Mutation Updated

7/30/2019 Mutation Updated

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DNA Mutation andDiseases

Dr Anand Mohan


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Define a Mutation

A mutation is a stable change in DNA structure

heritable Base sequence

is changed

Mutations are classified into several types


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Molecular Nature of Mutation

(what types of mutations arise) Point mutations (base substitutions): one base

substituted for another

Transition mutations = Ato G or C to T (purine to

purine or pyrimidine to pyrimidine)

Transversion mutations = purine to pyrimidine or vice

versa Can arise from mispairing, insertion of base analogs,

or chemical mutagens (nitrous acid, hydroxylamine

and alkylating agents)


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Consequences of Point mutations

A missense mutation: This results in one wrongcodon and one wrong amino acid.


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A nonsense mutation: If the change in the basesequence results in a stop codon, the protein would

be terminated at that point in the message.


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A sense mutation: This occurs when the change inthe DNA sequence results in a new codon still coding

for the same amino acid.


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Other types of mutations

Insertions and deletions: result inframeshift mutations (less common)

Insert a G here


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DNA Mutation - Reasons

1) Through mistakes during DNA replication

2) Spontaneous mutations (deamination,

depurination that occur naturally)

3) Induced mutations caused by

environmental agents (chemical mutagens,UV radiation)


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Spontaneous Mutagenesis

Watson and Crick originally suggested that mutation could occur

spontaneously during DNA replication if pairing errors occurred.

If a base of the DNA underwent a proton shift into one of its rare tautomeric

forms (tautomeric shift) during the replication process, aninappropriate pairing of bases would occur.

Normally, adenine and cytosine are in the amino (NH2) form. Theirtautomeric shifts are to the imino (NH) form. Similarly, guanine and thymine

go from a keto (C=O) form to an enol (COH) form.

During DNA replication, a tautomeric shift in either the incoming base

(substrate transition) or the basealready in the strand (template transition)results in mispairing.

The mispairing will be permanent and result in a new base pair after an

additional round of DNA replication. The original strand is unchanged


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Adenine can pair with adenine if one of the bases undergoes a

tautomeric shift while the other rotates about its base-sugar

(glycosidic) bond.

The normal configuration of the base is referred to as the anti

configuration; the rotated form is the syn configuration. Since we

now believe that as many as 10% of bases may be in the syn

configuration at any moment, the transversion mutagenesis rateshould be about 10% of the transition mutagenesis rate, a value not

inconsistent with current information.

Some base-pair mutations can have serious results. If guanine

undergoes an oxidation to 8-oxoguanine, it pairs with adenine. A

GC base pair is converted to a TA base pair through an 8-

oxoguanineadenine intermediate. This transversion has been found

to be common in cancers.


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Base Analogs may get incorporated during DNA replication

= thymine analog

gets incorporated during

DNA replication opposite an A

frequent tautomer change

-now pairs with G

Compounds that chemically resemble a nucleotide base closely enough that

during DNA replication, they can be incorporated into the DNA in place of

the natural base.


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The base analog,2-AP pairs with T.

But 2-AP canalso pair with C

via a singe H-bond.

Therefore, if 2-AP gets in DNA,

then uponreplication a C issometimesinserted.

Another base analog


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Spontaneous DNA Mutation

Deamination:- C to U (now pairs with T) (100/cell/day)

- A to Hypoxanthine (now pairs with C)

- G to Xanthine (still pairs with C) Depurination/Depyrimidination:

- removes purine base from DNA(104 bases

/cell/day) or pyrimidine bases (less frequent)

Oxygen radical damage

-breaks sugar rings

Oxidative deamination


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Oxidative deaminationThe primary amino groups of nucleic acid bases aresomewhat unstable. They can be converted to keto groups

in reactions like the one pictured here:

In a mammalian cell, ~100 uracils are generated per cell per dayin this fashion. Other reactions include conversion of adenine tohypoxanthine, and guanine to xanthine.

Oxidative deamination


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Oxidative deaminationother examples


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Spontaneous Base Loss

The glycosyl bond linking DNAbases with deoxyribose is labile

under physiological conditions.

Within a typical mammalian cell,

several thousand purines andseveral hundred pyrimidines arespontaneously lost per cell per

day.

Loss of a purine or pyrimidinebase creates an

apurinic/apyrimidinic (AP) site

(also called an abasic site)


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Oxygen radical damage


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Mutation through Exposure to

Environmental Agents

Chemical mutagens:

- Nitrous acid (HN02) formed from nitrites (in

preserved meats) reacting with stomach acid,causes oxidative deamination

-Alkylating agents add methyl or ethyl groups to

bases and change their pairing properties

- Intercalating agents (ethidium bromide, acridineorange) distort the helix causing frameshift

insertions or deletions.


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Nitrous acid

Agents causing oxidative deamination

Nitrosamines


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Agents causing alkylation

The first report of mutagenic action ofa chemical was in 1942 by CharlotteAuerbach, who showed that nitrogenmustard (component of poisonousmustard gas used in World Wars I andII) could cause mutations in cells.


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Intercalating Agents



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Environmental Agents

Ultraviolet Radiation: classified in terms ofits wavelength

UV-C (180-290 nm)--"germicidal"--most energetic and lethal, it

is not found in sunlight because it is absorbed by the ozone layer

UV-B (290-320 nm)--major lethal/mutagenic fraction of sunlight

UV-A (320 nm--visible)--"near UV"--also has deleterious

effects (primarily because it creates oxygen radicals) but itproduces very few pyrimidine dimers.

Tanning beds have UV-A and UV-B.


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Ultraviolet Radiation:

- causes dimers to form between adjacent T

bases on a strand of DNA (or between adjacent

T-C bases, hence the term pyrimidine dimer)

- a cyclobutyl ring links the adjacent Ts

- this interferes with the ability of the Ts to

base pair to the opposite strand, and

blocks DNA replication


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The most frequentphotoproducts are bondsformed between adjacent

pyrimidines within one strand

The most frequent are CPDs,cyclobutane pyrimidine dimers

T-T CPDs are formed mostreadily, followed by T-C or C-T;C-C dimers are least abundant.

CPDs cause a distortion in theDNA chain structure. The twoadjacent pyrimidines are pulledcloser to each other than in

normal DNA.

T-T CPD with

cyclobutyl ring in blue


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Nucleotide Excision Repair defects in HumansXeroderma pigmentosum (XP)

Genetic disorder with symptoms:

-extreme sensitivity to sunlight (by ~age 2),

and >1000X higher risk of skin cancer

(by ~age 8)

Defect is in repair of UV damage

Gene mapping identified several

repair proteins (called XP proteins)XP-C and XP-A recognize pyrimidine dimers

XP-B and XP-D have helicase activity

XP-G and XP-F have nuclease activity

Real world biochemistry


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Real-world biochemistryIn 1997, NASA

developed a PrototypeUV garment forchildren with XP,Porphyria and othersun Related Disorders

to have a Quality of lifeand Freedom. ThisNASA UV ProtectiveProject designed for XP

was completed in 1998and the UV garmentsare being supplied tochildren of parents thathave requested them.


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Sickle cell disease

haemoglobin

Haemoglobin is made up of 4 polypeptide chains (2 alpha & 2 beta chains).

Gene for beta chain is found on chromosome 11 and consists of 438 bases.

A mutation occurs in the gene coding for the beta chain.

The mutation is a substitution where adenine replace thymine on the DNA

template strand.

As a result the amino acid valine replaces glutamic acid.

This change the properties of the haemoglobin and results in distorted red

blood cells.

normal red blood cell

sickle cell

HBB gene on

chromosome 11

Haemoglobin is made up of 4 polypeptide chains (2 alpha & 2 beta chains).

Gene for beta chain is found on chromosome 11 and consists of 438 bases.

A mutation occurs in the gene coding for the beta chain.

The mutation is a substitution where adenine replace thymine on the DNA

template strand.

As a result the amino acid valine replaces glutamic acid.

This change the properties of the haemoglobin and results in distorted red

blood cells.


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This single point mutation has a dramatic effect.

Individuals have many health problems, eg weakness,jaundice, anaemia, heart & kidney defects, brain

damage, skin lesions and inflamed spleen.

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