Presented by: Rajesh K. Bhoge

Introduction

A sudden heritable change in genetic material of

an organism are called as mutation.

Mutation term was first used by De Vries in 1901.

Any physical or chemical treatment or agent

capable of producing genetic mutation are called

as mutagen.

An organism that exhibit a novel phenotype

resulting from a mutation is called a mutant.

Types of Mutation

1) Spontaneous mutation

2) Induced mutation

Spontaneous Mutation

On the basis of tautomeric shift,

mutation are devided into

1)Transition

2)Transversion

3)Frameshift mutation

These all are point mutation.

Transition

The replacement of purine in one strand of DNA

with other purine and pyrimidine replace by

other pyrimidine, such base pair substitution are

called transition.

Transversion

Base pair substitution involving the

replacement of purine with pyrimidine and

vice versa.

There are three substitution, one transition

and two transversionfor every base pair.

A total of four transition and eight

transversion are possible.

Frameshift Mutation

It is another type of point mutation involves

addition or deletion are collectively reffered as

frameshift mutation.

Because they alter the reading frame of all

base pair triplet in gene & mutation occur.

These frameshift mutation almost always

result in the synthesis of non functional

protein gene product.

Induced Mutation

In 1927 Hermann J. Muller discovered that x-

ray induced mutation in Drosophila.

Muller demonstrate that x- ray are mutagenic.

Mustard gas was the first chemical shown to

mutagenic. It was discovered by Charlotte

Aurbach and her associate.

Mutation are induced by chemical and

radiation.

Chemical Mutagen

Chemical mutagen are divided into two

groups.

1)Those that are mutagenic to both replicating

and non replicating DNA.

2)Those that are mutagenic only to replicating

DNA. Such as base analog.

Chemical mutagen are base analog, nitrous

acid, acridine dye, alkylating agent.

Base Analog

Mutagenic base analog are similar to the

normal bases and incarporated into DNA

during replication. But there structure are

different from normal bases in DNA that they

increase frequency of mispairing.

Commonly used base analog are 5-

bromouracil and 2-aminopurine

Nitrous acid

Nitrous acid is a potent mutagen that acts on

either replicating or non replicating DNA.

It causes oxidative deamination of amino group

in adenine, guanine and cytosine.

This reaction converts the amino group to keto

group and changes the hydrogen bonding

potentialof the modified bases.

Nitrous acid produces transition in both direction

Acridine dye

The acridine dye such as proflavin, acridine

orange and a whole series of potent mutagen

that induce frameshift mutation.

When DNA molecules containing intercalated

acridine replicates, addition and deletion one

to few base pair occur.

Alkylating agents

Alkylating agents are chemicals that donate alkyl group

to other molecule.

It induce all type of mutation, including transition,

tranversion frameshift and even chromosome

abberation.

Hydroxylating agent hydroxylamine has a specific

mutagenic effect. When DNA is treated with

hydroxylamine, the amino group of cytosine is

hydroxylated, resulting hydroxylamino cytosine base pair

with adenine.

Mutation induced by Radiation Ionizing radiation induce gross changes in

chromosome structure, including deletion,

duplication, inversion and translocation. These

chromosome abberation results from radiation

induced breaks in chromosome.

Ultraviolet radiation does not posses sufficient

ionization. Uv rays penetrate tissue only slightly.

In multicelluler organism only epidermal layer of cell

is exposed to effect of uv. Uv light is a potent

mutagen for unicelluler organism.

DNA REPAIR MECHANISM

The mechanism in which living organism contain

many enzyme that scan their DNA for damage and

initiate repair processes when damage is detected.

1) Light – dependent repair

2) Excision repair

3) Mismatch repair

4) Post replication repair

5) Error prone repair system

Light –dependent repair

Light dependent repair or photoreactivation of

DNA in bacteria is carried out by a light activated

enzyme called DNA photolyase.

When DNA exposed to uv light, thymine dimer are

produced by covalent cross linkage between

adjacent thymine residues. DNA photolyase

recognise and binds to thymine dimers in DNA and

uses light energy to cleave the co-valent cross

links.

Excision repair

Excision repair are of mainly two types

1)Base excision repair

2)Nucleotide excision repair

Base excision repair

In base excision repair, a repair glycosylase

enzyme recognises the damage base & remove

it from the DNA by cleaving the bond between

base & deoxyribose sugar.

Other enzyme then cleave the sugar and

leaving a gap in the DNA chain.

The gap is filled with correct nucleotide by a

repair DNA polymerase & DNA ligase, with

opposite DNA strand used as a template

Nucleotide excision repair

It is called as dark repair system.

In NER system, E. coli correct not only thymine

dimers, but also other serious damage induced

ditortion of DNA helix.

In system involves four protein UvrA, UvrB, UvrC

& UvrD that are encoded by genes UvrA, UvrB,

UvrC & UvrD.

DNA polymerase I fills in a gap in the 5’ to 3’

direction and DNA ligase seal the final gap.

Mismatch repair

Mismatched base repair corrected by methyl directed

mismatch repair.

In E. coli, products of three genes mutS, mutL & mutH

are involved in initial stage.

The mutS protein bound to mismatch forms a complex

with , mutL & mutH encoded protein. These bring

unmethylated GATC sequence close to the mismatch.

mut H protein nicks the unmethylated DNA strand at

GATC site, mismatch is removed by exonuclease & gap

is repaired by DNA polymerase III and ligase

Translesion DNA synthesis & sos response

This process are involve a special class of DNA

polymerase that are synthesize in response to DNA

damage. In E. coli such DNA damage activates a

complex system called SOS response.

SOS response allows the cell to servive othewise

lethal events although often at the expense of

generating new mutation.

In E. coli, two genes are key to controlling the SOS

system; lex A & rec A.