Mutagenesis and Its Effects Lecture Notes PDF

Republic of Iraq Ministry of Higher Education and Stage:S2 Scientific Research Module: MGD Wasit Universty College of medicine :Lecture Title Mutagenesis and Its effects Lecturer Name: Dr –Dham...

Republic of Iraq Ministry of Higher Education and Stage:S2 Scientific Research Module: MGD Wasit Universty College of medicine :Lecture Title Mutagenesis and Its effects Lecturer Name: Dr –Dhamyaa K. Kadhim Mutations Defined 1. A mutation is a change in a DNA base-pair or a chromosome. a. Somatic mutations affect only the individual in which they arise. b. Germ-line mutations alter gametes, affecting the next generation. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 2 Types of Mutations Point Mutations Base Pair Substitutions Silent Missense – new protein (Amino Acid Substitutions) Nonsense – stop codon Base Pair Insertions and deletions Triplet Repeats Frameshift Mutations 台大農藝系 遺傳學 601 20000 Chapter 19 slide 3 台大農藝系 遺傳學 601 20000 Chapter 19 slide 4 台大農藝系 遺傳學 601 20000 Chapter 19 slide 5 The point mutation that causes sickle cell disease is a missense mutation. The DNA sequence CTC encodes the mRNA codon GAG, which specifies glutamic acid. In sickle cell disease, the mutation changes the DNA sequence to CAC, which encodes GUG in the mRNA, which specifies valine. This mutation changes the protein’s shape, which alters its function. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 6 台大農藝系 遺傳學 601 20000 Chapter 19 slide 7 For example, in factor XI deficiency, a blood clotting disorder, a GAA codon specifying glutamic acid is changed to UAA, signifying “stop.” The shortened clotting factor cannot halt or stop the profuse bleeding that occurs during surgery or from injury. (Nonsense). Many common inherited disorders result from deletions, including male infertility caused by tiny deletions in the Y chromosome. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 8 In insertion mutation adds DNA. In one form of Gaucher disease, for example, an inserted single DNA base prevents production of an enzyme that normally breaks down glycolipids in lysosomes. The resulting buildup of glycolipid enlarges the liver and spleen and causes easily fractured bones and neurological impairment. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 9 Triplet Repeats mutation: a-(deletion of a codon) In Cystic fibrosis: there is a three nucleotide deletion from the coding sequence. This will causes a deletion of Phenylalanine. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 10 Triplet Repeats mutation: b-(duplication of a codon) 台大農藝系 遺傳學 601 20000 Chapter 19 slide 11 Point mutations are divided into two classes based on their effect on phenotype: a. Forward mutations change the genotype from wild type to mutant. b. Reverse mutations (reversions or back mutations) change the genotype from mutant to wild-type or partially wild-type. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 12 Spontaneous and Induced Mutations: Most mutations are spontaneous, rather than induced by a mutagen. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 13 台大農藝系 遺傳學 601 20000 Chapter 19 slide 14 achondroplasia 台大農藝系 遺傳學 601 20000 Chapter 19 slide 15  Induced Mutation A-Intentional Use of Mutagens B- Accidental Exposures to Mutagens C-Natural Exposure to Mutagens 台大農藝系 遺傳學 601 20000 Chapter 19 slide 16  A- Intentional Use of Mutagens Chemicals or radiation are used to induce mutation. Chemicals mutagens: 1- Alkylating agents, for example, are chemicals that remove a DNA base, which is replaced with any of the four bases—three of which are a mismatch against the complementary strand. *Dyes called acridines add or remove a single DNA base. (coal tar) It can be used in medicated shampoo, soap and ointment as treatment of dandruff and psoriasis. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 17 2 - Base analogs : like 5-bromouracil (5BU). It is used mainly as an experimental mutagen, but its deoxyriboside (BROMODEOXYURIDINE) is used to treat neoplasms or cancer. 3- Base-modifying agents : methylmethane sulfonate (MMS) used in cancer treatment, and HNO2. 4- Intercalating agents insert themselves between adjacent bases in DNA like ethidium bromide and proflavine (are molecules that may insert between bases in DNA). proflavine Topical antiseptic ethidium bromide 台大農藝系 遺傳學 601 20000 Chapter 19 slide 18 Physical mutagens: 1- X rays and other forms of radiation delete a few bases or break chromosomes. 2-Ultraviolet (UV) causes photochemical changes in the DNA. - UV has lower-energy wavelengths than X rays, and so has limited penetrating power. However, UV in the 254–260 nm range is strongly absorbed by purines and pyrimidines, forming abnormal chemical bonds. A common effect is dimer formation between adjacent pyrimidines, commonly thymines (designated T^T) or thymine dimer. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 19  B-Accidental Exposures to Mutagens Some mutagen exposure is unintentional. from industrial accidents; from medical treatments such as chemotherapy and radiation; and from exposure to weapons that emit radiation. An environmental disaster that released mutagenic radiation was a steam explosion at a nuclear reactor in the former Soviet Union on April 25, 1986. Between 1:23 and 1:24 a.m., Reactor 4 at the Chernobyl Nuclear Power Station in Ukraine exploded, sending a great plume of radioactive isotopes into the air that spread for thousands of miles. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 20 Researchers learned of a new type of mutation from a young man conceived within a week of the Chernobyl accident, near the disaster site. He has extra digits, an abnormal epiglottis, and a benign growth on the hypothalamus, a group of symptoms called Pallister-Hall syndrome. On his way to a camp for “children of Chernobyl” in the summer of 2002, he stopped at the National Institutes of Health to provide a DNA sample. Researchers indeed found a mutation in the gene on chromosome 7 known to cause “stop” codon to form, shortening the encoded protein. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 21 C-Natural Exposure to Mutagens Ionizing radiation is of three major types: -Alpha radiation is the least energetic and most short-lived, and the skin absorbs most of it. Uranium and radium emit alpha radiation. -Beta radiation can penetrate the body farther, and emitters include tritium (a form of hydrogen), carbon-14, and strontium-70. Both alpha and beta rays tend not to harm health, although they can do damage if inhaled or eaten. - Gamma rays. These can penetrate the body, damaging tissues. Plutonium and cesium isotopes used in weapons emit gamma rays, and this form of radiation is used to kill cancer cells. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 22  How mutation cases diseases ?? In the Familial hypercholesterolemia (FH) is an example of incomplete dominance in humans that can be observed on both the molecular and whole-body levels. A person with two disease causing alleles lacks receptors on liver cells that take up the low density lipoprotein (LDL) form of cholesterol from the bloodstream. A person with one disease-causing allele has half the normal number of receptors. Someone with two wild type (the most common) alleles has the normal number of receptors. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 23 台大農藝系 遺傳學 601 20000 Chapter 19 slide 24 台大農藝系 遺傳學 601 20000 Chapter 19 slide 25  How to explore mutation?? 1- Visible Mutations - Some mutations affect the appearance of an organism LIKE HUMAN PHENOTYPES and (Drosophila eyes or wing-shape, coat color in animals, colony size in yeast, plaque morphology of phages). 台大農藝系 遺傳學 601 20000 Chapter 19 slide 26 2-Auxotrophic Mutations - Auxotrophic mutants are easily detected for microorganisms that normally can grow on minimal medium, using methods that have been developed for selection and screening. -An example is replica plating. Cells are first grown on supplemented medium, and then the colonies transferred to minimal medium, as well as to a control plate of supplemented medium. Colonies that grow on supplemented, but not minimal, media are selected for further study. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 27 3-Conditional Mutations Heat sensitivity is a common conditional mutation, in which a normal protein is produced at permissive temperature, and a nonfunctional protein results at the non-permissive temperature. Screening is generally by replica plating and incubation at different temperatures. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 28 4-Resistance Mutations Microorganisms like E. coli and yeast are easily screened for resistance to viruses, chemicals or drugs, because resistant cells will grow when wild-type cells will not. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 29 DNA Repair Mechanisms -Both prokaryotes and eukaryotes have enzyme-based DNA repair systems that prevent mutations and even death from DNA damage. - Repair systems are grouped by their repair mechanisms. Some directly correct, while others excise the damaged area and then repair the gap. 台大農藝系 遺傳學 601 20000 Chapter 19 slide 30