MUTATIONS AND MUTAGENS in animals and the causitive agents

Term mutation was given by Hugo 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

1. MUTATIONS AND ITS
CAUSES (MUTAGENS)
Rayan Bhattacharya
Sushil Lukram

2. INTRODUCTION
Term mutation was given by Hugo
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

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

4. 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

5. 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

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

7. 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

8. 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

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

10. 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

11. GENE MUTATIONS

12. 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

13. Base substitution
Insertions & deletions

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

15. 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
CGT
A 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

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

19. 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

20. 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.

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

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

30. 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

31. 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

32. Gene Mutation Animation

33. CHROMOSOMAL
MUTATIONS/ABBERATIONS

34. Chromosome Mutations
•May Involve:
–Changing the
structure of a
chromosome
–The loss or gain
of part of a
chromosome

35. Chromosome Mutations
•Five types exist:
–Deletion
–Inversion
–Translocation
–Nondisjunction
–Duplication

36. Deletion
•Due to breakage
•A piece of a chromosome
is lost

37. Inversion
•Chromosome segment
breaks off
•Segment flips around
backwards
•Segment reattaches

38. Duplication
•Occurs when a gene
sequence is
repeated

39. Translocation
•Involves two
chromosomes that are
NOT homologous
•Part of one chromosome
is transferred to another
chromosome

40. Translocation

41. Non-disjunction
•Failure of chromosomes to separate
during meiosis
•Causes gamete to have too many or too
few chromosomes
•Disorders:
–Down Syndrome – three 21
st
chromosomes
–Turner Syndrome – single X chromosome
–Klinefelter’s Syndrome – XXY chromosomes

43. Chromosome Mutation Animation

44. MUTAGENS

45. A mutagen is any
substance or agent that
can cause a mutation,
or change in the
sequence or structure
of DNA.

46. Effects of mutagens
Mutagens cause changes to the DNA that can affect the
transcription and replication of the DNA, which in
severe cases can lead to cell death.
Powerful mutagens may result in chromosomal
instability, causing chromosomal breakages and
rearrangement of the chromosomes such as
translocation, deletion, and inversion. Such mutagens
are called clastogens.
Some mutagens can cause aneuploidy and change the
number of chromosomes in the cell.
Accumulation of mutations may lead to cancer.
Many mutagens are not mutagenic by themselves, but
can form mutagenic metabolites through cellular
processes. Such mutagens are called promutagens.

47. The first report of mutagenic action of a chemical
was in 1942. First discovery of a chemical mutagen
was made by Charlotte Auerbach.
There are many hundreds of known chemical
mutagens.
Chemical mutagens are categorized into four
general groups, based on the mechanism by which
they interact with DNA.
1.Base analogs
2.Base altering chemicals
3.Intercalating agents
4.Agents altering DNA structures
Chemical Mutagens

48. 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

49. 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

50. 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

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

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

53. 2-amino purine (P)
Base analog of adenine
Normally pairs with thymine
May mispair with cytosine
Causes a transition mutation

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

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

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

57. 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
Mutations produced by EMS can be reversed by
additional treatment with EMS.
Mustard gas is another alkylating agent.

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

59. 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

60. Possible effects of nitrous acid
•Causes change of CG base pair into TA base
pair
•Adenine changes into Hypoxanthin which
then pairs with Cytosine
•Guanine changes into Xanthin which pairs
with Cytosine. Xanthin can also pair with
Thymine.

61. 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

62. Hydroxyl amine
Specific base modifying mutagen which
adds a hydroxyl group to cytosine
producing hydroxylamine cytosine which
pairs with adenine instead of guanine
This Leads to C.G T.A transitions
Acts only on cytosine thus can not revert
the mutation produced
Cytosine changes into hydroxylamine
Cytosine which pairs with Adenine
instead of Guanine

63. 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

65. 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

66. Physical Mutagens
Radiation was the
first mutagenic agent
known; its effects on
genes were first
reported in the
1920's.
Radiations are of two
types.
I
.
EM
radiations
II.Ionizing
radiations

67. Electromagnetic Radiations(Non ionizing):
Visible light and other forms of radiation are
all types of electromagnetic radiation (consists of
electric and magnetic waves).
The portion of light which is biologically
significant is UV and higher energy radiation.
UV radiation is not ionizing but can react with
DNA and other biological molecules
UV radiation: Thymidine dimers (T-T)

69. 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.

70. Effects of various ionizing radiations on DNA molecule