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Bio1107k, Week 6 Notes

by: Bethany B.

Bio1107k, Week 6 Notes BIOL 1107K

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Bethany B.


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These notes cover the beginning of Chapter 3 and some material from other chapters. We are learning how the discovery of DNA and its properties came about.
Principles of Biology I w/Lab
Latanya Hammonds
Class Notes
Biology, DNA, Watson, and, Crick
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This 3 page Class Notes was uploaded by Bethany B. on Wednesday September 21, 2016. The Class Notes belongs to BIOL 1107K at Georgia Gwinnett College taught by Latanya Hammonds in Fall 2016. Since its upload, it has received 6 views. For similar materials see Principles of Biology I w/Lab in Biology at Georgia Gwinnett College.


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Date Created: 09/21/16
9.21.16 Biology1107KNotes[Week6] Chapter3,12, and14 Lecture Notes DNA & Replication  DNA Structures DNA RNA  History SUGAR Deoxyribose Ribose BASES A, T, C, G A, U, C, G  Replication 5’ END Monophosphate Triphosphate o Process SIZE Very large Smaller o Practical  Repair & Mutations STRANDS Double Single Review  Nucleotide includes nitrogenous base,sugar andphosphate  Ribonucleotide vs.deoxyribonucleotide (fig2.18)  Pyrimidine vs.purine bases (Fig2.19)  Base pairing (Fig 2.21)  Phosphodiester bond(Fig2.20) and sugar phosphate backbone Chromatin Structure  Genomic DNA is complexedwith proteins to form chromatin  The DNA is wrappedaroundhistone proteins o Histone proteins have manybasic amino acids (with positive charge (lysine andarginine)) o High levelof conservation in sequence of histone proteins,usedto labelDNA across species  Nucleosome o 150 bp of DNA wraps aroundan octomer of histoneproteins ((H2A,H2B,H3,and H4) x 4)  Chromatin with an average diameter of: o 30-nm fiber  Interactions between nucleosomescause the thin fiber to coilor foldinto this thicker fiber o 10-nm fiber (contains actively transcribedgenes)  DNA winds aroundhistones to form nucleosome“beads”  Nucleosomeare strungtogether like beads on astringby linker DNA o 300-nm fiber  The 30-nm fiber forms looped domainsthat attach to proteins o Metaphasechromosome  The loopeddomains coilfurther  The width of a chromatidis 700 nm DNA as the Genetic Material—A Battle BetweenDNA andProtein  Early in the 20 century,the identification of the molecule of inheritanceloomedas a major challenge to biologists  When T.H.Morgan’s groupshowedthat genesare locatedon chromosomes,the two components of chromosomes— DNA andprotein—became candidatesfor the geneticmaterial o Morgan workedwith fruit flies so he couldobserve the genes  The key factor in determiningthe genetic materialwas choosingappropriate experimentalorganisms  The role of DNA in heredity was first discoveredby studyingbacteriaandthe viruses that infect them  Biologists knew chromatinwas the moleculefor inheritance—but was it the DNA or the protein? 4 bases or 20 amino acids? Griffith’s Experiments:Transformation of Bacteria  The discoveryof the genetic roleof DNA began with research by FrederickGriffith in 1928  Griffith workedwith two strains of a bacterium,one pathogenic andone harmless  When he mixedheat-killedremains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic  He called this phenomenontransformation,now definedas a change in genotypeandphenotypedue to assimilation of foreign DNA Evidence that DNA can Transform Bacteria(Avery,McCarty, and MacLeod,1944)  In 1944, Oswald Avery,Maclyn McCarty,andColin MacLeodannouncedthat the transformingsubstancewas DNA usingGriffith’s experimentalprocedureas abasis.  They refinedthe workof their mentor,Griffith,by usingglycosidases,lipases,ribonucleases (RNAses), deoxyribonucleases (DNAses) andproteasesbeforeinjecting“debris” into mice  Their conclusion was basedon experimentalevidencethat only DNA workedin transformingharmless bacteriainto pathogenic bacteria  Many biologists remainedskeptical,mainly because little was known about DNA structure Evidence That ViralDNA Can Program Cells (Hershey andChase Experiments—1952)  More evidence for DNA as the genetic materialcame from studies of viruses that infect bacteria  Such viruses,called bacteriophages(or phages),are widely usedin molecular genetics research  In 1952, Alfred Hershey andMarthaChase performedexperiments showingthat DNA is the genetic materialof a phage known as T2  To determine the source of genetic materialin the phage,they designedan experiment showingthat only one of the two components of T2(DNA or protein) enters an E.colicellduringinfection  They concludedthat the injectedDNA of the phage provides the genetic information [The radioactive materialwas foundin the cellnot on the virus,so we know that it was in the nucleic acidnot the proteins.] 2 Chargaff’s Rules,1950 • It was known that DNA is a polymer of nucleotides,each consistingof anitrogenous base,asugar,and a phosphate group • In 1950, Erwin Chargaff reportedthat DNA composition varies from one species to the next • This evidence of diversity made DNA amore credible candidate for the genetic material • Chargaff’s rules state that in any species there is an equalnumber of A andT bases,and an equal number of G and C bases Buildinga StructuralModelof DNA • After most biologists became convincedthat DNA was the genetic material,the challenge was to determine how its structure accounts for its role • Maurice Wilkins andRosalindFranklin were usingatechniquecalledX-ray crystallography to study molecular structure • Franklin producedapicture of the DNA molecule usingthis technique • Franklin’s X-ray crystallographic images of DNA enabledWatson to deduce that DNA was helical • The X-ray images also enabledWatson to deduce the width of the helix andthe spacingof the nitrogenous bases • The width suggestedthat the DNA molecule was made upof two strands, forminga doublehelix • Watson andCrick built models of a double helix to conform to the X-rays andchemistry of DNA • Franklin hadconcludedthat there weretwo antiparallelsugar-phosphatebackbones,with the nitrogenous bases paired in the molecule’s interior • At first, Watson andCrick thought the bases pairedlike with like (A with A, and so on),but such pairings did not result in a uniform width • Instead,pairing a purine with a pyrimidine resultedin auniform width consistent with the X-ray • The Watson-Crick modelexplains Chargaff’s rules:in any organism the amount of A = T,and the amount of G = C • The relationshipbetween structureandfunctionis manifest in the double helix • Watson andCrick notedthat the specific base pairing suggesteda possible copyingmechanism for genetic material Shout-out to Rosalind Franklin: Helped/guided Watson and Crick in the discovery of the DNA double helix in the 1950’s. Being a woman, she did not get the credit she should have for her work with Watson and Crick. 3


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