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Active Lecture PowerPoint® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino Chapter 13 Translation and Proteins Copyright © 2010 Pearson Education, Inc. Outline • Overview of translation • Components of translation machinery • The process of translation • Eukaryotic translation • Relationship between genes and proteins • Protein structure • Summary (animation) Gene Expression Step 1 Transcription Step 2 Translation mRNA Components of Translation Machinery • mRNA transcribed from DNA • Amino acids • tRNAs • Ribosomes Translation - Components Codon Ribosome • Location of protein synthesis in cells • Composed of RNA and proteins • Composed of a large and a small subunit –Prokaryotes 50S and 30S –Eukaryotes 60S and 40S tRNA (Clover leaf model) Charging tRNA by Aminoacyl tRNA Synthetase Translation of mRNA Can Be Divided into Three Steps • Initiation • Elongation • Termination Various Proteins Assist Translation Ribosome Binding Sites The ribosome has three binding sites for tRNA. A site: Acceptor site tRNA bringing amino acid binds P site: Peptidyl site tRNA carrying the peptide chain binds E site: Exit site Translation Initiation • Small subunit of the ribosome binds to the start codon of mRNA. Initiation factors help. • The first amino acid attached to an initiator tRNA, binds to mRNA • The first amino acid – Prokaryotes: Formyl-methionine (f-Met) – Eukaryotes: methionine • The large subunit of the ribosome binds Translation Initiation Translation Elongation • Ribosome advances one codon at a time • tRNA in A site is translocated to the P site so that the A site is available for the next charged tRNA to bind • A peptide bond is formed between amino acid in A site and the amino acid in the P site by peptidyl transferase, a ribozyme composed of RNA in the large subunit Translation Termination • Ribosome binds to the stop codon of mRNA. • Ribosome releases mRNA & the polypeptide chain synthesized • Release factors assist this • Ribosome subunits dissociate Translation is More Complex in Eukaryotes In eukaryotes: • the ribosomes are larger than in bacteria • transcription and translation are spatially and temporally separated • ribosomes scan for the initiator tRNA that is in the proper context, as identified by the Kozak sequence • translation generally requires more factors for initiation, elongation, and termination than translation in bacteria does Relationship between Genes & Proteins • George Beadle (1933) provided evidence that genes are directly responsible for enzymes. • Work by Beadle & Edward Tatum on Neurospora led to one gene:one enzyme hypothesis • Studies on human hemoglobin established one gene: one polypeptide hypothesis Experiments of Beadle & Tatum Biochemical Pathways A Enzyme 1 D C B Enzyme 2 Enzyme 3 Defective enzyme is a result of mutation in the gene Hypothesis: One Gene - One Enzyme Sickle-cell Anemia Sickle-cell Anemia • Patients suffer from lack of oxygen to tissue and severe damage • Inherited as a recessive Mendelian trait Affected individuals are homozygous for Hbs • Heterozygots (HbAHbS) are carriers but not largely affected • HbS hemogloblin has a different rate of electrophoretic migration than HbA hemoglobin Protein Structure (self study) Four levels of protein structure – Primary structure (amino acid chain) – Secondary structure (α-helices & β-sheets) – Tertiary structure (a single subunit) – Quarternary structure (group of subunits) Tertiary Level Protein Respiratory pigment myoglobin Quarternary Level Protein Hemoglobin Diversity in Protein Function • Proteins play diverse roles in the body • Examples: Hemoglobin, Collagen, Keratin, Actin, Myosin, Immunoglobulins, Transport proteins, some hormones and their receptors, histones, enzymes Important features of proteins • Some proteins may be posttranslationally modified • Protein function is directly related to the structure • Proteins consist of functional domains Genetics, Technology & Society p 297 Prions (protenaceous infectious agents) • Mad cow • Creutzfeldt –Jakob disease • Kuru disease • Scrapie in sheep • BSE in cattle