412U2 - Gene Expression • Transcription produces a mRNA...

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Unformatted text preview: Gene Expression • Transcription produces a mRNA that has the ―same‖ base sequence as the corresponding gene(s). • Translation is the process in which the base sequence of the mRNA is used on the ribosome to synthesize a protein (polypeptide) with the corresponding AA sequence . Translation • Crick proposed that there is an ―adapter‖ molecule which: – carries the AA, and – H- bonds to mRNA to ―read‖ the codon for that AA. • > 20 different adaptors for the 20 AAs • The adaptor is transfer RNA (tRNA), which has: • a site for AA attachment (ribose-OHs), and • a site for recognizing codons (bases of anticodon loop base pair w/codon) • There is a different enzyme to link each AA to the correct tRNA for that AA (20 different amino acyl t-RNA synthetases) tRNAs ―read‖ the code contained in mRNA The Genetic Code • Francis Crick and Sydney Brenner deduced the triplet basis of the code from the properties of a series of insertion and deletion mutants. These mutations cause frame shifts: all subsequent codons are altered. • Consider a sentence (gene) consisting of the words (codons): TH E BIG R E D D O G ATE ALL THE PIE. • Insertion of one letter (base pair) yields: THE BI M GRE DDO GAT EAL LTH EPI E. • Deletion of a single letter (bp) from the original sentence yields: THE BIG RDD OGA TEA LLT HEP IE . • But deletion of the same letter from the insertion mutant yields: THE BI M GRD DOG ATE ALL THE PIE . The second mutation suppresses the first. The Genetic Code • They had identified a series of mutants FC0, FC1, FC2, FC3, FC4, FC5. They first had FC0, which behaved as a mutant and then found FC1, which suppressed FC0 (as in the double mutant above). • Then working with FC1, they found FC2, which suppressed FC1. But when combined with FC0, FC2 did not suppress FC0. • And so on, they found FC3 that suppressed FC2 and FC0, but not FC1. They reasoned that FC0, FC2, FC4, etc, were either insertion or deletion mutants and that FC1, FC3, FC5, etc were the other type. (ORIGINAL: TH E BIG R E D D O G ATE ALL THE PIE) • Combinations of three mutations of either type also were mutually suppressive. ( THB IGE DDG ATE ALL THE PIE , or THE BI M GRE Q DD O S G ATE ALL THE PIE) . The Genetic Code • In addition to discerning the triplet characteristic, they reasoned that • i. there are 4x4x4 = 64 ways to combine four bases in threes. With 64 possible codons to specify 20 AAs, there would be many AAs that had more than one codon. (Degeneracy) • ii. there is no ―punctuation‖ that demarcates each codon • iii. the codons do not over lap. This was also know from the fact that one common type of mutation involves a single base substitution resulting in a single AA substitution The Genetic Code • The code is degenerate- there is more than one codon for most of the AA's. Various code words for AAx are called synonyms • In most cases, synonyms have same bases in 1st and 2nd positions, different bases in 3rd position (Wobble, later.) The Genetic Code...
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This note was uploaded on 04/22/2011 for the course BIOCHEM 412 taught by Professor Thomas during the Spring '11 term at Tennessee Martin.

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412U2 - Gene Expression • Transcription produces a mRNA...

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