DNA Damage And Repair Flashcards
what are some short term consequences of DNA damage?
Reduced proliferation, altered gene expression,and apoptosis
What are some long term results of DNA damage?
Aging and cancer/other diseases
What is the difference between DNA damage and DNA mutations?
DNA damage occurs a lot, but is usually repaired. DNA mutations are the result of DNA damage not being recognized and being replicated
What are the two types of spontaneous mutations?
Errors or replication and spontaneous lesions
Errors of Replication
Mistakes made during replication. Only occur during the S phase of cell division.
Spontaneous Lesions
Chemical changes that occur spontaneously and occur in the resting cell
Tautomerism
Ability of certain chemicals to exist as a mixture of two interconvertible isomers (e.g., thymine has an enol form that can pair with G and its normal form which pairs with A)
What is the exonuclease activity of DNA polymerase?
3’—>5’
Bloom Syndrome
Defect in the BLM gene which is a DNA helicase enzyme, causing the DNA to not be properly unwound. This results in many chromosomal breaks and sister chromatid exchanges.
What are some symptoms of Bloom syndrome?
Small, narrow chin and prominent ears/nose, facial rash upon exposure to the sun, diabetes and immune system defects
Fanconi Anemia
Autosomal Recessive rare disorder. Multiple genes are involved in the disorder (locus heterogeneity), all genes are in the FANC family which are related to DNA repair. Disease causes spontaneous chromosome breakage and increased risk of neoplasia
What are some symptoms of Fanconi anemia?
radial Ray defects, mental development problems, and short stature.
Frameshift Mutations
Tend to occur at positions where there are base repeats. DNA loops are kinks at these points and some of the bases are not copied or are copied twice.
Spontaneous Lesions
Changes that occur in a resting cell due to the chemical nature of DNA. Very common, and increased by exposure to mutagens. Three main types are depurination, deamination, oxidative damage.
Depurination
Most common spontaneous lesion. Caused by breaking of glycosidic bond between base and sugar in purine nucleotides
Deamination
Second most common spontaneous lesion. Loss of amine group from a base (usually cytosine) which deaminates to form uracil. This one is easy to fix because your body knows that you don’t want uracil in DNA
Which deamination is hard to detect?
5-methyl cytosine can deaminate to thymidine which results in a mutation hotspot because your body doesn’t know which nucleotide is the right one. This is seen frequently in the CG islands
Oxidative damage
Results from the production of reactive oxidative compounds due to oxidative metabolism. This causes a mis-pairing with A and a potential transversion.
What kind of damage does UV light cause?
Can form covalent linkages between bases on the same strand which will interfere with normal pairing and block replication.
Nucleotide Excision
Remove about 30 bases around a damaged site. Repairs pyrimidine diners formed by UV damage
Base Excision
Repairs a single (or just a few) damaged bases by removing it (when it’s damaged by methylation, oxidation, etc)
Mismatch Repair
Post replication repair. Repairs mismatched bases that were formed due to tautomerism.
Steps to excision repair
1) Damage recognized
2) Endonucleases are recruited
3) regain is excised
4) DNA polymerase fills gaps
5) ligase seals the nick
Xeroderma Pigemetosum (XP)
Mutations in 9 different NER genes can lead to XP. Locus heterogeneity that causes extreme sun sensitivity. DNA damage is cumulative and irreversible.
