Exam 3 - RNA Translation \u25cf The template DNA strand that is transcribed is complementary and antiparallel to the mRNA codons \u25cf The non-template DNA

Exam 3 - RNA Translation u25cf The template DNA strand...

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RNA Translation The template DNA strand that is transcribed is complementary and antiparallel to the mRNA codons The non-template DNA strand has the same sequence as the mRNA and is called the “coding strand” U instead of T The genetic code specifies which amino acids will be used to build a protein Codon A sequence of three bases, something like a three-letter “word” Each codon specifies a particular amino acid Genetic code is not ambiguous but it is redundant Each codon specifies only one amino acid 61 codons code for 20 amino acids Translation = mRNA → Protein Transfer RNA (tRNA) links mRNA codons with specific amino acids There is at least one specific tRNA molecule for each of the 20 amino acids Each tRNA has three function It binds to a specific enzyme that attaches it to only one amino acid It is then charged Binds to mRNA at a triplet called the anticodon which is complementary to an mRNA codon Interactions with ribosomes (noncovalent) Wobble Specificity for the base at the 3’ end of the codon is not always observed tRNA may have modified nucleotides (such as Inosine) Example Codons for alanine (GCA, GCC, and GCU) are recognized by the same tRNA Allows cells to produce fewer tRNA species (about 30) Genetic code is still not ambiguous tRNA are charged by aminoacyl-tRNA synthetases Specific for one amino acid and its corresponding tRNA Charging requires ATP Translation occurs at a ribosome Holds mRNA and charged tRNAs in the correct position to allow assembly of the polypeptide Eukaryotes Large subunit Three different molecules rRNA and 49 different proteins A (aminoacyl tRNA) site binds with anticodon of charged tRNA P (peptidyl tRNA) site where tRNA adds its amino acid to the growing chain E (exit) site where tRNA sits before being released from the ribosome Small subunit One rRNA and 33 proteins Steps of Translation Initiation An initiation complex forms A charged tRNA and small ribosomal subunit both bound to mRNA Small subunit Small subunit binds to mrna In prokaryotes: Shine-Dalgamo sequence In eukaryotes: the 5’ cap Small subunit moves along mRNA until start codon (5’-AUG-3’) A tRNA charged with methionine (anticodon 3’-UAC-5’) binds to complete the initiation complex First amino acid in a polypeptide is always methionine which may be removed after translation Large subunit Joins the complex
The charged tRNA is now in the P site The A site is aligned with the second mRNA codon Initiation factors are responsible for assembly of the initiation complex Elongation Another charged tRNA enters A site and the large subunit catalyzes two reactions Bond between tRNA in P site and its amino acid is broken Peptide bond forms between the amino acid and the amino acid on tRNA in the A site

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