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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.