DNA damage and repair uploaded by Abbas W Abbas
D Repair and NA amageBy : Ayah TuffahaSupervisor : Dr. Mustafa Ghanim
DNA Damage vs. DNA Mutation• Physical abnormalities or • Change in normal abnormal chemical nucleotide sequence in modifications DNA• Cannot be inherited • Can be inherited• Effects :Prevents replication and transcription or may cause mutation if damaged DNA is replicated
DNA Damage Dna Damage occurs:• Spontaneously Interactions with water molecules causing hydrolysis of bonds – Deamination – Depurination• Mutagens – Radiation e.g. UV Causes pyrimidine dimers – Chemicals e.g. base analogs, base modifying agents(DNA adducts), intercalating agents
Translesion Synthesis Damage Tolerance MechanismOriginal DNA strand is still damaged but the newly synthesized DNA strand is normal. Specialized DNA polymerasesynthesizes DNA across regions in which DNA template is damaged
Excision Repair 3 Step Process “Cut and Patch” Each step requires a specific enzyme 1-Removal of defective nucleotides via endonucleaseThe phosphoester bonds on both sides of the damaged location are broken by the endonuclease and it removed by exonuclease or DNA helicase 2-The missing nucleotides are replaced with the correct ones by a DNA polymerase Other DNA strand is used as a template during the process3- DNA Ligase form phosphoester bond between repaired strand and newly synthesized nucleotides
Excision RepairThere are two types :• Base excision: Single damaged basee.g. Deaminated base -> DNA glycosylase Deaminated base is removed via glycosylase leaving behind ribose phosphate on DNA strand and sugar ribose is then removed via excision process.• Nucleotide excision (NER): Multiple base damage Transcription coupled repair When transcription stops because of DNA damage the NER is recruited to that damaged location and this process speeds up the repairing process for ACTIVE GENES.
Mismatch Repair • Wrong base pairing e.g. A with C • Improper hydrogen bondingAlso “Cut and Patch” like excision and nearly the same mechanism(3 step removal process).Differs in type of abnormality repaired because here the nucleotides are normal (not abnormal like excision) but they have been improperly paired during replication. How does the cell recognize which is the wrong nucleotide? How is that nucleotide removed? The cell recognizes the nucleotide that should be removed by recognizing the newly synthesized strand from the original strand. The newly synthesized strand is the one that has the wrongly incorporated nucleotide.
Why doesn’t DNA contain any Uracil?Deamination of cytosine gives uracilSo if uracil was normally present in DNA the repair system will NOT be able to recognize the normally present uracil from the abnormally present uracil and therefore won’t be able to repair the damage caused.
Double-Strand DNA Breaks• DNA helix is broken to two fragments• Repair Mechanisms: • Non-homologous end joining : error proneRepairs the break without template • Homologous recombinationUses homologous chromosome (All chromosome are present as homologous pairs) as template