Microbio 310 1st Edition Lecture 11 Outline of Last Lecture I. 9.1 General Properties of VirusesII. 9.2 Nature of the VirionIII. 9.3 The Virus HostIV. 9.4 Quantification of VirusesV. 9.5 General Features of Virus ReplicationVI. 9.6 Viral Attachment and PenetrationVII. 9.7 Production of Viral Nucleic Acid and ProteinVIII. 9.8 Overview of Bacterial VirusesIX. 9.9 Virulent Bacteriophages and T4X. 9.10 Temperate Bacteriophages, Lambda, and P1XI. 9.11 Overview of Animal VirusesXII. 9.12 RetrovirusesXIII. 9.13 Defective VirusesXIV. 9.14 ViroidsOutline of Current Lecture I. 9.15 PrionsII. 10.1 Mutations and MutantsIII. 10.2 Molecular Basis of MutationThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.IV. 10.3 Mutation RatesV. 10.4 MutagenesisVI. 10.5 Mutagenesis and Carcinogenesis: The Ames TestCurrent Lecture9.15 Prions• Prions: infectious proteins whose extracellular form contains no nucleic acid– Known to cause disease in animals (transmissible spongiform encephalopathies)o Ex: Mad Cow Disease, Scrapie– Host cell contains gene (PrnP) that encodes native (normal) form of prion protein that is found in healthy animals– Prion misfolding results in neurological symptoms of disease (e.g., resistance to proteases, insolubility, and aggregation)o Misfolded prions act as templates and bind to normal prions and convert them to misfolded/pathogenic prionso Produces plaques/clumps in the brain• Prion disease occurs by three distinct mechanisms:– Infectious prion disease: pathogenic form of prion protein is transmitted between animals or humans– Sporadic prion disease: random misfolding of a normal, healthy prion protein in an uninfected individual– Inherited prion disease: mutation in prion gene yields a protein that changes more often into the disease-causing form (misfolded protein)10.1 Mutations and Mutants• Mutation– Heritable change in DNA sequence that can lead to a change in phenotype (observableproperties of an organism); comes from a mistake in replication• Mutant– A strain of any cell or virus differing from parental strain in genotype (nucleotide sequence of genome)• Wild-type strain– Typically refers to strain isolated from nature• Selectable mutations– Those that give the mutant a growth advantage under certain conditionso Ex: Antibiotic resistance: an antibiotic-resistant mutant can grow in the presence of antibiotic concentrations that inhibit or kill the parent– Useful in genetic research• Nonselectable mutations– Those that usually have neither an advantage nor a disadvantage over the parento Ex: color loss in a pigmented organism– Detection of such mutations requires examining a large number of colonies and looking for differences (screening)• Screening is always more tedious than selection – Methods are available to facilitate screening• Example: replica plating– Replica plating is useful for identification of cells with a nutritional requirement for growth (auxotroph); allows you to isolate a mutant bacteria10.2 Molecular Basis of Mutation• Induced mutations– Those made environmentally or deliberately– Can result from exposure to natural radiation or oxygen radicals• Spontaneous mutations– Those that occur without external intervention• Point mutations – Mutations that change only one base pair– Can lead to single amino acid change in a protein, an incomplete protein, or no changeat all• Silent mutation– Does not affect amino acid sequence; codes for the same amino acid– Shows that the code is redundant• Missense mutation– Amino acid changed; polypeptide altered; faulty protein• Nonsense mutation– Codon becomes stop codon; polypeptide is incomplete/shortened• Deletions and insertions cause more dramatic changes in DNA• Frameshift mutations– Deletions/subtraction or insertions/addition that result in a shift in the reading frame– Often result in complete loss of gene function10.3 Mutation Rates• For most microorganisms, errors in DNA replication occur at a frequency of 10-6 to10-7 per kilobase• DNA viruses have error rates 100–1000x greater• The mutation rate in RNA genomes is 1000-fold higher than in DNA genomes– Some RNA polymerases have proofreading capabilities– However, RNA repair mechanisms similar to DNA repair mechanisms do not exist10.4 Mutagenesis• Mutagens: chemical, physical, or biological agents that increase mutation rates• Several classes of chemical mutagens exist:– Nucleotide base analogs: molecules that resemble nucleotide bases of DNA in structure but can lead to incorrect base pairing– Chemical mutagens that induce chemical modifications• For example, alkylating agents like nitrosoguanidine– Chemical mutagens that cause frameshift mutations• For example, intercalating agents like acridines: They become inserted betweentwo DNA base pairs and push them apart. During replication, this abnormal conformation can lead to single base insertions or deletions in acridine-containing DNA, resulting in a frameshift.• Several forms of radiation• Two main categories of mutagenic electromagnetic radiation:– Non-ionizing (i.e., UV radiation)• Purines and pyrimidines strongly absorb UV • Pyrimidine dimer is one effect of UV radiation– Ionizing (i.e., X-rays, cosmic rays, and gamma rays)• Ionize water and produce free radicals• Free radicals damage macromolecules in the cell; fragmentation of DNA• Three Types of DNA Repair Systems– Direct reversal: mutated base is still recognizable and can be repaired without referringto other strand– Repair of single strand damage: damaged DNA is removed and repaired using oppositestrand as template– Repair of double strand damage: a break in the DNA• Requires more error-prone repair mechanisms• When DNA damage is large scale, the cell may use a different type of repair system (i.e., damage interferes with DNA replication)– Mechanism called the SOS regulatory system• This system is more error prone• Allows replication to proceed and cell to replicate, but errors are more likely– Translesion synthesis allows DNA to be synthesized with no template; DNA lesions remain in the DNA, but are bypassed by specialized DNA polymerases that can move past DNA damage• Perfect fidelity in organisms is counterproductive because it prevents evolution• The mutation rate of an organism is subject