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MUTATIONS
Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany Deptt.
P.G.G.C.G. -11,Chandigarh
Mutations
 Any change in the DNA sequence of an organism is a
mutation.
 Mutations are the source of the altered versions of
genes that provide the raw material for evolution.
 Most mutations have no effect on the organism,
especially among the eukaryotes, because a large
portion of the DNA is not in genes and thus does not
affect the organism’s phenotype.
 Only a small percentage of mutations causes a visible
but non-lethal change in the phenotype.
Somatic Vs.Germinal Mutations
Some people may have mutations in
their skin cells or hair. Such mutations
are termed Somatic.
Germ mutations occur only in the sex
cells& called Germinal Mutations
These mutations are more threatening
because they can be passed to offspring
(forever).
Somatic Mutation
 In normal body tissue
 May have vast effects on individual
 Not passed on to offspring
 Not of evolutionary significance
Germinal Mutation
 Germinal or germ line
 Gametes or cells producing gametes
 Little or no effect on individual originating
mutation
 Passed on to all cells of zygote formed
 Passed on to subsequent generations
 Source of new alleles
Mutation
 Not necessarily harmful!
 Dependent on: 1) nature of mutation
2) environment
 Spontaneous
 occur randomly throughout genome
 rates: 10-6 - 10-4 /gene/cell
 Reversible
Types of Mutations
 Point Mutations
 Base Pair Substitutions
 Silent
 Missense – new protein (Amino Acid Substitutions)
 Nonsense – stop codon
 Base Pair Insertions and deletions
 Triplet Repeats
 Frameshift Mutations
 Variation in Chromosome Number
 Variation in Chromosome structure
 Deletions
 Translocations
 Duplications
 Inversions
Point mutations
 Base subsitution :
 neutral
 no change in final protein
 silent
 no change in amino acid
 missense
 amino acid substitution
 nonsense
 stop codon substitution
Point Mutations:
G
G
C
C
Glu
(a) Silent mutation
(d) Run-on mutation
Point Mutations
changes in one or a few
nucleotides
 Substitution
THE FAT CAT ATE THE RAT
THE FAT HAT ATE THE RAT
 Insertion
THE FAT CAT ATE THE RAT
THE FAT CAT XLW ATE THE RAT
 Deletion
THE FAT CAT ATE THE RAT
THE FAT ATE THE RAT
Point 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
H ATE THE RAT
TEF ATC ATA TET GER AT
H
Summary of Mutation Types
Run-on mutation
(can also produce
nonsense and run-ons)
Chromosome Mutations
Changes in structure of entire chromosomes
 Original Chromosome ABC * DEF
 Deletion
AC * DEF
 Duplication
ABBC * DEF
 Inversion
AED * CBF
 Translocation
ABC * JKL
GHI * DEF
Changes in number Of
Chromosome
 Down Syndrome
 Chromosome 21 does
not separate correctly.
 They have 47
chromosomes in stead
of 46.
 Children with Down
Syndrome develop
slower, may have heart
and stomach illnesses
and vary greatly in their
degree of inteligence.
Changes in structure Of
Chromosome
 Cri-du-chat
 Deletion of material on 5th
chromosome
 Characterized by the cat-like
cry made by cri-du-chat
babies
 Varied levels of metal
handicaps
Sex Chromosome
Abnormalities
 XYY Syndrome
 Normal male traits
 Often tall and thin
 Associated with antisocial and behavioral
problems
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
Spontaneous and Induced
Mutation
 Spontaneous mutation rate = 1 in 109 (a billion)
replicated base pairs or 1 in 106 ( a million) replicated
genes. Mistakes occur during DNA Replication just before
cell division. This is natural error rate of DNA
polymerase.
 Induced Mutations: Caused by mutagens. Mutagens
increase mistakes to 10–5 (100 thousand) or 10–3 ( a
thousand) per replicated gene
( Induced Mutation : Mutagens
Mutagens are things that cause mutations
Chemical mutagens
Base pair changers (nitrous acid)
Base analogues (e.g.. 5 bromouracil)
Frameshift mutagens (aflatoxin, benzpyrene)
Radiation
X rays, gamma rays break DNA, bases
UV light causes knots in DNA strand
Many common place items are capable of causing mutations:
microwave, fruit from the store, radar, cellular phones….
Chemical Mutagens:
Base pair changers
(base modifiers)
deaminators like nitrous acid,
nitrosoguanidine, or alkylating
agents like cytoxan
cytoxan
Nitrous acid
Base analogues “mimic”
certain bases but pair with
others - E.g. 5-fluorouracil,
cytarabine
Incorporated as a “T”
cytarabine
Acts like a “C”
Ionizing Radiations as mutagens
 Ionizing radiation (X rays, gamma rays, UV
light) :
causes the formation of ions that can react
with nucleotides and the deoxyribosephosphate backbone.
 Nucleotide excision repairs mutations
X-rays and Gamma Rays
Cause Breaks in DNA
Ionizing Radiation: UV
 UV radiation
causes thymine
dimers, which
block replication.
 Light-repair
separates thymine
dimers
 Sometimes the
“repair job”
introduces the
wrong nucleotide,
leading to a point
mutation.
Figure 8.20
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