Chapter 3 - Gene Expression

Chapter 3 - Gene Expression - Gene expression Enzymes and...

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Gene expression Enzymes and genetics - George Beadle and Edward Tatum, 1941 o Studied the fungus Neurospora crassa o Used x-rays to damage the DNA in cells - Isolate cells w/mutant alleles of arg genes o Arg genes used in biochemical synthesis of the aminio acid arginine o Mutants identified by failure to make Arg Media contains Arg = growth, no Arg = no growth Biochemical pathways (figure 15.1) - Correlate with biochemical pathway o Biochemical pathways involve multiple steps o Multiple enzymes used in a specific order o Identify each arg mutant with media containing intermediates Genes encode enzymes - Beadle and Tatum proposed that each enzyme of the arginine pathway was encoded by a separate gene - They proposed the one gene – one enzyme hypothesis - Today we know this as the one gene – one polypeptide hypothesis (we know that one gene is made up of multiple polypeptides) The central dogma (figure 15.2) - The central dogma of molecular biology states that information flows in one direction:
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- DNA -> RNA -> protein - Transcription is the flow of information from DNA to RNA - Translation is the flow of information from RNA to protein Gene expression - Accessing and using information encoded on a particular stretch of DNA (often for proteins) - Timing and location tightly controlled - Leads to conversion of genotype to phenotype Overview - DNA stores information copied into RNA using RNA polymerase (RNA synthesis enzyme) - RNA used to direct synthesis of protein - ncRNA (new-coding) regulates expression, ribosome structure - Reverse transcription (DNA from RNA) Genetic code - Order of string and nucleotides is information needed to make protein - Nucleotides are read in groups of three How many bases needed? - 20 amino acids - 4 bases - Mathematically o 1 base = 1 amino acid = 4 total o 2 bases = 1 amino acid = 16 total
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o 3 bases = 1 amino acid = 64 possible How to identify code? - First made synthetic homopolymer: Poly U (uracil) UUU o Translate w/cell extract o Makes Phe, UUU = Phe - Later used triplet binding assay o Synthesize specific triplets o Test with tRNA/amino acid combinations Nature of code - Universal: virtually same in all organisms on planet - 3 bases = 1 codon = 1 amino acid - Code is redundant (64 codons for 20 on, 61 used) o More than 1 codon for 1 amino acid o Redundancy usually in 3 rd position - 1 start codon (AUG; Met) - 3 stop (UGA, UAG, UAA) Terminology - Codon: set of 3 nucleotides for one amino acid - Reading frame: series of nucleotides read in sets of 3 - Coding strand: strand of DNA that codes for a particular protein - Template strand: strand of DNA used to make RNA Transcription
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- Transfer of information from DNA -> RNA - Includes: o Initiation: RNA polymerase identifies where to begin transcription; attaches to DNA o Elongation: RNA nucleotides are added to 3’ end of new RNA
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This note was uploaded on 10/24/2011 for the course BIOL 111 taught by Professor Dr.zielinski during the Spring '08 term at Purdue University-West Lafayette.

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Chapter 3 - Gene Expression - Gene expression Enzymes and...

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