DNA Repair

• DNA replication steps:

  1. DNA helicase unwinds the DNA
  2. single stranded DNA binding proteins stabilize the unwound DNA
  3. DNA polymerase gets held on by a sliding clamp
  4. the leading strand gets synthesized continuously (5' to 3')
  5. the lagging strand is replicated in Okazaki fragments (RNA primase adds primers to the 3' end and DNA ligase seals the nicks)
• the end replication problem refers to how the lagging strand cannot be fully replicated
• telomerase adds repeating sequences to the 3' end of the lagging strand and carries a piece of RNA complementary to this sequence to act as template for DNA polymerase.
• DNA polymerase has an editing site for exonuclease activity (3' to 5')
• DNA polymerase's exonuclase activity occurs by

  1. excision (damage DNA is cut out)
  2. resynthesis (original DNA sequences is restored by a repair DNA polymerase)
  3. DNA ligase seals the nick
• depurination can remove a G or A from DNA
• deamination most commonly converts C to U
• both deamination and depurination occur on double stranded DNA, causing no breaks in the backbone
• base excision repair is used for small, non helix distorting lesions
• base excision repair requires

  1. specific recognition of a damaged base by glycosylase enzymes
  2. damaged base is excised and corrected
  3. DNA ligase seals the nick
• nucleotide excision repair is used for noticeable damage via DNA legions
• nucleotide excision repair occurs as

  1. recognition of a DNA damage lesion
  2. a protein complex verifies the presence of damage and initiates repair
  3. the DNA gets locally unwound
  4. an incision is made on both sides of the lesion by endonucleases
  5. DNA fragments with the lesion are removed
  6. DNA polymerase refills the strand using the complementary strand
  7. DNA ligase seals the nicks
• non-homologous end joining is a quick, error prone method (often loses nucleotides) of DNA double stranded break repair
• homologous recombination can occur if a break happens in 1 of 2 duplicated DNA helices and the chromosome copies have not been separated
• homologous recombination uses the undamaged helix as a template to repair the damaged helix