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lecture_23_notes - Lecture 23 The genetic code After...

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Lecture 23 The genetic code After discovery of DNA structure there was a race to discover how the 4-nucleotide code of DNA could specify the 20-amino acid code of proteins We now know the Genetic Code, illustrated on the side: The unit of the code are words of three letters, called codons. The first, second, and third letter of each codon are, respectively, on the left, top and right side of the square. The combinations of four letters (U, C, A, G) in three- letter words provides 64 possible words (4x4x4). Note that a single letter word, or a two-letter word would have been insufficient to specify the 20 amino acids. one-letter words: 4 possible two-letter words: 4x4=16 possible Discovery of the code: Nirenberg and Matthaei, 1961 Use a simple synthetic RNA as input for in vitro translation system UUUUUUU -> PhePhePhe, thus UUU = Phe AAAAAAA -> LysLysLys, thus AAA = Lys CCCCCCC -> ProProPro, thus CCC = Pro other combinations tried until the code was solved [NOT required: interesting review on genetic code discovery, http://genome.cshlp.org/content/ 17/4/401.full]
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