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Unformatted text preview: -1-LECTURE 1220 September 2010 (P. J. Hollenbeck)BIOL231The Genetic Code and Protein TranslationRead: Chap. 7, pp. 246-258Probs: 31-34 + Exam III, '05, #1I. Information flow (A) You already know the “central dogma” of molecular genetics, that DNA => RNA =>protein, with limited exceptions (one is the “reverse transcription” of retroviral RNA intoDNA). And you have seen diagrams like this one:DNA:3' - GTG GTA CAA AAG TAC TGC GAA - 5'9(Transcription)9RNA:5' - CAC CAU GUU UUC AUG ACG CUU - 3'9(Translation) 9protein: N - his his val phe met thr leu – CToday let’s figure out in more detail what this means...(B) Transcription vs translation.“Transcription” and “translation” mean, in biology, prettymuch what they mean in the English language. Transcription is the conversion of informationfrom one form to another (i.e., written to spoken, as a court reporter does) without changinglanguages. Translation is the conversion of information between two different languages. Inour cells, transcription copies information from DNA to RNA, keeping it in the language ofnucleic acids. Translation copies information from RNA to protein, converting the informationto a totally different molecular language. How?II. The code for translating RNA into protein(A) Codons & degeneracy (1) There are just 4 bases in DNA and RNA, but 20 common amino acids. This allowedGeorge Gamow, among others, to predict (long before the code was cracked) that eachamino acid in a protein must be specified by a set of 3 bases. That is because 4 = 162(too few), while 4 = 64 (enough, in fact, more than necessary). The “more than3necessary” part means that each amino acid can in principle be encoded by more thanone set of 3 bases. This kind of code – in which one set of elements does NOT mapuniquely into another set – is called a DEGENERATE code. With 64 base tripletsavailable to encode just 20 amino acids, we say that the genetic code is highlydegenerate. This has important consequences for how changes (mutations) in the DNAfind expression in proteins as well as for how mRNA is translated into protein. find expression in proteins as well as for how mRNA is translated into protein....
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