in which
only certain cells in an otherwise diploid
organism are polyploid.
Numerous examples of naturally occurring
endopolyploidy have been observed.
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Vertebrate liver cell nuclei, 4n, 8n, or 16n
The proliferation of chromosome copies often
occurs in cells where high levels of certain
gene products are required.
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Allopolyploid 异源多倍体
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双二倍体
Doubled diploid
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Three species
of Brassica
(blue boxes)
and their
allopolyploids
(pink boxes),
showing the
importance of
allopolyploidy
in the
production of
new species.
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Some allopolyploids have agriculturally
desirable traits derived from two species
X
2n1(= 4X1) = 28
doubling
2n2(= 2X2) = 14
黑小麦
2n1 + 2n2 = 42
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G. Karpechenko in 1928
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2. Aneuploid (非整倍体)
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双体(disomic)：2n ABCD/ABCD
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单体(monosomic): 2n-1 ABCD/ABC
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缺体(nullisomic): 2n-2 ABC/ABC
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三体(trisomic): 2n+1 ABCD/ABCD/A
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四体(tetrasomic): 2n+2 ABCD/ABCD/AA
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双三体(double trisomic): 2n+1+1 ABCD/ABCD/AB
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Capsule phenotypes of the fruits of the jimson weed（曼陀罗）
Trisomic strains for each chromosome of rice ( Oryza sativa )
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Nondisjunction （不分离）in mitosis or
meiosis is the cause of most aneuploids.
See movie1
movie2
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Aneuploid and human diseases
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Down syndrome
47, 21+
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Genetic studies of Down syndrome
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Down syndrome critical region (DSCR)
of chromosome 21 contains the genes that
are dosage sensitive
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A mouse model was created in 2004
The presences of three copies of the genes in this
region are necessary, but themselves not
sufficient for the syndrome.
DSCR1 gene: decreased risk of developing a
number of cancers involving solid tumors
The Origin of the Extra 21st
Chromosome in Down Syndrome
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The ovum is the source in about 95% 47,21+
trisomy cases
Nondisjunction during meiosis I: 75%
Genetic counseling
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Amniocentesis羊膜穿刺
Chorionic villus sampling (CVS)
绒膜绒毛取样
Noninvasive prenatal genetic
diagnosis (NIPGD)
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Why the nondisjunctional event clearly
increases with maternal age?
Arrested at the diplotene
substage of prophase I
初级卵母细胞
卵原细胞
Familial Down Syndrome
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Patau syndrome
47, 13+
1/15000
About half of children with trisomy
13 die within the first month of life,
and 95% die by the age of 3.
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Child with trisomy 18,
Edward syndrome,
1/8000
Few live for more than a year after birth
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Whether aneuploid occurred only on
these three autosomal chromosomes?
Approximately 20% conceptions terminate in
spontaneous abortion
About 30% spontaneously aborted fetuses
demonstrate some forms of chromosomal
imbalance.
6% conceptions contain an abnormal chromosome
Turner syndrome
(XO)
Klinefelter syndrome
(XXY)
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Origin of a
human sexual
mosaic(嵌合体）
(XY)(XO)
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第三节 基因突变
Gene mutation
一、基因突变的类型( Types of mutation)
Mutation may be classified in various ways
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Germinal and Somatic mutation
Spontaneous and induced mutation
Forward mutation and reverse mutation
Phenotypic effects:
(morphological形态, biochemical, behavior,
regulatory, lethal, conditional……)
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二、突变的特点
Characteristics of mutations
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1. Mutation: spontaneous or induced
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Spontaneous mutations
Induced mutations
Mutation frequencies vary among
organisms and genes
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10-8 to 10-9 detectable mutation per nucleotide pair
per generation (phage and bacteria) ; 10-7 to 10-9
(eukaryotes)
10-4 to 10-7 per gene per generation
Mutagenic agents can increase mutation frequencies
by orders of magnitude
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Mutation rates of different genes in different organisms
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2. Random, but exists hot spots of
mutation
deamination
5-methylcytosine
T
5-甲基胞嘧啶
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The distribution of 140 spontaneous mutations in lacI . Boxes indicate
position of point mutations, with red designating fast-reverting mutants.
Deletions are represented in gold. Circles designate larger deletion and insertion
mutants. Allele numbers correspond to mutants that have been sequenced.
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3. Mutation: usually a random,
nonadaptive process
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Random, nonadaptive
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Fluctuation test
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Adaptive mutation
Cairns J. et.al
1988 Nature 335:142-145
The origin of mutants
Lac－
Lac＋
Minimal medium
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4. Mutation: a reversible process
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Forward mutation
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Multiple directions
Reverse mutation
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Back mutation
Suppressor mutation
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三、突变的分子基础
(Molecular basis of mutation)
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1. Spontaneous mutation（自发突变）
① Errors in DNA replication （复制错误）
Consequences of point mutations within genes
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② Spontaneous
lesions (自发损伤)
Depurination
（脱嘌呤）
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Deamination （脱氨基）
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Oxidatively damaged bases
（氧化性损伤碱基）
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③ Spontaneous mutations in human:
trinucleotide-repeat diseases
A specific area along the
metaphase chromosomes
prepared from human cell
culture failed to stain, giving the
appearance o f a gap. Such
areas eventually became
known as fragile sites, which
were susceptible to
chromosome breakage when
cultured in the absence of
certain chemicals such as folic
acid, which is normally present
in the culture medium.
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Fragile X syndrome
(脆性X染色体综合症)
 Inherited mental retardation
 Affects about 1 in 4000 males and 1 in
8000 females
 Affected females usually carry only one
fragile X chromosome
 Penetrance is not complete
 FMR-1 (fragile-X-associated-mentalretardation)
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The possible mechanism of trinucleotide
repeat amplification（三核苷酸重复扩增）
Steisinger model
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Examples of genetic diseases caused by
expanding trinucleotide repeats
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2. Induced mutations 诱发突变
① Radiation 射线
 Ultraviolet（UV） irradiation 紫外线
 Ionizing radiation 离子射线
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UV light–generated photoproducts
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② Chemical Mutagens
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Base analogs 碱基类似物
5-bromouracil (5-BU) (T) 5-溴尿嘧啶
5-BU pairs with G
2-amino-purine (2-AP) (A) 2-氨基嘌呤
2-AP pairs with C
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Alkylating agents（烷化剂）
ethylmethanesulfonate (EMS)
乙基甲烷磺酸
nitrosoguanidine (NTG) 亚硝基胍
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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
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Aflatoxin B1 (AFB1)
黄曲霉毒素
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③ The Ames test :
evaluating mutagens in our environment
1974, Bruce Ames
Salmonella typhimurium
DNA repair system has been inactivated
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四、定点诱变
（site-directed mutations）
Reverse Genetics
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See movie
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第三节 DNA的修复机制
Mechanisms of DNA repair
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1. Proofreading and mismatch repair

Proofreading:
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DNA Polymerase III (bacteria) : error rate
of 10-5.
Proofread: 3’ to 5’ exonuclease, a final
error rate of 10-7.
Mismatch repair:

Mismatched bases and small loops
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Model for mismatch
repair in E. coli
Mismatch repair reduces the
error rate to less than 10-9
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2. Photoreactivation repair (光复活)
in prokaryotes
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Direct reversal of
damaged DNA
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3. Excision Repair（切除修复） in
Prokaryotes and Eukaryotes

Base-Excision Repair

Nucleotide-Excision Repair (NER)
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Minor base damage is
detected and repaired by
Base-Excision Repair
DNA glycosylase
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Nucleotide-Excision Repair remove larger
defects like thymine dimers and bases with
bulky side-groups
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Excision Repair in E.coli
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Nucleotide-Excision Repair
and human disease
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NER in eukaryotes involves many
more proteins
Xeroderma pigmentosum
着色性干皮病
Autosomal recessive, XPA-XPG
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4. Recombinational repair (postreplication repair)
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5. Error prone repair
SOS repair
(error prone)
Translesion DNA synthesis
(跨损伤DNA合成)
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Problems and Discussion Questions
1. What is the difference between a chromosomal
aberration and a gene mutation?
2. Inversions are said to “suppress crossing over”. Is
this terminology technically correct? If not,
restate the description accurately.
3. Why are translocation heterozygotes semisterile?
Why are translocation homozygotes fully fertile?
4. How could you isolate a mutant strain of bacteria
that is resistant to penicillin (an antibiotic)?
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作业：
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
P305：4，8
P325：10
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The figures and tables are cited from:
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Genetics (From genes to genomes), Leland
Hartwell, Mcgraw-Hill Companies, Inc
Concept of Genetics, William S.Klug, Prentice Hall,
Inc
Introduction to Genetics Analysis, Anthony J.F.
Griffiths, W.H.Freeman, Inc
Principle of Genetics, D.Peter Snustad, John Wiley
& Sons, Inc
Genetics-A Conceptual Approach, Benjamin A.
Pierce, W. H. Freeman
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