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DNA Mutation and Repair
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DNA
Primary function permanent storage ofinformation
Does not normally change
Mutations do occur
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Mutation Heritable change in the genetic
material
Permanent structural change of
DNA
Alteration can be passed on to
daughter cells (somatic cell)
Mutations in reproductive cells
can be passed to offspring (germ
line)
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SOMATIC VS. GERM-LINE
The timing of mutations in multicellular
organisms plays an important role
Mutations may occur in gametes or a fertilized egg
Mutations may occur later in life
Embryonic or adult stages
Timing can affect
The severity of the genetic effect
The ability to be passed from parent to offspring
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SOMATIC VS. GERM-LINE
Animals possess germ-line and somatic cells
Germ-line cells
Cells giving rise to gametes
Somatic cells
All cells of the body
excluding the germ-line cells
e.g., Muscle cells, nerve cells,
etc.
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SOMATIC VS. GERM-LINE
Germ-line cells
Germ-line mutations can occur in
gametes Germ-line mutations can occur in a
precursor cell that produces
gametes
All cells in the resulting offspring
will contain the mutation
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SOMATIC VS. GERM-LINE
Somatic cells
Somatic mutations in embryonic
cells can result in patches of tissues
containing the mutation
Size of the patch depends on the
timing of the mutation
Individual is a genetic mosaic
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Mutations
Provide allelic variation
Ultimate source of genetic variation
Foundation for evolutionary change
Various phenotypic effects
Neutral
Harmful
Beneficial
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TYPES OF MUTATIONS
Types of mutations
Chromosome mutations
Changes in chromosome structure
Genome mutations Changes in chromosome number
Single-gene mutations
Relatively small changes in DNA
structure Occur within a particular gene
Focus of study in this chapter
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TYPES OF MUTATIONS
Mutations involve the permanent alteration of
a DNA sequence
Alteration of base sequence
Removal or addition of one or more nucleotides
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MUTATIONS
Point mutations
Change in a single base pair within the DNA
Two main types of point mutations
Base substitutions
Transition
Transversion
Small deletions or insertions
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MUTATIONS
Two types ofbase substitutions
Transition
Pyrimidine changed to another pyrimidine
e.g., C
T Purine changed to another purine
e.g., A G
Transversion
Purines and pyrimidines areinterchanged
e.g., A C
More rare than transitions
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EFFECTS OF MUTATIONS
What can happen when a mutation occurs in the DNA
(e.g. sickle cell anemia)
Figure 1. Concept of a mutation in the protein-coding region of a gene. (Note that not
all mutations lead to altered proteins and that not all mutations are in protein-coding
regions.)
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Silent mutations Amino acid sequence is not altered
e.g., CCC CCG (pro pro)
Genetic code is degenerate
Alterations of the third base of a codonoften do not alter the encoded amino
acid
Phenotype is not affected
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EFFECTS OF MUTATIONS
Missense mutations
Amino acid sequence is altered
e.g., GAAGTA (glu val)
Phenotype may be affected
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EFFECTS OF MUTATIONS
Neutral mutations Type of missense mutation
Amino acid sequence is altered
e.g., CTTATT (leu ile)
e.g., GAAGAC (glu asp)
No detectable effect on protein
function
Missense mutations substitutingan amino acid with a similar
chemistry to the original is likely
to be neutral
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EFFECTS OF MUTATIONS
Nonsense mutations
Normal codon is changed into a stop
codon
e.g., AAA AAG (lys stop)
Translation is prematurely terminated
Truncated polypeptide is formed
Protein function is generally affected
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A nonsense mutation and its effect on translation
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EFFECTS OF MUTATIONS
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EFFECTS OF MUTATIONS
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EFFECTS OF MUTATIONS
Mutations occasionally produce a polypeptidewith an enhanced ability to function
Relatively rare
May result in an organismwith a greater likelihoodto survive and reproduce
Natural selection may
increase the frequency ofthis mutation in thepopulation
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MUTATION TYPES
Genetic terms to describe mutations
Wild-type
Relatively common genotype
Generally the most common allele
Variant
Mutant allele altering an organisms phenotype
Forward mutation
Changes wild-type allele into something else
Reverse mutation
Reversion
Restores wild-type allele
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MUTATION TYPES
Genetic terms to describe mutations Deleterious mutation
Decreases an organisms chance ofsurvival
Lethal mutation Results in the death of an organism
Extreme example of a deleteriousmutation
Conditional mutants Affect the phenotype only under a
defined set of conditions
e.g., Temperature-sensitive (ts) mutants
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MUTATION TYPES
Genetic terms to describe mutations
Suppressor mutation
Second mutation that restores the wild-type phenotype
Intragenic suppressor Secondary mutation in the same gene as
the first mutation
Differs from a reversion
Second mutation is at a different site
than the first Intergenic suppressor
Secondary mutation in a different genethan the first mutation
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MUTATION TYPES
Intergenic suppressors involving a mutantstructural gene Usually involve altered expression of one gene that
compensates for a loss-of-function mutation affecting
another gene Second gene may take over the functional role of the first
May involve proteins participating in a common cellularfunction
Sometimes involve mutations in genetic regulatory
proteins e.g., Transcription factors activating other genes that can
compensate for the mutation in the first gene
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TRINUCLEOTIDE REPEATS
DNA trinucleotide repeats
Three nucleotide sequences repeated in tandem
e.g., CAGCAGCAGCAGCAGCAG
Generally transmitted normally from parent tooffspring without mutation
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TRINUCLEOTIDE REPEATS
Trinucleotide repeat expansion (TNRE)
Number of repeats can readily increase from one
generation to the next
Cause of several human genetic
diseases
Length of a repeat has increased
above a certain critical size
Becomes prone to frequent
expansion
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TRINUCLEOTIDE REPEATS
TNRE disorders
Fragile X syndrome (FRAXA)
FRAXE mental retardation
Myotonic muscular dystrophy (DM)
Spinal and bulbar muscular atrophy (SBMA)
Huntington disease (HD)
Spinocerebellar ataxia (SCA1)
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TRINUCLEOTIDE REPEATS
TNRE disorders
Expansion may be within a coding sequence of a
gene
Most expansions are of a CAG repeat
Encoded proteins possess long tracts of glutamine
CAG encodes a glutamine codon
Presence of glutamine tracts causes aggregation of the
proteins
Aggregation is correlated with the progression of the
disease
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TRINUCLEOTIDE REPEATS
TNRE disorders
Expansion may be in a noncoding region of a gene
Two fragile X syndromes
Repeat produces CpG islands that become methylated
Methylation can lead to chromosome compaction
Can silence gene transcription
Myotonic muscular dystrophy
Expansions may cause abnormal changes in RNA structure
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TRINUCLEOTIDE REPEATS
TNRE disorders
Severity of the disease tends to worsen in future
generations
Anticipation
Severity of the disease depends on the parent
from whom it was inherited
e.g., In Huntingdon disease, TNRE likely to occur if
mutation gene is inherited from the father
e.g., In myotonic muscular dystrophy, TNRE likely to
occur if mutation gene is inherited from the mother
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TRINUCLEOTIDE REPEATS
TNRE disorders
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TRINUCLEOTIDE REPEATS
TNRE disorders
Cause of TNRE is not well understood
Trinucleotide repeat may produce alterations in
DNA structure
e.g., Stem-loop formation
May lead to errors in DNA replication
TNRE within certain genes alters gene expression Disease symptoms are produced
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CHROMOSOME STRUCTURE
Altered chromosome structure can alter gene
expression
Inversions and translocations commonly have no
obvious phenotypic effects
Phenotypic effects sometimes occur
Position effect
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CHROMOSOME STRUCTURE
Altered chromosome structure can alter geneexpression and phenotype
Breakpoint may occur within a gene
Expression of the gene is altered
Breakpoint may occur near a gene
Expression is altered when moved to a new location
May be moved next to regulatory elements influencing theexpression of the relocated gene
i.e., Silencers or enhancers
May reposition a gene from a euchromatic region to a highlycondensed (heterochromatic) region
Expression may be turned off
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CAUSES OF MUTATIONS
Two causes of mutations
Spontaneous mutations
Result from abnormalities in biological
processes Underlying cause lies within the cell
Induced mutations
Caused by environmental agents
Cause originates outside of the cell
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Spontaneous
Chemical Changes
Figure 7.9 (a) Deamination of cytosine to uracil. (b) Deamination of 5-methylcytosine
(5mC) to thymine.
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CAUSES OF MUTATIONS
Induced mutations are caused by mutagens
Chemical substances or physical agents originating
outside of the cell
Enter the cell and then alter the DNA structure
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CAUSES OF MUTATIONS
Spontaneous mutations are random events
Not purposeful
Mutations occur as a matter of chance
Some individuals possess beneficial mutations
Better adapted to their environment
Increased chance of surviving and reproducing
Natural selection results in differential reproductive
success The frequency of such alleles increases in the population
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Spontaneous mutations: Depurination
Most common type of naturally occurring
chemical change Reaction with water removes a purine (A or G)
from the DNA
Apurinic site
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Spontaneous mutations: Depurination
~10,000 purines lost per 20 hours at 37oC in atypical mammalian cell
Rate of loss increased by agents causing certain basemodification
e.g., Attachment of alkyl(methyl, ethyl, etc.) groups
Generally recognized by
DNA repair enzymes Mutation may result if
repair system fails
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Spontaneous mutations: Deamination of
cytosines
Methylation of cytosine occurs in many eukaryotic
species as well as prokaryotes
Removal of an amino group from the 5-methyl
cytosine produces thymine DNA repair enzymes cannot determine which is
the incorrect base
Hot spots for mutations are produced
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Spontaneous mutations: Tautomeric shifts
Common, stable form of T and G is the keto form
Interconvert to an enol form at a low rate
Common, stable form of A and C is the amino
form
Interconvert to an imino form at a low rate
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Spontaneous mutations: Tautomeric shifts
Enol and imino forms do not conform to normal
base-pairing rules
AC and GT base pairs are formed
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Certain non-mutagenic chemicals can be
altered to a mutagenically active form after
ingestion
Cellular enzymes such as oxidases can activatesome mutagens
Certain foods contain chemicals acting as
antioxidants Antioxidants may be able to counteract the effects
of mutagens and lower cancer rates
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CAUSES OF MUTATIONS
Mutagens alter DNA structure in various ways
Nitrous acid (HNO3) replaces amino groups with
keto groups
-NH2 =O
Can change cytosine
to uracil
Pairs with A, not G
Can change adenineto hypoxanthine
Pairs with C, not T
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Mutagens alter DNA structure in various ways
Alkylating agents covalently attach methyl or ethyl
groups to bases
e.g., Nitrogen mustards, ethylmethanesulfonate (EMS)
Appropriate base pairing is
disrupted
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CAUSES OF MUTATIONS
Mutagens alter DNA structure in various ways
Some mutagens directly interfere with the DNA
replication process
e.g., Acridine dyes such as proflavin Flat, planar structures interchelate into the double helix
Sandwich between adjacent base pairs
Helical structure is distorted
Single-nucleotide additions and deletions can result
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CAUSES OF MUTATIONS
Mutagens alter DNA structure in various ways
Some mutagens are base analogs
e.g., 2-aminopurine
e.g., 5-bromouracil (5BU)
Become incorporated into
daughter strands during
DNA replication
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CAUSES OF MUTATIONS
Mutagens alter DNA structure in
various ways
Some mutagens are base analogs
5-bromouracil (5BU) is a thymineanalog
Incorporated in place of thymine
5BU can base-pair with adenine
Can tautomerize and base-pair withguanine at a relatively high rate
AT A5BU G5BU GC
Transition mutations occur
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CAUSES OF MUTATIONS
Mutagens alter DNA structure in various ways
DNA molecules are sensitive to physical agents
such as radiation
e.g., Ionizing radiation such as X rays and gamma rays Short wavelength and high energy
Can penetrate deeply into biological materials
Creates free radicals
Chemically reactive molecules Free radicals alter DNA structure in a variety of ways
Deletions, single nicks, cross-linking, chromosomal breaks
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CAUSES OF MUTATIONS
Mutagens alter DNA structure in various ways DNA molecules are sensitive to physical agents such as
radiation
e.g., Nonionizing radiation such as
UV light Contains less energy
Penetrates only the surface of materialsuch as the skin
Causes the formation of thymine dimers
May be repaired through one of numerousrepair systems
May cause a mutation when that DNAstrand is replicated
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Effect of UV light on base DNA
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CAUSES OF MUTATIONS
Many different kinds of testes can determine
if an agent is mutagenic
Ames test is commonly used
Developed by Bruce Ames
Uses his- strains ofSalmonella typhimurium
Mutation is due to a point mutation rendering an
enzyme inactive
Reversions can restore his+ phenotype
Ames test monitors rate of reversion mutations
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CAUSES OF MUTATIONS
Ames test Suspected mutagen is mixed with rat liver extract
and his-Salmonella typhimurium
Rat liver extract provides cellular
enzymes that may be required toactivate a mutagen
Bacteria are plated on minimalmedia
his+ revertants can be detected
Mutation frequency calculated
Compared to control
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DNA REPAIR
Most mutations are deleterious DNA repair systems are vital to the survival
Bacteria possess several different DNA repairsystems
Absence of a single system greatlyincreases mutation rate
Mutator strains
Humans defective in a single DNA
repair system may manifest variousdisease symptoms
e.g., Higher risk of skin cancer
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DNA REPAIR
Living cells contain several DNA repair systems
Able to fix different types of DNA alterations
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