Lec14+notes

Lec14+notes - Translation, and the Translation Machinery....

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1 Translation, and the Translation Machinery. RNA plays several central roles Genetic code tells how nucleic acid sequence relates to protein sequence tRNAs and aminoacyl synthetases couple a sequence to an amino acid The ribosome provides the platform and activity for mRNA directed protein synthesis Controling initiation and termination of translation
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2 The translational apparatus is heavily dependent on folded RNAs for its activity: tRNA and the ribosome. * The ribosome is about 70% RNA by mass. * The ribosomal RNA is heavily involved in many different steps of translation. * tRNAs are a key intermediate in information transfer during translation and a key biochemical intermediate as well. * Translation is dependent on messenger RNA, copied from a gene.
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3 Informational aspects of translation. The genetic code is triplet, that is three bases specify an amino acid. This was suggested on informational grounds early on: a singlet code can specify 4 1 =4 amino acids a doublet code can specify 4 2 =16 amino acids a triplet code can specify 4 3 =64 amino acids. There are 20 amino acids, thus a triplet code is the smallest that will suffice. Nonoverlapping and not "punctuated": each base is part of only a single codon, and there are no bases that are not part of a codon.
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4 Cracking the code. Solving the genetic code is considered one of the most important contributions to 20th century biology. The code was cracked by a number of groups working with in vitro translation systems and purified ribosomes and tRNAs, as well as synthetic mRNAs . First observation: ribosomes given polyU as an mRNA can synthesize polyphenylalanine under in vitro conditions. Second observation, under certain conditions, the binding of tRNA is dependent on the presence of a messenger RNA bound to the ribosome. Using repeating polymers of different sequence, a restricted subset of tRNAs could bind, and or a repeating polypeptide was produced: Example GUGUG. ..codes for VCVCVCV. ... These types of experiments allowed the code to be deduced.
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5 What's the code? Sense: sense codons specify an amino acid. "Nonsense" or stop codons: Three codons (UGA, UAA, UAG) usually indicate that protein synthesis should stop, that is that no amino acid should be inserted. start codons, AUG or sometimes GUG codons in a special context indicate that translation should start. In these cases, methionine is always the first amino acid.
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6
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7 Degeneracy : Some amino acids are specified by more than one codon, in some cases there are six different codons specifying an amino acid. –The degeneracy is commonly found in the third position of the codon (Example: R is encoded by CGU, CGC, CGA, CGG, or CGN). –In addition, there are not enough tRNAs to read every codon. wobble hypothesis
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Lec14+notes - Translation, and the Translation Machinery....

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