BIO130 Section One Guide (6B)

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University of Toronto St. George
Kenneth Yip

From RNA to Protein An mRNA Sequence Is Decoded in Sets of Three Nucleotides - the conversion of RNA into protein represents a translation - translation into the amino acid sequence of a protein is done through the genetic code - RNA is a linear polymer of 4 different nucleotides, and since each codon (3 nucleotides) produces one amino acid; there are 444=64 possible combinations of three nucleotides o The difference between 20 amino acids, and a potential for 64 is due to the fact that the code is redundant and some amino acids are specified by more than one codon - In principle, an RNA sequence can be translated in any one of three reading frames, depending on where the decoding processbegins tRNA Molecules Match Amino Acids to Codons in mRNA - The translation of mRNA into protein depends on adaptor molecules that recognize and bind to both the codon and the amino acid o These adaptors are called tRNA (transfer RNAs); roughly 80 nucleotides in length - tRNA folds into a cloverleaf-like shape - Two regions of unpaired nucleotides situated at either end of the L-shaped molecule are crucial to the function of tRNA in protein synthesis - One of these regions forms the anticodon, a set of 3 consecutive nucleotides that pair with the complementary mRNA codon - The other region is the single-stranded 3 end where amino acids that match the codon attaches to the tRNA - Due to the redundancy of the genetic code, some amino acids have more than one tRNA and some tRNAs can base-pair with multiple codons rd - Certain tRNAs can tolerate a mismatch (wobble) at the 3 codon position; explains why somany alternative codons for an amino acid differ only at the 3 nucleotide tRNAs Are Covalently Modified Before They Exit the Nucleus - Eukaryotic tRNAs are synthesized by RNA polymerase III o Originally synthesized as larger precursor tRNAs, trimmed to produce mature tRNAs o Some tRNA precursors contain introns that must be spliced out
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