lecture 4 dna repair&mutation
TRANSCRIPT
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DNA MUTATION/DAMAGE
The most common lesion that occurs in DNA is
depurination.
Molecules in the gene do undergo major changes due to
thermal fluctuations. About 5,000 purine bases (adenine and
guanine) are lost per day from the DNA of each human cell
because of the thermal disruption of their N-glycosyl linkagesto deoxyribose (depurination)
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DNA MUTATION/DAMAGE
Similarly spontaneous deaminations of cytosine
to uracil in DNA are estimated to occur at a rate of100 per genome per day.
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DNA MUTATION/DAMAGE
DNA bases are also subject to change by reactive
metabolites that alter their base-pairing abilitiesand by ultraviolet light from the sun, which can
promote a covalent linkage of two adjacent
thymine bases in DNA (forming thymine dimers).
These changes lead to either deletion of one or
more base pairs in the daughter DNA chain after
DNA replication or to a base-pair substitution(e.g., each C U deamination would eventually
change a C-G base pair to a T-A base pair, since
U closely resembles T and forms a
complementary base pair with A).
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DNA REPAIR
The altered position of a damaged or mutatedstrand is recognised and removed by one set of
enzymes and then replaced in its original by
another enzyme, DNA polymerase (which copies
the information stored in the good strand bymeans of complementary base-pairing).
DNA ligase then seals the nick that remains in the
DNA helix to complete restoration of an intactDNA strand.
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DNA REPAIR
Depurination is very efficiently repaired. First, the
presence of the missing base is recognised by a
repair nuclease that cuts the phosphodiesterbackbone at the altered site. The neighbouring
nucleotides (including the damage one) have
been removed by further cuts around the initial
site of incision, an undamaged DNA sequence isrestored.
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DNA REPAIR
Another important repair pathway involves a
battery of different enzymes called DNAglycoylases, each of which recognises a single
type of altered base in DNA and catalyses its
hydrolytic removal from the deoxyribose sugar.
At least 20 different such enzymes are thought to
exist, including those for removing deaminated
Cs, deaminated As, different types of alkylatedbases, bases with opened rings, and bases in
which a carbon-carbon double bond has been
accidentally converted to a carbon-carbon single
bond.
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The reaction catalysed by all DNA glycosylase enzymes.
There are many different DNA glycosylases, each recognising
a different altered base.
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Excision and Restoration-fundamental to DNA repair.
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DNA REPAIR
In cases where the C bases are methylated to produce
5-methylcytosine, at specific points in the DNAsequence it yields a T residue which is a normal
component of DNA, rather than a U residue. As a
result, these particular C deaminations are not
recognised by the cells uracil DNA glycosylases, andthey are consequently not repaired.
The deamination of a methylated cytosine
residue in DNA produces thymine instead of
uracil, which cannot be recognised and
removed by uracil DNA glycosylase
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DNA REPAIR
Multiple-step excision pathway capable ofremoving almost any type of DNA damage that
creates a very large lesion. Such bulky lesions
include those created by the covalent reaction of
DNA bases with large hydrocarbons, such asbenzpyrene, that have carcinogenic potential and
the thymine dimers caused by sunlight.
In such cases, a large multi-enzyme complexrecognises the large distortion in the DNA double
helix that results, rather than a specific base
change.
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Pyrimidine dimer- Two adjacent
pyrimidine residues in the same
strand of DNA, which have
become covalently cross-linked(e.g by UV radiation)
The first step is cleavage of the
phosphodiester backbone nextto the distortion.
The second step is excision of
the lesion and resynthesis of
DNA in its place
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Excision and Restoration
Restoration
reaction: two
steps
A filling by
DNApolymerase of
the gap
created by
excision
events.
The sealing by
DNA ligase of
a nick left in
the repaired
strand
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DNA REPAIR
The importance of these repair processes to life isreflected in the large investment that cells make in
DNA repair enzymes.
For example, a genetic analysis of human patientswith xeroderma pigmentosum suggest that at
least five different enzymes are required for the
excision of bulky lesions alone.
In yeast there are more than 50 different types of
DNA repair enzymes, each monitoring the DNA of
each cell at all times.
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DNA REPAIR
Genetic information can be stably stored in DNA
sequences only because a large variety of different
DNA repair enzymes are continuously scanning the
DNA and removing damaged nucleotides.
The process of DNA repair depends on the fact that
a separate copy of the genetic information is stored
in each strand of the DNA double helix.
An accidental lesion on one strand can therefore be
cut out by a repair enzyme and a good strand
resynthesised from the information in the undamaged
strand.