Post on 04-Jan-2016
12.6 DNA Repair12.6 DNA Repair
DNA RepairDNA Repair Errors in DNA replication Errors in DNA replication or or damagedamage to DNA to DNA
create mutations create mutations - May result in cancer - May result in cancer
Fortunately, Fortunately, mostmost errors and damage are errors and damage are repairedrepaired
Type of repair depends upon the type of Type of repair depends upon the type of damage or errordamage or error
Organisms vary in their ability to repair DNAOrganisms vary in their ability to repair DNA
What damages DNA?What damages DNA?
DNA DNA polymerasepolymerase proofreads DNA, but proofreads DNA, but repair enzymesrepair enzymes correct errors in other correct errors in other ways.ways.
Types of DNA RepairTypes of DNA Repair
In many modern species, three types of In many modern species, three types of DNA repair peruse the genetic material DNA repair peruse the genetic material
1) Photoreactivation repair1) Photoreactivation repair
2) Excision repair2) Excision repair
3) Mismatch repair3) Mismatch repair
Photoreactivation RepairPhotoreactivation Repair
Photoreactivation repairPhotoreactivation repair uses light energy uses light energy to split to split pyrimidine dimerspyrimidine dimers that that kinkkink the the DNA.DNA.Pyrimidine dimersPyrimidine dimers - bonds between C’s - bonds between C’s
and/or T’s on the same strand.and/or T’s on the same strand.PhotoPhotolyases - lyases - enzymes enzymes that absorb light that absorb light
energy and use it to energy and use it to detectdetect and and bind bind to to pyrimidine dimers, then pyrimidine dimers, then breakbreak the extra bond. the extra bond.
Humans Humans do not do not have this type of repairhave this type of repair
Excision RepairExcision Repair
Pyrimidine dimers and surrounding Pyrimidine dimers and surrounding bases are removed and the area is filled bases are removed and the area is filled in correctly.in correctly.Enzymes cut the bond between the Enzymes cut the bond between the sugar sugar
and baseand base and snip out (excise) the and snip out (excise) the pyrimidine dimerpyrimidine dimer..
DNA polymeraseDNA polymerase fills in the correct fills in the correct nucleotides.nucleotides.
Mismatch RepairMismatch Repair
Mismatch repairMismatch repair proofreads newly proofreads newly replicated DNA for replicated DNA for loopsloops that that indicate indicate noncomplemetarynoncomplemetary base pairing base pairing
The incorrect base is excised and The incorrect base is excised and replaced.replaced.
Mutations in repair genes cause Mutations in repair genes cause disorders characterized by disorders characterized by broken broken chromosomeschromosomes and and increased risk of increased risk of cancercancer..
Figure 12.4
Repair Disorders: Trichothiodystrophy
Photosensitivity, Icthyosis, Brittle hair and nails, Intellectual impairment, Decreased fertility and Short stature.
At least five genes are involved
Symptoms reflect accumulating oxidative damage
Faulty nucleotide excision repair or base excision repair or both
Mild case
Figure 12.4
Repair Disorders:Inherited Colon Cancer
Hereditary nonpolyposis colon cancer
Affects 1/200 individuals
Defect in mismatch repair
HNPCC gene is on chromosome 2
Figure 12.4
Repair Disorders:Xeroderma Pigmentosum
• Autosomal recessive; Seven genes involved
• Malfunction of excision repair
• Thymine dimers remain and block replication
• Must avoid sunlight• Only 250 cases
worldwide
Figure 12.4
Repair Disorders:Ataxia Telangiectasis
• Autosomal recessive disorder• Defect in cell cycle checkpoint kinase• Cells continue through cell cycle without
pausing to inspect DNA• Individuals with AT have 50X the risk of
developing cancer over general population• Heterozygotes have a two- to sixfold
increase in cancer risk