DNA repair mechanisms

What is the effect of a single-stranded damage to DNA on transcription?

• if only 1 of the strands of the double helix has a defect the other strand can be used as a template

What are the different types of DNA repair mechanisms?

• base excision repair

• nucleotide excision repair

• mismatch repair

What is base excision repair?

• damaged base is excised by a DNA glycosylase with the gap being recognised by AP endonuclease

• before the missing base is resynthesized by a DNA polymerase

What is nucleotide excision repair?

• recognises and removes bulky DNA adducts

• typically caused by UV light before the missing segment is resynthesized by a DNA polymerase

What is the importance of nucleotide excision repair?

• repairs thymidine dimers caused by UV light damage

• defect found in xeroderma pigmentosum

What is mismatch repair?

• inspects newly formed DNA

• looking for and removing mispaired (but undamaged) nucleotides

What is the clinical relevance of mismatch repair?

• defect found in hereditary non-polyposis colorectal cancer

What is an example of a double-stranded break?

• non-homologous end joining

What is non-homologous end joining?

• a DNA ligase forms a complex with XRCC4 joints the 2 ends of DNA fragments