Chapter 17-OverviewThe information content of DNA is in the form of specific sequences of nucleotidesDNA inherited by an organism leads to specific traits by dictating the synthesis of proteinsProteins are links between genotypes and phenotypesPhenotype: Physical and physiological trait of an organism, determined by its genetic make up (Genotype)Gene expression: The process which DNA directs protein synthesis, includes 2 stages, transcription and translation-Concept 17.1: Genes specificity proteins via transcription and translationHow was the fundamental relationship between genes and proteins discovered?-Evidence from the study of metabolic defects1902 British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions.He thought symptoms of inherited reflect an inability to synthesize a certain enzymeHe reasoned that the condition alkaptonoria (causes urine to turn black in the presence of oxygen) is caused by an inability to produce an enzyme that metabolized alkapton.Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway.-Nutritional mutants in nuerosporaGeorge beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium since they became unable to synthesize certain moleculesUsing crosses they and their coworkers identified 3 classes of arginineDeficient mutants, each lacking a different enzyme necessary for synthesizing arginineThey devolved one gene – one enzyme hypothesis, which states that each gene dictates production of a specific enzyme-The products of Gene expressionSome proteins aren’t enzymes so researchers later revised the hypotheses to one-gene= one proteinMany proteins = many polypeptides, each with its own gene, so its now One gene=One polypeptide-Basic Principles of transcription and translationRNA is the bridge between genes and the proteins for which they codeTranscription: is the synthesis of RNA using information in DNATranscription: synthesis of a polypeptide, using the information stored in the mRNASpecific sequence of nucleotide bases in mRNA codes for synthesis of a specific sequence of amino acids in a particular polypeptide (primary structure)Ribosomes are the sites of translationRibosomes and RNA form a complexIn prokaryotes, translation of mRNA can begin before transcription has finishedmRNA produced by transcription is immediately translated withoutmore processing in prokaryotesIna eukaryotic cell, the nuclear envelope separates transcription from translationIn a eukaryotic cell, the nuclear envelope separates transcription from translationEukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNAA Primary transcript: The initial RNA transcript from any gene prior to processingCentral Dogma (Termed by Francis Crick in 1956) is the concept that cells are governed by a cellular chain of command (DNARNAProtein)Information stored in DNA is transcribed onto RNA. That information is then used by ribosomes to create a chain of amino acids (polypeptides)-Concept 17.2Transcription is the first stage of gene expression-Molecular components of transcriptionRNA synthesis is catalyzed by RNA polymerase which pried the DNA strands apart and hooks together the RNA nucleotidesRNA synthesis follows the same base pair rules except uracil replaces thymineDNA sequence where RNA polymerase attaches is called the promoter in bacteria the sequence signaling the end of transcription is called the terminator.The stretch of DNA that is transcribed is called transcription unit.-Synthesis of an RNA transcript3 Stages1) Initiation2) Elongation3) Termination-Step 1 Initiation; ProkaryotesOccurs in the nucleoid regionEnzyme RNA polymerase binds to the promoter region of the DNA (gene)Bind of RNA polymerase spread apart the 2 strands of DNANote: Bacteria only have a single type of RNA polymerase that works to produce mRNA, and other types of RNA (such as ribosomal RNA)-Step 2 Initiation: ProkaryotesRNA polymerase moves along one strand of DNA (that strand is called the template strand)mRNA assembles by adding nucleotides to the 3’ end of the elgating mRNA moleculeremember elongation of nucleic acids occur in the 5’ to 3’ directionBase sequence in the DNA strand determines the base sequence in the mRNA strand, follows base pairing rulesNucleoside Triphosphates are added initially, pyrophosphate are hydrolyzed to provide energy for bond formationStep 3: Termination of transcription; ProkaryotesThe RNA polymerase strops transcription when it reaches the end of the terminator regionWhen the RNA polymerase reaches the terminator:mRNA molecule is completeRNA polymerases dissociated from the DNA templates2 DNA strands are able to reform hydrogen bonds between nitrogenous bases.-Transcription in EukaryotesOccurs in the nucleusPromoters signal the transaction start point (promoters usually extend several dozen nucleotide pairs upstream from the start point)Promoter called TATA box is crucial in forming the initiation complex in eukaryotesTranscription factors mediate the bind of RNA polymerase 2 and the initiation of transcriptionCompleted assembly of transcription factors and RNA polymerase 2bound to a promoter is called a transcription initiation complex-Transcription FactorsProteins than enhance RNA polymerase binding to increase the rate of transcriptionCan control which genes are express by regulating RNA polymerase attachment to the promotersNote eukaryotes have at least 3 types of RNA polymerase. RNA polymerase2 is used for mRNA synthesis.Other RNA polymerases are used for transcribing RNA molecules that are not translated into proteins-Elongation of the RNA strandAs RNA polymerase moves along the DNA, it untwists the double helix.Transcription progresses at a rate of 40 nucleotides per second in eukaryotesA gene can be transcribed simultaneously by several RNA polymeraseNucleotide are added to the 3’ end of the growing RNA molecule-Termination of transcription in EukaryotesTermination of transcription occurs when RNA polymerase transcribes a DNA sequence called the polyadeneylation signal sequenceCodes for polyadeneylation signal
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