MOLECULAR MECHANISM OF SPONTANEOUS AND INDUCE MUTATION

PRESENTED BY: ACHYUT BORAROLL NO :130209003SEMESTER: 4th sem.DEPT: BS MEDICAL BIOTECHGUIST

PRESENTATION OVERVIEW

INTRODUCTION

TYPES OF MUTATION

SPONTANEOUS MUTATION

INDUCED MUTATION

MUTATION RATE AND FREQUENCY

MUTATION

A sudden change occurring in hereditary materials is called mutation.

The process by which mutation is produced is called mutagenesis .

An organism exhibiting a novel phenotype as a result of the presences of a mutation is referred to as a mutant.

General characteristics of Mutation Mutation are generally recessive, but dominant mutations also occur. Mutation may be harmful, beneficial or neutral. Mutation are random, occur at any time and in any cell of an organism. Mutations are recurrent i.e. the same mutations may occur again and

again.

ROLE OF MUTATION Ultimate source of all genetic variation and it provides the raw material

for evolution.

Mutation results into the formation of alleles, without mutation all genes would exists in only one from.

Organism would able to evolve and adapt to environmental change.

In multicellular organism, two broad categories of mutations:

1)Somatic mutations &

2)Germ line mutations

Somatic mutations

Arise in the somatic cells Passed on to other cells through the process of mitosis Effect of these mutations depends on the type of the

cell in which they occur & the developmental stage of the organism

If occurs early in development, larger the clone of the mutated cells.

E.g. Skin cancers and leukemia

Germ line mutations

They occur in the cells that produce gametes.

Passed on to future generations.

In multicellular organisms, the term mutation is generally used for germ line mutations.

Some Facts

Term mutation was given by Devries in 1901 while studying evening primerose Oenothera lamarckiana.

Most of these were chromosomal variations

Some were point variations

Cont. Originally the term mutation was given to both chromosomal as well as

point mutation.

Recently chromosomal mutations are studied separately.

The term mutation is now given only to point mutations.

Types of gene mutation

Number of ways to classify gene mutations:

On the basis of the molecular nature of the defect,

On the nature of the phenotypic effect-- amino acid sequence of the protein is altered or not,

On the basis of the causative agent of the mutation

Mutations on the basis of the molecular nature of the defect: Base substitution

Insertions & deletions

Base substitution: Simplest type of gene mutation.

Involves the alteration of a singlenucleotide in the DNA.

Base substitution is of two types:

Transition: Purine is replaced with a purine

Pyrimidine is replaced with a pyrimidine

A

TTCC

GG A

Transversions: A purine is replaced by a pyrimidine

ACAT

GG

CT

CGCA

T GT A

or a pyrimidine is replaced by a purine

Insertions & deletions: 2nd major class of gene mutation

Addition or the removal , respectively, of one or more nucleotide pair

Usually changes the reading frame, altering all amino acids encoded by codons following the mutation

Also called as frame shift mutations

cont.

Additions or deletions in the multiples of three nucleotides will lead to addition or deletion of one or more amino acids.

These mutations are called in-frame insertions and deletions, respectively.

Forward mutation:

a mutation that alters the wild type phenotype

Reverse mutation (reversion):

a mutation that changes a mutant phenotype back in to the wild type

Missense mutation: a base is substituted that alters a codon in the mRNA resulting in a different amino acid in the protein product

TCAAGT

UCA

TTAAAT

UUA

Ser Leu

Nonsense mutation: changes a sense codon into a nonsense(stop) codon. Nonsense mutation early in the mRNA sequence produces a greatly shortened & usually nonfunctional protein, i.e. stop codon (UAG,UAA,UGA).

TCAAGT

UCA

TGAACT

UGA

Ser

Stop codon

Neutral mutation: mutation that alters the amino acid sequence of the protein but does not change its function as replaced amino acid is chemically similar or the affected aa has little influence on protein function.

AAATTT

AAA

AGATCT

AGA

Lys Arg

Silent mutation: alters a codon but due to degeneracy of the codon, same amino acid is specified

TCAAGT

UCA

TCGAGC

UCG

Ser Ser

Suppressor mutation:

Suppresses the effect of other mutation

Occurs at a site different from the site of original mutation

Organism with a suppressor mutation is a double mutant but exhibits the phenotype of un mutated wild type

Different from reverse mutation in which mutated site is reverted back into the wild type sequence

On the basis of Causative agent of mutation:Spontaneous:

Mutations that result from natural changes in DNA. All the types of point mutation. It occurs in DNA replication and G1 and G2 phase of cell cycles.

Induced:

Results from changes caused by environmental chemicals & radiations Any environmental agent that increases the rate of mutation

above the spontaneous is called a mutagen such as chemicals & radiations

SPONTANEOUS MUTATION

DNA Replication Errors.

Spontaneous Lesions.

Transposition.

1) DNA REPLICATION ERRORS Each of the common bases in DNA can spontaneously undergo a

transient rearrangement of bonding. i.e. Base pairs substitution mutations

Chemically each base can exists in alternatives states called Tautomers.

A proton shifts in nitrogen base forms on its rare tautomeric form i.e. Tautomeric shift.

In DNA, H- atoms usually prefer specific atomic location in purine and pyrimidine bases .

• The N- atoms attached to C , G and A are in the amino form ( - NH2 )• The O- atom in G and T are in Keto form ( = O )• Keto (C=O), enol (C-OH)• Amino - (NH2), imino (=NH)

Eg. In the example diagramed, a guanine undergoes a tautomeric shifts to its rare enol from at the time of replication .in its enol form, it pairs with thymine, during the next replication the guanine shifts back to its more stable keto form. The thymine incorporated opposite the enol form of guanine directs the incorporation of adenine during the subsequent replication . The net result is a GC to AT mutation.

2) SPONTANEOUS LESIONS Naturally occurring damage to the DNA also can generate

mutations. Three types of spontaneous lesions:

Depurination and Depyrimidiation.

Deamination.

Oxidative damage.

Depurination and Depyrimidiation Depurination is the loss of purine due to breaking of glycosidic

bond of nucleotides in DNA. Similarly depyrimidiaton is the loss of pyrimidine base.

AP: APURINIC SITE (or baseless site). If such lesions are not repaired, there is no base to specify a complementary

base during DNA replication, and the DNA polymerase may stall or dissociate from DNA.

Deamination Removal of amino group from a molecules i.e. from cytosine,

adenine and guanine .Chemicals are Nitrous acid(HNO2),sodium bisulfite(NaHSO3).

e.g. Xanthine result form deamination of guanine selectively base pairs with thymine instead of cytosine. Hypoxanthine selectively base pairs with cytosine instead of thymine.

OXIDATIVELY DAMAGE Attack by reactive oxygen species to the DNA. Radicals like superoxide(O2), hydrogen peroxide (H2O2) and hydroxyl

radicals (OH).

E.g. 8-oxo-7-hydrodeoxyguanosine(8-oxodG) and thymidine glycol are products of oxidative damage. Results 8-oxodG mispairs with A, resulting in high level of G-T Transversions.

Thymidine glycol blocks the DNA replication.

3) TRANSPOSITION A transposons is a DNA sequence that is able to insert itself(or copy

itself) at a new location in the genome, without having any sequence relationships with target locus.

Insertion of a transposable elements into or near a functional gene can alter its expression by causing loss of gene function or by changing the gene expression (insertion mutation).

INDUCED MUTATION Physical Mutagens

Radiation (a) Ionizing( e.g. X-ray, gama ray, cosmic ray), (b) Non- ionizing (e.g. UV ray) Heat

Breaks the N-glycosidic bond in DNA. Results form apurinic site or baseless site. Leads to unstable and rapidly degrade the DNA.

Chemical Mutagens Base Analogs Alkylating agents Intercalating agents

Radiations:Ionizing radiations: In 1927, Herman Muller demonstrated that mutations could

be induced by X-rays. X-rays, gamma rays, and cosmic rays are all capable of

penetrating tissues and damaging DNA.

They remove electrons from the atoms that they encounter, changing stable molecules into free radicals and reactive ions which then alter the structures of bases and break phosphodiester bonds in DNA.

Ionizing radiation also frequently results in double-strand breaks in DNA.

Non-ionizing UV radiation causes thymine dimers, which block replication. Light repair separates thymine dimers. Sometimes the repair jobs introduces the wrong nucleotide,

leading to a point mutation.

Chemical mutagen:Base Analogs

Chemicals with structures similar to that of any of four standard bases of DNA.

DNA polymerases cannot distinguish these analogs. They may be incorporated into newly synthesized DNA molecules. Both base analogs produce transition mutations. Mutations by base analogs can be reversed by treatment with the same

analogs or different analog.

2-Amino-purine: Base analog of adenine. Normally pairs with thymine. May mispairs with cytosine. Causes a transition mutation.

Alkylating agents: Chemicals that donate alkyl groups e.g.

Ethylmethanesulfonate(EMS). It adds an ethyl group to guanine and produces 6-ethylguanine,

which pairs with thymine and leads to CG:TA transitions Also adds an ethyl group to thymine to produce 4-ethylthymine,

which then pairs with guanine, leading to a TA:CG transition Mustard gas is another alkylating agent.

Intercalating agents Proflavin, acridine orange, ethidium bromide, and dioxin . They are about the same size as a nucleotide They produce mutations by sandwiching themselves (intercalating) between

adjacent bases in DNA They distort the three-dimensional structure of the helix and cause single-

nucleotide insertions and deletions in replication These insertions and deletions frequently produce frameshift mutations.

E.g.

Mutation Rates

The frequency with which a gene changes from the wild type to a mutant is reffered to as the mutation rate.

Expressed as the number of mutations per biological unit i.e. mutations per cell division, per gamete per round of replication

e.g. mutation rate for achondroplasia (hereditary dwarfism) is about 4 mutations per 100,000 gametes.

Mutation Frequency

Incidence of specific type of mutation with in the group of individual organism.

e.g. for achondroplasia, the mutation frequency in united state is about 2x10⁻⁴.

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