Lecture 4: Errors, Repair, SNP Flashcards
Types of DNA damage
- Single strand breaks
- Double strand breaks
- Pyrimidine Dimer
- Nucleotide base oxidation
Cause of DNA damage
Exogenous
Thermal disruption
UV light exposure
Ionizing radiation
Exposure to mutagens, carcinogens, and viruses
Cause of DNA damage
Endogenous
Cellular metabolism
Hydrolysis
Nuclease Digestion
AGENTS THAT DAMAGE DNA
Certain wavelengths of radiation
- ionizing radiation such as gamma rays and X-rays
- ultraviolet rays, especially the UV-C rays (~260 nm) that are absorbed strongly by
DNA but also the longer-wavelength UV-B that penetrates the ozone shield.
AGENTS THAT DAMAGE DNA
Chemicals in the environment
- many hydrocarbons, including some found in cigarette smoke
- some plant and microbial products, e.g. the aflatoxins produced in moldy peanuts
- Chemicals used in chemotherapy, especially chemotherapy of cancers
AGENTS THAT DAMAGE DNA
Intrinsic Spontaneous mutation
- most error during DNA replication by error of polymerase 3’ to 5’ exonuclease activity
- MMR enzyme mutation caused mismatch repair failure
- Highly-reactive oxygen radicals produced during normal cellular respiration as well as
by other biochemical pathways.
REPAIRING DAMAGED BASES
The recent publication of the human genome has revealed _________ genes whose
products participate in DNA repair. More is expected to be discovered
130
REPAIRING DAMAGED BASES
- Damaged or inappropriate bases can be repaired by several mechanisms:
- Direct chemical reversal of the damage
- Excision Repair, in which the damaged base or bases are removed and then replaced with
the correct ones in a localized burst of DNA synthesis. There are three modes of excision
repair, each of which employs specialized sets of enzymes. - Base Excision Repair (BER)
- Nucleotide Excision Repair (NER)
- Mismatch Repair (MMR)
REPAIRING DAMAGED BASES
Excision Repair
In which the damaged base or bases are removed and then replaced with
the correct ones in a localized burst of DNA synthesis. There are three modes of excision
repair, each of which employs specialized sets of enzymes.
REPAIRING DAMAGED BASES
The 2015 Nobel Prize in chemistry was shared by three researchers for their
pioneering work in DNA repair: Tomas Lindahl (BER), Aziz Sancar (NER), and
Paul Modrich (MMR).
DIRECT REPAIR GENES
DNA photolyase
- Natural repair system for pyrimidine dimers caused by UV damage
- Directly reverse cyclobutane pyrimidine dimer (CPD) via photochemical reactions
DIRECT REPAIR GENES
O6-methylguanine-DNA methyltransferase (MGMT)
- naturally occurring mutagenic DNA lesion O6-methylguanine back to guanine
- prevents mismatch and errors during DNA replication and transcription
- the methylation state of the MGMT gene promotor determined whether tumor cells
would be responsive to temozolomide drug therapy
BASE EXCISION REPAIR (BER)
The damaged base estimated to occur some _______ times a day in each cell in
our body!
20,000
BASE EXCISION REPAIR (BER)
Remove it by a __________. There are at least 8 genes encoding different
DNA glycosylases:
Remove it by a DNA glycosylase. There are at least 8 genes encoding different
DNA glycosylases
BASE EXCISION REPAIR (BER)
There are at least 8 genes encoding different
DNA glycosylases.
- Each enzyme responsible for identifying and removing a specific kind of base damage.
- Two genes encoding enzymes (AP endonuclease and DNA Exonuclease) function to
removal deoxyribose phosphate in the backbone, producing a gap
BASE EXCISION REPAIR (BER)
Replacement with the correct nucleotide. This relies on DNA polymerase _________, one of at least _____ DNA polymerases encoded by our genes
Replacement with the correct nucleotide. This relies on DNA polymerase
beta, one of at least 11 DNA polymerases encoded by our genes
BASE EXCISION REPAIR (BER)
______ of the break in the strand. Two enzymes are known that can do this;
both require ______ to provide the needed energy.
Ligation of the break in the strand. Two enzymes are known that can do this;
both require ATP to provide the needed energy.
BASE EXCISION REPAIR
One stand of DNA contains __________
base, such as _________
One stand of DNA contains deaminated
base, such as Uracil
BASE EXCISION REPAIR
DNA glycosylases scans _______
the DNA and
removes Uracil, leaving AP site
BASE EXCISION REPAIR
AP endonuclease locates _________
AP site and nicks backbone
BASE EXCISION REPAIR
DNA Exonuclease removes
nucleotides near the nick, leaving gap
BASE EXCISION REPAIR
DNA Polymerase synthesizes to _______
fill in gap
BASE EXCISION REPAIR
DNA Ligase seals the
backbone