MUTATIONS
Dr. Muhammad Imtiaz Shafiq
http://imtiaz.pk
Institute of Biochemistry and Biotechnology,
University of the Punjab
Mutations
 Any sudden change occurring in
hereditary material is known as
mutation
 They may be harmful, beneficial
or neutral
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
Or
 Changes in DNA that affect genetic
information
Mutation ?
Significance of Mutations
• Most are neutral
• Eye color
• Birth marks
• Some are harmful
• Sickle Cell Anemia
• Down Syndrome
• Some are beneficial
• Sickle Cell Anemia to Malaria
• Immunity to HIV
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
 Recently chromosomal mutations
are studied separately
 The term mutation is now given
only to point mutations
Gene Mutations
• Point Mutations – changes in
one or a few nucleotides
• Substitution
• Insertion
• Deletion
Point Mutations
• Point Mutations
• Base Pair Substitutions
• Silent
• Missense – new protein (Amino Acid Substitutions)
• Nonsense – stop codon
• Point Mutations
• Base Pair Insertions and deletions
• Frameshift Mutations
• Frameshift Mutations – shifts
the reading frame of the
genetic message so that the
protein may not be able to
perform its function.
• Insertion
• THE FAT CAT ATE THE RAT
• THE FAT HCA TAT ETH ERA T
• Deletion
• THE FAT CAT ATE THE RAT
• TEF ATC ATA TET GER AT
Mutations: genetic material changes in a cell
• Point mutations….
Changes in 1 or a few base
pairs in a single gene
• Base-pair substitutions:
• •silent mutations
•
•
•
•
•
no effect on protein
•missense
a different amino acid (different
protein)
•nonsense
a stop codon and
a
nonfunctional protein
Base-pair insertions or
deletions:
additions
or losses of nucleotide pairs in a
gene; alters the ‘reading frame’ of
triplets~frameshift mutation
• Mutagens: physical and
chemical agents that change
DNA
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
UCA
Ser
TGA
ACT
UGA
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
Missense Mutation
• One Amino Acid Substituted for Another
• Sickle Cell Anemia
• Valine is replaced with Glutamic Acid
Sickle Cell Anemia
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
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
Chromosome Mutations
• Changes in number and structure of entire chromosomes
• Original Chromosome
• Deletion
• Duplication
• Inversion
• Translocation
ABC * DEF
AC * DEF
ABBC * DEF
AED * CBF
ABC * JKL
GHI * DEF
Variation in Chromosome Number
• Having any
number of
chromosomes
that does not
equal 23 pairs.
• Ex) Down
Syndrome – 3
Chromosome
21’s
• In multicellular organism, two broad
categories of mutations: Somatic mutations &
germ line mutations
Gametic and somatic mutations
• Gametic – testis of males, ovaries of females, inherited
• Somatic – in normal body cells occuring beyond zygote
formation, not inherited but may effect the person during
their lifetime. Chimaeras
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
What Causes Mutations?
• There are two ways in which DNA can become
mutated:
• Mutations can be inherited.
• Parent to child
• Mutations can be acquired.
• Environmental damage
• Mistakes when DNA is copied
What causes mutation
• Spontaneous
• Increases caused by environmental factors
• UV light
• X-rays
• Benzene, formaldehyde, carbon tetrachloride
Mutagens and their effects
• Ionising radiation – Nuc
radiation, xrays, gamma rays
(e.g. medical treatment)
associated with development of
cancers (e.g. leukaemia, thyroid
cancer and skin cancer
Mutagens and their effects
• Viruses and microorganisms
– integrate into human
chromosome, upset genes
and can trigger cancer
Mutagens and their effects
• Environmental poisons
– Organic solvents
such as formaldehyde,
tobacco, coal tars,
benzene, asbestos,
some dyes
Mutagens and their effects
• Alcohol and diet – High
alcohol intake increase the
risk of some cancers. Diet
high in fat and those
containing burned or highly
preserved meat
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’GA 5’
C
CB
5’ G 3’
3’GA 5’
C
3’ GA 5’
CT
C
5’ G 3’
3’ GA 5’
CB
5’CB 3’
C
G
5’ G 3
GG
Incorporated error
3’GA5’
’
3’ C 5’
Strand
C
CT
seperation
5 CB 3’ 3’GG5’
G
5’ 3’
’ G
C
3’GA5’
3’GA5’
3’GG5’
CT
C
CT
C
C
CB
CC
5’ G 3’
5’ G 3’
replication
5’ G 3’ 5’ G 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
C.G
Incorporated error
3’ 5’
3’
5’
GTC
CAG
5’
3’
GTC
Strand
separation
CAG
5’
3’
3’ 5’
3’ 5’
3’ 5’
GTC
GTC
GTC
CAG
5’ 3’
5’ 3’
CPG
5’
3’
3’
5’
GTC
CPG
CPG
5’ 3’
GCC
3’
CAG
5’
3’
replication
5’ 3’
CPG
3’
5’
5’
3’
5’
GCC
3’
5’
GTC
GCC
CAG
CGG
5’
3’
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
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
 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
’
3’
C
G 5’
HNO2
5’
5’
5’
3’
3’
U
3’
U
G 5’
3’
5’
3’
5’
3’
G
C
G
5’
C.G
U
A
3’
5’
3’
U
5’
3’
3’
5’
3’
5’
A
3’
U
A
5’
TA
3’
5’
5’
T
A
3’
Adenine changes into Hypoxanthin which then pairs with Cytosine
5’
3
’
3’
A
T 5’
HNO2
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
’
3’
G
C 5’
HNO2
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
’
3’
C
NH₂OH
G 5’
5’
3’
5’
5’
hC
G
3’
hC
A
hC
3’
3’
5’
5’
3’
G
5’
3’
C.G
3’
5’
3’
C
G
T.A
5’
3’
5’
3’
A
hC
5’
3’
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
The effect of muations
• Not all are harmful
• Survival advantage
• Most common among bacteria and viruses but also seen
in insects
• If no selective pressure may remain in population
Harmful mutations
• Cystic fibrosis and sickle cell anaemia
• Disfunctional proteins
• Albinism – caused by mutation in gene of enzyme
pathway of melanin
Beneficial mutations
• Bacteria – antibiotic
resistance through mutation,
transfer between bacterial
species
• Superbugs such as MRSA
have arisen this way
• RNA viruses – such as HIV –
mutates it’s protein coat so
that the host human is unable
to make antibodies quick
enough against it
Neutral mutations
• Neither harmful or beneficial to the organism but may be
important in an evolutionary sense
• Silent mutations
• Virtually impossible to detect because no observable
effect