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Biochem Brandl 5

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Biochemistry 2280A
Eric Ball

Biochem - Lecture 5 - Nov. 17th Translation: • mRNA to protein Genetic Code: • the genetic code “spells” out the amino acid sequence in 3 “letter” “words” called codons • 20 amino acids are used in protein synthesis in the cell • therefore we must have at least 20 codons • there are 64 different 3 base codons (AGUC = 4 letters.......4 x 4 x 4 = 64) Key Features of the Genetic Code: • UNIVERSAL - every organism uses almost the same code • code evolved once • non-overlapping • overlap would allow more coding in the genome, but it would [;ace signifi- cant restrictions on what amino acid residues could follow each other • no gaps - no codons in the middle that aren’t used (ie. no “skipping”) • redundancy • some codons specify the same amino acid • redundancy often occurs at the third position of the codon • generally amino acids found less frequently in proteins have fewer codons • ex. Met - AUG, Trp - UGG • one with a lot of codons are usually more prevalent • functionally related amino acids have similar codons • ex. Asp - GAC/GAU, Glu - GAG/GAA, Gln - CAG/CAA • Glu and Gln are very similar in structure (codons only differ at first letter) • advantage: increases the chance of a functional protein in the case of a single base mutation • 3 stop codons: UAA, UAG, UGA • 1 start codon: AUG Types of Mutations: • missense mutation: • single base change • one base swapped out for another • may have very little effect, or very large effect depending on the impor- tance of the base • frameshift mutation: • insertion or removal of 1, 2 or 3 bases • changes the encoded protein completely from the point of mutation • may end up with a stop codon which ends protein synthesis too quickly • most likely that it will not be deleterious when frameshift mutation is a multiple of 3 bases (results in the addition or deletion of entire amino acids leading to the first part and last part of the protein being un- changed) • nonsense mutation: • a single base change that results in a stop codon being created • results in premature termination of the protein • makes it a null-type allele tRNA (transfer): • bring the amino acid to the growing polypeptide chain • all roughly 80 bases in length • contain non
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