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
NUCLEOTIDE EXCISION REPAIR (NER)
NER differs from BER in several ways:
- It uses different enzymes (XP products).
- Even “though there may be only a single “bad” base to correct, NER removes a large “patch around
the damage such as to removes DNA damage induced by ultraviolet light (UV), such as thymine dimer.
NUCLEOTIDE EXCISION REPAIR (NER)
The steps and some key players:
The damage is recognized by ______________, also functions in normal transcription) and
may be more protein factors that assemble at the location.
Transcription Factor IIH, (TFIIH)
NUCLEOTIDE EXCISION REPAIR (NER)
The steps and some key players:
The DNA is unwound producing a “bubble”. The enzyme system (Numerous proteins, including _____
products (____________), make cut both the ___’ side and the _____’ side of the damaged area so the
tract containing the damage can be removed.
XP ( XPA, XPB, XPF, XPG)
3’
5’
NUCLEOTIDE EXCISION REPAIR (NER)
The steps and some key players:
A fresh burst of DNA synthesis — using the intact (opposite) strand as a template — fills in the correct
nucleotides with ___________
A fresh burst of DNA synthesis — using the intact (opposite) strand as a template — fills in the correct
nucleotides with DNA polymerase delta and epsilon
XERODERMA PIGMENTOSUM (XP)
Type of disease
A rare inherited autosomal recessive
disease of humans in which a deficiency of
excinuclease occurs
XERODERMA PIGMENTOSUM (XP)
XP can be caused by
XP can be caused by mutations in any one
of several genes (XPA, XPB, XPF, XPG), all
of which have roles to play in NER
XP resulting in
- XP resulting in skin discolouration and
multiple tumours on exposure to UV light.
XP unprepared __________ in humans may lead to ___________
- Unrepaired pyrimidine dimers in humans
may lead to melanoma
MISMATCH REPAIR (MMR)
- DNA mismatch repair is a system for recognizing and repairing erroneous insertion,
deletion, and mis-incorporation of bases that can arise during ____________________, as well as repairing some forms of ____________
- DNA mismatch repair is a system for recognizing and repairing erroneous insertion,
deletion, and mis-incorporation of bases that can arise during DNA replication and
recombination, as well as repairing some forms of DNA damage
MISMATCH REPAIR (MMR)
Mismatch repair deals with correcting mismatches of the __________; that is, failures to
maintain normal ________ base pairing (AT, CG)
Mismatch repair deals with correcting mismatches of the normal bases; that is, failures to
maintain normal Watson-Crick base pairing (AT, CG)
MISMATCH REPAIR (MMR)
It can enlist the aid of enzymes involved in both ______________ and ____________ as well as using enzymes specialized for this function.
It can enlist the aid of enzymes involved in both base-excision repair (BER) and nucleotide-
excision repair (NER) as well as using enzymes specialized for this function.
MISMATCH REPAIR (MMR)
- Recognition of a mismatch requires several different proteins including one encoded by
__________ known as ________________, a caretaker gene
- Recognition of a mismatch requires several different proteins including one encoded by
MSH2 known as MutS protein homolog 2, a caretaker gene
MISMATCH REPAIR (MMR)
Cutting the mismatch out also requires several proteins, including one encoded by ______
known as ______ homologs. It forms a complex with _____ and _______, increasing the _______
footprint on the DNA.
Cutting the mismatch out also requires several proteins, including one encoded by MLH1
known as MutL homologs. It forms a complex with MutS and MutH, increasing the MutS
footprint on the DNA.
MISMATCH REPAIR (MMR)
Mutations in either of these genes predisposes the person to an inherited form of colon
cancer. So these genes qualify as _____________
Tumor suppressor genes
Mismatch Repair in Prokaryotes and Eukaryotes
Mut S recognizes
and binds mismatch
Mismatch Repair in Prokaryotes and Eukaryotes
Mut L links
S to H
Mismatch Repair in Prokaryotes and Eukaryotes
Mut H recognizes the
CH3-parental strand and makes nick on daughter strand
Mismatch Repair in Prokaryotes and Eukaryotes
In human: MutS =
hMSH (1-6); MutL = hMLH1 and hPMS2; MutH = GTBP
REPAIRING STRAND BREAKS
Single-Strand Breaks (SSBs)
- Breaks in a single strand of the DNA molecule are repaired using the
same enzyme systems that are used in Base-Excision Repair (BER). - BER, NER and MMR are all strand specific SSB repairing system
REPAIRING STRAND BREAKS
Double-Strand Breaks (DSBs)
There are two mechanisms by which the cell attempts to repair a
complete break in a DNA molecule:
- Direct joining of the broken ends. This requires proteins that recognize and bind
to the exposed ends and bring them together for ligating. They would prefer to
see some complementary nucleotides but can proceed without them so this
type of joining is also called Nonhomologous End-Joining (NHEJ). - Homologous Recombination (next page)
Direct joining
Direct joining of the broken ends. This requires proteins that recognize and bind
to the exposed ends and bring them together for ligating. They would prefer to
see some complementary nucleotides but can proceed without them so this
type of joining is also called Nonhomologous End-Joining (NHEJ)
REPAIRING OF DOUBLE STRAND BREAKS
Homologous Recombination (also known as Homology-
Directed Repair HDR).
Sister chromatids
available in G2 after chromosome duplication
REPAIRING OF DOUBLE STRAND BREAKS
Homologous Recombination (also known as Homology-
Directed Repair HDR).
Homologous chromosome
(in G1; that is, before each chromosome
has been duplicated). This requires searching around in the nucleus
for the homolog - a task sufficiently uncertain that G1 cells usually
prefer to mend their DSBs by NHEJ.
REPAIRING OF DOUBLE STRAND BREAKS
Homologous Recombination (also known as Homology-
Directed Repair HDR).
Same chromosome
if there are duplicate copies of the gene on the
chromosome oriented in opposite directions (head-to-head or back-to-
back).