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

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
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
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
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.
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 )
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.
(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.
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.
recombination can be divided into three types：
➢
Homologous recombination；
➢
Site-specific recombination；
➢
Transpositional recombination
4.3.1 Homologous recombinantion
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.
In a broad sense, any cause of genotype changes in gene exchange is recombination.
According to the difference of the
mechanisms and protein factors required,
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.
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.
1 Types of gene mutation
(1) Induced mutation and spontaneous mutation (2) Single-point mutation and multi-point mutation (3) Synonymous mutation and missense mutation (4) Nonsense mutation, stop codon mutation and
Also known as general recomination, this process involve the exchange of homologous regions between two DNA molecules.
Example: The eukaryotic nonsister chromatid
Schematic ofλ phage integrated
4.4 Transposition
Replicated copies of transposable DNA elements can insert themselves anywhere in the genome.
All transposons encode a transposase which catalyzes the insertion.
repair
Comparing of 5 - methyl cytosine deamination and cytosine deamination
4.2 DNA repair
1 Mismatch repair
2 Uracil-N-glucosidic bond system
Glycosylase
suppressor mutation (5) Other types
Two adjacent TT by ultraviolet radiation to produce the dimer
Base chemical modifiers
base mutation by alkylation agent
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.
5 Recombination repair
Recombi源自文库ation repair
6 SOS (Emergency repair system)
4.3 Recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one.
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
chr.9
CI
SH Bz
Wx
Ac
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.