If we have an error one in a million this mean we will have 3000 errors during each replication cycle of the genome The genome of a typical mammalian cell accumulates many thousands of lesions during a 24hour period ID: 928398
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Slide1
DNA Repair
Slide2The human genome contains 3 billion base pairs .
If we have an error one in a million this mean we will have 3000 errors during each replication cycle of the genome.
Slide3The genome of a typical mammalian cell
accumulates many
thousands of lesions during a 24-hour period.
However, as a result of DNA repair, fewer than 1 in
1,000 becomes
a mutation. DNA is a relatively stable molecule
, but
in the absence of repair systems, the
cumulative effect
of many infrequent but damaging
reactions would
make life impossible.
Slide4After
synthesis, the DNA may undergo some damages during the cell life-span, but naturally there are certain repair systems present inside the body that can provide protections against DNA damages.
The
rate of DNA repair is dependent on many factors, including the cell type, the age of the cell, and the extracellular
environment.
Slide5A
cell that has accumulated a large amount of DNA damage, or one that no longer effectively repairs damage incurred to its DNA, can enter one of three possible states:
an irreversible state of dormancy, known as
senescence
( getting old )
cell suicide, also known
as
apoptosis
or
programmed cell death
unregulated cell division, which can lead to the formation of a
tumor
that is
cancerous
Slide6The 2015
Nobel Prize in Chemistry
was awarded to
Tomas
Lindahl
,
Paul
Modrich
, and
Aziz
Sancar
(
Turkish-American in the field of genetics, was born in
Mardin
in 1946 ), for
their work on the molecular mechanisms of DNA repair
processes.
There are two types:
nucleotide excision repair
and
base excision repair
Slide7I
. Mismatch Repair System during DNA replication
1.Exonucleolytic proofreading of DNA polymerase usually occurring simultaneously with DNA replication. In these processes DNA molecules with single mismatched 3’ OH ends do not become effective templates because polymerase cannot extend when 3’ OH is not base paired. DNA polymerase has a separate
exonuclease
catalytic site that removes unpaired residues at the terminus and replace them by the correct base pair.
Slide82
. An
alternative DNA mismatch repair is a system which recognizes and repairs major insertion, deletion and
mis
-incorporation of bases
. During
DNA synthesis the newly synthesized (daughter) strand often includes errors and such defective copy of the mismatch repair gene could have serious consequences on human health.
Slide9The mismatch repair system carries out the following corrections:
Removes replication errors which are not
recognized
by the replication
machine.
Detects structural changes in the DNA
helix.
The newly synthesized strand is preferentially nicked to be distinguished from the parental strand. Next step is the binding of mismatch proofreading complex at the defective base pair of the new strand, followed by removal of segment from this strand to be correctly replaced using the parental strand as a template
.
Slide10II
. Repair
of DNA damage after DNA maturation
Main
Causes of DNA damage that occur after DNA synthesis:
1. Chemical
pollutants
2
. Radiation
Industrial chemicals such as vinyl chloride or hydrogen peroxide, and environmental chemicals such as polycyclic hydrocarbons found in smoke and tar create a large chemical modifications in DNA leading to the formation of oxidized bases, alkylated bases such as
methyl bases
or conversion of one base into another type (conversion of
cytidine
into
uridine
).
Slide11Thermal disruption at elevated temperature increases the rate of
depurination
(loss of purine bases from the DNA backbone) and single strand breaks. For example, hydrolytic
depurination
is seen in the
thermophilic
bacteria, which grow in hot springs above 80 °C
.
Slide12The main types of DNA damage caused by these environmental factors are:
base
loss
base
modification
strands breakage.
Slide13Thymine
oxidation
Repair
of DNA damages
Despite
the exposure of DNA to large damaging environmental factors each day, very few damages actually have serious effects on human
chromosomal
DNA due to highly efficient repair mechanisms. An inactivation or loss of functions in these DNA repair systems may cause errors in replication and lead to genetic diseases such as :
Slide14Slide15Fanconi
anaemia
(
FA
)
It is
a rare
genetic disease
. Among those affected the majority develop
cancer
, most often
acute
myelogenous
leukemia
, and 90% develop
bone marrow failure
(the inability to produce blood cells) by age 40. About 60–75% of people have
congenital defects
, commonly
short stature
, abnormalities of the skin, arms, head, eyes, kidneys, and ears, and developmental disabilities.
Slide16Lynch syndrome
(
HNPCC
or
hereditary
nonpolyposis
colorectal cancer
)
It is
an
autosomal dominant
genetic condition that has a high risk of
colon
cancer
as well as other cancers including
endometrial
cancer
(second most common),
ovary
,
stomach
,
small intestine
,
hepatobiliary
tract
, upper
urinary tract
,
brain
, and
skin
. The increased risk for these cancers is due to inherited mutations that impair
DNA mismatch repair
. It is a type of
cancer syndrome
.
Slide17Werner syndrome
(WS)
It is also
known as "adult
progeria
", is
a rare,
autosomal
recessive
progeroid
syndrome
(PS), which is characterized by the appearance of premature
aging
. The
median and mean ages of death are 47–48 and 54 years,
respectively. The
main cause of death is
cardiovascular disease
or
cancer.
Slide18Types of DNA repair systems
Base
Excision Repair (BER)
BER is a cellular mechanism that repairs damaged DNA throughout the cell cycle. It is
primarily responsible for removing small, non-helix distorting base lesions from
the genome
. BER is important for removing damaged bases that could otherwise
cause mutations
by
mispairing
or lead to breaks in DNA during replication. The repair
system containing
the following system:
Slide19First
, a
glycosylase
enzyme recognizes a specific type of incorrect base., which
then cleaves
the N-
glycosyl
bond to remove the defective nitrogen base and generating
an
apurinic
or
apyrimidinic
(AP) site.
Different DNA
glycosylases
recognize different
types of
defective bases
.
Each DNA
glycosylase
is generally specific for one
type of lesion.
Uracil DNA
glycosylases
, for example, found in most cells, specifically remove from DNA the uracil that results from spontaneous deamination of cytosine. Mutant cells that lack this enzyme have a high rate of
G
C to A=T
mutations.
2
. The AP sites generated by
glycosylase
action are then recognized by
AP endonuclease
, which cleaves the
phosphodiester
backbone immediately 5' to the AP site, leaving a 3'-OH and 5'-deoxyribose-phosphate terminus (incision step).
Slide213. The 5'
deoxyribose
phosphate is then removed by specific
exonuclease
called DNA
deoxyribo-phosphodiesterase
(
dRpase
), leaving a nick (gap).
4. The second strand is used as a template to fill in the gap. This process is carried out by a DNA polymerase and a DNA
ligase.
Slide22Slide23UV-B light causes covalent cross-linking between adjacent thymine bases creating
cyclobutane
thymine dimmers
. Ionizing radiation such as that created by radioactive or x-rays causes breaks in DNA strands. When thymine dimers are present, the double helix is distorted (bent), as the
thymines
are pulled toward each other. Hydrogen bonding to adenines on the opposite strand is weakened .The distortion causes a loss of template information. In addition, it can prevent the advancing replication fork or inhibiting the process of RNA transcription.
Slide24Fig. 10-13, p.252
UV irradiation causes dimerization of adjacent thymine bases
Slide25Fig. 10-14, p.252
Oxidation
damage ( oxygen radicals in the presence of metal ions such as Fe
2
+ can destroy sugar rings in DNA,
breaking
the
strand.)
Slide26Fig. 10-15, p.253
Missmatch
repair
in
E.coli
Slide27BB
Fig. 10-16, p.254
Base –
excition
repair
Slide28Nucleotide
Excision Repair (NER)
The
related nucleotide excision repair pathway repairs bulky helix-distorting lesions
1. Specific
endonuclease cleaves damaged DNA
on
either side of a lesion thereby producing a
short
gap.
2. Highly
processive
DNA polymerase fills in the
gap
and DNA ligase completes the repair
process
.
Slide29Fig. 10-17, p.254
Nucleotide-excision
repair
Slide30Slide31p.255
Recombination can be used to repair infrequent lesions
Slide32Thymine dimer excision repair
system
A photo-reactivating enzyme system that can recognizes the thymine dimer and
in response
to visible light this enzyme system is stimulated to cleave the thymine
rings. The
system contains
UV
endonuclease
that
cleaves the damaged DNA on either side
of the
dimer to produce a short gap (about 100 nucleotides) on a single strand that can
be filled
by DNA polymerase using
opposite strand as a
template. Then the
DNA ligase
is used
to complete the repair
process.
Slide33Slide34Xeroderma
pigmentosum
It is
a rare autosomal recessive
genetic disorder
of
DNA repair
in which the ability to repair damage caused by
ultraviolet
(UV) light is
deficient.
Slide35Nearly 90% of these individuals develop skin carcinomas Most XP individuals also suffer from neurologic disorders including mental retardation. XP individuals are extremely sensitive to sunlight (UV light) and are unable to efficiently repair thymine dimers and other types of DNA damages
Slide36END