Transcript Molecular Mechanism of Mutation

Shikha Yashveer1, Jayanti Tokas2, Shalini Jain3 and Hariom Yadav4
1Department
of Molecular Biology and Biotechnology, 2Department of Biochemistry, CCS HAU,
Hisar, Haryana, India
3Department
of Biochemistry, PGIMER, Chandigarh, India
4National Agri-Food Biotechnology, Mohali, Punjab, India
Email: [email protected]
 Any sudden change occurring in
hereditary material is called as
mutation
 They may be harmful, beneficial or
neutral
 In multicellular organism, two broad
categories of mutations: Somatic
mutations & 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
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
 Originally the term mutation was
given to both chromosomal as well as
point mutations
Cont.
 Recently chromosomal mutations
are studied separately
 The term mutation is now given only
to point mutations
Definition
 DNA is a highly stable molecule that
replicates with amazing accuracy
 Some errors of replication do occur
 A mutation is defined as an inherited
change in genetic information
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


Base substitution
Insertions & deletions
Base substitution:
 Simplest type of gene mutation
 Involves the alteration of a single
nucleotide in the DNA
A base substitution usually leads to base pair
substitution
GGG AGT GTA GAT CGT
CCC TCA CAT CTA GCA
GGG AGT GCA GAT CGT
A base substitution
CCC TCA CAT CTA GCA
First cycle of DNA replication
GGG AGT GCA GAT CGT
CCC TCA CAT CTA GCA
CCC TCA CGT CTA GCA
GGG AGT GTA GAT CGT
Base substitution is of two types:
Transition:
Purine is replaced with a purine
Pyrimidine is replaced with a pyrimidine
Transversions:
A purine is replaced by a pyrimidine
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.
Mutations on the basis of the Phenotypic effects of
mutations:

Most common phenotype in natural populations of
the organism is called as wild type phenotype

The effect of mutation is considered with reference
to wild type phenotype
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
TCA
AGT
TTA
AAT
UCA
UUA
Ser
Leu
Nonsense mutation: changes a sense codon into a
nonsense codon. Nonsense mutation early in the mRNA
sequence produces a greatly shortened & usually
nonfunctional protein
TCA
AGT
TGA
ACT
UCA
UGA
Ser
Stop codon
Silent mutation: alters a codon but due to degeneracy of
the codon, same amino acid is specified
TCA
AGT
TCG
AGC
UCA
UCG
Ser
Ser
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.
CTT
GAA
ATT
TAA
CUU
AUU
Leu
Ile
Loss of function mutations:
 Complete or partial loss of the normal function
 Structure of protein is so altered that it no longer
works correctly
 Mutation can occur in regulatory region that
affects transcription , translation or spilicing of
the protein
 Frequently recessive
Gain of function mutations:
 Produces an entirely new trait
 Causes a trait to appear in inappropriate tissues
or at inappropriate times in development
 Frequently dominant
Conditional mutations:
 Expressed only under certain conditions
Lethal mutations:
 Cause the death of the organism
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
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
Chemical Mutagens:
 First discovery of a chemical mutagen was made by
Charlotte Auerbach
Base Analogs:
 Chemicals with structures similar to that of any of the
four standard bases of DNA
 DNA polymerases cannot distinguish these analogs
 They may be incorporated into newly synthesized
DNA molecules
5-bromouracil
an analog of thymine
O
O
4
4
N3
2
O
5
Br
N3
5BU
2
6
1
N
O
5
T
6
1
N
CH₃
OH
O
4
4
N3
5BU
2
O
5
6
1
N
Keto
pairs with A
N3
Br
5
5BU
2
O
6
1
N
Enol
mispair with G
Br
T
A
5dBU
5dBU
A
5dBU
G
C
G
TRANISITION
T
C
A
G
3’
5’
GAC
3’
5’
GAC
CTG
5’
3’
3’
5’
GAC
3’
5’
CBG
5’
3’
GAC
CBG
5’
3’
CBG
5’
3’
3’ 5’
Incorporated error
GGC
3’
5’
GAC Strand
CTG seperation
3’ 5’
5’
3’
5’
3’
CBG
GGC
5’
CTG
3’
replication
3’
5’
GAC
CTG
5’
3’
3’
5’
GAC
CBG
5’
3’
3’
5’
GGC
CCG
5’
3’
G
C
5dBU
G
5dBU
5dBU
A
A
T
TRANISITION
G
A
C
T
2-amino purine (P)




Base analog of adenine
Normally pairs with thymine
May mispair with cytosine
Causes a transition mutation
T.A
Incorporated error
3’
5’
GTC
CAG
5’
3’
3’ 5’
GTC
Strand
separation
CAG
5’
3’
3’ 5’
GTC
CPG
5’
3’
3’
5’
GTC
CAG
5’
3’
replication
C.G
3’ 5’
GTC
3’ 5’
GTC
CAG
5’ 3’
5’ 3’
CPG
GCC
3’
5’
CPG
5’ 3’
5’ 3’
CPG
3’
5’
GTC
CAG
5’
3’
3’
GCC
5’
3’
5’
GCC
CGG
5’
3’
T
A
2AP
T
2AP
C
2AP
C
G
TRANISITION
T
C
A
G
C
G
2AP
C
2AP
T
2AP
T
A
TRANISITION
C
T
G
A


Both base analogs produce transition
mutations
Mutations by base analogs can be
reversed by treatment with the same
analog or different analog
Alkylating agents:
 Chemicals that donate alkyl groups e.g.
ehylmethanesulfonate(EMS)
 It adds an ethyl group to guanine and produces 6ethylguanine, which pairs with thymine and leads to
CG:TA transitions
 Also adds an ethyl group to thymine to produce 4ethylthymine, which then pairs with guanine,
leading to a TA:CG transition
 Mutations produced by EMS can be reversed by
additional treatment with EMS.
 Mustard gas is another alkylating agent.
C
G
T
A
EMS
EMS
T
6EG
4ET
T
A
C
G
G
Nitrous acid: causes deamination
Cytosine
Uracil
NH2
o
4
N 3
N 3
5
HNo2
2
O
4
2
6
1
N
H
CYTOSINE
5
O
6
1
N
H
URACIL
5’
3’
C
G
3’
HNO2
5’
5’
5’
5’
3’
U
G
3’
U
3’
3’
5’
5’
3’
G
5’
3’
C
G
5’
C.G
U
A
3’
5’
3’
U
5’
3’
3’
5’
3’
A
3’
U
A
5’
5’
TA
3’
5’
5’
T
A
3’
Adenine changes into Hypoxanthin which then pairs with Cytosine
5’
3’
A
T
3’
HNO2
5’
5’
5’
5’
3’
H
T
3’
H
3’
3’
5’
5’
3’
T
5’
3’
A.T
T
A
G.C
5’
H
C
3’
5’
3’
H
5’
3’
3’
5’
3’
C
3’
H
C
5’
5’
3’
5’
5’
C
G
3’
Guanine changes into Xanthin which pairs with Cytosine.
Xanthin can also pair with Thymine
5’
3’
G
C
3’
HNO2
5’
5’
5’
5’
3’
X
C
3’
X
3’
3’
5’
5’
3’
C
5’
3’
G.C
G
C
A.T
5’
X
T
3’
5’
3’
X
5’
3’
3’
5’
3’
T
3’
X
T
5’
5’
3’
5’
5’
T
A
3’
 Nitrous acid produces exclusively
transition mutations
 Both C.G
T.A & T.A C.G transitions
are produced
 Thus mutations can be reversed with the
nitrous acid
Hydroxl amine



Specific base modifying mutagen which
adds a hydroxyl group to cytosine
producing hydroxlamine cytosine which
pairs with adenine instead of guanine
This Leads to C.G T.A tranisitions
Acts only on cytosine thus can not revert
the mutation produced
Cytosine changes into hydroxlamine Cytosine which pairs
with Adenine instead of Guanine
5’
3’
C
G
3’
NH₂OH
5’
5’
5’
5’
3’
hC
G
hC
A
3’
hC
3’
3’
3’
G
5’
3’
C.G
5’
5’
5’
C
G
T.A
5’
3’
3’
hC
5’
3’
3’
5’
3’
A
3’
hC
A
5’
5’
3’
5’
5’
A
T
3’
Oxidative reactions:
 Reactive forms of oxygen like superoxide
radicals, hydrogen peroxide and hdroxyl
radicals produced in the course of normal
aerobic metabolism or by radiation, ozone,
peroxides, and certain drugs Cause damage
to DNA & induce mutations by chemical
changes
 Oxidation converts guanine into 8-oxy-7,8dihydrodeoxyguanine which mispairs with
adenine leading to G.C
T.A transversion
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
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.
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 a specific type of
mutation with in a group of
individual organism
e.g. for achondroplasia, the
mutation frequency in united states
is about 2x10⁻⁴