8.5 DNA Repair8.5 DNA Repair

At the end of S phase, DNA At the end of S phase, DNA polymerase has copied both strands polymerase has copied both strands of template DNAof template DNA

DNA polymerase does make mistakesDNA polymerase does make mistakes

-1 mistake every 10,000 bases-1 mistake every 10,000 bases

What is a mutation?What is a mutation?

• MutationMutation– any change in DNA sequenceany change in DNA sequence

• Results varyResults vary– Silent (very minor)Silent (very minor)– Harmful (major)Harmful (major)– Lethal (death)Lethal (death)

• These mutations will be passed on to These mutations will be passed on to next generation of cellsnext generation of cells

DNA Repair MechanismsDNA Repair Mechanisms

• Body must fix errors(mutations) in DNABody must fix errors(mutations) in DNA

• Two typesTwo types

– 1. DNA polymerases proofreading1. DNA polymerases proofreading

• Most common way of repairMost common way of repair

– 2. Excision Repair2. Excision Repair

Polymerase ProofreadingPolymerase Proofreading

• DNA polymerase makes a mistake every DNA polymerase makes a mistake every 10,000 bases10,000 bases

• As DNA polymerase adds nucleotide, it As DNA polymerase adds nucleotide, it proofreadsproofreads– Will remove wrong nucleotide Will remove wrong nucleotide

– Add correct nucleotideAdd correct nucleotide

After it is done, only 1 error every After it is done, only 1 error every 10 million 10 million basesbases

22ndnd method: Excision Repair method: Excision Repair

• Other mutations arise Other mutations arise

– Not during DNA replicationNot during DNA replication

– Other times during cell lifeOther times during cell life

• Caused by outside Caused by outside Mutagenic Mutagenic factorsfactors

– ChemicalChemical

– radiationradiation

Chemical Mutagenic FactorsChemical Mutagenic Factors

• Anything that causes a mutation, a Anything that causes a mutation, a change or error in DNA sequencechange or error in DNA sequence– 1. Chemical Factors 1. Chemical Factors

•Chemical molecules physically bind to DNAChemical molecules physically bind to DNA– Ex. BenzopyreneEx. Benzopyrene– Hetrocyclic aminesHetrocyclic amines– pesticidespesticides

2. Radiation2. Radiation•Physically damages DNAPhysically damages DNA

Radiation Mutagenic FactorsRadiation Mutagenic Factors

• Anything that causes a mutation, a Anything that causes a mutation, a change or error in DNA sequencechange or error in DNA sequence2. Radiation: Physically damages DNA2. Radiation: Physically damages DNA

• Ionizing RadiationIonizing Radiation– X-raysX-rays

Actually punch holes through DNAActually punch holes through DNA

•UV RadiationUV Radiation– SunlightSunlight

Causes kink between two thyminesCauses kink between two thymines

UV Radiation Damage to UV Radiation Damage to DNADNA

• Causes portions of DNA to be bound Causes portions of DNA to be bound together together

N T T N

N A A N

UV Radiation Damage to UV Radiation Damage to DNADNA• Creates “Thymine Dimers”Creates “Thymine Dimers”

• Does sunblock prevent mutations?Does sunblock prevent mutations?

N T T N

N A A N

Excision RepairExcision Repair

• The mutations The mutations must be detected must be detected and correctedand corrected

Excision RepairExcision Repair

• Most common Most common mutations are mutations are mismatches(wrong mismatches(wrong bases paired up)bases paired up)

• Or a chemical Or a chemical molecule binds molecule binds DNADNA

Excision RepairExcision Repair

• Special enzymes constantly Special enzymes constantly circulatecirculate

• Travel up and down DNA Travel up and down DNA til they find “damaged til they find “damaged DNA”DNA”

• Mismatched bases don’t Mismatched bases don’t bond well-bond well-– form humpform hump

Excision RepairExcision Repair

• 22ndnd step: Incision step: Incision– Enzyme breaks Enzyme breaks

sugar-phosphate sugar-phosphate bonds between bonds between damaged DNA and damaged DNA and normal DNAnormal DNA

• 33rdrd step: Excision step: Excision– Damaged DNA region Damaged DNA region

is removedis removed

Excision RepairExcision Repair

• 44thth step: Re-synthesis step: Re-synthesis– DNA polymerase fills in DNA polymerase fills in

deleted regiondeleted region

• Last step:Last step:– “ “Ligase” forms bonds Ligase” forms bonds

between new region between new region and rest of DNA strandand rest of DNA strand

DNA RepairDNA Repair

• What would happen if What would happen if your body couldn’t repair your body couldn’t repair mutations as efficiently?mutations as efficiently?

• What if you couldn’t fix What if you couldn’t fix UV damage?UV damage?

• Anyone ever watch “The Anyone ever watch “The Others?”Others?”

http://www.wwlp.com/news/segments/sybersy/others.jpg

Xeroderma PigmentosaXeroderma Pigmentosa

• Autosomal recessive disease(very rare)Autosomal recessive disease(very rare)

• Mutation in DNA repair proteinMutation in DNA repair protein– Protein responsible for replacing nucleotides Protein responsible for replacing nucleotides

in DNA of skin damaged by UV radiationin DNA of skin damaged by UV radiation

Cells expose to sunlight have damage to DNACells expose to sunlight have damage to DNA

Now they can’t fix itNow they can’t fix it

Xeroderma PigmentosaXeroderma Pigmentosa

• Cannot be exposed to sunlightCannot be exposed to sunlight• Not even strong fluorescent Not even strong fluorescent

lightlight• Must stay indoors with Must stay indoors with

windows covered to block out windows covered to block out light.light.

• Only a few seconds of Only a few seconds of exposureexposure– Cause 3Cause 3rdrd degree burns degree burns– Skin cancerSkin cancer