C4-Gene mutation and exchange 分子生物学双语课程PPT

l IS
target site
target sequence repeats directly flanked the IS
General pattern of DNA transposon
4.4.1.2 Composite transposon
A composite transposon has protein coding DNA segments flanked by inverted, repeated sequences that can be recognized by transposase enzymes.
repair
Comparing of 5 - methyl cytosine deamination and cytosine deamination
4.2 DNA repair
1 Mismatch repair
2 Uracil-N-glucosidic bond system
Glycosylase
4.4.1 Transposons and their structural characteristics
➢ Insertion sequences, IS ➢ Composite transposon ➢ TnA family
4.4.1.1 IS element
Insertion sequences are small relative to other transposable elements and only code for proteins (usually the transposase ) implicated in the transposition activity, allowing the IS to move, and also one regulatory protein which either stimulates or inhibits the transposition activity.
4.4.2 Transposons in eukaryotes 4.4.2.1 Transposons in maize
Autonomous element Nonautonomous element
（1）Ac-Ds system
11bp Ac
Ds
transposable element 是引起玉米糊粉层花斑不稳定 现象的遗传因子
糖基酶
Mismatch base
3 Photoreactivation
The pyrimidine dimer blocks DNA replication
photolyase binding energy Cut C-C bond
release
4 Excision repair(dark repair )
4.4.2.2 Transposons in Drosophila melanogaster
There are two kinds: ➢P ➢ Copia
31bp
The copia element is ~5000 bp long, with identical direct terminal repeats of 276 bp. Each of the direct repeats itself ends in related inverted repeats. A direct repeat of 5 bp of target DNA is generated at the site of insertion. The divergence between individual members of the copia family is slight, <5%; variants often contain small deletions.
Chapter Ⅳ
Gene mutation and exchange
4.1 Mutation
Mutations are heritable permanent changes (Addition, deletion, transition etc.) in the base sequence of DNA. It is the main factor in biological evolution.
(6) Isomerization (7）Branch migration
Meslson-Radding Model
2 Site-specific recombination
This involves the exchange of nonhomologous but specific pieces of DNA and is mediated by proteins (enzyme) that recognize specific DNA sequences.
Also known as general recomination, this process involve the exchange of homologous regions between two DNA molecules.
Example: The eukaryotic nonsister chromatid
Definition: A region of DNA that exhibits an
unusually high propensity to mutate. One of the main reasons is the modifications of the
bases, that deamination of 5- methylcytosine forming thymine.
chr.9
c
Ds
CI CcI i
SH Bz
Ac
Ds
cCiI
SsHh Bz
large colored sectors
Wx
Wx
（2）Spm-dSpm system
13bp
Spm/dSpm has two genes. tnpA consists of 11 exons that are transcribed into a spliced 2500 base mRNA. tnpB may consist of a 6000 base mRNA containing ORF1 + ORF2.
Transcripts of copia are found as abundant poly(A)+ mRNAs, representing both full-length and part-length transcripts. The mRNAs have a common 5' terminus. Several proteins are produced, probably involving events such as splicing of RNA.
base mutation by hydroxylamine
Structure of base analog
keto form enol form
5-BU caused the A-T to the G-C conversion
Pairing of 2 – amino purine and cytosine
recombination can be divided into three types：
➢
Homologous recombination；
➢
Site-specific recombination；
➢
Transpositional recombination
4.3.1 Homologous recombinantion
Conversion the A-T to the G-C caused by 2 – aminopurine(2-AP)
Some mutagenicities intercalate agent
Mutation mechanism induced by intercalate agent
3 Hot spot of mutation
exchange; Transformation of bacteria; bacterial transduction, etc.
4.3.1.1 Holliday model
4.3.1.2 Meslson-Radding model
There are 7 steps：
（1）Nicking （2）Strand displacement （3）Single-strand invasion （4）Loop cleavage （5）Strand assimilation
Composite transposons will also often carry one or more genes conferring antibiotic resistance.
4.1.1.3 TnA family
TnA family is about 5000bp, which is much greater than the insertion sequence. The same as composite transposon, TnA family also carries the gene who is responsible for its own transposition and other gene such as resistance gene β-amine acyl enzyme (AmpR) . It has no IS, but there are terminal repeat sequences of about 37-38bp in the end.