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DNA Structure & Replication By: Tina Yao
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DNA Structure & Replication

DNA Structure & ReplicationBy: Tina YaoWhat is DNA?RECALL:

Deoxyribonucleic Acid or DNA is responsible for the transfer of genetic information from one generation to the next

DNA structureThe chemical composition of DNA1860s: MiescherDNA is slight acidicContains high percentages of phosphorus & nitrogen

1920sDNA is made of deoxyribose sugar, phosphate group, and nitrogenous base

1949: Erwin Chargaff# of Adenine = # of Thymine# of Guanine = # of CytosineStructure of nucleotidesSimple building blocks of DNAEach block contains:A nitrogenous baseA phosphate groupA deoxyribose sugar

Nitrogenous basesThere are 4 types:Adenine (A)Thymine (T)Cytosine (C)Guanine (G)A and G are called purinesdouble-ringed structuresT and C are called pyrimidinessingle-ringed structures,

Deoxyribose sugarAttached to phosphate and nitrogenous base

The carbons are numbered from 1 to 51 connects to a nitrogenous base3 contains a hydroxyl (OH) group5 connects to a phosphate group

Glycosyl bondSugar and nitrogenous base

Phosphodiester bondBetween sugar and phosphate

The double helixDiscovered by James Watson & Francis Crick with help of Rosalind Franklins photo 51

Physical description of a DNA molecule:Diameter: 2 nanometres (nm)Length of one turn: 3.4 nm10 nitrogenous base pairings

Antiparallel

Clockwise directionPhosphate groupDeoxyribose sugarHydrogen bondsNitrogenous baseGlycosyl bondPhosphodiester bond

Complementary base pairingA purine is always bonded to a pyrimidine via Hydrogen bondingAdenine (A) with Thymine (T) via 2 H-bondsCytosine (C) with Guanine (G) via 3 H-bonds

Significant to DNAs storage and transfer of genetic informationAllows whole sequence to be known with only one strand

9DNA Replication & repairDNA replicationThe DNA of the parent cell and its daughters must be identical

Due to complementary base pairing, identical DNA molecules can easily be built from single strands of DNA

DNA replicates SEMICONSERVATIVELYSemiconservative vs conservativeSemiconservative replicationTwo identical copies are made from:1 parental DNA strandAnd 1 newly synthesized strand

Conservative replicationTwo identical copies are made, with:1 copy being the parent or original molecule And the second copy being a new copy of the parent molecule

http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0073402354/930240/Bidirectional_DNA_Replication.swf::Bidirectional%20DNA%20Replication

12Process of DNA replicationSeparating DNA strands

- Building the complementary strands

Separating the DNA strandsA protein binds to the REPLICATION ORIGINOnly one for prokaryotic DNAMultiple for eukaryotic DNA

DNA gyraseBacterial enzyme that minimize tension during replication

DNA helicaseBreaks H-bonds to unravel or unzip parental DNASeparating the DNA strands Cont.Single-stranded binding proteins (SSBs)SSBs are proteins that binds to exposed single-stranded DNAPrevent the formation of H-bonds between bases to keep the single strands of DNA apart

Replication happens in two directions from the replication originDNA too large to be fully unravelledSeparating the DNA strands cont.Replication forkPoint where 2 DNA strands are still connectedReplication happens in opposite directions for each strandReplication bubbleArea where 2 replication forks are close to each other, creating a bubble

Building the Complementary strandsDNA polymeraseEnzyme that replicates DNA

DNA polymerase III is responsible for synthesizing complementary strands

DNA polymerase III only synthesizes in 5 to 3 directionBuilding the Complementary strands cont.Leading strandSynthesizes continuously towards replication forkDNA polymerase III adds deoxyribonucleoside triphosphates to 3 endHowever, it cannot synthesize without a Ribonucleic acid (RNA) primerRNA PRIMASE builds the RNA primersLeading strand only need the primers ONCEDNA polymerase breaks 2 extra phosphates from the deoxyribonucleoside triphosphatePhosphates are recycledThe energy from breaking the bond drives the condensation reaction that adds complementary nucleotidesBuilding the Complementary strands cont.Lagging strandRecall DNA strands are antiparallelSynthesizes discontinuously, away from the replication forkRNA primers are needed and continuously added throughout replicationOkazaki fragmentsDNA polymerase III synthesizes new strand in short fragments as primers are addedThese are called Okazaki fragmentsReplication Diagram

Final touch-upsDNA polymerase IGoes through two daughter DNA strandsRemoves RNA primers from both leading and lagging strandsReplaces them with appropriate complementary nitrogenous basesDNA ligaseCreates phosphodiester bonds in gaps between Okazaki fragmentsConnects and joins them into a complete strandThe two daughter DNAs twist naturally into a double helical shapehttp://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf

21DNA repairExonucleaseAny enzyme that can repair misplaced nitrogenous basesDNA polymerase I & III are examplesProofreads after synthesization is completeThey cut out nucleotides that are wrongly paired on a DNA strand, replaces them and continue to add new nucleotidesFinal Reviewhttps://www.youtube.com/watch?v=8kK2zwjRV0MWork CitedGiuseppe, M., Vavitsas, A., Ritter, B., Fraser, D., Arora, A., & Lisser, B. (2003). Nelson biology 12. Toronto: Nelson Thomson Canada Limited.