BIOL10221 2024 Molecular Biology Lecture 1: Causes of DNA Mutation PDF

BIOL10221 Molecular Biology Module 8 - Lecture 1: Causes of DNA mutation Ray O’Keefe [email protected] What will we cover? What causes DNA mutations? Errors in replication Replication errors from tautomeric shifts and base analogs Chemical and physical mu...

BIOL10221 Molecular Biology Module 8 - Lecture 1: Causes of DNA mutation Ray O’Keefe [email protected] What will we cover? What causes DNA mutations? Errors in replication Replication errors from tautomeric shifts and base analogs Chemical and physical mutagens Effects of deaminating agents, intercalating agents, UV radiation and heat. Why is it important? DNA mutations, if not repaired, can cause in some cases disease or death Knowing how DNA is damaged is important for its prevention and knowing how damage is repaired BIOL10221 Module 8 - Lecture 1 2 A mutation is an alteration in the nucleotide sequence of a DNA molecule There are two ways in which mutations can occur 1. Some mutations result from errors in DNA replication These are called spontaneous mutations Not common – most DNA polymerases have proofreading activities so are very accurate – but sometimes they do make mistakes! BIOL10221 Module 8 - Lecture 3 1 BIOL10221 Module 8 - Lecture 4 1 Some replication errors are caused by the DNA polymerase making a mistake One type of mistake is Strand-Slippage Regions of DNA containing many copies of small repeated sequences are particularly prone to this type of error. BIOL10221 Module 8 - Lecture 5 1 Some replication errors are caused by the DNA polymerase making a mistake Most mistakes occur because of base tautomerism – polymerase is tricked! Tautomers: isomers with slightly different chemical structures BIOL10221 Module 8 - Lecture 6 1 Normal base Less common tautomer amino-adenine (base pairs with T) imino-adenine (base pairs with C) keto-thymine (base pairs with A) enol-thymine (base pairs with G) keto-guanine (base pairs with C) enol-guanine (base pairs with T) amino-cytosine (base pairs with G) imino-cytosine (base pairs with G) BIOL10221 Module 8 - Lecture 7 1 A mutation is an alteration in the nucleotide sequence of a DNA molecule There are two ways in which mutations can occur 1. Some mutations result from errors in DNA replication These are called spontaneous mutations 2. Some mutations are caused by mutagens Mutagens are chemical or environmental agents that cause changes in DNA molecules BIOL10221 Module 8 - Lecture 8 1 Two important ways in which mutagens can cause mutations BIOL10221 Module 8 - Lecture 9 1 An important base analog is 5-bromouracil (5bU) 5bU is an analog of thymine Can be used to induce mutations in the lab Also used in cancer chemotherapy BIOL10221 Module 8 - Lecture 10 1 An important base analog is 5-bromouracil (5bU) 5bU is an analog of thymine keto-5bU base pairs with adenine It might be incorporated into DNA instead of T during replication This is fine BIOL10221 Module 8 - Lecture 11 1 The problem with 5bU is that the enol tautomer is very common enol-5bU pairs with G not A BIOL10221 Module 8 - Lecture 12 1 Two important ways in which mutagens can cause mutations Example: 5-bromouracil BIOL10221 Module 8 - Lecture 13 1 Deaminating agents change the structures of some nucleotides examples are nitrous acid and sodium bisulphite Deamination of adenine gives hypoxanthine hypoxanthine pairs with C not T BIOL10221 Module 8 - Lecture 14 1 Effect of adenine deamination BIOL10221 Module 8 - Lecture 15 1  Summary of the effects of deaminating agents Deamination of cytosine gives uracil, pairs with A not G Deamination of adenine gives hypoxanthine, pairs with C not T Deamination of guanine gives xanthine, blocks DNA replication Thymine not deaminated – no amine group BIOL10221 Module 8 - Lecture 16 1 Other agents causing structural changes to nucleotides Alkylating agents – add alkyl groups example is ethylmethane sulphonate (EMS) Causes a transition mutation BIOL10221 Module 8 - Lecture 17 1 Other agents causing structural changes to nucleotides Intercalating agents - insert between base pairs example is ethidium bromide Causes insertion mutations BIOL10221 Module 8 - Lecture 18 1  Other agents causing structural changes to nucleotides Ultraviolet Radiation causes Base Dimerization and Photoproducts Most DNA damage easy to repair - UV damage not easy to repair! cyclobutyl dimer BIOL10221 Module 8 - Lecture 19 1  Other agents causing structural changes to nucleotides Heat causes detachment of bases - gives rise to an AP site AP = apurinic, apyrimidinic (depurination or depyrimidination) Happens about 1000 times a day, but easy to repair (AP site) BIOL10221 Module 8 - Lecture 20 1 Summary of the effects of deaminating agents Deamination of adenine gives hypoxanthine, pairs with C not T Deamination of cytosine gives uracil, pairs with A not G Deamination of guanine gives xanthine, blocks DNA replication Other agents causing structural changes to nucleotides Alkylating agents – add alkyl groups example is ethylmethane sulphonate Intercalating agents - insert between base pairs example is ethidium bromide Ultraviolet radiation cause base dimerization and photoproducts Heat causes detachment of bases give rise to an AP site AP = apurinic, apyrimidinic BIOL10221 Module 8 - Lecture 21 1 Resources Reading BROWN, Chapter 16 pages 313–323 ALBERTS (5th ed), Chapter 6 pages 215-216 ALBERTS (6th ed), pages 225-226 Blackboard PDF files with Objectives, Terms, Questions and Answers BIOL10221 Module 8 - Lecture 22 1  BIOL10221 Molecular Biology Module 8 - Lecture 1: Causes of DNA mutation Ray O’Keefe [email protected]