10- Mutagens and Mutagenesis

7/27/2019 10- Mutagens and Mutagenesis

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1 0 TH WEEK

DNA damage, repair & Mutagenesis


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Mutagenesis

DNA damage, repair & mutagenesis

DNA damage

Mutation: replication fidelity, mutagens,

mutagenesis

DNA repair

DNA lesions: oxidative damage, alkylation, bulkyadducts

Photoreaction, alkyltransferase, excision repair,

mismatch repair, hereditary repair defects


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DNA damage, repair & Mutagenesis

1 Mutagenesis

Mutation

Replication fidelity

Mutagens: chemical & physicalMutagenesis: direct & indirect


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Mutation

ReplicationFidelity

Mutagens

Mutagenesis


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1-1 Mutation

1 Mutaagenesis

Permanent, heritablealterations

in the base sequence of DNA

Reasons

1. Spontaneous errors in DNA replication or meiotic

recombination

2. A consequence of the damaging effects of physical

or chemical mutagens on DNA


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Transition: Purine or pyrimidine is replacedby the other AG TC

Transversion: a purine is replaced by apyrimidine or vice verse

AT or C TA or GGT or C CA or G

Point mutation

(a single base change)

1 Mutaagenesis


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Noncoding DNA

Nonregulatory DNA

3rd position of a codon

Silent mutation

Coding DNA altered Missense mutation

Phenotypiceffects

No

Coding DNA stop codon truncated protein Nonsense mutation

Effects of a point mutation

1 Mutaagenesis

Yes or No

Yes


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Insertions or deletions

Frameshift mutationsThe translation of a protein encoded gene is

frameshifted , then changed the C-terminal side of the

mutation is completely changed.

The addition or loss of one or more bases in a DNA region

1 Mutaagenesis


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Examples of deletion mutations


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1-2 Replication fidelity1 Mutaagenesis

Mutation relevant

1. Spontaneous errors in DNA replication is very rare, one

error per 1010 base in E. coli.

Important for preserve the genetic informationfrom one generation to the next


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Molecular mechanisms for the

replication fidelity

1. DNA polymerase:Watson-Crick base pairing

2. 35 proofreading exonuclease.3. RNA priming: proofreading the 5 end of the

lagging strand

4. Mismatch repair

1 Mutaagenesis


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by

E. colipolymerase

Proofreading

1 Mutaagenesis


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Mutagens

1 Mutaagenesis

Mutation relevant

Cause DNA damage that can be converted to mutations.


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Physical mutagens

High-energy ionizing radiation: X-rays and g-rays

strand breaks and base/sugar destruction

Nonionizing radiation : UV light pyrimidine dimers

Chemical mutagens

Base analogs: direct mutagenesis

Nitrous acid: deaminates C to produce U

Alkylating agents

Intercalating agentsLesions-indirect mutagenesis

1 Mutaagenesis


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Base analogs:derivatives of the normal bases

incorporated in DNA, altering base pairing properties.

Nitrous acid: deaminates Cto produce U, resulting in GC

AU


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Mutagenesis1 Mutaagenesis

The molecular process

in which the mutation is generated.

Note: the great majority of lesions introduced bychemical and physical mutagens are repaired byone or more of the error-free DNA repairmechanisms before the lesions is encounter by a

replication fork


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Direct mutagenesis

The stable, unrepaired base withaltered base pairing properties in theDNA is fixed to a mutation during DNAreplication.

1 Mutaagenesis

1 M i


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5-BrU

: G

: A

enol form

Br

OH

H

O

Br

Keto form

H

O

AGCTTCCTA

TCGAAGGAT

AGCTBCCTA

TCGAAGGAT

1. Base analog

incorporation

AGCTBCCTA

TCGAGGGAT

AGCTTCCTA

TCGAAGGAT

2. 1st round

of replication

AGCTBCCTA

TCGAAGGAT

AGCTCCCTA

TCGAGGGAT

3. 2nd round

of replication

ATGC transition

1 Mutaagenesis

1 M i


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Indirect mutagenesis

The mutation is introduced as a resultof an error-prone repair.Translesion DNA synthesis tomaintain the DNA integrity but notthe sequence accuracy: when damageoccurs immediately ahead of an advancingfork, which is unsuitable for recombination

repairthe daughter strand is synthesizedregardless of the the base identity of the

damaged sites of the parental DNA.

1 Mutaagenesis


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E. coli translession replication: SOS

response: Higher levels of DNA damageeffectively inhibit DNA replication and trigger astress response in the cell, involving a regulated

increase (induction) in the levels of a number of

proteins. This is called the SOS response.

1. Some of the induced proteins, such as the UvrA and

UvrB proteins, have roles in normal DNA repair

pathways.2. A number of the induced proteins, however, are part

of a specialized replication system that can

REPLICATE PAST the DNA lesions that block

DNA polymerase III. back

1 Mutaagenesis


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Proper base pairing is often impossible and not

strictly required at the site of a lesion because

of the SOS response proteins, thistranslesionreplication is error-prone.

The resulting increase in mutagenesis doesnot contradict the general principle that replicationaccuracy is important (the resulting mutationsactually kill many cells). This is the biological price

that is paid, however, to overcome the generalbarrier to replication and permit at least a fewmutant cells to survive.

DNA d i d t i


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DNA damage and repair

Mutagen

Completely repaired

DNA damage(lesions)

chemical reactivity of the

bases

Error-free

Repairing

mutations

Indirect

mutagenesis

DNA damage, repair and mutagenesis

minor ormoderate

Extensive, rightbefore Replication

Fork (not repairable)Direct

mutagenesis

10- Mutagens and Mutagenesis

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