Translation and Genetic Code (Chapter 12) Protein Structure Amino Acids and Polypeptides Proteins made of polypeptides; every polypeptide encoded by a gene Polypeptide is a long chain of amino acids Amino acids have free amino group, free carboxyl group, and a side group R Highly varied side groups provide diversity of properties in proteins A peptide is a compound composed of two or more amino acids 20 amino acids amino acids joined by peptide bonds o carboxyl group covalently attached to amino group o goes from N to C terminus conventionally Levels of Protein Structure 1= amino acid sequence, which is specified by the nucleotide sequence of a gene shape determination 2=spatial interrelationships of the a.a in segments of the polypeptide 3=overall folding in 3D space maintained primarily by large number of weak noncovalent bonds only covalent bonds that play a significant role in protein conformation is S-S 4=association of two or more polypeptides in a multimeric protein e.g hemoglobin Protein Folding Most polypeptides fold spontaneously into specific conformations dictated by their primary structure If denatured most proteins will reform their original conformations when denaturing agent removed Key Points Most genes exert their effect on the phenotype of an organism through proteins The a.a. sequence of each polypeptide is specified by nucleotide sequence of each gene Vast functional diversity of proteins results from their complex 3D structures One Gene- One Co-Linear Polypeptide Co-linear-lying in the same straight line or linear sequence
The sequence of nucleotide pairs in a gene specifies a co-linear sequence of amino acids in its polypeptide product Tatum’s and Beadle’s Experiment with Neurospora One gene-one enzyme hypothesis, modified to one gene-one polypeptide concept Metabolic block is in the synthesis of one specific vitamin-pantothenic acid o growth only when pantothenic acid added Mutations in different triplets (nucleotide sequences) = different corresponding amino acids The sequences of nucleotide pairs in a gene and a.a in its polypeptide product are co- linear Protein Synthesis Genetic info in mRNA is translated into the a.a. sequences of polypeptides according to the specification of the genetic code Polypeptide synthesis involves mRNA, ribosomes, rRNA, transfer RNA, enzymes, and energy sources Translation occurs in ribosomes o Complex macromolecular structures located in the cytoplasm 3-5 rRNA + 50 polypeptides, 40-60 tRNA, 20 amino acid activating enzymes multiple small proteins are also involved in translation (aminoacyl-tRNA synthetases) multiple soluble proteins involved in initiation, elongation, and termination Ribosomes Approx. half protein and half RNA Composed of two subunits, one large, one small, dissociate when translation of an mRNA molecule is completed and reassociate during initiation of translation Each subunit contains large folded RNA molecule on which the ribosomal proteins assemble
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