Bài giảng Biochemistry 2/e - Chapter 29: DNA: Genetic Information, Recombination, and Mutation

CHAPTER 29DNA: Genetic Information, Recombination, and Mutation to accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline29.1 DNA Carries Genetic Information29.2 Genetic Information in Bacteria29.3 Molecular Mechanism of Recombination29.4 The Immunoglobulin Genes29.5 Molecular Nature of Mutation29.6 RNA as Genetic Material29.7 Transgenic AnimalsThe Transforming Principle is DNAGriffith in 1928 injected bacteria into miceCombination of heat-killed type S and non-virulent type R killed the miceShowed that type S had been transformed by the type R!In 1944, Oswald Avery showed that the active transforming agent was DNA!Further ProofIn 1952, Hershey and Chase, studying bacteriophages, labelled DNA with 32P and protein with 35SBacteriophage progeny produced by infection of bacteria contained 32P (thus DNA from the original phage), but not 35S (from the protein)!Genetic Information Can Be Transferred Between BacteriaIn 1946, Lederberg and Tatum showed that two different strains of bacteria with different growth requirements could exchange genesLederberg and Tatum surmised that the bacterial cells must interact with each other - the process is now known as sexual conjugationChromosome Mappingby Interrupted MatingPassage of the ‘F factor’ from Hfr cells to F- cells also brings adjacent genes Genes from the Hfr chromosome are transferred in a fixed orderThis order can be mapped by ‘interrupted mating’ methodsThe genetic map obtained reveals a circular arrangement of genesMechanism of RecombinationGeneral recombination: any pair of homologous DNA segments as substratesIn 1964, Robin Holliday proposed a model involving single-stranded nicks at homologous sitesDuplex unwinding, strand invasion and ligation create a Holliday junctionEnzymology of RecombinationRecBCD initiates recombination in E.coliRecA forms nucleoprotein filament for strand invasion and homologous pairingRuvA, RuvB, RuvC drive branch migration and process the Holliday junction into recombination productsEukaryotic systems are probably similarThe RecA Protein38 kD enzyme that catalyzes ATP-dependent DNA strand exchange, leading to formation of Holliday junctionRecA forms a helical filament with a groove to accommodate DNARecA:ssDNA complex binds dsDNA at secondary site and searches for regions homologous with the bound ssDNA, then forms the desired duplexResolving Holliday JunctionsRuv proteins resolve the junction into recombination productsRuvA and RuvB act as a helicase that dissociates the RecA filament and catalyzes branch migrationRuvC is an endonuclease that binds at the junction and cuts pairs of DNA strands of similar polarity. Splice and patch recombinants result.TransposonsIn 1950, Barbara McClintock showed that activator genes in corn could move freely about the genome.This was at first viewed as heresy and McClintock was ostracizedMolecular biologists in the late 1970s rediscovered what McClintock knewShe received a MacArthur Award in 1981 and a Nobel in 1983Immunoglobulin GenesImmunoglobulin genes maximize protein diversity with a limited amount of genetic informationDNA rearrangement (genetic recombination) during assembly of IG genes accounts for this diversityThree separate genes are combined to form the L-chain geneFour genes combine to form H-geneMolecular Nature of MutationPoint mutations - one base for another - arise from mispairing, insertion of analogs or chemical mutagensMutagens include nitrous acid, hydroxylamine and alkylating agentsInsertions and deletions result in frameshift mutationsPrionsProteins as Genetic Agents?Prions are ‘protein infectious particles’Agents composed only of proteinResponsible for kuru, Creutzfeld-Jacob disease, mad-cow disease, etc.The ‘infection’ appears to involve a change of secondary structure and conformation (or conformations!) in the prion proteinA Nobel for Stanley Prusiner in 1997Transgenic AnimalsGenes can be introduced into animals by transfection - injection of plasmid DNA into recipient cellsPlasmids can be injected into fertilized eggs in miceExpression is usually variable, because the gene is inserted randomlyGrowth hormone transfection produces mice that are very large!