ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Fifteen Outline INTRODUCTION Proteins Are Tools of Heredity Genes Code for Particular Polypeptides and Proteins CELLS USE RNA TO MAKE PROTEIN Polypeptides Assembled on Ribosomes in Cytoplasm fig 15.1 Ribosomes composed of RNA and proteins RNA similar in structure to DNA fig 15.2 Cells Contain Three Classes of RNA Ribosomal RNA (rRNA) With proteins, make up the ribosomes Site of polypeptide synthesis Transfer RNA (tRNA) Transport amino acid molecules to ribosome Position amino acid along growing polypeptide chain Smaller in size than rRNA, 40 different kinds Messenger RNA (mRNA) Long strand of RNA copied from DNA Passes from nucleus to cytoplasm Conveys information from chromosomes to ribosomes AN OVERVIEW OF GENE EXPRESSION Basic Apparatus of Gene Expression Shared by All Organisms fig 15.3 Transcription Production of mRNA copy of DNA gene fig 15.4 Initiated by RNA polymerase enzyme Binds to promotor at beginning (5' end) of DNA strand mRNA complementary to DNA assembled Adenine and thymine pair Guanine and cytosine pair New RNA strand contains uracil not thymine At stop signal polymerase disengages, mRNA is released mRNA made is primary RNA transcript of DNA information Translation fig 15.5 Synthesis of polypeptide by ribosomes mRNA directs choice of amino acids Nucleotide sequence translated into amino acid sequence Initiated by rRNA molecule of ribosome that binds to mRNA "start" Ribosome moves along mRNA chain in three nucleotide groups Disengages at stop signal, polypeptide is released HOW GENES ENCODE INFORMATION Crick Determined Nature of Genetic Code Blocks of information corresponding to amino acids Group of nucleotides called a codon Postulated code was three nucleotides long Two nucleotide block would code for only 16 amino acids 20 known amino acids Three nucleotide block would code for 64 amino acids Questioned Whether Code Was Simple or Punctuated In simple code, each nucleotide is part of a codon Punctuated code has spacer nucleotide between codons Experimental process involved altering reading frame fig 15.6 Change in three nucleotides restored reading frame Change of less than three caused nonsense reading Concluded code was simple triplet code, not punctuated Determination of words of code Added artificial RNA to cell-free RNA and protein Poly-U resulted in synthesis of polyphenylalanine Concluded UUU coded for phenylalanine Repeated for all other triplets tbl 15.1 64 codons possible for only 20 amino acids Some amino acids coded by more than one codon THE GENETIC CODE Deciphering the Genetic Code Examine process of translation in prokaryotes Initial portion of mRNA binds to rRNA in ribosome fig 15.7 Single mRNA codon exposed at polypeptide-making site tRNA with complementary anticodon binds to mRNA fig 15.8 Anticodon three nucleotides long Each tRNA specific for an amino acid Amino acid added to growing string of polypeptides Activating enzymes specify amino acid to be added to tRNA fig 15.9 Binds amino acid to tRNA One aminoacyl-tRNA synthetase enzyme for each amino acid Recognizes nucleotide-sequence information Recognizes protein-sequence information Code word is three nucleotides long Each recognizes different identities and numbers of tRNA's Special, non-amino acid associated codons Nonsense codons are stop signals: UAA, UAG, UGA AUG is the start signal Deviations From the "Universal Genetic Code" Most of the code is similar Examples of differences in stop signals Mammalian mitochondria Chloroplasts Some single-celled ciliates THE MECHANISM OF PROTEIN SYNTHESIS In Prokaryotes Synthesis Begins with Initiation Complex fig 15.10 Met-tRNA binds to small ribosomal subunit Initiation factors position met-tRNA Positioning critical to reading frame of mRNA Initiation complex binds to mRNA mRNA beginning marked by sequence complementary to rRNA on ribosome Allows base pairs to form between mRNA and rRNA Bacteria and eukaryotes differ in number of genes per mRNA transcript Several genes in one bacterial transcript (polycistronic) One gene per eukaryotic transcript Synthesis of Polypeptide Proceeds fig 15.11 Ribosome exposes codon adjacent to initiating AUG Appropriate tRNA briefly binds to its exposed mRNA site tRNA positioned by elongation factors Amino acid on tRNA adjacent to initial methionine The two amino acids chemically react with one another Methionine released from its tRNA Attached by peptide bond to adjacent amino acid Translocation occurs fig 15.12 Ribosome moves along mRNA to next codon Ejects prevoius tRNA from site Repositions tRNA with growing polypeptide Exposes next codon for incoming tRNA Process continues repeatedly from step B.2. Process stops when chain terminating code reached fig 15.13 No tRNA binds to nonsense codons Recognized by special release factors PROTEIN SYNTHESIS IN EUKARYOTES Slight Differences Between Prokaryotes and Eukaryotes tbl 15.2 Primary Difference in Eukaryotic Protein Synthesis Eukaryotic genes much longer than necessary Stretches of nucleotides cut out of mRNA transcript fig 15.14 Stretches called introns not translated Do not correspond to any portion of a polypeptide Exons are remaining, polypeptide specifying portions Exons are shorter than and scattered among introns RNA splicing cuts introns out of primary transcript "Processed" mRNA then translated