Post on 20-Dec-2015
1. Switch of one base for another:
purine
pyrimidine
(transition) (transversion)
2. insertion or deletion of a nucleotide
The nature of mutations: Point mutation
Replication errors and their repair
Drastic changes in DNA
DeletionInsertionRearrangement of chromosome
By insertion of a transposon, or aberrant actions of recombinationProcess.
Mismatch repair removes errors escape proofreading 1. It must scan the genome. 2. The system must correct the mismatch accurately.
Scan DNA
Distortion in the backbone
Embracing mismatch;Inducing a kick in DNA;Conformational change inMutS itself
(III)
Nicking is followed by Helicase (UvrD) and one of exonucleases
MutL activate MutH
DNA methylation to recognize the parental strain
Once activated, MutH selectively nicks the Unmethylated strand.
Mismatch repair system in Eukaryotics
MutS MutLE. coli
MSH(MutS homolog)
Eukaryotics MLH or PMS
Hereditary nonpolyposis colorectal cancer(mutations in human homologes of Muts and MutL)
DNA damageRadiation, chemical mutagens, and spontaneous damage
deamination
spontaneous damage due to hydrolysis and deamination
Base pair with A
depurination
Deamination converts adenine to hypoxanthine, base pair with CDeamination converts Guanine to xanthine, base with C but only two H bonds
DNA damage
spontaneous damage to generate natural base
deamination
Methylated Cs are hot spot for spontaneous mutation in vertebrate DNA
Damaged by alkylation and oxidation
Alkylation at the oxygen of carbon atom 6 of G : O6-metylguanine,often mispairs with T.
Oxidation of G generates oxoG, it can mispair with A and C. a G:C to T:A transversion is one of the most common mutation in human cancers.
Gamma radiation and X-rays
• Cause double-strand breaks in the DNA, which are difficult to repair.
• Ionizing radiation and agents like bleomycin that cause DNA to break are said to be clastogenic (p245).
DNA damage by UV
Thymine dimer
These linked bases are incapable of base-pairing and cause DNA polymerase to stop.
Mutations caused by base analogs and intercalating agents
Base analogs
Analogs mispair to cause mistakes during replication
Thymine analog
Mutations caused by intercalating agents
Intercalating agents
flat molecules
Causing addition or deletion of bases during replication
Repair of DNA Damage: DNA repair system
Excision repair systems: the damaged nucleotide is not repaired but removed from the DNA, the other undamaged strand serves as a template for reincorporation of the correct nt by DNA polymeraseRecombination repair: both strands are damaged. Sequence information is retrieved from a second undamaged copy of the chromosome.
Direct reversal of DNA damagephotoreactivation
breaking covalent bond
O6-metylguanine To its own cytosine
Capture energy from light
Base excision repairDNA glycosylases are lesion-specific and cells have multiple DNA glycosylases 1. Uracil glycosylase 2. Another specific glycosylase is responsible for removing oxoG
AP: apurinic or apyrimidinic
Base excision repair
If a damaged base is not removed by base excision before DNA replication: a fail-safe systemoxoG:A repair
Nucleotide Excision Repair
UvrA detecting distortion
UvrB melting DNA
Recognizing distortions to the shape of the DNA (thymine dimer or bulky chemical adduct)
8 nt away from 5’4-5 nt away from 3’
In E.coli: 4 proteins involved
Nucleotide Excision Repair
The principles of nucleotide excision repair in higher cells is much the same as in E. coli but us moer complicated, involving 25 or more polypeptides.The UVR proteins are needed to mend damage from UV light; Mutants of uvr genes are sensitive to UV light, and lack the capacity toremove T-T or T-C adducts.
In human, xeroderma pigmentosum patients have mutations in seven genes(XP genes). These XP proteins are corresponding to proteins involved innucleotide excision repair.
Transcription-coupled repair
Involves recruitment to the stalled RNA polymerase of nucleotide excision repair proteins
It focuses repair on genes being actively transcribed.
TFIIH unwinds the DNA template during the initiation of transcription. Subunits of TFIIH include the DNA helix-opening proteins XPA and XPD.