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Unformatted text preview: -1-Four different depictions of a tRNA molecule: a flattened “cloverleaf” cartoonstructure that shows the base pairs and different portions clearly; two views of amodel of the actual molecule (90Efrom each other); and a linear diagram of thenucleotide sequence.LECTURE 1322 September 2010 (P. J. Hollenbeck)BIOL231Protein Translation: Ribosomes Read: pp. 246-258; Study DVD: 7.6, 7.7, 7.8 (v important), 7.10Probs: 35, 36; Exam III'05, #2; exam III'06, #3, #4 I. transfer RNA [DVD 7.6](A) adaptor between mRNA and protein(1) Because of what we know about the structure of nucleic acids, we can see thattranscription (like DNA replication) proceeds because of the direct interaction betweencomplementary bases. But to translate an mRNA into protein, there is no simplestructural interaction between amino acids and nucleic acids that can map a DNAsequence accurately into an amino acid sequence. Instead, the cell has an adaptormolecule, transfer RNA, that interacts with mRNA and brings amino acids to theprocess of translation.(2) tRNA molecules turn out to have a conserved structure, represented below as athree-leaf clover shape. There are four “stems,” in which the tRNA chain folds back onitself in antiparallel fashion and forms intramolecular base pairs over short stretches. Atthe end of 3 of the stems there is a “loop” in which 7-8 bases form an unpaired, single-stranded circle. After the tRNA is transcribed, some of its bases are modified intoforms not typically found in DNA or mRNA. The 3' end of a tRNA is covalentlycoupled to the appropriate amino acid by an enzyme called aminoacyl-tRNA synthase, aprocess called “charging” the tRNA molecule. The “anticodon” – the triplet of basesthat interacts with mRNA – is in one of the loops (see below).-2-(B) Anticodons, wobble, and mutation (1) The triplet sequence on a tRNA that interacts with a codon on an mRNA is calledan anticodon. Recall, when nucleic acid chains interact, they are always antiparallel,so that an mRNA codon, running 5'=>3' pairs up with an anticodon, running 3'=>5'.(2) Although there are 61 codons that encode amino acids, there are only about 50tRNA species in most eucaryotic cells. That means that some tRNA molecules mustrecognize MORE THAN ONE CODON! How does that work – how can translationoccur accurately when some anticodons don’t need to match the codon perfectly? occur accurately when some anticodons don’t need to match the codon perfectly?...
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