DNA Repair Lecture Notes PDF

DNA Repair by Dr.Ezat Mersal Types of DNA repair Photo activation Repair in E. coli Excision Repair Mistakes in DNA replication Post-Replication Repair Diseases in which DNA repair is damage  Because each cell contains only one or two copies of its DNA, the DNA sequence is...

DNA Repair by Dr.Ezat Mersal Types of DNA repair Photo activation Repair in E. coli Excision Repair Mistakes in DNA replication Post-Replication Repair Diseases in which DNA repair is damage  Because each cell contains only one or two copies of its DNA, the DNA sequence is highly protected from harm. DNA is a relatively stable molecule, but Earth’s natural environment is quite toxic, and damage to DNA is inevitable. DNA can also be altered by mistakes made during its own replication or recombination. Damage and sequence alterations to DNA are often quickly repaired, but when they are not, the DNA becomes permanently altered and harbors a mutation. Mutations are changes in DNA sequence, and when mutations occur in germ-line cells, these changes are inheritable. © 2015 John Wiley & Sons, Inc. All rights reserved.  A cancer cell has mutations that prevent cell death, resulting in loss of cell cycle control and unregulated cell division, which leads to malignant tumors that can end the life of the entire organism. The cell has a limited amount of time to fix the initial alteration and restore the DNA to its normal sequence (DNA Repair), before replication converts the alteration into a mutation that will be passed on to the next generation. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  Exposing UV treated cells to blue light results in a reversal of the thymine dimer formation Enzyme, photoactivation repair enzyme (PRE) absorbs a photon of light (from blue light) and is able to cleave the bond forming the thymine dimer. Once bond is cleaved, DNA is back to normal © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  Conserved throughout evolution, found in all prokaryotic and eukaryotic organisms Three steps process: 1. Error is recognized and enzymatically clipped out by a nuclease that cleaves the phosphodiester 2. f bonds (uvr gene products operate at this step) DNA Polymerase I fills in the gap by inserting the appropriate nucleotides 3. DNA Ligase seals the gap © 2015 John Wiley & Sons, Inc. All rights reserved. Two know types of excision repair Base excision repair (BER) corrects damage to nitrogenous bases created by the spontaneous hydrolysis of DNA bases as well as the hydrolysis of DNA bases caused by agents that chemically alter them Nucleotide excision repair (NER) Repairs “bulky” lesions in DNA that alter or distort the regular DNA double helix Group of genes (uvr) involved in recognizing and clipping out the lesions in the DNA Repair is completed by DNA pol I and DNA ligase © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  DNA replication is not perfect. Errors occur in DNA replication, when the incorrect base is incorporated into the growing DNA strand. This leads to mismatched base pairs, or mispairs. more  DNA polymerases have proofreading activity, and a DNA repair enzymes have evolved to correct these mistakes.  Occasionally, mispairs survive and are incorporated into the genome in the next round of replication. © 2015 John Wiley & Sons, Inc. All rights reserved. These mutations may: Have no consequence. I Or Result in the: death of the organism, or 8 genetic disease or cancer. Or Give the organism a competitive advantage over its neighbors, which leads to evolution 3 by natural selection. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  Post-replication repair– Discovered in E. coli by Miroslav Radman Responds when damaged DNA escapes repair and the damage disrupts replication Recognize damage Rec A protein stimulates recombination between donor strand and new strand Creates gap in donor strand which can be repaired DNA Polymerase and DNA Ligase involved © 2015 John Wiley & Sons, Inc. All rights reserved. Case study A 6- year- old child was brought to the clinic because his parents were concerned with excessive lesions and Click to see larger picture blistering in the facial and neck area. The parents noted that the lesions did not go away with typical ointments and creams and often became worse when the child was exposed to sunlight. The physician noted excessive freckling through the child's body, as well as slight stature and poor muscle tone. © 2015 John Wiley & Sons, Inc. All rights reserved.  Xeroderma pigmentosum (XP): Patients are hypersensitive to UV light; patients often develop malignancies of the skin. Ataxia telangiectasia (AT): Patients are sensitive to gamma irradiation; patients develop neurological and skin lesions. Fanconi’s anemia: Patients demonstrate aplastic anemia, growth retardation, and congenital anomalies; related to a deficiency in repair of DNA cross-links. Lynch Syndrome (LS): © 2015 John Wiley & Sons, Inc. All rights reserved.  XP is an autosomal recessive disease associated with dry skin, freckling, corneal ulceration, and skin tumors Many patients die before age 30 from metastases of malignant skin tumors One form of XP is produced by a defect in the human endonuclease that removes pyrimidine dimers Mutations in at least seven other genes involved in repairing UV-damaged DNA can cause XP © 2015 John Wiley & Sons, Inc. All rights reserved.  Symptoms include: - Extreme sensitivity to sunlight - Early on set of skin cáncer © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  Lynch syndrome (LS), also known as Hereditary Non-Polyposis Colorectal Cancer, (HNPCC), is an inherited cancer predisposition syndrome Individuals with LS have a greatly increased risk for certain cancers. LS accounts for about 0.7-3.6% of cases of CRC. Research on LS-related endometrial cancer is still emerging; current data suggest that in North America between 1.8% and 4.5% of cases are attributed to LS LYNCH SYNDROME-RELATED CANCERS Colorectal Endometrial Renal Gastric Ovarian Ureter Small bowel Hepato-biliary Pancreatic Brain Sebaceous (adenoma or carcinoma) © 2015 John Wiley & Sons, Inc. All rights reserved.  Autosomal dominant Variable expressivity Signs and symptoms/type and onset of cancer will vary between affected family members (variable expressivity) Mismatch repair genes play an important role in a cell’s ability to repair DNA damage as a cell grows and divides, by identifying and removing single nucleotide mismatches, insertions and deletion loops. Defects in the MMR pathway lead to an accumulation of mutations in a cell which may result in a malignancy Caused by an inherited mutation in at least one of four mismatch repair (MMR) genes* (MLH1, MSH2, MSH6 or PMS2) or in EPCAM © 2015 John Wiley & Sons, Inc. All rights reserved.  Basic genetics : a human approach / BSCS. Dubuque, IA, Kendall/Hunt Pub. Co., c1999. 147 p. QH431.B305 1999 Genes, ethnicity, and ageing. Edited by Lincoln H. Schmitt, Leonard Freedman, Rayma Pervan. Nedlands, Australia, Centre for Human Biology, University of Western Australia ; Singapore, River Edge, NJ, World Scientific, c1995. 100 p.QH455.G45 1995 Genetic polymorphisms and susceptibility to disease. Edited by M. S. Miller and M. T. Cronin. New York, Taylor & Francis, 2000. 266 p. Ogawa, T., & Okazaki, T. (1980). Discontinuous DNA replication. Annual review of biochemistry, 49(1), 421- 457. Kunkel, T. A., & Bebenek, K. (2000). DNA replication fidelity. Annual review of biochemistry, 69(1), 497-529. Benkovic, S. J., Valentine, A. M., & Salinas, F. (2001). Replisome-mediated DNA replication. Annual review of biochemistry, 70(1), 181-208.