DNA Repair

causes of DNA damage

• internal factors

• environmental factors like radiation

effects of DNA damage

• structural damage to DNA

• affect a cell’s ability to transcribe such genes

• harmful mutations affect survival rates of daughter cells

what happens if DNA repair is not regulated?

• if neither repair nor apoptosis occurs, then permanent damage can lead to malignant tumors/cancer

3 mechanisms of DNA repair

• Direct reversal

• Excision repair

• Postreplication repair

what is direct reversal used for?

• when consecutive pyrimidine bases are fused (dimerize) when exposed to UV light

how does direct reversal work?

• an enzyme called photolyase reacts to UV light to reverse dimerization (does not break the DNA backbone)

• this enzyme is found in bacteria and fungi, not humans

• humans instead use a nucleotide excision repair method

what is excision repair used for?

• when one strand of a double helix is damaged, the other strand can be used to identify the damaged bases

• then, the damaged nucleotide is removed and replaced

base excision repair (BER)

• repairs single-base damage via glycosylases, which remove the altered base which is replaced by DNA polymerase

nucleotide excision repair (NER)

• less specific than BER

• used when a large portion of the helix is damaged

• three-step process: recognition of damage, excision, resynthesis

• NER is used in almost every organism

what is postreplication (translesion) repair used for?

• occurs when replication continues past DNA lesions (small breaks in the structure); these gaps are fixed later after replication

• a gap is left at the lesion when the Okazaki fragments are synthesized

how does postreplication repair work?

• recombination repair: a sequence from a sister chromosome is used to repair

• error-prone repair (translesion synthesis): uses the damaged strand as a sequence template (inaccurate)