Molecular Mechanisms of Mutation PowerPoint Presenation

Molecular Mechanisms of Mutation PowerPoint Presentation

1. Shikha Yashveer
1
, Jayanti Tokas
2,
Shalini Jain
3
and Hariom Yadav
4
1
Department of Molecular Biology and Biotechnology,
2
Department of Biochemistry, CCS HAU,
Hisar, Haryana, India
3
Department of Biochemistry, PGIMER, Chandigarh, India
4
National Agri-Food Biotechnology, Mohali, Punjab, India
Email: [email protected]

2. Any sudden change occurring in
hereditary material is called as
mutation
They may be harmful, beneficial or
neutral

3. In multicellular organism, two broad
categories of mutations: Somatic
mutations & germ line mutations

4. 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

5. 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

6. 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

7. Cont.
Recently chromosomal mutations
are studied separately
The term mutation is now given only
to point mutations

8. 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

9. 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

10. Base substitution
Insertions & deletions

11. Base substitution:
Simplest type of gene mutation
Involves the alteration of a single
nucleotide in the DNA

12. A base substitution usually leads to base pair
substitution
GGG AGT GTA GAT CGT
CCC TCA CAT CTA GCA
CCC TCA CAT CTA GCA
GGG AGT GCA GAT CGTA base substitution
CCC TCA CGT CTA GCA
GGG AGT GCA GAT CGT
GGG AGT GTA GAT CGT
CCC TCA CAT CTA GCA
First cycle of DNA replication

13. Base substitution is of two types:
Transition:
Purine is replaced with a purine
Pyrimidine is replaced with a pyrimidine

14. Transversions:
Apurineisreplacedbyapyrimidine
orapyrimidineisreplacedbyapurine

16. Insertions & deletions:
2
nd
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

17. 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.

18. 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

19. 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

20. 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
UCA
TTA
AAT
UUA
Ser
Leu

21. 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
TGA
ACT
UGA
Ser
Stop codon

22. Silent mutation: alters a codon but due to degeneracy of
the codon, same amino acid is specified
TCA
AGT
UCA
TCG
AGC
UCG
Ser Ser

23. 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
CUU
ATT
TAA
AUU
Leu Ile

24. 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 spilicingof
the protein
Frequently recessive

25. 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

26. Conditional mutations:
Expressed only under certain conditions
Lethal mutations:
Cause the death of the organism

27. 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

28. 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

29. 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

30. 5-bromouracil
an analog of thymine
N
N
1
2
3
6
5
4
O
O
5BU
Br N
N
1
2
3
6
5
4
CH₃
T
O
O

31. N
N
1
2
3
6
5
4
Br
O
5BU
O
N
N
1
2
3
6
5
4
Br
O
5BU
OH
Keto
pairs with A
Enol
mispair with G

32. T
A
5dBU
A
5dBU
5dBU
G
C
G
TRANISITION
T C
A G

33. GAC
CTG
Strand
seperation
3’
5’3’
5’
GAC
3’ 5’
CTG
5’ 3’
GAC
3’
5’
CBG
5’
3’
CTG
5’
3’
GAC
3’
5’
GAC
3’5’
CBG
5’3’
GAC
3’ 5’
CTG
5’3’
CBG
5’3’
GGC
3’5’
CBG
5’3’
GGC
3’5’
CBG
5’3’
GAC
3’5’
GGC
3’
5’
CCG
5’
3’
replication
Incorporated error

34. G
C
5dBU
5dBU
G
5dBU
A
A
T
TRANISITION
G A
C T

35. 2-amino purine (P)
Base analog of adenine
Normally pairs with thymine
May mispair with cytosine
Causes a transition mutation

36. GTC
CAG
Strand
separation
3’
5’3’
5’
GTC
3’
5’
CAG
5’
3’
GTC
3’5’
CPG
5’3’
CAG
5’3’
GTC
3’5’
GTC
3’5’
CPG
5’3’
GTC
3’5’
CAG
5’3’
CPG
5’3’
GCC
3’5’
CPG
5’3’
GCC
3’5’
CAG
5’3’
GTC
3’5’
GCC
3’5’
CGG
5’3’
replication
Incorporated error
T.A C.G

37. T
A
2AP
2AP
T
C
2AP
C
G
TRANISITION
T C
A G

38. C
G
2AP
2AP
C
T
2AP
T
A
TRANISITION
C T
G A

39. Both base analogs produce transition
mutations
Mutations by base analogs can be
reversed by treatment with the same
analog or different analog

40. Alkylatingagents:
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 alkylatingagent.

41. C
G
EMS
6EG
T
T
A
T
A
EMS
G
4ET
C
G

42. Nitrous acid: causes deamination
Cytosine Uracil
N
N
CYTOSINE
1
2
3
6
5
4
NH2
O
H
N
N
1
2
3
6
5
4
O
o
HNo2
URACIL
H

43. U
G
U
G
U
A
G
C
U A
U
A A
T
C.G TA
C
G
HNO2
5’3’
3’5’
5’3’
3’ 5’
5’3’
3’5’
5’3’
3’5’
3’ 5’
5’3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’

44. H
T
H
T
H
C
A
T
H C
H
C G
C
A.TG.C
A
T
HNO2
5’3’
3’5’
5’3’
3’ 5’
5’3’
3’5’
5’3’
3’5’
3’ 5’
5’3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
Adenine changes into Hypoxanthin which then pairs with Cytosine

45. X
C
X
C
X
T
C
G
X T
X
T A
T
A.TG.C
G
C
HNO2
5’3’
3’5’
5’3’
3’5’
5’3’
3’5’
5’3’
3’5’
3’ 5’
5’3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
Guanine changes into Xanthin which pairs with Cytosine.
Xanthin can also pair with Thymine

46. 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

47. Hydroxlamine
Specific base modifying mutagen which
adds a hydroxyl group to cytosine
producing hydroxlaminecytosine 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

48. hC
G
hC
G
hC
A
G
C
hC A
hC
A T
A
T.AC.G
C
G
5’3’
3’5’
5’3’
3’5’
5’3’
3’5’
5’3’
3’5’
3’ 5’
5’3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
Cytosine changes into hydroxlamine Cytosine which pairs
with Adenine instead of Guanine
NH₂OH

49. 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-7,8-
dihydrodeoxyguanine which mispairs with
adenine leading to G.C T.A transversion

50. 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

52. 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
phosphodiesterbonds in DNA.
Ionizing radiation also frequently results in
double-strand breaks in DNA.

53. 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

54. 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