University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2020
Views:

296

Pages:

8
Author:

Deandre Simmons

Chapter 19 Protein Synthesis Overview n n n Protein Synthesis – genetic info encoded in nucleic acids translated into standard amino acids Genetic code – dictionary defining meaning for base sequence Codon – tri-nucleotide sequence for amino acid Protein Synthesis §Requires ribosomes, mRNA, tRNA, and protein factors §Formation of aminoacyl-tRNA §Aminoacyl-tRNA synthetases – amino acid activation §Formation of polypeptide chain §Chain initation – binding of 1st aminoacyl-tRNA at start site §Chain elongation – formation of peptide bond §Chain termination – release of protein Section 19.1: The Genetic Code §Translation – conversion of nucleic acid sequence to a amino acid sequence §Genetic code is the dictionary that specifies a meaning for each base sequence (codon) §tRNA - adaptor molecules must mediate translation process §Three base sequence with four different bases (43 = 64) can code for 20 amino acids §Codons - Nirenberg, Matthaei, and Khorana show that it was a triplet code §64 possible trinucleotide sequences, 61 code for amino acids; four codons serve as punctuation §UAA, UGA, and UAG - stop codons; AUG codes for methionine - start codon Section 19.1: The Genetic Code Properties of Genetic Code: §Triplet: continuous sequence of three bases (a codon) specifies one amino acid §Non-overlapping: bases are NOT shared between consecutive codons §Commaless: no intervening bases between codons §Degenerate: more than one triplet can code for the same amino acid §Leu, Ser, and Arg - each coded for by six triplets §Universal: same in viruses, prokaryotes, and eukaryotes Section 19.1: The Genetic Code Section 19.1: The Genetic Code Figure 19.1 Codon-Anticodon Base Pairing of Cysteinyl-tRNAcys §Codon-Anticodon Interactions §tRNA carries specific amino acid and possesses an anticodon §Codon-anticodon pairing is antiparallel §1st base anticodon (5’ end) pairs with 3rd base mRNA codon (3’end) §Wobble hypothesis: 1. First two base pairings confer most of the specificity 2. Interaction of the third codon position and anticodon nucleotide is less stringent §G in the wobble position can interact with C or U §U in the wobble position can interact with A or G §Nontraditional bases can be found (e.g., inosinate) §I can interact with U, A, or C 3. Minimum 32 tRNAs translate all 61 codons. Section 19.1: The Genetic Code Wobble Base Pairing Alternatives Section 19.1: The Genetic Code Figure 19.3 Formation of Aminoacyl-tRNA Aminoacyl-tRNA Synthetase § Increases accuracy of translation (1 error/104 aa) § § 1 aminoacyl-tRNA for each amino acid Specific for both amino acid & tRNA, requires Mg2+ Reaction: 1. Activation - synthetase catalyzes formation of aminoacyl-AMP (activates amino acid) § ATP provides energy for bond formation 2. tRNA linkage - specific tRNA bound in the active site becomes covalently bound to the aminoacyl site § § Class I – ester linkage between amino acid & 2’-OH on ribose ClassII – ester linkage between amino caid &3’-OH on ribose