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Lecture

Translation.doc

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Department
Biology
Course
BIO1140
Professor
Kathleen Gilmour
Semester
Winter

Description
Translation -overview: -tRNAs are small RNAs of highly distinctive structure that brings amino acids to the ribosome adaptor hypothesis -hypothesizes that there had to be something that allowed information recognized as nucleic acid to become information recognized as protein; this is found in the form of tRNA, which has an anti-codon and has an amino acid (known as the cognate tRNA) -ribosomes are rRNA-protein complexes that work as automated protein assembly machines -translation initiation brings the ribosomal subunits, an mRNA, and the first aminoacyl-tRNA together -ribosomes exist as large and small subunits and only assemble as a ribosome with the message --multiple ribosomes simultaneously translate a single mRNA; this is known as a polysome -polypeptide chains grow during the elongation stage of translation -occurs by the transfer peptide chain onto another amino acid -termination releases a completed polypeptide from the ribosome and the ribosome fall apart as their respective subunits -newly synthesized polypeptides are processed and folded into finished form -processing is associated with translocation of the protein to the correct spot -finished proteins contain sorting signals that direct them to cellular locations -base-pair mutations can affect protein structure and function -within the cell are multiple sites of protein synthesis (i.e.: cytoplasm, membrane, organelles, etc.) -the location and function are related 1) met-tRNA with GTP bound to it and the small ribosomal subunit form a complex -assembly of amino acids into polypeptides occurs on ribosomes -the complex has 3 sites: A, P, and E and are used for stepwise additions of amino acids to the polypeptide as directed by mRNA -the tRNA enters at the A site (acceptor site) -the tRNA exits at the E site (exit site) -the first tRNA enters at the P site (peptidyl site) in order to avoid having to come into A site and then having to shift to the P site -all reactions in translation (codon / anti-codon reactions; tRNA and site interactions) all involve additional protein factors -initiation factors (IF or eIF in eukaryotes; help stabilize at the A site) -elongation factors (EF or eEF in eukaryotes; help stabilize at the P site) -the enzyme that helps the reaction is known as peptidyl transferase -the energy for movement is supplied by GTP 2) the complex binds to the 5' cap of the mRNA and scans along it until it reaches the AUG start codon 3) the large ribosomal subunit binds and GTP is hydrolyzed, completing initation Translation and the Genetic Code -a 3 letter code with 4 bases allows 64 combinations which is more than enough to code for 20 amino acids and start / stop codons -DNA 3-letter codes are referred to as triplets -RNA 3-letter codes are referred to as codons -the codon in DNA is written the same as the codon in RNA except that T becomes U -e.g.: 5'GAC3' in DNA is 5'GAC3' in RNA -e.g.: 5'GTC3' in DNA is 5'GUC3' in RNA -if the codon is 5'GAC3', then the anticodon is 5'GUC3' to maintina H-bonding rules -by convention, 5'GAC3' is written as GAC -the start codon / initiator codon is the first amino acid recognized during translation -specified by the amino acid "methionine"; cleaved off from post-translational modifications -establishes the reading frame -sense codons -61 codons specify different amino acids -most amino acids are specific by several codons (known as degeneracy or redundancy) -as seen in the codon table -in recognition, the first 2 nucleotides in the codon (the last 2 in the anti-codon) are important and the 3rd nucleotide (1st position on the anti-codon) is of lesser importance and is known as the wobble position (it can be variable) -this led to the wobble hypothesis -e.g.: tRNA carrying phenylalanine matches codons UUU and UUC -e.g.: tRNA carrying leucine matches codons CUC and CUU -e.g.: tRNA carrying isoleucine matches codons AUU, AUC, and AUA -protein factors stabilize structures that don’t have the correct hydrogen bonding -helps explain some mutations -if the 3rd position is mutated, the amino acid is not changed -the principle of co-linearity breaks down at this level -e.g.: if lyseine is changed to asparagine, only the side chains differ but the chemical properties are similar and the mutated protein still has function -known as conservative mutations -stop codons / termination codons are found at the end of a polypeptide-encoding mRNA sequence -involves UAA, UAG, UGA -the main players in translation: tRNAs -tRNA contains anticodon sequences t
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