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Chapter 15

Chapter 15 Notes.docx

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Fiona Rawle

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Chapter 15 Notes How Genes WorkMendel discovered that alleles dont change when transmitted from parent to offspringBeadle and Tatum worked on Neurospora crassa Beadle said one ought to be able to discover what genes do by making them defective Idea was to knock out a gene by damaging it and then infer what the gene does by observing the phenotype of the mutant individualToday genes that dont function at all are called knockout null or lossoffunction alleles Beadle and Tatum exposed N crassa cells to radiation and examined The results inspired their onegene oneenzyme hypothesisProposed that the mutant N crassa individual could not make pyridoxine because it lacked an enzyme required to synthesize the compound and the lack of the enzyme was due to a genetic defect Concluded each gene contains the info needed to make an enzymeGenetic screen Any technique for picking certain types of mutants out of many thousands of randomly generated mutantsGenes dictate the structures of all the proteins produced by an organism not just the structures of enzymesMost genes contain the instructions for making proteinsIn many cases proteins are made of different polypeptides each of which is a product of a different gene Thus best called onegene onepolypeptide hypothesisCrick proposed that the sequence of bases in DNA might act as a code Idea was that DNA was only an information storage molecule Crick was proposing that different combinations of bases could specify the 20 amino acids However later it was apparent that info encoded in the base sequence of DNA is not translated into the amino acid sequence of proteins directly The messenger RNA hypothesis Jacob and Monod predicted that shortlived molecules of RNA mRNA carry info from DNA to the site of protein synthesis One important piece of evidence was discovery of enzyme RNA polymerase because it polymerizes ribonucleotides into strands of RNA RNA polymerase synthesizes RNA molecules according to the info provided by the sequence of bases in a particular stretch of DNA RNA polymerase does not require primer to begin connecting ribonucleotides together to produce strand of RNA like DNA polymerase does
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