BIOL 1181st Edition Lecture 14 Outline of Last Lecture I DNA s Primary Structure II DNA Has Directionality III DNA s Secondary Structure IV DNA Strands Are Templates for DNA Synthesis V How Do the New DNA Strands Form VI A Comprehensive Model for DNA Synthesis VII Characteristics of DNA Polymerases VIII How Does Replication Get Started IX How is the Helix Opened Stabilized X How is the Leading Strand Synthesized XI Primase XII The Lagging Strand XIII How is the Lagging Strand Synthesized XIV The Discontinuous Replication Hypothesis XV The Discovery of Okazaki Fragments XVI DNA Synthesis Enzymes Are Well Organized XVII Replicating the Ends of Linear Chromosomes XVIII Repairing Mistakes DNA Damage These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute XIX If DNA is Damaged How is It Detected XX What if DNA Polymerase Misses an Error XXI Repairing Damaged DNA XXII Nucleoside Excision Repair System Outline of Current Lecture I Introduction II Gene Expression III What Do Genes Do IV The One Gene One Enzyme Hypothesis V The Central Dogma of Molecular Biology VI RNA The Intermediary Between Genes and Proteins VII The Central Dogma VIII The Roles of Transcription and Translation IX Exceptions to the Central Dogma X The Genetic Code XI How Long Is A Word in the Genetic Code XII What is a Codon XIII Cracking the Genetic Code XIV Important Properties of the Code XV Using the Code XVI What is the Molecular Basis of Mutation XVII Mutations Have Varying Effects on Organisms XVIII Chromosome Mutations Current Lecture Introduction The work of early geneticists illuminated the structure of DNA genes The method of inheritance including Mendel Watson Crick others Biologists still did not understand how gene expression occurred Gene Expression The process of translating the information in DNA into functioning molecules within the cell What Do Genes Do George Beadle Edward Tatum proposed that genes carry the instructions for making and maintaining an individual how o Damage a gene by creating a mutant o Then observe the resulting effect on the mutant s phenotype o Nonfunctioning alleles are now called knock out null or loss of function alleles The One Gene One Enzyme Hypothesis To test hypothesis Beadle Tatum damaged genes in the bread mold Neurospora crassa Observed that defects in particular genes resulted in the mold s inability to produce specific proteins The results of their experiments inspired their one gene one enzyme hypothesis Proposed that each gene contains the information needed to make an enzyme The Central Dogma of Molecular Biology Francis Crick proposed DNA is an information storage molecule Sequence of bases in DNA is a code different combinations of bases specify the 20 amino acids Gene a particular stretch of DNA that contains the information to specify the amino acid sequence of one protein o Information encoded in the base sequence of DNA is not directly translated into the amino acid sequence of proteins Central Dogma DNA converted to RNA transcription RNA in converted to protein amino acid sequence translation RNA The Intermediary Between Genes and Proteins Francois Jacob Jacques Monod proposed RNA molecules act as a link between genes found in the cell s nucleus and the protein manufacturing centers located in the cytoplasm Messenger RNA mRNA was found to carry information from DNA to the site of protein synthesis The enzyme RNA polymerase synthesizes RNA according to the information provided by the sequence of bases in a particular stretch of DNA The Central Dogma The central dogma summarizes the flow of information in cells DNA codes for RNA which codes for proteins DNA RNA Proteins The sequence of bases in a stretch of DNA specifies the sequence of bases in an RNA molecule which in turn specifies the sequence of amino acids in a protein Genes ultimately code for proteins The Roles of Transcription Translation DNA is transcribed to mRNA by RNA polymerase Transcription the process by which the hereditary information in DNA is copied to RNA mRNA is then translated to protein Translation the process where the order of the nucleotide bases is converted to the order of amino acids DNA info storage Transcription mRNA information carrier Translation Proteins active cell machinery According to central dogma an organism s genotype is determined by the sequence of bases in its DNA Phenotype is a product of the proteins it produces Alleles of the same gene differ in their DNA sequence Proteins produced by different alleles of the same gene frequently differ in their amino acid sequence Exceptions to the Central Dogma Many genes code for RNA molecules that do not function as mRNAs are not translated into proteins These other RNAs perform important functions in the cell Sometimes information flows in the opposite direction from RNA back to DNA Ex Some viral genes are composed of RNA use reverse transcriptase a viral polymerase to synthesize a DNA version of the virus s RNA genes The Genetic Code How does the sequence of bases in a strand of mRNA code for the sequence of amino acids in a protein The genetic code contains the rules that specify the relationship between a sequence of nucleotide bases in DNA or RNA The corresponding sequence of amino acids in a protein How Long Is a Word In the Genetic Code George Gamow predicted each word in the genetic code contains 3 bases There are 20 amino acids but only 4 RNA bases A 3 base code is the least that could specify enough amino acids could code for 64 amino acids 3 base code provides more than enough messages to code for all 20 amino acids 3 base code triplet code What is a codon The group of 3 bases specifies a particular amino acid Francis Crick Sydney Brenner found that the reading frame sequence of codons of a gene could be destroyed by mutation and then restored if the total number of deletion or additions were multiples of 3 Cracking the Genetic Code There is one start codon AUG signifies the starts of the protein encoding sequence in mRNA There are 3 stop codons UGA UAA UAG in the genetic code signal the end of the protein coding sequence Important Properties of the Code It is redundant o All amino acids except 2 are encoded by more than one codon It is ambiguous o One codon never codes for more than one amino acid It is nearly universal o All codons specify the same amino acids in all organisms with a few minor exceptions It is
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