Molecular Biology: DNA Repair Mechanisms (BIOL10221 2024)

BIOL10221 Molecular Biology Module 8 - Lecture 2: Repair of DNA mutations Ray O’Keefe [email protected] What will we cover? Direct repair of a damaged nucleotide Repair by excision and resynthesis of a damaged region of DNA Mismatch repair Nonhomologous end...

BIOL10221 Molecular Biology Module 8 - Lecture 2: Repair of DNA mutations Ray O’Keefe [email protected] What will we cover? Direct repair of a damaged nucleotide Repair by excision and resynthesis of a damaged region of DNA Mismatch repair Nonhomologous end joining Diseases caused by defective DNA repair Why is it important? DNA repair is important for preserving the composition and coding potential of the genome DNA is constantly being damaged and repaired Damage to DNA repair systems can cause disease BIOL10221 Module 8 - Lecture 2 2 DNA damage occurs at an estimated frequency of approximately 20,000 – 50,000 lesions per cell per day in humans This roughly adds up to 10 - 40 trillion lesions per second in the human body!! BIOL10221 Module 8 - Lecture 3 3 Repair of mutations BIOL10221 Module 8 - Lecture 2 4 Direct repair - an enzyme corrects a nucleotide alteration caused by a mutagen is quite uncommon The effects of some alkylating agents can be repaired in E. coli, the ADA enzyme can remove alkyl groups attached to position 4 of T and position 6 of G in humans, the MGMT enzyme can remove alkyl groups from position 6 of G Base dimers formed by UV radiation can be directly repaired e.g. by DNA photolyase in E. coli (humans don’t have this enzyme, humans rely solely on the more complex and less efficient nucleotide excision repair system. That’s why UV is so dangerous!) BIOL10221 Module 8 - Lecture 5 2 Excision repair - the damaged nucleotide is removed and the gap filled by DNA synthesis Two types base excision repair – a single altered base is removed nucleotide excision repair – a longer piece of DNA containing the altered bases is removed BIOL10221 Module 8 - Lecture 6 2 Base excision repair in E. coli begins with removal of the damaged base by a DNA glycosylase enzyme BIOL10221 Module 8 - Lecture 7 2 Other agents causing structural changes to nucleotides Heat causes detachment of bases give rise to an AP site AP = apurinic, apyrimidinic (depurination or depyrimidination) (AP site) BIOL10221 Module 8 - Lecture 8 2 Base excision repair in E. coli the resulting AP site is then filled in (AP site - apurinic/apyrimidinic site, also known as an abasic site) BIOL10221 Module 8 - Lecture 9 2 Repair of mutations BIOL10221 Module 8 - Lecture 2 10 Nucleotide excision repair in E. coli is carried out by the UvrABC endonuclease BIOL10221 Module 8 - Lecture 11 2 Nucleotide excision repair in E. coli the region containing the damaged nucleotide is excised and resynthesised BIOL10221 Module 8 - Lecture 12 2 Repair of mutations BIOL10221 Module 8 - Lecture 2 13 Mismatch repair - corrects errors in DNA replication The parent strand contains the correct nucleotide The daughter strand contains the mismatch how are the two distinguished? In E. coli, the parent strand is methylated BIOL10221 Module 8 - Lecture 14 2  mismatch repair occurs during this period BIOL10221 Module 8 - Lecture 15 2 Mismatch repair in E. coli mismatch is recognised by the MutH and MutS enzymes BIOL10221 Module 8 - Lecture 16 2 Mismatch repair in E. coli the mismatch is excised and the DNA resynthesised BIOL10221 Module 8 - Lecture 17 2 Mismatch repair in Humans Less understood and more complex! Methylation not involved Base-base mismatches recognized by MutS like proteins Don’t have a MutH like protein but another protein acts as an endonuclease Human mismatch repair enzymes tightly associated with the replication fork so can recognize the daughter strand as it is being synthesized BIOL10221 Module 8 - Lecture 18 2 Nonhomologous end joining - corrects DNA breaks Important to distinguish real breaks from the natural ends of chromosomes the telomeres mark the natural ends BIOL10221 Module 8 - Lecture 19 2 Nonhomologous end joining in humans DNA ligase comes in and joins the two ends together BIOL10221 Module 8 - Lecture 20 2 BIOL10221 Module 8 - Lecture 21 2  Xeroderma pigmentosum (XP) Rare human skin and neurodegenerative disease in which exposure to sunlight can result in a high incidence of skin and mucous membrane cancer. Severe sunburn and blistering occurs in ~50% of patients, and all show early extensive freckling. Cancer incidence for XP individuals under 20 years of age is 2,000 times higher than general population. Mutations causing XP found in nucleotide excision repair genes required for repairing UV damage like base dimers and photoproducts BIOL10221 Module 8 - Lecture 22 3 Resources Reading BROWN, Chapter 16 pages 323–331 ALBERTS (5th ed), Chapter 6 pages 215-223 ALBERTS (6th ed), pages 227-233 Blackboard PDF files with Objectives, Terms, Questions and Answers BIOL10221 Module 8 - Lecture 23 2  BIOL10221 Molecular Biology Module 8 - Lecture 2: Repair of DNA mutations Ray O’Keefe [email protected]