7
Mechanisms of
Mutation and DNA
Repair
Mutations
• Spontaneous mutation: occurs in
absence of mutagenic agent
• Rate of mutation: probability of change
in DNA sequence during a single
generation
• Induced mutation: caused by exposure
to mutagenic agent=mutagen
Mutations: Phenotypic Effects
• Mutations can be classified
by their phenotypic effects
• Germ-line mutations:
affect gametes (inherited)
• Somatic mutations: may
affect any type of body
cell except gametes
(not inherited)
Mutations
• Conditional mutations: produce
phenotypic changes under specific
(restrictive) conditions but not others
(permissive conditions)
• Temperature-sensitive mutations:
conditional mutation whose expression
depends on temperature
Mutations: Genotypic Effects
• Mutations can be classified by their
effect on gene function
• Loss-of-function (null): totally
nonfunctional gene product
• Hypomorphic: reduces level of
expression
Mutations: Genotypic Effects
• Hypermorphic: above normal level of
expression
• Gain-of-function: many are dominant
and may cause expression at an
abnormal time or in an abnormal
place
Molecular Basis of Mutation
Mutations result from changes in the base
sequence of DNA:
• Base substitutions -one pair of of DNA
nucleotides is replaced by another pair :
-Transition
mutations- a purine
is substituted for a
purine or a
pyrimidine is
substituted for a
pyrimidine
Molecular Basis of Mutation
-Transversion mutations- a purine replaces
a pyrimidine or vice versa
• Base substitutions are point mutations
which alter one DNA base pair without
adding or deleting any base pairs
• Point mutations may affect gene
expression in several ways
Point Mutations
Types of point mutations:
• Silent substitutions are base substitutions
which do not alter the amino acid
composition of the protein encoded by a
gene:
-silent mutations may affect the noncoding
portion of a gene or may occur in the
coding portion but may not alter codon
usage
Point Mutations
• Missense mutations change a single
amino acid as a result of a change in
codon specification:
-missense mutations can have serious
consequences on the biological properties
of a protein
- sickle cell anemia results from a single
amino acid substitution in hemoglobin
which alters its structure
Point Mutations
Point mutations can also alter signals used
to regulate gene expression:
• Promoter mutations may block
transcription
• Splice site mutations may block splicing
or create new splice signals
• Nonsense mutations change a codon to a
stop codon which results in a premature
termination of translation
Insertions and Deletions
• Insertions add one or more nucleotide
pairs to DNA sequence
• Deletions remove one or more nucleotide
pairs from DNA sequence
• Insertions or deletions involving a multiple
of 3 DNA base pairs = in-frame since they
do not alter the reading frame of the
genetic code
Insertions and Deletions
• Insertions or deletions which involve a
non-multiple of 3 DNA base pairs =
frameshift mutations since they alter the
codon translation reading frame
• Large deletions may remove genes-no
gene product is made
• Insertions can result from gene
amplification which can result in the
overproduction of gene products
Insertions and Deletions
• Deletion mutations in the dystrophin gene
cause muscular dystrophy
• Gene amplifications are often observed in
human malignancies
• Insertion and deletion mutations may
result from unequal crossing-over during
recombination or replication slippage
during replication of simple tandem repeat
sequences
Transposable Elements
• Transposable elements are found in
prokaryotes and eukaryotes
• Transposable
elements are called
selfish DNA because
these elements
maintain themselves
through replication
and transposition
Transposable
Elements
• Transposition=
movement of genetic elements from
one chromosome location to another
• transposase=enzyme which
catalyzes movement of genetic
element
Transposable Elements
• Steps in transposition:
-transposase binds to terminal inverted
repeat sequence
-enzymatic cleavage results in transfer to
different chromosomal site
-insertion site is random and involves
duplication of 2-12 base pairs
• Transposable elements cause mutations
by inactivating genes at sites of insertion
Transposable Elements
• Reverse transcriptase: enzyme using RNA
transcript as a template for a DNA
daughter strand
• LTR retrotransposons: long terminal
repeats
• Non-LTR retrotransposons: no terminal
repeats
– LINE and SINE: most abundant transposable elements
in mammalian genomes
Spontaneous Mutations
Lederberg’s replica plating:
• bacterial colonies are
transferred to velvet
pad and from pad to
new plate to test for
the frequency of
phage resistant
colonies in a
• population
Mutation Hot Spots
Mutation hot spots have a higher mutation
rate than most DNA:
• Cytosine deamination to uracil is often
detected at hot spots
• Sites of cytosine methylation result in
deamination which converts 5methylcytosine to thymine
• Both mutations result in GC to AT
transitions
Mutation Hot Spots
• Cytosine deamination can be repaired by
DNA uracil glycosylase which recognizes
the incorrect GU base pair and removes
uracil
• AP endonuclease then removes the ribose
sugar
• Single-strand gap is repaired by DNA
polymerases and nick is sealed by ligase
Induced Mutations
• Base analogs such as 5-bromouracil may
be incorporated into DNA during
replication instead of thymine and pairs
with guanine resulting in AT to GC
transition
• Nucleotide analogs can inhibit DNA
replication
Chemical Mutagenesis
• Nitrous acid converts amino groups to
keto groups altering the base pairing
properties of the bases to produce
transition mutations
• Alkylating agents add alkyl groups to
bases resulting in transition mutations or
depurination = loss of guanine
Radiation Mutagenesis
• Ultraviolet radiation (UV) causes adjacent
thymines to become covalently linked =
pyrimidine dimers
• Ionizing radiation causes formation of free
radicals, highly reactive ions which can
damage DNA producing serious
mutagenic effects
DNA Repair Mechanisms
• Mismatch Repair consists
of the excision of a segment
of DNA that contains a base
mismatch followed by repair
synthesis
• Photoreactivation repairs UV- induced
pyrimidine dimers by breaking the
covalent linkage between the thymine
bases
Excision Repair
• Excision repair is a
multistep process in
which a segment of
damaged DNA is
removed and replaced by
Resynthesis using the undamaged
strand as a template
DNA Repair Mechanisms
• Postreplication repair involves
replication of damaged DNA
strand which results in a gap at
the damaged DNA site
• A segment of the template
DNA from the other strand is
inserted to repair the gap by
recombination
• The gap in the template is
repaired