Chapter 17 Overview nucleotides synthesis of proteins The information content of DNA is in the form of specific sequences of DNA inherited by an organism leads to specific traits by dictating the Proteins are links between genotypes and phenotypes Phenotype 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 translation How was the fundamental relationship between genes and proteins discovered Evidence from the study of metabolic defects 1902 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 enzyme He 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 nuerospora George 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 molecules Using crosses they and their coworkers identified 3 classes of arginine synthesizing arginine Deficient mutants each lacking a different enzyme necessary for They devolved one gene one enzyme hypothesis which states that each gene dictates production of a specific enzyme The products of Gene expression Some proteins aren t enzymes so researchers later revised the hypotheses to one gene one protein One gene One polypeptide Many proteins many polypeptides each with its own gene so its now Basic Principles of transcription and translation RNA is the bridge between genes and the proteins for which they code Transcription is the synthesis of RNA using information in DNA Transcription synthesis of a polypeptide using the information stored in the mRNA finished translation from translation processing Specific 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 translation Ribosomes and RNA form a complex In prokaryotes translation of mRNA can begin before transcription has mRNA produced by transcription is immediately translated without more processing in prokaryotes Ina eukaryotic cell the nuclear envelope separates transcription from In a eukaryotic cell the nuclear envelope separates transcription Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA A Primary transcript The initial RNA transcript from any gene prior to Central Dogma Termed by Francis Crick in 1956 is the concept that cells are governed by a cellular chain of command DNA RNA Protein 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 2 Transcription is the first stage of gene expression Molecular components of transcription RNA synthesis is catalyzed by RNA polymerase which pried the DNA strands apart and hooks together the RNA nucleotides RNA synthesis follows the same base pair rules except uracil replaces thymine DNA 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 transcript 3 Stages 1 Initiation 2 Elongation 3 Termination Step 1 Initiation Prokaryotes Occurs in the nucleoid region Enzyme RNA polymerase binds to the promoter region of the DNA gene Bind of RNA polymerase spread apart the 2 strands of DNA Note 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 Prokaryotes RNA polymerase moves along one strand of DNA that strand is called the mRNA assembles by adding nucleotides to the 3 end of the elgating template strand mRNA molecule remember elongation of nucleic acids occur in the 5 to 3 direction Base sequence in the DNA strand determines the base sequence in the mRNA strand follows base pairing rules Nucleoside Triphosphates are added initially pyrophosphate are hydrolyzed to provide energy for bond formation Step 3 Termination of transcription Prokaryotes The RNA polymerase strops transcription when it reaches the end of the terminator region When the RNA polymerase reaches the terminator mRNA molecule is complete RNA polymerases dissociated from the DNA templates 2 DNA strands are able to reform hydrogen bonds between nitrogenous bases Transcription in Eukaryotes Occurs in the nucleus Promoters 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 eukaryotes the initiation of transcription Transcription factors mediate the bind of RNA polymerase 2 and Completed assembly of transcription factors and RNA polymerase 2 bound to a promoter is called a transcription initiation complex Transcription Factors transcription attachment to the promoters 2 is used for mRNA synthesis Proteins than enhance RNA polymerase binding to increase the rate of Can control which genes are express by regulating RNA polymerase Note eukaryotes have at least 3 types of RNA polymerase RNA polymerase Other RNA polymerases are used for transcribing RNA molecules that are not translated into proteins Elongation of the RNA strand As RNA polymerase moves along the DNA it untwists the double helix Transcription progresses at a rate of 40 nucleotides per second in eukaryotes A gene can be transcribed simultaneously by several RNA polymerase Nucleotide are added to the 3 end of the growing RNA molecule Termination of transcription in Eukaryotes Termination of transcription occurs when RNA polymerase transcribes a DNA sequence called the polyadeneylation signal sequence Codes for polyadeneylation signal AAUAAA 10 35 nucleotides downstream the AAUAA signal proteins associated with the growing RNA transcript cut it free from the RNA polymerase Transcription can processed for 100 nucleotides further but an enzyme moves
View Full Document
Unlocking...