mutagenesis - Mutation s Define Mutation Are changes in the...

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Unformatted text preview: Mutation s Define Mutation Are changes in the genetic material Can be change in the base pair sequences or chromosomes too Two types of Mutations: 1. Somatic Mutations 2. Germline Mutations Somatic Mutations Affects only the individual It cannot be transmitted to the descendant Can be transmitted to the descendants Occurs in the Gametes Germline Mutations DNA DNA is the genetic material Purine bases Adenine, Guanine Pyrmidine bases Thymidine, Cytosine Complementary base pairing. GC base pair with three hydrogen bonds, an AT base pair with two hydrogen bonds Genetic Code Genetic code trinucleotide sequences called codons. Each triplet code specifies sequence of amino acid Genetic code is degenerate (most amino acids are coded for by several alternative codons), the resulting new codon may still code for the same amino acid 1. Point Mutations Point mutation, or single base substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide. Transitions Replacement of a purine base with another purine or replacement of a pyrimidine with another pyrimidine Transversions Replacement of a purine with a pyrimidine or vice versa. 1. 2. 3. 4. Nonsense mutations: changes a codon that specified an amino acid to one of the STOP codons (TAA, TAG, or TGA). Missense mutations: code for a different amino acid. Replacement of A by T at the 17th nucleotide of the gene for the beta chain of hemoglobin changes the codon GAG (for glutamic acid) to GTG (which encodes valine). Thus the 6th amino acid in the chain becomes valine instead of glutamic acid Frameshift mutation: change in the reading frame, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon Silent Mutations: Most amino acids are encoded by several different codons. Example third base in the TCT codon for serine is changed to any one of the other three bases, serine will still be encoded. Such mutations are said to be silent because they cause no Types 2. Mutations in chromosomal structure, including: Amplifications (or gene duplications) leading to multiple copies of all chromosomal regions, increasing the dosage of the genes located within them. Deletions of large chromosomal regions, leading to loss of the genes within those regions. Irreversible Insertion Addition of one or more extra nucleotides into the DNA. They are usually caused by transposable elements, or errors during replication of repeating elements (e.g. AT repeats) Mutagenesis 1. 2. Creation of a mutation There can be two types to it: Spontaneous mutation Induced mutation Mutagen 1. 2. Is a physical or chemical agent that changes the genetic information (usually DNA) of an organism . They can be of two types: Chemical Mutagens Natural Synthetic Radiation Mutagens based on mode of action: 1. BASE ANALOGS A base analog is a compound sufficiently similar to one of the four DNA bases but have different pairing properties. For example, 5bromouracil is the analog of thymine but sometimes pairs with guanine 2. BASE MODIFYING AGENTS Nitrous Acid affects DNA complementation. The acid randomly modifies the base adenine so that it will pair with cytosine instead of thymine. ALKYLATING AGENTS Chemical mutagens that react with bases and add methyl or ethyl groups. Examples: nitrosoguanidine, methyl methanesulfonate, ethyl methanesulfonate 3. INTERCALATING AGENTS Inserts in between nucleotides because of their planar configuration Radiation causing Mutation 1. Ionizing Radiation Examples: X rays , gamma rays 2. Non Ionizing Radiation Example: UV rays Photo Reactivation Photo reactivation Brief exposure to blue light after UV exposure can reverse the effects of the UV radiation. The blue light can cause a thymine dimer to be corrected. Enzyme called photolyase or photo reactivation enzyme (PRE), cleaves the covalent bonds linking the thymine dimers using the energy from a photon of blue light. Types of Mutants Description Auxotroph Nature of change Organismthat has de lope a nutritional ve d re quire e through m m nt utation Drug Re sistant Alte ration of pe e rm ability to drug/ drug targe t/ de toxification of drug Alte ration of an e ntial prote so organism sse in is m he se ore at nsitive Te p se m nsitive 1 Tape the grid page onto your benchtop. 2. Place a YPD plate, lid up, onto each of the grid circles. NOTE: Keep your lids on your plates when you aren't using them. 3.Set out one of the irradiated plates and a beaker of toothpicks. 4. Pull out one toothpick from the beaker being careful not to touch the other end of the toothpick to anything. 5. Now select one of your irradiated yeast colonies and pick up the colony onto the sterile end of the toothpick, being careful not to press too hard on the agar. NOTE: Even a small colony has a few thousand cells. 6.Once you have picked a colony, gently streak the cells onto both plates sitting on the sitting on the grid template being careful to streak across the same square on both grids. NOTE: Always streak the same plate first. 7.Repeat the process until you run out of squares on the grid or colonies, whichever comes first. 8.Incubate the first plate you streaked at 37 degrees, incubate the second at 28 degrees. Wait two days and then look to see if you have any temperature sensitive mutant ...
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