DNA Damage and Repair Lecture Notes PDF

The Human Body PJ1311 Lecture 4: DNA Damage and Repair Dr Alina Oknianska 1  Learning Objectives and Exam Revision Advice You should be able to discuss and give examples of multiple DNA repair mechanisms necessary to cor...

The Human Body PJ1311 Lecture 4: DNA Damage and Repair Dr Alina Oknianska 1  Learning Objectives and Exam Revision Advice You should be able to discuss and give examples of multiple DNA repair mechanisms necessary to correct errors arising during replication and able to repair DNA damage You should be able to discuss different types of mutations 2  Introduction : Mutations Heritable change in the genetic material Mutations provide allelic variations evolutionary change can be harmful and often cause disease Organisms developed ways to repair damaged DNA 3  Types of Mutations 3 main types of mutations 1. Chromosome mutations Changes in chromosome structure 2. Genome mutations Changes in chromosome number 3. Gene mutations Small change in DNA structure – affects a single gene 4  Gene Mutations Mutation Point Larger mutation mutation Change in one base Insertion Deletion Inversion Addition 1 Removal 1 Flipping a or > or > fragment nucleotide nucleotide of the DNA 5 s s  Point Mutations May be introduced by addition of incorrect base in 1st round of replication 6  Point Mutations 2nd round replication - mutation permanent part of DNA 7  Translation of a Normal Gene DNA sequence transcribed into messenger RNA 3bp of mRNA = codon Every codon encodes an amino acid Normal Gene 8  Point Mutations : Silent Change in bp change mRNA same amino acid Normal Gene Mutated Gene 9  Point Mutations : Neutral Change in bp change mRNA amino acid substitution no detectable change – amino acid similar properties Normal Gene Mutated Gene 10  Point Mutations : Neutral Normal Gene Mutated Gene The most hydrophilic R groups are + ve or – ve charged 11  Point Mutations : Missense Change in bp change mRNA codon different amino acid Normal Gene Mutated Gene 12  Point Mutations : Nonsense Change in bp change mRNA codon to STOP (UAG, UAA or UGA) no amino acid Normal Gene Mutated Gene 13  Gene Mutations Mutation Point Larger mutation mutation Change in one base Insertion Deletion Inversion Addition 1 Removal 1 Flipping a or > or > fragment nucleotide nucleotide of the DNA14 s s  Larger Mutations : Insertion/Deletion Insertion or deletion of bp change mRNA reading frame downstream May generate new STOP codons, shorter/longer polypeptides Normal Gene Mutated Gene 15  Mutations Can Occur From DNA Replication Errors REMINDER : DNA Replication 16 https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/molecular-mechanism-of-  Mutations Can Occur From DNA Replication Errors Proof reading activity of DNA polymerase 17 https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/dna-  Mutations Can Occur From DNA Replication Errors Replication machinery makes 1 mistake per 107 nucleotides copied Mismatch – incorrect base Errors must be corrected to avoid mutations Backup system to recognise and correct mistakes DNA mismatch repair 18  Mutations Can Occur From DNA Replication Errors DNA Mismatch Repair (MMR) Mismatch repair proteins recognise mismatch error New strand cut – remove region of DNA with error 19 https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/dna-  Mutations Can Occur From DNA Replication Errors DNA Mismatch Repair (MMR) DNA replaced by DNA polymerase New DNA joined together DNA ligase 20 https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/dna-  Mutations Can Occur From DNA Replication Errors DNA Ligase 21  DNA Damage Can Occur Spontaneously Rare mistakes in DNA replication corrected by mismatch repair mechanism DNA damaged in other ways – require different repair mechanisms Random interactions between DNA and surrounding molecules can cause damage Depurination and deamination chemical reactions 22  DNA Damage Can Occur Spontaneously Depurination and deamination affect DNA bases 23  DNA Damage Can Occur Spontaneously If chemical modification of nucleotides not repaired they result in mutation 24  DNA Damage Can Occur in Response to Mutagens DNA damage also caused by mutation- causing agents (mutagens) Environmental mutagens Radiation (UV and X-rays) Chemicals 25  DNA Damage Can Occur in Response to Mutagens : Ionising radiationRadiation e.g. X-rays Single (ss) and double strand (ds)breaks 26  DNA Damage Can Occur in Response to Mutagens : Non-ionising radiation Radiation e.g. UV Thymine dimers 27  DNA Damage Can Occur in Response to Chemical Mutagens Mutagenic chemicals alter DNA in variety of ways Mutagen Consequence Resemble DNA bases in Base Analogues structure React with DNA bases Base Modifying Agents to alter structure Intercalate into the Intercalating Agents DNA double helix, distorting it 28  DNA Damage Can Occur in Response to Chemical Mutagens : Base Analogues Resemble DNA bases Incorporated into new strand during DNA replication E.g. 5-bromouracil – thymine analogue 29  DNA Damage Can Occur in Response to Chemical Mutagens : Base Analogues 5-bromouracil mispairs with guanine 30  DNA Damage Can Occur in Response to Chemical Mutagens Mutagenic chemicals alter DNA in variety of ways Mutagen Consequence Resemble DNA bases in Base Analogues structure React with DNA bases Base Modifying Agents to alter structure Intercalate into the Intercalating Agents DNA double helix, distorting it 31  DNA Damage Can Occur in Response to Chemical Mutagens : Base Modifying Agents React with DNA bases to alter structure 32  DNA Damage Can Occur in Response to Chemical Mutagens Mutagenic chemicals alter DNA in variety of ways Mutagen Consequence Resemble DNA bases in Base Analogues structure React with DNA bases Base Modifying Agents to alter structure Intercalate into the Intercalating Agents DNA double helix, distorting it 33  DNA Damage Can Occur in Response to Chemical Mutagens : Intercalating Agents Intercalate into the DNA double helix, distorting it Problems during DNA replication 34  Repair of DNA Damage Two general types of repair systems 1. Direct reversal systems correct damaged areas by reversing damage 35 https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/dna-  Repair of DNA Damage Two general types of repair systems 2. Excision repair systems cut out damage repair gap with new DNA synthesis 36 Repair of DNA Damage 37  Repair of DNA Damage : Base Excision Repair (BER) Enzyme recognises damage Sugar phosphate backbone removed Gap filled and repaired by DNA polymerase and ligase 38 Repair of DNA Damage 39  Repair of DNA Damage : Nucleotide Excision Repair (NER) Damage detected e.g. pyrimidine dimer DNA Helicase unwinds surrounding DNA 40 https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/dna-  Repair of DNA Damage : Nucleotide Excision Repair (NER) DNA Helicase Uses energy from ATP to break hydrogen bonds DNA becomes single stranded 41  Repair of DNA Damage : Nucleotide Excision Repair (NER) Damaged region removed by enzyme Gap filled and repaired by DNA polymerase and ligase https:// www.khanacademy.org/ science/biology/dna-as- the-genetic-material/dna- 42 replication/a/dna- Repair of DNA Damage 43  Repair of DNA Damage : Non-Homologous End Joining (NHEJ) ds breaks – no intact template Not repaired – fragmentation of chromosomes and loss of genes NHEJ – “quick and dirty” mechanism Results in short 44  Repair of DNA Damage : Homologous Recombination (HR) Error–free strategy for ds breaks Exchange of information from homologous chromosomes Undamaged DNA used as template 45 DNA Damage and Repair 46  DNA Repair and Disease Mutations in genes that encode DNA repair proteins associated with heredity cancers Hereditary nonpolyposis colorectal cancer mismatch repair Xeroderma pigmentosum nucleotide excision repair 47  Summary DNA mutations can occur spontaneously or as a result of damage by environmental influences DNA repair mechanisms exist in all organisms to maintain the fidelity of the DNA sequence. Multiple repair mechanisms necessary to correct errors arising during replication and to repair DNA damage 48