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From Gene to Protein Chapter 17 Hon. Adv. Biology Notes 12/01/06 • Connection between Genes and Proteins – – – – Evidence that genes specify proteins Overview: “Central Dogma of Molecular Genetics” “The Code” – nucleotide triplets specify a.a.’s Evolution of the code • Synthesis and Processing of RNA – A closer look: Transcription – RNA processing occurs in eukaryotic cells • Synthesis of Protein – – – – A closer look: Translation Signal peptides – target specific cell destinations Review: Roles of RNA in the cell Review: Comparison of protein synthesis in prokaryotes and eukaryotes – Point mutations can affect protein structure and function – Wrap-up: “So, What is a gene?” Lecture Objectives • Compare the structure of RNA with that of DNA. • Summarize the process of transcription. • Relate the role of codons to the sequence of amino acids that results after translation. • Outline the major steps of translation. • Discuss the evolutionary significance of the genetic code. Figure 17.1 Beadle and Tatum’s evidence for the one gene-one enzyme hypothesis RNA vs. DNA • there are 3 major differences between the 2 forms of nucleic acids – Composition of sugar backbone (reflects name) • RNA - ribonucleic acid (ribose) • DNA - deoxyribonucleic acid (deoxyribose) – number of strands: RNA = 1 and DNA = 2 – RNA does not have thymine as a nitrogen base, instead uracil (U) is substituted in its place (back) Central Dogma of Molecular Genetics • Explains the flow of information from gene to protein. – Replication: flow of gene from one generation to the next – Transcription: flow of info from DNA to RNA – Translation: conversion of info from nucleic acid language into protein •Transfer of Information from DNA to RNA • Transcription: mRNA production, or the synthesis of other types of RNA • Short segment of DNA (gene) is transcribed into mRNA for movement out of the nucleus to the ribosome • DNA is used as a template to make a complementary piece of “messenger RNA”. This requires the aid of the RNA polymerase enzyme. Translation: Protein Synthesis • mRNA transcript arrives at the ribosome to act as the genetic code for protein assembly • proteins are composed of a unique sequence of amino acids. • mRNA carries the code for the order and type of amino acids to be included in the protein. The “genetic code” is the unique order of triplet codons located on the mRNA strand. (back) The Genetic Code: ThreeNucleotide “Words” • Genetic Code The genetic code (codons) used by most organisms to translate mRNA is nearly universal. Sometimes called a triplet code. Figure 17.3 The triplet code Figure 17.4 The dictionary of the genetic code What does a “Gene” really do? • It is simply a blueprint in the DNA that tells how to build a protein. • “WHO” we are genetically is really just a collection of “WHAT” proteins are expressed in our phenotype. • Figure 17.5 A tobacco plant expressing a firefly gene – (back) Evolution of the Genetic Code • must have evolved very early in the history of life…. Why? • Answer: nearly universal, shared by organisms from the simplest bacteria to complex plants and animals • Ex. CCG codes for the amino acid proline in every organism ever studied (back) The Synthesis/Processing of RNA • The Stages of Transcription – initiation – elongation – termination • RNA processing occurs in eukaryotes – – – – alteration of mRNA ends RNA splicing ribozymes introns (back) Protein Synthesis (Translation) • • • • structure and function of tRNA aminoacyl-tRNA synthetases ribosomes building a polypeptide – initiation – elongation – termination • signal peptides target protein destinations Point Mutations • types of point mutations – base-pair substitutions – insertions – deletions • Results of point mutations – – – – silent mutation missense mutations nonsense mutations frameshift mutations (back) Key Terms for Review!!