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DNA repair PowerPoint Presentation

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Description : Check out this medical presentation on Ataxia-Telangiectasia, which is titled "DNA repair", to know about DNA repair.

Tags : Ataxia-Telangiectasia, DNA repair, mutation, Xeroderma pigmentosum, Fanconi’s anemia, Cockayne’s Syndrome

Published on : May 10, 2021 | Views : 2687 | Downloads : 1

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Slide 1-

DNA repair
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spontaneous vs. induced mutation gametic vs. somatic mutation lethal or conditional mutation Classification of the mutation
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Spontaneous mutations are those that happen naturally no specific agents are associated with their occurence and they are generally assumed to be random changes in the nucleotide sequences of genes
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Induced mutations those that result from the influence of any artificial factor various forms of radiation a wide spectrum of chemical agents biological agents (e.g. viruses)
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Gametic vs. somatic mutations Mutation arising in somatic cells are not transmitted to future generations Mutations in gametes or gamete-forming tissue are of greater significance because they are transmitted to offspring as part of the germ line dominant recessive X-linked
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Lethal vs. conditional mutations Mutation may interrupt a process that is essential to the survival of the organism - in this case, it is reffered to as a lethal mutation Conditional mutation is present in the genome of an organism, but it is expressed and can be detected only under certain conditions
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Induced mutagenesis Can be caused by environmental agents that damage DNA: • UV light • X-rays and γ-rays • Chemical carcinogens e.g. cigarette smoke DNA damage can lead to mutations unless it is removed by DNA repair enzymes Unrepaired damage can have serious consequences
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IMPORTANCE OF DNA REPAIR Hoeijmakers , 2001
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Photoactivation Repair in E. coli • 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
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Excision Repair • Conserved throughout evolution, found in all prokaryotic and eukaryotic organisms • Three step process: – 1. Error is recognized and enzymatically clipped out by a nuclease that cleaves the phosphodiester bonds (uvr gene products operate at this step) – 2. DNA Polymerase I fills in the gap by inserting the appropriate nucleotides – 3. DNA Ligase seals the gap
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Excision Repair • 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
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Proofreading and Mismatch Repair • In bacterial systems, proofreading decreases the error rate in DNA replication by two orders of magnitude – from 1 mismatch in every 105 nucleotide pairs to 1 in every 107 base pairs • Mismatch repair is another mechanism by which mismatches can be fixed in the DNA strand • In bacteria, mismatch repair is based on the process of DNA Methylation, which labels one strand, providing a basis for the mismatch repair.
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Post-Replication Repair • Post-replication repair– Discovered in E. coli by Miroslav Radman – Responds when damaged DNA escapes repair and the damage disrupts replication – 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
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• 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. Diseases in which DNA repair is damage
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Xeroderma Pigmentosum (XP) and DNA Repair Defects • 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
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DNA Repair and Clinical Syndromes: Increased Sensitivity; Chromosomal Instability and Increased Cancer Risk
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Xeroderma Pigmentosum (XP) Symptoms include: --- Extreme sensitivity to sunlight --- Early onset of skin cancer
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XP cases Sunlight-induced dermatologic abnormalities in a patient with xeroderma pigmentosum. Typical skin manifestation of xeroderma pigmentosum with numerous areas of hypopigmentation and freckles (ie, solar lentigines) with different intensities of pigmentation.
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NER Healthy DNA Why are XP patients sensitive to sunlight? Healthy DNA UV Damaged DNA 8 gens
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NER Healthy DNA Why are XP patients sensitive to sunlight? Healthy DNA UV Damaged DNA 8 gens
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XP: Skin Cancer Incidence Rapid Secondary to NER Faulty Repair of UV-induced DNA Damage and Genetic NER Mutations
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Molecular Genetics of Xeroderma Pigmentosum Data are compiled from the following standard references: Gene symbol from HUGO; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from Swiss-Prot.
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Fanconi anemia (FA) • Fanconi anemia (FA) is an autosomal recessive disease characterized by progressive bone marrow failure due to defective stem cell function . • FA cells are hypersensitive to DNA cross - linking agents such as mitomycin C (MMC) resulting in cytogenetic aberrations, G2 - M cell cycle arrest, apoptosis, and cell death. • Seven complementation groups (termed FANCA - G) are identified. Group A (FANCA) mutations are the most prevalent (70%). • The function of the FANC genes are still unclear. Involvement in DNA repair system is suggested. There are at least seven FA genes: A, C, D2, E, F, G and BRCA2 •
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Cockayne’s Syndrome Occurrence: 1 per million population Sensitivity: ultraviolet radiation (sunlight) Disorder: arrested development, mental retardation, dwarfism, deafness, optic atrophy, intracranial calcifications; (no increased risk of cancer) Biochemical: defect in NER Genetic: autosomal recessive, five genes (A, B and XPB, D & G)
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Cockayne’s Syndrome
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Trichothiodystrophy Occurrence: 1-2 per million population Sensitivity: ultraviolet radiation (sunlight) in subset of patients Disorder: sulfur deficient brittle hair, mental and growth retardation, peculiar face with receding chin, ichthyosis; (no increased cancer risk) Biochemical: defect in NER Genetic: autosomal recessive, three genes (TTDA, XPB, XPD)
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Trichothiodystrophy
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Ataxia telangiectasia (AT):