DNA Damage & Repair

Question 1

UV damaging DNA and cells

Formation of pyrimidine dimers

Answer 1

Damage:

• Exposure to UV causes covalent bonds between 2 thymine bases to form
• Thymine dimers are created
• Resulting in distortion in the sugar-phosphate backbone

Repair:

• Photoreactivation occurs
• Photolyase uses the energy from sunlight to break the covalent bonds between thymine dimers
• The distortion of the backbone is removed

Question 2

UV damaging DNA and cells

Breaking DNA strands

Answer 2

Damage:

• Exposure to UV causes the phosphodiester bonds between 2 nucleotides to break
• Creating nicks in the sugar-phosphate backbone and DNA strand breaks
• DNA fragments are formed

Repair:

• DNA ligase help to repair the broken DNA strands in the DNA backbone
• DNA ligase will seal the broken DNA strand

Question 3

Causes of DNA damage

Answer 3

Mutagens:

• Ionizing radiation
• Replication errors
• Cellular mechanism
• UV light
• Exposure to chemicals

Question 4

Direct repair of DNA damage

pyrimidine dimers

Answer 4

• Photolyase cleaves the 2 covalent bonds between the thymine dimers
• Normal structure of the 2 thymine dimers are restored

Question 5

Direct repair of DNA damage

Methylated guanine

Answer 5

• Alkyltransferase catalyzes the removal of the methyl group onto itself
• Normal structure of guanine is restored

Question 6

Mismatch repair

Answer 6

During DNA synthesis, DNA polymerase will fix most of the mispaired bases

• DNA polymerase adds an incorrect nucleotide to the new strand of DNA
• DNA polymerase detects the bases are mispaired
  DNA polymerase uses 3’ - 5’ exonuclease activity to remove the incorrect nucleotides

Question 7

Base excision repair

Deamination, cytosine –> uracil

Answer 7

• Damaged base s detected and removed by a glycosylase enzyme, leaving an intact phosphodiester backbone with a missing base
• Endonuclease will break the phosphodiester backbone, at the site with the missing base
• The hole is filled with the correct base by DNA polymerase and gap is sealed by ligase

Question 8

Nucleotide excision repair

pyrimidine dimers and DNA adducts

Answer 8

• Exonuclease system detects the pyrimidine dimer/DNA adduct and nicks DNA strand containing it
• Helicase will release the damaged DNA fragment
• A DNA polymerase replaces the excised DNA and ligase seals the DNA backbone