Lecture 33 Genes and how they work: transription.. Fig. 13-7, p. 286 RNA polymerase Eukaryotes Prokaryotes Fig , p. 294 Fig. 13-8, p. 287 Fig. 13-9, p. 287 Lecture 34 Genes and how they work: Processing of the primary RNA transcript.. Fig , p. 295 Lecture 35 Genes and how they work: The genetic code. Unraveling the Genetic Code : 1.Prediction: Only a 3-letter code can code for 20 amino acids using 4 different nucleotides (A,U,C, G). 1-letter code: 4 codons = 4 possible AA 2-letter code: 4 x 4 codons = 16 possible AA 3-letter code: 4 x 4 x 4 codons = 64 possible AA Francis Crick Frame-shift mutations: - A 3-letter code - No punctuations (Acridine orange- induced frame-shift mutations in bacteria) w/o punctuation (all nucleotides function in specification of amino acids): WHY DID THE RED CAT EAT THE FAT RAT Delete T: WHY DID HER EDC ATE ATT HEF ATR AT NONSENSE (reading frame shift) With punctuation: WHYzDIDzTHEzREDzCATzEATzTHEzFATzRAT Delete T WHYzDIDzHEzREDzCATzEATzTHEzFATzRAT NONSENSE READING FRAME W/O PUNCTUATION. Deletion of three bases restores sense: WHY DID THE RED CAT EAT THE FAT RAT Delete T, R and C: WHY DID HEE DAT EAT THE FAT RAT NONSENSE SENSE Adding a base negates the effect of a deletion: WHY DID THE REDA CAT EAT THE FAT RAT Delete T and add A: WHY DID HER EDA CAT EAT THE FAT RAT NONSENSE A THREE-LETTER CODE: Marshall Nirenberg tRNA ribosomes energy (ATP) Enzymes Poly-UracylPoly-Phenylalanine Conclusion: the codon GUU specifies valine. Many other codons were determined using this method. Nirenberg and Leder (1964) 2009) Lecture 36 Genes and how they work: Translation. tRNA charging by AA tRNA synthetase AA tRNA synthetase tRNA Anti-codon Acceptor stem tRNA Anti-codon AA tRNA synthetase A-siteP Meth tRNA Anti-codon mRNA AA 2 Meth AA 2 P-site tRNA A-siteP Anti-codon mRNA AA 3 Meth AA 3 Meth AA 2 P-site tRNA Meth AA 2 Meth AA 2 AA 3 Fig , p. 294 The End.