Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsPowerPoint Lectures forBiology, Seventh EditionNeil Campbell and Jane ReeceLectures by Chris RomeroChapter 17From Gene to ProteinCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsOverview: The Flow of Genetic Information• The information content of DNA is in the form ofspecific sequences of nucleotides• The DNA inherited by an organism leads tospecific traits by dictating the synthesis of proteins• Gene expression, the process by which DNAdirects protein synthesis, includes two stages:transcription and translation• The ribosome is part of the cellular machinery fortranslation, polypeptide synthesisCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 17.1: Genes specify proteins viatranscription and translation• How was the fundamental relationship betweengenes and proteins discovered?Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsEvidence from the Study of Metabolic Defects• In 1909, British physician Archibald Garrod firstsuggested that genes dictate phenotypes throughenzymes that catalyze specific chemical reactions• He thought symptoms of an inherited diseasereflect an inability to synthesize a certain enzyme• Linking genes to enzymes required understandingthat cells synthesize and degrade molecules in aseries of steps, a metabolic pathwayCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsNutritional Mutants in Neurospora: ScientificInquiry• Beadle and Tatum exposed bread mold to X-rays,creating mutants that were unable to survive onminimal medium as a result of inability tosynthesize certain molecules• Using crosses, they identified three classes ofarginine-deficient mutants, each lacking a differentenzyme necessary for synthesizing arginine• They developed a “one gene–one enzyme”hypothesis, which states that each gene dictatesproduction of a specific enzymeLE 17-2Class IMutants(mutationIn gene A)Wild typeClass IIMutants(mutationIn gene B)Class IIIMutants(mutationIn gene C)PrecursorOrnithineAEnzymeACitrullineArginineGene AGene BGene CPrecursor Precursor PrecursorAAB B BOrnithine Ornithine OrnithineEnzymeBEnzymeCArginine Arginine ArginineC C CCitrulline Citrulline CitrullineMinimalMedium(MM)(control)Wild typeClass IMutantsClass IIMutantsClass IIIMutantsMM +OrnithineMM +CitrullineMM +arginine(control)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Products of Gene Expression: A DevelopingStory• Some proteins aren’t enzymes, so researcherslater revised the hypothesis: one gene–one protein• Many proteins are composed of severalpolypeptides, each of which has its own gene• Therefore, Beadle and Tatum’s hypothesis is nowrestated as one gene–one polypeptideCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsBasic Principles of Transcription and Translation• Transcription is the synthesis of RNA under thedirection of DNA• Transcription produces messenger RNA (mRNA)• Translation is the synthesis of a polypeptide,which occurs under the direction of mRNA• Ribosomes are the sites of translationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• In prokaryotes, mRNA produced by transcription isimmediately translated without more processing• In a eukaryotic cell, the nuclear envelopeseparates transcription from translation• Eukaryotic RNA transcripts are modified throughRNA processing to yield finished mRNA• Cells are governed by a cellular chain ofcommand: DNA → RNA → proteinLE 17-3-1TRANSCRIPTIONDNAProkaryotic cellLE 17-3-2TRANSCRIPTIONDNAProkaryotic cellRibosomePolypeptidemRNAProkaryotic cellLE 17-3-3TRANSCRIPTIONTRANSLATIONDNAmRNARibosomePolypeptideDNAProkaryotic cellNuclearenvelopeTRANSCRIPTIONEukaryotic cellLE 17-3-4TRANSCRIPTIONTRANSLATIONDNAmRNARibosomePolypeptideDNAPre-mRNAProkaryotic cellNuclearenvelopemRNATRANSCRIPTIONRNA PROCESSINGEukaryotic cellLE 17-3-5TRANSCRIPTIONTRANSLATIONDNAmRNARibosomePolypeptideDNAPre-mRNAProkaryotic cellNuclearenvelopemRNATRANSLATIONTRANSCRIPTIONRNA PROCESSINGRibosomePolypeptideEukaryotic cellCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Genetic Code• How are the instructions for assembling aminoacids into proteins encoded into DNA?• There are 20 amino acids, but there are only fournucleotide bases in DNA• So how many bases correspond to an amino acid?Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCodons: Triplets of Bases• The flow of information from gene to protein isbased on a triplet code: a series ofnonoverlapping, three-nucleotide words• These triplets are the smallest units of uniformlength that can code for all the amino acids• Example: AGT at a particular position on a DNAstrand results in the placement of the amino acidserine at the corresponding position of thepolypeptide to be producedCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• During transcription, a DNA strand called thetemplate strand provides a template for orderingthe sequence of nucleotides in an RNA transcript• During translation, the mRNA base triplets, calledcodons, are read in the 5′ to 3′ direction• Each codon specifies the amino acid to be placedat the corresponding position along a polypeptideLE 17-4DNAmoleculeGene 1Gene 2Gene 3DNA strand(template)3′TRANSCRIPTIONCodonmRNATRANSLATIONProteinAmino acid3′5′5′Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCracking the Code• All 64 codons were deciphered by the mid-1960s• The genetic code is redundant but not ambiguous;no codon specifies more than one amino acid• Codons must be read in the correct reading frame(correct groupings) in order for the specifiedpolypeptide to be producedLE 17-5Second mRNA baseFirst mRNA base (5′ end)Third mRNA base (3′ end)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsEvolution of the Genetic Code• The genetic code is nearly universal, shared bythe simplest bacteria to the most complex animals• Genes can be transcribed and translated afterbeing transplanted from one species to anotherCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 17.2: