09-15 16 centrl dogma transcrip translat no complete bw

09-15 16 centrl dogma transcrip translat no complete bw -...

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Central Dogma, Transcription, Translation
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Main Ideas Most genes code for proteins. In cells, information flows from DNA to RNA to proteins. DNA is transcribed by RNA polymerase to messenger RNA, and then messenger RNA is translated by ribosomes to proteins. Each amino acid in a protein is specified by a group of three bases in RNA.
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One Gene – One Enzyme Hypothesis Beadle and Tatum (1941) “…one ought to be able to discover what genes do by making them defective…” Various arg loss-of-function mutants were created using X- rays Experimental  organism:   Neurospora crassa bread mold
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Dissecting the Pathway to Arg Biosynthesis Isolate a series of mutants incapable of growing without arginine Precursor orn or cit orn or cit arginine What are the intermediates at each step??? >> Defective ornithine biosynthesis gene >> Defective citrulline biosynthesis gene >> Defective arginine biosynthesis gene
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One Gene – One Enzyme Hypothesis Each gene is responsible for encoding a different protein Most of these proteins function as enzymes
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How do Genes Work? 1) biochemical pathway with separate steps Precursor     intermediate     final product C chromosome 4 chromosome 7 P ‘precursorase’ ‘intermediase’
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How do Genes Work? 2) Different subunit-polypeptides of a multimeric enzyme C chromosome 4 chromosome 7 P Precursor     intermediate     final product Functional holoenzyme  ‘intermediase’ p c
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Genetic Code? Crick’s hypothesis: Sequence of nucleotide bases in DNA encoded instructions for enzyme (and other protein) sequences The ability to break the weak hydrogen cross- linking bonds, thus opening the double helix, would expose either strand to be “read” Problem with this hypothesis is spatial: DNA is found only in the nucleus; ribosomes (protein synthesis) are only found in the cytoplasm
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DNA Proteins? Messenger RNA Hypothesis Jacob and Manod RNA “messenger” must be involved Messenger RNA (mRNA) Fig. 15.4
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Experimental Evidence Fig. 15.5 Poly T DNA  “template” Poly A RNA  synthesized RNA synthesis is determined  by complementary base- pairing, template   product
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The Genetic Code Gamow hypothesis: each code contains three bases 64 combinations possible from {A,T/U, G, C} 3 Able to specify each of the 20 amino acids, with redundancy Nirenberg et al. experiments: Polynucleotide phosphorylase + UTP 5’-UUUU…-3’ 5’-UUUU…-3’ + in vitro translation system Phe-Phe- Phe… 5’-AAAA…-3’ + “ Lys-Lys-Lys… 5’-CCCC…-3’ + “ Pro-Pro-Pro… They later figured out the other codons based on probabilities
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09-15 16 centrl dogma transcrip translat no complete bw -...

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