Chapter 12 Presented by Dr Laurie M Erickson Chair of the Department of Health Sciences Blitstein Institute of Hebrew Theological College Chicago IL USA 121122 Mutations Mutation any heritable change in the genetic material ID: 918263
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Lecture 5:
Mutation and DNA Repair(Chapter 12)
Presented by Dr. Laurie M. Erickson
Chair of the Department of Health Sciences
Blitstein Institute of Hebrew Theological College
Chicago, IL, USA.
Slide212.1-12.2 Mutations
Mutation: any heritable change in the genetic materialClassified in a variety of ways. Overlapping.
Origin: spontaneous vs induced
Cell type: somatic vs germ line
Expression: unconditional vs conditionalEffect on protein function: more activity or less activityMolecular effect: change in DNAEffect on translation: change in amino acid sequence.Mutagen: any agent that causes mutation.Mutation rate: the probability of a change in DNA sequence in one generation.Some genes have more mutation than others.Direct vs indirect measurement.
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Slide31. OriginSpontaneous:
random, unpredictable events. Most common.Induced: treatment with a chemical mutagen or radiation higher mutation rate.
2. Cell type
Germ-line
mutations: in cells that form gametes. Inherited.Somatic mutations: all other cells. Not inherited. Mosaic of normal and mutant tissue
Classification of Mutations
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Slide43. Expression levelUnconditional:
always expressedConditional: causes phenotypic changes under certain conditions but not always.
Effects can be turned on or off by the scientist
Very useful for genetic analysis
Permissive conditions: mutation is expressedRestrictive conditions: not expressed
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Temperature-sensitive
mutations: conditional mutation whose expression depends on temperature.
Slide54. Effects on protein functionLoss-of-function
mutation (a knockout or null): complete gene inactivation or in a completely nonfunctional gene product.
Hypomorphic
mutation: reduced expression of a gene or reduced activity of a product.
Hypermorphic mutation: greater-than-normal level of gene expressionGain-of-function mutation: qualitative change. New function. May become active in a different cell type or tissue. Rare.
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Slide65. Molecular changes in DNA
Base substitution replaces one nucleotide pair with anotherTransition
mutations replace one
pyrimidine base
with the other or one purine base with the other. C to T T to C A to G G to A.Transversion mutations replace a pyrimidine with a purine or the other way around. Eight possible transversion mutationsTransitions are more frequent than transversions in spontaneous mutations. Ratio approximately 2:1
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Slide76. Effects on translation:
mutations in protein-coding regionsSilent or Synonymous substitutions: no change in the amino acid sequence.
Genetic code is redundant.
Missense
or nonsynonymous substitutions: amino acid is replaced with a different amino acid.Nonsense mutation: creates a new stop codon.Frameshift mutations: shift the reading frame of the codons in the mRNA. Any addition or deletion that is not a multiple of three. AUGAUG codes for Met-Met
insertion AUUGAUG codes for Ile-Asp
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Slide8Mutation Example: Sickle-cell anemia
Mutant gene for b-globin.
part of hemoglobin that carries oxygen in the blood
severe genetic disease
premature death.Missense mutation: codon 6.Normal GAG, which codes for glutamic acidMutant GUG (GTG in DNA), which codes for valineSickle-cell carriers show resistance to malaria.Sickle-cell disease is common in regions with malaria in Middle East or Africa.
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Slide9Mutation Example: Trinucleotide Repeats
Dynamic mutation: the mutation becomes worse in succeeding generations.extraordinary genetic instability
Molecular change:
Trinucleotide Repeat Expansion
due to replication slippage during DNA replicationGenetic diseases associated with dynamic mutation: Fragile X syndrome (CGG) (next slide)Myotonic dystrophy (CTG)Kennedy disease (AGC)Friedreich ataxia (AAG)Spinocerebellar ataxia type 1 (AGC)Huntington disease (AGC)
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Slide10Dynamic Mutation on the X Chromosome
Fragile-X syndrome: Moderate mental retardation and testicular abnormalities in men, mild effects in women.X chromosomes easily damaged in cells grown in lab culture.Highly variable symptoms, overlapping with autism and others.
1 in 2500 children
Loss of function
of FMR1 (fragile-site mental retardation-1)FMR1 is expressed primarily in the brain and testes
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Human Autosomes:
chromosome pairs 1-22.
Human Sex chromosomes:
chromosome pair 23
determines sex of organism.
Human female: XX Human male: XY
Slide1111
Normal X chromosomes have 6–54 copies of CGG.
Expansion occurs in female (carrier).
Affected persons have 230–2300 copies.
CpG islands become methylated and have
reduced
expression.
Loss of function
Dynamic mutation in upstream region of FMR1.
CGG
Slide1212.3 Transposable Elements cause Mutations
Transposable elements (TE): genetic elements that can move (transpose) within the genome.Classified into “families” based on DNA sequence.
Have distinctive ends: Short terminal inverted repeats (IR) or Long Terminal Repeats (LTRs, direct repeats) or both.
New insertions can cause mutations.
Interrupt coding sequence to create non-functional proteins.Modify expression of nearby genes.
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Terminal inverted repeat
Terminal inverted repeat
Slide13TE cause mutations by recombination.
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Two copies of same TE can pair and have cross-overs during replication.
Outcome: Deletion
Outcome: Inversion
Slide14Types of Transposable Elements
DNA transposable element: uses a DNA intermediateCarries gene for transposase enzyme.
Cut-and-paste mechanism: the TE is cleaved from one position in the genome and the
same
molecule is inserted somewhere else. Retrotransposon: uses an RNA intermediateLong Terminal Repeats and Inverted RepeatsCarries gene for reverse transcriptase enzymeReverse transcriptase: makes a cDNA strand from an RNA template. cDNA: complementary DNANon-LTR retrotransposonsSINE: Short Interspersed Nuclear ElementLINE: Long Interspersed Nuclear Element
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Slide1515
Nearly Half of the Human Genome is TE.
Most TE are inactive.
Inactive elements can still cause mutations by recombination.
Slide1612.4 Spontaneous Mutations
Mutations are statistically random events.no way to predict when, or in which cell.not caused by evolutionary advantage.A potentially favorable mutation does
NOT
arise because the organism has a “need” for it.
Mutations are already present in population, demonstrated by replica plating experiment in text.Scientists can select for particular types of mutants in order to detect them.Mutational hotspots: some locations in genes are more likely to undergo mutation.
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Slide1712.5 Mutagens and Chemical Damage.
Slide18Base Mispairing causes Mutations.
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5-bromouracil (part of 5-bromodeoxyuridine) is similar to Thymine.
Has keto and enol forms.
The
keto
form pairs with A.
no mutation,
The
enol
form pairs with G.
mutates to GC pair.
Slide19Ionizing radiation (x-rays, others) generates free radicals inside the tissues.
Free radicals: active ions that damage other molecules. breaking the backbone of DNAsingle-strand or double-strand breaksSieverts (Sv): unit of radiation dose. Higher doses cause more mutations.
Human exposure to Radiation is very low.
Natural background radiation is 3.00 mSv (0.003 Sv) per year.
Human-produced radiation is 0.63 mSv per year.Ionizing Radiation causes Mutations.
Mutations in Fruit Flies.
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12.6 DNA Repair Mechanisms
During DNA replication, errors can occur.
Different types of errors get corrected by different systems.
May need two or more systems.DNA ligase enzyme repairs nicks in the DNA backbone.Base excision removes bases that were chemically modified.
Mismatch repair
fixes incorrectly matched basepairs.
The AP endonuclease system repairs nucleotide where the nitrogenous base has been lost.Nucleotide excision repair removes a section of damaged DNA.UV repair enzymes repair damage from ultraviolet light.DNA damage bypass system skips over damaged bases.Double-stranded gaps are repaired by two mechanisms:Rejoin broken ends, sometimes creating a deletion.
Use other chromosome to fill in correct sequence.
Slide21Mismatch Repair
DNA replication allows mismatch errors at a low rate.Most errors are corrected during the replication process.Proofreading repair function of DNA polymerase
Mismatch repair:
fixes incorrectly matched base pairs if proofreading has failed.
“last chance” to correct errors in replication.Uses DNA polymerase and other enzymes.Mechanism: cuts out mismatched segment of DNA.The “hole” is repaired by enzymes.Recognizes the degree of methylation in each strand.The undermethylated strand is the daughter strand.
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Slide22Nucleotide Excision Repair
Removes damaged DNA strand from a duplex molecule.Replaced by re-synthesis, using the undamaged strand as a template.Repairs many types of damage.
Non-specific
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Slide23Other Mechanisms
DNA ligase enzyme: permanently joins the nucleotidesrepairs the backbone of DNA strand.AP endonuclease enzyme: repairs apurinic and apyrmidinic sites.
Loss of purine base = apurinic site
Loss of pyrimidine base = apyrmidinic site
DNA damage bypass: skips over damaged bases.Repair is completed later.
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UV repair
enzymes: repair damage from UV radiation.
Xeroderma pigmentosum
: inherited defect in UV damage repair enzymes.
Slide24Genetic Testing for Mutagens and Carcinogens
Chemicals are found as environmental contaminants, or used in medicine, manufacturing, or food.Must be tested for mutagenicity.Most agents that cause cancer
(carcinogens)
are also mutagens,
mutagenicity is an initial screening for potential carcinogens.Ames test: is a genetic test to measure the rate of mutation. Bacterial experiments.If a chemical causes more mutations in bacteria, it is likely mutagenic in humans.
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Slide25ENdNext Lecture: Cancer and the Cell Cycle
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