Mutation
Mutation
• Means change
• Definition: An event that gives rise to an
alteration in the genotype
• It also can be the change itself
• Mutation does not mean “bad”
• Mutations are IMPORTANT for a variety of
reasons
Mutation ‘Levels’
• DNA Level
• Chromosomal Level
• Genome Level
DNA Level
• At or below the gene level
• 1) Addition or Deletion of one or two
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nucleotides (+ and – frameshifts)
2) Substitution of one nucleotide for
another
3) Inversion within a gene
4) Crossingover within a gene
– Equal
– Unequal
1 and 2 are often referred to as POINT
MUTATIONS
Chromosomal Level
• 1) Duplications
• 2) Deletions
• 3) Translocations
– Reciprocal
– Non-reciprocal
• 4) Inversions
• ? ‘Standard’ crossing over ?
• 3 and 4 may result in POSITION EFFECTS
Genome Level
• These result in ploidy changes
• Aneuploidy
• Plus or minus one or a few chromosomes
• Euploidy (true polyploidy)
• Involves whole SETS of chromosomes
– Autopolyploidy
– Allopolyploidy
Additional Points
• ALL genes can mutate
• Observed levels are usually low
• Some genes have very high rates of
mutation (Mutable Genes)
• Some genes seem to increase the rate of
mutation in adjacent genes (Mutator
Genes)
What Causes Mutations ?
• 1) Spontaneous (?????)
• 2) Chemically induced
• 3) Radiation induced
• Most (if not all) really come down to a
chemical level
• No such thing as a “spontaneous”
mutation
• “Spontaneous” means…
Chemicals
• Many chemicals are mutagens; many others
are promutagens
• Chemicals (just like radiation) can cause point
mutations.
• Chemicals (just like radiation) can cause
chromosome breakage (e.g. - LSD, Mercury,
Caffeine --- at least in cell culture)
DNA Level Mutations
• + and – Frameshifts and their effects
• + or – one nucleotide
• + or – a whole codon
• Multiple frameshifts
• The effects depend on WHERE it occurs
• The effects depend on what the change is
• ?????? One change equals one change, or
does it ??????
DNA Level Mutations
• Substitutions
• Transitions (more common)
• Purine (A, G) for a purine OR pyrimidine (T,
C) for a pyrimidine
• Transversions (less common)
• Purine for a pyrimidine (or pyrimidine for
a purine
• GENERALLY have less chance of an effect
than frameshifts. WHY?
Other DNA Level Mutations
• Inversions within a gene
• Crossingover within a gene
Substitution Mutations
• Results from a temporary type of
isomerization (tautomerization) during
replication
• Base enters a “rare” state (unstable)
• Can be in a base already in the DNA strand
• Can be in one of the incoming bases
• We will look at both for a TRANSITION
• Requires two divisions to get the “stable”
mutation
Substitution Mutations
• One nucleotide is replaced by another
• Transition --- insertion of a one purine base for
the other or one pyrimidine base for the other
(results in an “odd” purine-pyrimidine pair)
• Transversion --- replacement of a purine base
with a pyrimidine or vice versa (results in a
temporary purine-purine pair or pyrimidinepyrimidine pair)
Substitution Mutations
• Base analogs increase the rate of formation
of substitution mutations
• 5-bromouracil (5BU)
• Use of 5-fluorouracil
Radiation Induced Mutations
• By non-ionizing radiation (UV)
• Dimer formation (especially ThT dimers)
• Light Repair System (enzyme breaks dimers)
Radiation Induced Mutations
• By non-ionizing radiation (UV)
• Dimer formation (especially ThT dimers)
• Dark Repair System (excision repair)
– Endonuclease, exonuclease,
DNA polymerase, ligase
Radiation Induced Mutations
• By ionizing radiation
• Mechanisms
- “Target Hypothesis”
- “Physical-Chemical” Hypothesis
• Temperature, hydration, and oxygen content
all have an effect
Mechanism
Mechanism
Ionizing Radiation and Mutation
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Late 1920s
Stadler – working on barley and corn
Muller – working on Drosophila
Muller later received a Nobel Prize (1948)
Why H. Muller and not Stadler?
SOME GENERALIZATIONS FOR
IONIZING RADIATION
• Mutation frequency is DIRECTLY proportional
to the dose
• Doesn’t matter if given in one large dose or
several small ones
• No lower limit (no threshold)
• Safe amount?????
Additions and Deletions
• Chemical basis
• Caused by intercalation (insertion) of some
nucleotide like chemical into the chain
• Before replication --- increases length
• Results in a Plus (+) frameshift mutation
• During replication --- decreases length
• Results in a Minus (-) frameshift mutation
• Acridines are capable of doing this
Additions and Deletions
• Requires 2 divisions to get the “stable” mutation
• Proflavin
• Acridine Orange (very potent)
Additions and Deletions
- frameshift
Intercalation
of Proflavin
DURING
DNA
replication
+ frameshift
Intercalation
of Proflavin
PRIOR TO
DNA
replication
Chemicals Besides Acridines
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Too many to mention…but…
Mustard gas (adds a methyl group)
Formaldehyde
Phenol
Nitrous Oxide (replaces NH2 with O)
Mutation Concluded
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Temperature has an effect
Carcinogens and Mutagens
Somatic vs. germinal mutations
Somatic are “dead-end” in animals
In plants somatic can be “passed on”
asexually by vegetative propagation
(runners, bulbs, corms, cuttings, grafting)
• In plants somatic mutations can get into the
germ line!
Testing for Mutagens & Promutagens
• Ames test
• Uses a histidine deficient (dependent) strain
of Salmonella
• Looks for reverse mutation to histidine
independence above the background rate
• For suspected promutagens: first treat
substance with a liver (human or rat) extract
and then do the test
Teratogenesis
• Teratogen – an agent that induces a noninheritable defect during embryonic
development (Proteratogen)
• In humans --- about 4% of births
• How do they work?
– Interfere with mitosis or cell migration
– Interfere with differentiation (disturb gene
regulation, translation or activity of ultimate
gene products)
Teratogens
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Radiation
Viruses (Rubella and pregnant women)
Thalidomide (really a proteratogen)
Diethylstilbestrol (DES) (taken by pregnant
women - results in genital tract abnormalities
in their children)
• Dioxins (found as a contaminant in herbicides
such as Agent Orange)
• Hexachlorophene (a bacteriocide)
Teratogens
• Smoking (CO reduces O2 available to the fetal
circulation; nicotine constricts blood vessels and
reduces blood flow)
• Progestin (artificial progesterone-like compound;
causes masculinization of female fetuses)
• Alcohol – microcephaly, low birth weight, some
mental retardation)
• Caffeine, Aspirin and numerous legal and not so
legal drugs
Some Other Birth Defects
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Cleft lip and/or cleft palate
Spina bifida
Club foot
Heart disease and malformations
Cataracts
Deafness
Intestinal tract disorders
Low birth weight